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Handbook of Timing Belts and Pulleys

 

SECTION 1 INTRODUCTION

Timing belts are parts of synchronous drives which represent an important category of drives. Characteristically, these drives employ the positive engagement of two sets of meshing teeth. Hence, they do not slip and there is no relative motion between the two elements in mesh.

Due to this feature, different parts of the drive will maintain a constant speed ratio or even apermanent relative position. This is extremely important in applications such as automatic machinery in which a definite motion sequence and/or indexing is involved.

The positive nature of these drives makes them capable of transmitting large torques and withstanding large accelerations.

Belt drives are particularly useful in applications where layout flexibility is important. They enable the designer to place components in more advantageous locations at larger distances without paying a price penalty. Motors, which are usually the largest heat source, can be placed away from the rest of the mechanism. Achieving this with a gear train would represent an expensive solution.

Timing belts are basically flat belts with a series of evenly spaced teeth on the inside circumference, thereby combining the advantages of the flat belt with the positive grip features of chains and gears.

There is no slippage or creep as with plain flat belts. Required belt tension is low, therefore producing very small bearing loads. Synchronous belts will not stretch and do not require lubrication. Speed is transmitted uniformly because there is no chordal rise and fall of the pitch line as in the case of roller chains.

The tooth profile of most commonly known synchronous belts is of trapezoidal shape with sides being straight lines which generate an involute, similar to that of a spur gear tooth. As a result, the profile of the pulley teeth is involute. Unlike the spur gear, however, the outside diameter of a timing pulley is smaller than its pitch diameter, thus creating an imaginary pitch diameter which is larger than the pulley itself. This is illustrated in Figure 1. Backlash between pulley and belt teeth is negligible.

Fig. 1 Pulley and Belt Geometry

The trapezoidal shape timing belt was superseded by a curvilinear tooth profile which exhibited some desirable and superior qualities. Advantages of this type of drive are as follows:

  • Proportionally deeper tooth; hence tooth jumping or loss of relative position is less probable.
  • Lighter construction, with correspondingly smaller centrifugal loss.
  • Smaller unit pressure on the tooth since area of contact is larger.

NOTE:   Credit for portions of this technical section are given to: Gates Rubber Co., Sales Engineering Dept., Rubber Manufacturers Association (RMA), International Organization for Standardization (ISO).

Fig. 2 Stress Pattern in Belts

  • Greater shear strength due to larger tooth cross section.
  • Lower cost since a narrower belt will handle larger load.
  • Energy efficient, particularly if replacing a "V" belt drive which incurs energy losses due to slippage.
  • Installation tension is small, therefore, light bearing loads.

In Figure 2, the photoelastic pattern shows the stress distribution within teeth of different geometry. There is a definite stress concentration near the root of the trapezoidal belt tooth, with very low strains elsewhere. For the curvilinear tooth, there is a uniform, nearly constant, strain distribution across the belt. The load is largest in the direction of the tension member to which it is transferred. Because of their superior load carrying capabilities, the curvilinear belts are marketed under the name of Gates' HTD drives. This is an abbreviation of High Torque Drives. As a result of continuous research, a newer version of the curvilinear technology was developedby Gates, which was designated as Gates' PowerGrip® GT®2 belt drives.

SECTION 2 GATES POWERGRIP® GT®2 BELT DRIVES

The PowerGrip GT2 Belt Drive System is an advance in product design over the Gates' older, standard HTD system. The PowerGrip GT2 System, featuring a modified curvilinear belt tooth profile, provides timing and indexing accuracy superior to the conventional PowerGrip Trapezoidal Belt System. Plus, PowerGrip GT2 Belts have a higher capacity and longer belt life than trapezoidal belts. It's difficult to make a true quantitative comparison between the backlash of a trapezoidal tooth drive and PowerGrip GT2 drive due to the difference in "pulley to belt tooth" fit (see Figure 3). Trapezoidal belts contact the pulley in the root radius-upper flank area only, while the PowerGrip GT2 system permits full flank contact.

The main stress line in a trapezoidal tooth timing belt is at the base of the teeth. During operation, this stress greatly reduces belt life. The PowerGrip GT2 system overcomes this condition with its complete tooth flank contact which eliminates the tooth stress line area. This greatly increases belt life and prevents tooth distortion caused by drive torque. In addition, the conventional timing belt has a chordal effect as it wraps small pulleys. This is significantly reduced in the PowerGrip GT2 system because there is full tooth support along the pulley. Full support improves meshing, reduces vibration and minimizes tooth deformation.

On drives using a low installation tension, small pulleys, and light loads, the backlash of thePowerGrip GT2 system will be slightly better than the trapezoidal timing belt system. However, with increased tension and/or loads and/or pulley sizes, the performance of the PowerGrip GT2 system becomes significantly better than the trapezoidal timing belt system.

The PowerGrip GT2 system is an extension of the HTD system with greater load-carrying capacity. HTD was developed for high torque drive applications, but is not acceptable for most precision indexing or registration applications. The HTD design requires substantial belt tooth to pulley groove clearance (backlash) to perform.

As smaller diameter pulleys are used, the clearance required to operate properly is increased. HTD drive clearance, using small diameter pulleys, is approximately four times greater than an equivalent GT2 timing belt drive.

The PowerGrip GT2 system's deep tooth design increases the contact area which provides improved resistance to ratcheting. The modified curvilinear teeth enter and exit the pulley grooves cleanly, resulting in reduced vibration. This tooth profile design results in parallel contact with the groove and eliminates stress concentrations and tooth deformation under load. The PowerGrip GT2 design improves registration characteristics and maintains high torque carrying capability.

PowerGrip GT2 belts are currently available in 2 mm, 3 mm, 5 mm, 8 mm and 14 mm pitches. Specific advantages of the PowerGrip GT2 system can be summarized as follows:

  • Longer belt life: The strong fiberglass tensile cords wrapped in a durable neoprene body provide the flexibility needed for increased service life. The deep tooth profile provides superior load-carrying strength and greatly reduces ratcheting when used with pulleys provided by a licensed supplier.
  • Precision registration: PowerGrip GT2 belts provide timing and synchronization accuracy that make for flawless registration, with no loss of torque carrying capacity.
  • Increased load-carrying capacity: Load capacities far exceed HTD and trapezoidal belt capabilities making PowerGrip GT2 belts the choice for accurate registration, heavy loads and small pulleys.
  • Quieter operation: The PowerGrip GT2 belt's specially engineered teeth mesh cleanly with pulley grooves to reduce noise and vibration. Clean meshing and reduced belt width result in significant noise reduction when compared to Trapezoidal and HTD belts.
  • Precise positioning: PowerGrip GT2 belts are specifically designed for applications where precision is critical, suchas computer printers and plotters, laboratory equipment and machine tools.

Some of the many applications of PowerGrip GT2 belts are:

  • data storage equipment
  • machine tools
  • hand power tools
  • postage handling equipment
  • DC stepper/servo applications
  • food processors
  • centrifuges
  • printers
  • floor care equipment
  • money handling equipment
  • medical diagnostic equipment
  • sewing machines
  • automated teller machines
  • ticket dispensers
  • plotters
  • copiers
  • robotics equipment
  • vending equipment
  • vacuum cleaners
  • office equipment

SECTION 3 COMPARISON GRAPHS

In order to provide comparison of performances of different pitch drives, several graphs have been developed. Figure 4 shows numerical values, plotted in logarithmic scale, of Rated Horsepower vs. Speed (rpm) of faster shaft.

Figure 5 shows an illustrative graph representation of horsepower ratings over a wide speed range of the belt types commonly used. The graph assumes that belt widths and pulley diameters have been chosen such that they provide realistic comparison of product capability.

Figure 6 provides a comparison of the rated torque carrying capabilities of synchronous belts,on small diameter pulleys at low speeds. The pulley diameters and belt widths represent a realistic comparison.

SECTION 4 DRIVE COMPARATIVE STUDIES

The development of the PowerGrip GT2 belt has produced an impressive range of enhanced properties and subsequent design opportunities for engineers. Comparative studies, shown in Figures 7 through 10, allow designers to make quantitative assessments and to highlight the most significant improvements and design opportunities. Particularly significant points from the comparative studies follow:

4.1 Durability

The greatly increased durability of the PowerGrip GT2 design has resulted in power capacities far above those quoted for similar size belts of previous designs. The resulting small drive packages will increase design flexibility, space utilization and cost effectiveness.

4.2 Tooth Jump Resistance

The very significant improvement in tooth jump resistance of PowerGrip GT2 when compared to similar belts has several important advantages.

1.    Ratcheting resistance during high start-up torques.

2.    Reduced bearing loads, particularly in fixed-center drives. Lower average tensions can be used without encountering tooth jump at the low tension end of the tolerance ranges.

3.    Reduced system losses result from lower pre-tensioning, with less potential for tooth jumping.

4.3 Noise

The smoother meshing action of the PowerGrip GT2 belt, with its optimized design, produces significantly lower noise levels when compared with other similar sized belt types operating under similar speeds and tensions. These improvements are enhanced by the fact that narrower belts can be used due to increased power capacities.

4.4 Positioning Accuracy

The PowerGrip HTD belt tooth forms were primarily designed to transmit high torque loads. This requirement increased tooth to groove clearances which resulted in increased backlash when compared with the original trapezoidal designs. PowerGrip GT2 has reversed this problem with power capacities now exceeding those of PowerGrip HTD while giving equivalent or higher levels of positional accuracy than trapezoidal timing belts.

SECTION 5 DIFFERENT BELT CONFIGURATIONS

5.1 Double-Sided Twin Power Belt Drives

Timing belts are also available in double-sided designs, which offer an infinite number of new design possibilities on computer equipment, business machines, office equipment, textile machines and similar light-duty applications. Belts with driving teeth on both sides make it possible to change the direction of rotation of one or more synchronized pulleys with only one belt.  The inside and outside teeth are identical as to size and pitch and operate on standard pitch diameter pulleys. If the belts have nylon facing on both sides, then the same design parameters can be used for the drives on both sides of the belt. In case the outside teeth do not have nylon facing, the horsepower rating of the outside teeth is only 45% of the total load. For example: assuming the drive pulley and belt are capable of transmitting 1 horsepower, 0.55 hp can be transmitted from the inside teeth of the pulley (A), and 0.45hp can be transmitted by the outside teeth to pulley (B) for a total of 1 hp, the rated capacity of the driver pulley.

5.2  Long Length Timing Belt Stock

These belts are an excellent solution for drives that require belt lengths longer than those produced inconventional endless form. Long length belting has the same basic construction as conventional timing belts. These belts are usually produced by spiral cut of large diameter endless belts. These belts are creatively used in:

  • reciprocating carriage drives
  • rack and pinion drives
  • large plotters

An example of application is shown in Figure 13. A complete timing belt and a timing belt segment reduce vibration and chatter in this oscillating drive for a surface grinder.

SECTION 6 BELT CONSTRUCTION

The load-carrying elements of the belts are the tension members built into the belts (see Figure 14). These tension members can be made of:

  1. Spirally wound steel wire.
  2. Wound glass fibers.
  3. Polyester cords.
  4. Kevlar.

The tension members are embedded in neoprene or polyurethane.  The neoprene teeth are protected by a nylon fabric facing which makes them wear resistant.

The contributions of the construction members of these belts are as follows:

  1. Tensile Member – Provides high strength, excellent flex life and high resistance to elongation.
  2. Neoprene Backing – Strong neoprene bonded to the tensile member for protection against grime, oil and moisture. It also protects from frictional wear if idlers are used on the back of the belt.
  3. Neoprene Teeth – Shear-resistant neoprene compound is molded integrally with the neoprene backing. They are precisely formed and accurately spaced to assure smooth meshing with the pulley grooves.
  4. Nylon Facing – Tough nylon fabric with a low coefficient of friction covers the wearing surfaces of the belt. It protects the tooth surfaces and provides a durable wearing surface for long service.

6.1  Characteristics Of Reinforcing Fibers

Polyester

Tensile Strength: 160,000 lbs/in2

Elongation at break: 14.0%

Modulus (approx.): 2,000,000 lbs/in2

One of the main advantages of polyester cord over higher tensile cords is the lower modulus of polyester, enabling the belt to rotate smoothly over small diameter pulleys. Also, the elastic properties of the material enable it to absorb shock and dampen vibration. In more and more equipment, stepping motors are being used. Polyester belts have provenfar superior to fiberglass or Kevlar reinforced belts in these applications. High-speed applications with small pulleys are best served by polyester belts under low load.

Kevlar

Tensile Strength: 400,000 lbs/in2

Elongation at break: 2.5%

Modulus: 18,000,000 lbs/in2

High tensile strength and low elongation make this material very suitable for timing belt applications. Kevlar has excellent shock resistance and high load capacity.

Fiberglass

Tensile Strength: 350,000 lbs/in2

Elongation at break: 2.5 – 3.5%

Modulus: 10,000,000 lbs/in2

The most important advantages are:

1.    High strength.

2.    Low elongation or stretch.

3.    Excellent dimensional stability.

4.    Excellent chemical resistance.

5.    Absence of creep, 100% elongation recovery.

Disadvantages:

1.    High modulus (difficult to bend).

2.    Brittleness of glass. Improper handling or installation can cause permanent damage.

3.    Poor shock resistance. No shock absorbing quality when used in timing belts.

Steel

Tensile Strength: 360,000 lbs/in2

Elongation at break: 2.5%

Modulus (approx.): 15,000,000 lbs/in2

Additional characteristics of tension members and their effect on the drive design are shown intabulated form in Table 1.

6.2  Cord Twist And Its Effect On The Drive

There is a specific reason for not applying the yarn directly in the form of untwisted filaments around the mold. If the filament would be applied continuously, the top and bottom of the belt body would be prevented from being properly joined, and separation could result. See Figure 15.

Two strands each composed of several filaments are twisted around each other, thus forming a cord which is subsequently wound in a helical spiral around the mold creating a space between subsequent layers, which corresponds to the step of the helix. The two strands, however, can be twisted two ways in order to create an "S" or a "Z" twist construction. See Figure 16.

The "S" twist is obtained if we visualize the two strands being held stationary with our left hand on one end, while a clockwise rotation is imparted by our right hand to the two strands, thus creating a twisted cord. The "Z" twist is obtained similarly, if a counterclockwise rotation is imparted to the two strands. Different types of cord twist will cause side thrust in opposite directions. The "S" twist will cause a lateral force direction which will obey the "Right-Hand" rule as shown in Figure 17.

A "Z" type cord twist will produce a direction of lateral force opposite to that of "S" cord. Therefore, in order to produce a belt with minimum lateral force, standard belts are usually made with "S" and "Z" twist construction, in which alternate cords composed of strands twisted in oppositedirections are wound in the belt. This is illustrated in Figure 18. The lay of the cord is standard, as shown in Figure 18, and it is wound from left to right with the cord being fed under the mold. The smaller the mold diameter and the fewer the strands of cord per inch, the greater the helix angle will be, and the greater the tendency of the lay of the cord to make the belt move to one side. In general, a standard belt of "S" and "Z" construction, as shown in Figure 18, will have a slight tendency to behave as a predominantly "S" twist belt, and will obey the "Right-Hand" rule accordingly.

6.3  Factors Contributing To Side Travel

The pulleys in a flat belt drive are crowned to keep the belt running true. Since crowned pulleys are not suitable for a timing belt, the belt will always track to one side. Factors contributing to this condition include:

I. In the Drive

1.    Misalignment – A belt (any belt – any construction) will normally climb to the highend (or tight) side.

2.    Tensioning – In general, lateral travel can be altered or modified by changing tension.

3.    Location of plane – Vertical drives have a greater tendency to move laterally due to gravity.

4.    Belt width greater than O.D. of pulley – This condition creates an abnormal degree of lateral travel.

5.    Belt length – The greater the ratio of length/width of the belt, the less the tendencyto move laterally.

II.    In the Belt

1.    Direction of the lay of the cords in the belt. See Figure 18.

2.    Twist of the strands in the cord. See Figure 16.

6.4  Characteristics Of Belt Body Materials

Basic characteristics of the three most often used materials are shown in Table  2. The tabulated characteristics give rise to the following assessment of these materials:

Natural Rubber

•  High resilience, excellent compression set, good molding properties

•  High coefficient of friction; does not yield good ground finish

•  High tear strength, low crack growth

•  Can withstand low temperatures

•  Poor oil and solvent resistance;  unusable for ketones and alcohol

•  Ozone attacks rubber, but retardants can be added

Neoprene

•  High resilience

•  Flame resistant

•  Aging good with some natural ozone resistance

•  Oil and solvent resistance fair

Polyurethane

•  Excellent wear resistance, poor compression set

•  Low coefficient of friction

•  Oil and ozone resistance good

•  Low-temperature flexibility good

•  Not suitable for high temperatures

SECTION 7 BELT TOOTH PROFILES

There are several belt tooth profiles (Figure 19, Table 3) which are the result of different patented features, marketing and production considerations.

For the sake of completeness, the three additional belt profiles shown in Figure 19j, 19k and19l are used in Europe and are sometimes found on machinery imported from Europe and Japan. They are not produced in the U.S.A. and are not covered by RMA standards. The belts are made of polyurethane, and steel is usually used as the tension member.

As described in previous sections, the presently known most advantageous belt tooth configuration is the Gates PowerGrip GT2. This is a result of continuous improvement of the previous HTD tooth profile. The HTD profile is protected by U.S. Patent Number 4,337,056, whereas the GT2 profile is described in U.S. Patent Number 4,515,577.

Pulleys for these belt profiles are usually available from manufacturers licensed by Gates Rubber Company. Stock Drive Products is one of the companies who can supply a full range of these pulleys as standards or specials, per customers' drawings.

SECTION 8 PULLEY PITCH AND OUTSIDE DIAMETERS

Pulley and belt geometry as indicated in Figure 1 shows reference to a Pitch Circle, which is larger than the pulley itself. Its size is determined by the relationship:

where P is the belt tooth spacing (pitch) and N is the number of teeth on the pulley. The reinforcing cord centerline will coincide with the pulley pitch diameter while the belt is in contact with the pulley. At the same time, the outside diameter of the pulley will be in contact with the bottom of the belt tooth. Hence, the distance “U” between the reinforcing cord centerline and the bottom of the belt tooth will determine the outside diameters of pulleys for different pitches. See Table 4.

Due to the particular geometry of the 8 mm HTD belts, some corrections are needed for small-size pulleys only. Hence, consult pulley specifications tables given later in this text, pertaining to the 8 mm HTD pulley.

As previously noted, the pitch and the number of teeth will determine the pitch diameter of the pulley, whereas its outside diameter will depend on the “U” dimension (distance from tooth bottom to centerline of cord) as shown in Table 4.

The outside diameter, O.D., is then given by:

O.D. = pd – 2U       

In order to provide fast reference, the following tables show pitch and outside diameters of different pitch pulleys:

Table 5:   0.080" MXL Pitch

Table 6:   0.0816" (40 D.P.) Pitch

Table 7:   1/5" – XL Pitch

Table 8:   3/8" – L Pitch

Table 9:   3 mm Pitch HTD

Table 10:   5 mm Pitch HTD

Table 11:   8 mm Pitch HTD

Table 12:   2 mm Pitch GT2

Table 13:   3 mm Pitch GT2

Table 14:   5 mm Pitch GT2

Table 15:   T2.5 (2.5 mm Pitch)*

Table 16:   T5 (5 mm Pitch)*

Table 17:   T10 (10 mm Pitch)*

These tables enable the designer to judge immediately the space requirements for a particular drive. In many instances, the torque transmission capability of the drive can be satisfied by a less voluminous solution. This is one of the excellent features of the GT2 profile; it facilitates miniaturization. The size of the small pulley of the drive, however, is subject to some limitations. The suggested minimum size of the pulley related to a particular pitch and rpm is given in Table 18.

*NOTE:  T2.5, T5 and T10 series have O.D.s and Pitch Diameters which do not conform to equations (8-1) and (8-2).

SECTION 9 DESIGN AND INSTALLATION SUGGESTIONS

There are some general guidelines which are applicable to all timing belts, including miniature and double-sided belts:

1.    Drives should always be designed with ample reserve horsepower capacity. Use of overload service factors is important. Belts should be rated at only 1/15th of their respective ultimate strength.

2.    For MXL pitch belts, the smallest recommended pulley will have 10 teeth. For other pitches, Table 18, on the previous page, should be used.

3.    The pulley diameter should never be smaller than the width of the belt.

4.    Belts with Fibrex-glass fiber tension members should not be subjected to sharp bends or rough handling, since this could cause breakage of the fibers.

5.    In order to deliver the rated horsepower, a belt must have six or more teeth in mesh with the grooves of the smaller pulley. The number of teeth in mesh may be obtained by formula given in SECTION 24 TIMING BELT DRIVE SELECTION PROCEDURE. The shear strength of a single tooth is only a fraction of the belt break strength.

6.    Because of a slight side thrust of synchronous belts in motion, at least one pulley in the drive must be flanged. When the center distance between the shafts is 8 or more times the diameter of the smaller pulley, or when the drive is operating on vertical shafts, both pulleys should be flanged.

7.    Belt surface speed should not exceed 5500 feet per minute (28 m/s) for larger pitch beltsand 10000 feet per minute (50 m/s) for mini pitch belts. For the HTD belts, a speed of 6500 feet per minute (33 m/s) is permitted, whereas for GT2 belts, the maximum permitted speed is 7500 feet per minute (38 m/s). The maximum allowable operating speed for T series is 4000 feet per minute (20 m/s).

8.    Belts are, in general, rated to yield a minimum of 3000 hours of useful life if all instructions are properly followed.

9.    Belt drives are inherently efficient. It can be assumed that the efficiency of a synchronous belt drive is greater than 95%.

10.    Belt drives are usually a source of noise. The frequency of the noise level increases proportionally with the belt speed. The higher the initial belt tension, the greater the noise level. The belt teeth entering the pulleys at high speed act as a compressor and this creates noise. Some noise is the result of a belt rubbing against the flange, which inturn may be the result of the shafts not being parallel. As shown in Figure 9 (page T-9), the noise level is substantially reduced if the PowerGrip GT2 belt is being used.

11.    If the drive is part of a sensitive acoustical or electronics sensing or recording device, it is recommended that the back surfaces of the belt be ground to assure absolutely uniform belt thickness.

12.    For some applications, no backlash between the driving and the driven shaft is permitted. For these cases, special profile pulleys can be produced without any clearance between the belt tooth and pulley. This may shorten the belt life, but it eliminates backlash. Figure 10 (page T-9) shows the superiority of PowerGrip GT2 profile as far as reduction of backlash is concerned.

13.    Synchronous belts are often driven by stepping motors. These drives are subjected to continuous and large accelerations and decelerations. If the belt reinforcing fiber, i.e.,tension member, as well as the belt material, have high tensile strength and no elongation, the belt will not be instrumental in absorbing the shock loads. This will result in sheared belt teeth. Therefore, take this into account when the size of the smallest pulley and the materials for the belt and tension member are selected.

14.    The choice of the pulley material (metal vs. plastic) is a matter of price, desired precision, inertia, color, magnetic properties and, above all, personal preference based on experiences. Plastic pulleys with metal inserts or metal hubs represent a good compromise.

The following precautions should be taken when installing all timing belt drives:

1.    Timing belt installation should be a snug fit, neither too tight nor too loose. The positive grip of the belt eliminates the need for high initial tension. Consequently, a belt, when installed with a snug fit (that is, not too taut) assures longer life, less bearing wear and quieter operation. Preloading (often the cause of premature failure) is not necessary. When torque is unusually high, a loose belt may "jump teeth" on starting. In such a case, the tension should be increased gradually, until satisfactory operation is attained. A good rule of thumb for installation tension is as shown in Figure  20, and the corresponding tensioning force is shown in Table 19, both shown in SECTION 10 BELT TENSIONING. For widths other than shown, increase force proportionally to the belt width. Instrumentation for measuring belt tension is available. Consult the product section of this catalog.

2.    Be sure that shafts are parallel and pulleys are in alignment. On a long center drive, it is sometimes advisable to offset the driven pulley to compensate for the tendency of the belt to run against one flange.

3.    On a long center drive, it is imperative that the belt sag is not large enough to permit teeth on the slack side to engage the teeth on the tight side.

4.    It is important that the frame supporting the pulleys be rigid at all times. A nonrigid frame causes variation in center distance and resulting belt slackness. This, in turn, can lead to jumping of teeth – especially under starting load with shaft misalignment.

5.    Although belt tension requires little attention after initial installation, provision should be made for some center distance adjustment for ease in installing and removing belts.  Do not force belt over flange of pulley.

6.    Idlers, either of the inside or outside type, are not recommended and should not be used except for power takeoff or functional use. When an idler is necessary, it should be on the slack side of the belt. Inside idlers must be grooved, unless their diameters are greater than an equivalent 40-groove pulley. Flat idlers must not be crowned (use edge flanges). Idler diameters must exceed the smallest diameter drive pulley. Idler arc of contact should be held to a minimum. In addition to the general guidelines enumerated previously, specific operating characteristics of the drive must be taken into account. These may include the following:

9.1   Low-Speed Operation

Synchronous drives are especially well-suited for low-speed, high torque applications. Their positive driving nature prevents potential slippage associated with V-belt drives, and even allows significantly greater torque carrying capability. Small pitch synchronous drives operating at speeds of 50 ft/min (0.25 m/s) or less are considered to be low-speed. Care should be taken in the drive selection process as stall and peak torques can sometimes be very high. While intermittent peak torques can often be carried by synchronous drives without special considerations, high cyclic peak torque loading should be carefully reviewed. Proper belt installation tension and rigid drive bracketry and framework is essential in preventing belt tooth jumping under peak torque loads. It is also helpful to design with more than the normal minimum of 6 belt teeth in mesh to ensure adequate belt tooth shear strength. Newer generation curvilinear systems like PowerGrip GT2 and PowerGrip HTD should be used inlow-speed, high torque applications, as trapezoidal timing belts are more prone to tooth jumping, andvhave significantly less load carrying capacity.

9.2   High-Speed Operation

Synchronous belt drives are often used in high-speed applications even though V-belt drives aretypically better suited. They are often used because of their positive driving characteristic (no creep or slip), and because they require minimal maintenance (don't stretch significantly). A significant drawback of high-speed synchronous drives is drive noise. High-speed synchronous drives  will  nearly always produce more noise than V-belt drives. Small pitch synchronous drives operating at speeds in excess of 1300 ft/min (6.6 m/s) are considered to be high-speed. Special consideration should be given to high-speed drive designs, as a number of factors can significantly influence belt performance. Cord fatigue and belt tooth wear are the two most significant factors that must be controlled to ensure success. Moderate pulley diameters should be used to reduce the rate of cord flex fatigue. Designing with a smaller pitch belt will often provide better cordflex fatigue characteristics than a larger pitch belt. PowerGrip GT2 is especially well suited for high-speed drives because of its excellent belt tooth entry/exit characteristics. Smooth interaction between the belt tooth and pulley groove minimizes wear and noise. Belt installation tension is especially critical with high-speed drives. Low belt tension allows the belt to ride out of the driven pulley, resulting in rapid belt tooth and pulley groove wear.

9.3   Smooth Running

Some ultrasensitive applications require the belt drive to operate with as little vibration as possible, as vibration sometimes has an effect on the system operation or finished manufactured product. In these cases, the characteristics and properties of all appropriate belt drive products should be reviewed. The final drive system selection should be based upon the most critical design requirements, and may require some compromise. Vibration is not generally considered to be a problem with synchronous belt drives. Low levels of vibration typically result from the process of tooth meshing and/or as a result of their high tensilemodulus properties. Vibration resulting from tooth meshing is a normal characteristic of synchronous belt drives, and cannot be completely eliminated. It can be minimized by avoiding small pulley diameters, and instead choosing moderate sizes. The dimensional accuracy of the pulleys also influences tooth meshing quality. Additionally, the installation tension has an impact on meshing quality. PowerGrip GT2 drives mesh very cleanly, resulting in the smoothest possible operation. Vibration resulting from high tensile modulus can be a function of pulley quality. Radial run out causes belt tension variation with each pulley revolution. V-belt pulleys are also manufactured with some radial run out, but V-belts have a lower tensile modulus resulting in less belt tension variation. The high tensile modulus found in synchronous belts is necessary to maintain proper pitch underload.

9.4   Drive Noise

Drive noise evaluation in any belt drive system should be approached with care. There are many potential sources of noise in a system, including vibration from related components, bearings, and resonance and amplification through framework and panels. Synchronous belt drives typically produce more noise than V-belt drives. Noise results from the process of belt tooth meshing and physical contact with the pulleys. The sound pressure level generally increases as operating speed and belt width increase, and as pulley diameter decreases. Drives designed on moderate pulley sizes without excessive capacity (overdesigned) are generally the quietest. PowerGrip GT2 drives have been found to be significantly quieter than other systems due to their improved meshing characteristic (see Figure 9, page T-9). Polyurethane belts generally produce more noise than neoprene belts. Proper belt installation tension is also very important in minimizing drive noise. The belt should be tensioned at a level that allows it to run with as little meshing interference as possible. Drive alignment also has a significant effect on drive noise. Special attention should be given to minimizing angular misalignment (shaft parallelism). This assures that belt teeth are loaded uniformly and minimizes side tracking forces against the flanges. Parallel misalignment (pulley offset) is not as critical of a concern as long as the belt is not trapped or pinched between opposite flanges (see the special section dealing with drive alignment). Pulley materials and dimensional accuracy also influence drive noise. Some users have found that steel pulleys are the quietest, followed closely by aluminum. Polycarbonates have been found to be noisier than metallic materials. Machined pulleys are generally quieter than molded pulleys. The reasons for this revolve around material density and resonance characteristics as well as dimensional accuracy.

9.5   Static Conductivity

Small synchronous rubber or urethane belts can generate an electrical charge while operating on a drive. Factors such as humidity and operating speed influence the potential of the charge. If determined to be a problem, rubber belts can be produced in a conductive construction to dissipate the charge into the pulleys, and to ground. This prevents the accumulation of electrical charges that might be detrimental to material handling processes or sensitive electronics. It also greatly reduces the potential for arcing or sparking in flammable environments. Urethane belts cannot be produced in a conductive construction. RMA has outlined standards for conductive belts in their bulletin IP-3-3. Unless otherwise specified, a static conductive construction for rubber belts is available on a made-to-order basis. Unless otherwise specified, conductive belts will be built to yield a resistance of 300,000 ohms or less, when new. Nonconductive belt constructions are also available for rubber belts. These belts are generally built specifically to the customers conductivity requirements. They are generally used in applications where one shaft must be electrically isolated from the other. It is important to note that a static conductive belt cannot dissipate an electrical charge through plastic pulleys. At least one metallic pulley in a drive is required for the charge to be dissipated to ground. A grounding brush or similar device may also be used to dissipate electrical charges. Urethane timing belts are not static conductive and cannot be built in a special conductive construction. Special conductive rubber belts should be used when the presence of an electrical charge is a concern.

9.6   Operating Environments

Synchronous drives are suitable for use in a wide variety of environments. Special considerations may be necessary, however, depending on the application.

Dust:  Dusty environments do not generally present serious problems to synchronous drives as long as the particles are fine and dry. Particulate matter will, however, act as an abrasive resulting in a higher rate of belt and pulley wear. Damp or sticky particulate matter deposited and packed into pulley grooves can cause belt tension to increase significantly.  This increased tension can impact shafting, bearings, and framework. Electrical charges within a drive system can sometimes attract particulate matter.

Debris:    Debris should be prevented from falling into any synchronous belt drive. Debris caught in the drive is generally either forced through the belt or results installing of the system. In either case, serious damage occurs to the belt and related drive hardware.

Water:  Light and occasional contact with water (occasional wash downs) should not seriously affect synchronous belts. Prolonged contact (constant spray or submersion) results in significantly reduced tensile strength in fiberglass belts, and potential length variation in aramid belts. Prolonged contact with water also causes rubber compounds to swell, although less than with oil contact. Internal belt adhesion systems are also gradually broken down with the presence of water. Additives to water, such as lubricants, chlorine, anticorrosives, etc. can have a more detrimental effect on the belts than pure water. Urethane timing belts also suffer from water contamination. Polyester tensile cord shrinks significantly and experiences loss of tensile strength in the presence of water. Aramid tensile cord maintains its strength fairly well, but experiences length variation. Urethane swells more than neoprene in the presence of water. This swelling can increase belt tension significantly, causing belt and related hardware problems.

Oil: Light contact with oils on an occasional basis will not generally damage synchronous belts. Prolonged contact with oil or lubricants, either directly or airborne, results in significantly reduced belt service life. Lubricants cause the rubber compound to swell, breakdown internal adhesion systems,  and reduce belt tensile strength. While alternate rubber compounds may provide some marginal improvement in durability, it is best to prevent oil from contacting synchronous belts.

Ozone: The presence of ozone can be detrimental to the compounds used in rubber synchronous belts. Ozone degrades belt materials in much the same way as excessive environmental temperatures. Although the rubber materials used in synchronous belts are compounded to resis tthe effects of ozone, eventually chemical breakdown occurs and they become hard and brittle and begin cracking. The amount of degradation depends upon the ozone concentration and duration of exposure. For good performance of rubber belts, the following concentration levels should not be exceeded: (parts per hundred million)

Standard Construction:100 pphm

Nonmarking Construction: 20 pphm

Conductive Construction: 75 pphm

Low Temperatures Construction: 20 pphm

Radiation: Exposure to gamma radiation can be detrimental to the compounds used in rubberand urethane synchronous belts. Radiation degrades belt materials much the same way excessive environmental temperatures do. The amount of degradation depends upon the intensity of radiation and the exposure time. For good belt performance, the following exposure levels should not be exceeded:

Standard Construction: 108 rads

Nonm arking Construction: 104 rads

Conductive Construction: 106 rads

Low Temperatures Construction: 104 rads

Dust Generation: Rubber synchronous belts are known to generate small quantities of fine dust, as a natural result of their operation. The quantity of dust is typically higher for new belts, as they run in. The period of time for run in to occur depends upon the belt and pulley size, loading and speed. Factors such as pulley surface finish, operating speeds, installation tension, and alignment influence the quantity of dust generated.

Clean Room: Rubber synchronous belts may not be suitable for use in clean room environments, where all potential contamination must be minimized or eliminated. Urethane timing belts typically generate significantly less debris than rubber timing belts. However, they are recommended only for light operating loads. Also, they cannot be produced in a static conductive construction to allow electrical charges to dissipate.

Static Sensitive: Applications are sometimes sensitive to the accumulation of static electrical charges. Electrical charges can affect material handling processes (like paper and plastic film transport), and sensitive electronic equipment. Applications like these require a static conductive belt, so that the static charges generated by the belt can be dissipated into the pulleys, and to ground. Standard rubber synchronous belts do not meet this requirement, but can be manufactured in a static conductive construction on a made-to-order basis. Normal belt wear resulting from long term operation or environmental contamination can influence belt conductivity properties. In sensitive applications, rubber synchronous belts are preferred over urethane belts since urethane belting cannot be produced in a conductive construction.

9.7  Belt Tracking

Lateral tracking characteristics of synchronous belts is a common area of inquiry. While it is normal for a belt to favor one side of the pulleys while running, it is abnormal for a belt to  exert significant force against a flange resulting in belt edge wear and potential flange failure. Belt tracking is influenced by several factors. In order of significance, discussion about these factors is as follows:

Tensile Cord Twist: Tensile cords are formed into a single twist configuration during their manufacture. Synchronous belts made with only single twist tensile cords track laterally with a significant force. To neutralize this tracking force, tensile cords are produced in right- and left-handtwist (or "S" and "Z" twist) configurations. Belts made with "S" twist tensile cords track in the opposite direction to those built with "Z" twist cord. Belts made with alternating "S" and "Z" twist tensile cords track with minimal lateral force because the tracking characteristics of the two cords offset each other. The content of "S" and "Z" twist tensile cords varies slightly with every belt that is produced. As a result, every belt has an unprecedented tendency to track in either one direction orthe other. When an application requires a belt to track in one specific direction only, a single twist construction is used. See Figures 16 & 17, previously shown, on pages T-12 and T-13.

Angular Misalignment: Angular misalignment, or shaft nonparallelism, cause synchronous belts to track laterally. The angle of misalignment influences the magnitude and direction of the tracking force. Synchronous belts tend to track "downhill" to a state of lower tension or shorter center distance.

Belt Width: The potential magnitude of belt tracking force is directly related to belt width. Wide belts tend to track with more force than narrow belts.

Pulley Diameter: Belts operating on small pulley diameters can tend to generate higher tracking forces than on large diameters. This is particularly true as the belt width approaches the pulley diameter. Drives with pulley diameters less than the belt width are not generally recommended because belt tracking forces can become excessive.

Belt Length: Because of the way tensile cords are applied on to the belt molds, short beltscan tend to exhibit higher tracking forces than long belts. The helix angle of the tensile cord decreases with increasing belt length.

Gravity: In drive applications with vertical shafts, gravity pulls the belt downward. The magnitude of this force is minimal with small pitch synchronous belts. Sag in long belt spans should be avoided by applying adequate belt installation tension.

Torque Loads: Sometimes, while in operation, a synchronous belt will move laterally from side to side on the pulleys rather than operating in a consistent position. While not generally considered to be a significant concern, one explanation for this is varying torque loads within the drive. Synchronous belts sometimes track differently with changing loads. There are many potential reasons for this; the primary cause is related to tensile cord distortion while under pressure against the pulleys. Variation in belt tensile loads can also cause changes in framework deflection, and angular shaft alignment, resulting in belt movement.

Belt Installation Tension: Belt tracking is sometimes influenced by the level of belt installation tension. The reasons for this are similar to the effect that varying torque loads have on belt tracking. When problems with belt tracking are experienced, each of these potential contributing factors should be investigated in the order that they are listed. In most cases, the  primary problem will probably be identified before moving completely through the list.

9.8 Pulley Flanging

Pulley guide flanges are necessary to keep synchronous belts operating on their pulleys. As discussed previously in Section 9.7 on belt tracking, it is normal for synchronous belts to favor one side of the pulleys when running. Proper flange design is important in preventing belt edge wear, minimizing noise and preventing the belt from climbing out of the pulley. Dimensional recommendations for custom-made or molded flanges are included in tables dealing with these issues. Proper flange placement is important so that the belt is adequately restrained within its operating system. Because design and layout of small synchronous drives is so diverse, the wide variety of flanging situations potentially encountered cannot easily be covered in a simple set of rules without finding exceptions. Despite this, the following broad flanging guidelines should help the designer inmost cases:

Two Pulley Drives: On simple two pulley drives, either one pulley should be flanged on both sides, or each pulley should be flanged on opposite sides.

Multiple Pulley Drives: On multiple pulley (or serpentine) drives, either every other pulley should be flanged on both sides, or every pulley should be flanged on alternating sides around the system.

Vertical Shaft Drives: On vertical shaft drives, at least one pulley should be flanged on both sides, and the remaining pulleys should be flanged on at least the bottom side.

Long Span Lengths: Flanging recommendations for small synchronous drives with long beltspan lengths cannot easily be defined due to the many factors that can affect belt  tracking characteristics. Belts on drives with long spans (generally 12 times the diameter of the smaller pulley or more) often require more lateral restraint than with short spans. Because of this, it is generally a good idea to flange the pulleys on both sides.

Large Pulleys: Flanging large pulleys can be costly. Designers often wish to leave large pulleys unflanged to reduce cost and space. Belts generally tend to require less lateral restraint on large pulleys than small and can often perform reliably without flanges. When deciding whether or not to flange, the previous guidelines should be considered. The groove face width of unflanged pulleys should also be greater than with flanged pulleys. See Table 37, on page T-68 for recommendations.

Idlers: Flanging of idlers is generally not necessary. Idlers designed to carry lateral sideloads from belt tracking forces can be flanged if needed to provide lateral belt restraint. Idlers used for this purpose can be used on the inside or backside of the belts. The previous guidelines should also be considered.

9.9   Registration

The three primary factors contributing to belt drive registration (or positioning) errors are belt elongation, backlash, and tooth deflection. When evaluating the potential registration capabilities of a synchronous belt drive, the system must first be determined to be either static or dynamic interms of its registration function and requirements.

Static Registration: A static registration system moves from its initial static position to a secondary static position. During the process, the designer is concerned only with how accurately and consistently the drive arrives at its secondary position.  He/she is not concerned  with any potential registration errors that occur during transport. Therefore, the primary factor contributing to registration error in a static registration system is backlash. The effects of belt elongation and tooth deflection do not have any influence on the registration accuracy of this type of system.

Dynamic Registration: A dynamic registration system is required to perform a registering function while in motion with torque loads varying as the system  operates. In this case, the designer is concerned with the rotational position of the drive pulleys with respect to each other atevery point in time.  Therefore, belt elongation, backlash and tooth deflection will all contribute to registrational inaccuracies. Further discussion about each of the factors contributing to registration error is as follows:

Belt Elongation:  Belt elongation, or stretch, occurs naturally when a belt is placed under tension. The total tension exerted within a belt results from installation, as well as working loads. The amount of belt elongation is a function of the belt tensile modulus, which is influenced by the type of tensile cord and the belt construction. The standard tensile cord used in rubber synchronous belts is fiberglass. Fiberglass has a high tensile modulus, is dimensionally stable, and has excellentflex-fatigue  characteristics. If a higher tensile modulus is needed, aramid tensile cords can b econsidered, although they are generally used to provide resistance to harsh shock and impulse loads. Aramid tensile cords used in small synchronous belts generally have only a marginally higher tensile modulus in comparison to fiberglass. When needed, belt tensile modulus data is available from our Application Engineering Department.

Backlash: Backlash in a synchronous belt drive results from clearance between the belt teethand the pulley grooves. This clearance is needed to allow the belt teeth to enter and exit the grooves smoothly with a minimum of interference. The amount of clearance necessary depends upon the belt tooth profile. Trapezoidal Timing Belt Drives are known for having relatively little backlash. PowerGrip HTD Drives have improved torque carrying capability and resist ratcheting, but have a significant amount of backlash. PowerGrip GT2 Drives  have  even  further  improved torque carrying capability, and have as little or less backlash than trapezoidal timing belt drives. In special cases, alterations can be made to drive systems to further decrease backlash.  These alterations typically result in increased belt wear, increased drive noise and shorter drive life. Contact our Application Engineering Department for additional information.

Tooth Deflection: Tooth deformation in a synchronous belt drive occurs as a torque load is applied to the system, and individual belt teeth are loaded. The amount of belt tooth deformation depends upon the amount of torque loading, pulley size, installation tension and belt type. Of the three primary contributors to registration error, tooth deflection is the most difficult to quantify. Experimentation with a prototype drive system is the best means of obtaining realistic estimations of belt tooth deflection. Additional guidelines that may be useful in designing registration critical drive systems are as follows:

  • Select PowerGrip GT2 or trapezoidal timing belts.
  • Design with large pulleys with more teeth in mesh.
  • Keep belts tight, and control tension closely.
  • Design frame/shafting to be rigid under load.
  • Use high quality machined pulleys to minimize radial runout and lateral wobble.

SECTION 10 BELT TENSIONING

10.1 What Is Proper Installation Tension

One of the benefits of small synchronous belt drives is lower belt pre-tensioning in comparison to comparable V-belt drives, but proper installation tension is still important in achieving the best possible drive performance. In general terms, belt pre-tensioning is needed for proper belt/pulley meshing to prevent belt ratcheting under peak loading, to compensate for initial belt tension decay, and to prestress the drive framework. The amount of installation tension that is actually needed is influenced by the type of application as well as the system design. Some general examples of this are as follows:

Motion Transfer Drives: Motion transfer drives, by definition, are required to carry extremely light torque loads. In these applications, belt installation tension is needed only to cause the belt to conform to and mesh properly with the pulleys. The amount of tension necessary for this is referred to as the minimum tension (Tst). Minimum tensions, on a per span basis, are included inTable 19, on page T-51. Some motion transfer drives carry very little torque, but have a need for accurate registration requirements. These systems may require additional static (or installation) tension in order to minimize registration error.

Normal Power Transmission Drives: Normal power transmission drives should be designed in accordance with published torque ratings and a reasonable service factor (between 1.5 and 2.0). In these applications, belt installation tension is needed to allow the belt to maintain a proper fit with the pulleys while under load, and to prevent belt ratcheting under peak loads. For these drives, proper installation tension can be determined using two different approaches. If torque loads are known and well defined, and an accurate tension value  is desired, Equation  (10-1) or Equation (10-2) should be used. If the torque loads are not as well defined, and a quick value is desired formuse as a starting point, values from Table 20 can be used.  All static tension values are on a per-span basis.

Registration Drives: Registration drives are required to register, or position accurately. Higher belt installation tensions help in increasing belt tensile modulus as well as in increasing meshing interference, both reducing backlash. Tension values for these applications should be determined experimentally to confirm that desired performance characteristics have been achieved. As a beginning point, use values from Table 20 multiplied by 1.5 to 2.0.

Most synchronous belt applications often exhibit their own individual operating characteristics. The static installation tensions recommended in this section should serve as a general guideline in determining the level of tension required. The drive system should be thoroughly tested to confirm that it performs as intended.

10.2   Making Measurements

Belt installation tension is generally measured in the following ways:

Force/Deflection: Belt span tension can be measured by deflecting a belt span 1/64" per inch (0.4 mm per 25 mm) of span length at midspan, with a known force (see Figure 20). This method is generally convenient, but not always very accurate, due to difficulty in measuring small deflections and forces common in small synchronous drives. The force/deflection method is most effective on larger drives with long span lengths. The static (or installation) tension (Tst) can either be calculated from Equation (10-1) or Equation (10-2), or selected from Table 19 or Table 20. The deflection forces can be calculated from Equation (10-4) and Equation (10-5). The span length can either be calculated from Equation (10-3), or measured. If the calculated static tension is less than the minimum Tst values in Table 19, use the minimum values.

Shaft Separation: Belt installation tension can be applied directly by exerting a force against either the driver or driven shaft in a simple 2-point drives ystem (see Figure 21). The resulting belt tension will be as accurate as the force applied to driver or driven shaft. This method is considerably easier to perform than the force/deflection method and, in some cases, more accurate. In order to calculate the required shaft separation force, the proper static tension (on a per-span basis) should first be determined as previously discussed. This tension value will be present in both belt spans as tension is applied. The angle of the spans with respect to the movable shaf tshould then be determined. The belt spans should be considered to be vectors (force with direction), and be summed into a single tension vector force (see Figure 22).  Refer to SECTION 14  BELT

PULL AND BEARING LOADS for further instructions on summing vectors.

Idler Force: Belt installation tension can also be applied by exerting a force against an idler pulley within the system that is used to take up belt slack (see Figure 23). This  force can be applied manually, or with a spring. Either way, the idler should be locked down after the appropriate tension has been applied.

Calculating the required force will involve a vector analysis as described previously in the shaft separation section.

Sonic Tension Meter: The Sonic Tension Meter (Figure 24) is an electronic device that measures the natural frequency of a free stationary belt span and instantly computes  the static belt tension based upon the belt span length, belt width, and belt type. This provides accurate and repeatable tension measurements while using a nonintrusive procedure (the measurement process itself doesn't change the belt span tension). A measurement is made simply by plucking the belt while holding the sensor close to the vibrating belt span.

The unit is about the size of a portable phone (8-1/8" long x 3-3/4" wide x 1-3/8" thick or 206 mm long x 95mm wide x 35mm thick) so it can be easily handled. The sensor is about 1/2" (13mm) in diameter for use in cramped spaces, and the unit is either battery operated for portability or AC operated for production use.   The unit measures virtually all types of light power and precision belts. A gain adjustment allows measurements to be made in environments with high noise levels. Data can also be collected through an IBM Compatible RS-232 serial port, if desired. For additional details, see the product section of this handbook.

SECTION 11 DRIVE ALIGNMENT

11.1 Angular And Parallel

Drive misalignment is one of the most common sources of drive performance problems. Misaligned drives can exhibit symptoms such as high belt tracking forces, uneven belt tooth wear, high noise levels, and tensile cord failure. The two primary types of drive misalignment are angular and parallel. Discussion about each of these types are as follows:

Angular: Angular misalignment results when the drive shafts are not parallel (see Figure 25). As a result, the belt tensile cords are not loaded evenly, resulting in uneven tooth/land pressure and wear. The edge cords on the high tension side are often overloaded which may cause an edge cord failure that propagates across the entire belt width. Angular misalignment often results in high belt-tracking forcesas well which cause accelerated belt edge wear, sometimes leading to flange failure or belts tracking off of the pulleys.

Parallel: Parallel misalignment results from pulleys being mounted outof line from each other (see Figure 26). Parallel misalignment is generally moreof a concern with V-type belts than with synchronous belts because V-type belts run in grooves and are unable to free float on the pulleys. Synchronous belts will generally free float on the pulleys and essentially self-align themselves as they run. This self-aligning can occur as long as the pulleys have sufficient groove face width beyond the width of the belts. If not, the belts can become trapped between opposite pulley flanges causing serious performance problems. Parallel misalignment is not generally a significant concern with synchronous drives as long as the belts do not become trapped or pinched between opposite flanges. For recommendations on groove face width, see Table 37, on page T-68.

Allowable Misalignment: In order to maximize performance and reliability, synchronous drives should be aligned closely. This is not, however, always a simple task in a production environment. The maximum allowable misalignment, angular and parallel combined, is 1/4°.11.2  

Practical Tips

Angular misalignment is not always easy to measure or quantify. It is sometimes helpful to use the observed tracking characteristics of a belt, to make a judgment as to the system's relative alignment. Neutral tracking "S" and "Z" synchronous belts generally tend to track "down hill" or to a state of lower tension or shorter center distance when angularly misaligned. This may not always hold true since neutral tracking belts naturally tend to ride lightly against either one flange or the other due to numerous factors discussed in the section on belt tracking. This tendency will generally hold true with belts that track hard against a flange. In those cases, the shafts will require adjustment to correct the problem.

Parallel misalignment is not often found to be a problem in synchronous belt drives. If clearance is always observable between the belt and all flanges on one side, then parallel misalignment should not be a concern.

SECTION 12 INSTALLATION AND TAKE-UP

12.1 Installation Allowance

When designing a drive system for a manufactured product, allowance for belt installation must be built into the system. While specific installation allowances could be published, as they are for larger industrial belt drives, small synchronous drive applications are generally quite diverse, making it nearly impossible to arrive at values that apply in all cases. When space is at a premium, the necessary installation allowance should be determined experimentally using actual production parts for the best possible results.

12.2 Belt Installation

During the belt installation process, it is very important that the belt be fully seated in the pulley grooves before applying final tension. Serpentine drives with multiple pulleys and drives with large pulleys are particularly vulnerable to belt tensioning problems resulting from the belt teeth being only partially engaged in the pulleys during installation. In order to prevent these problems, the belt installation tension should be evenly distributed to all belt spans by rotating the system by hand. After confirming that belt teeth are properly engaged in the pulley grooves, belt tension should be rechecked and verified. Failure to do this may result in an under tensioned condition with the potential for belt ratcheting.

12.3   Belt Take-up

Synchronous belt drives generally require little if any retensioning when used in accordance with proper design procedures. A small amount of belt tension decay can be expected within the first several hours of operation. After this time, the belt tension should remain relatively stable.

12.4   Fixed Center Drives

Designers sometimes attempt to design synchronous belt drive systems without any means of belt adjustment or take-up. This type of system is called a Fixed Center Drive. While this approach is often viewed as being economical, and is simple for assemblers, it often results in troublesome reliability and performance problems in the long run. The primary pitfall in a fixed center design approach is failure to consider the effects of system tolerance accumulation. Belts and pulleys are manufactured with industry accepted production tolerances. There are limits to the accuracy that the center distance can be maintained on a production basis as well. The potential effects of this tolerance accumulation is as follows:

Low Tension:
Long Belt with Small Pulleys on a Short Center Distance

High Tension:
Short Belt with Large Pulleys on a Long Center Distance

Belt tension in these two cases can vary by a factor of 3 or more with a standard fiberglass tensile cord. This potential variation is great enough to overload bearings and shafting, as well as the belts  themselves. The probability of these extremes occurring is a matter of statistics, but however remote the chances may seem, they will occur in a production setting. In power transmission drives, the appearance of either extreme is very likely to impact drive system performance in a negative manner. The most detrimental aspect of fixed center drives is generally the potentially high tension condition. This condition can be avoided by adjusting the design center distance. A  common approach in these designs is to reduce the center distance from the exact calculated value by some small fraction. This results in a drive system that is inherently loose, but one that has much less probability of yielding excessively high shaft loads. NOTE: This approach should not be used for power transmission drives since the potentially loose operating conditions could result in accelerated wear and belt ratcheting, even under nominal loading. There are times when fixed center drive designs can't be avoided. In these cases, the following recommendations will maximize the probability of success.

1. Do not use a fixed center design for power transmission drives. Consider using a fixed center design only for lightly loaded or motion transfer applications.

2. Do not use a fixed center design for drives requiring high motion quality or registration precision.

3. When considering a fixed center design, the center distance must be held as accurately aspossible, typically within 0.002" – 0.003" (0.05 mm – 0.08 mm). This accuracy often requires the use of stamped steel framework. Molding processes do not generally have the capacity to maintain the necessary accuracy.

4. Pulleys for fixed center systems should be manufactured with a process that is capable of producing the required O.D. tolerances accurately enough.

5. The performance capabilities of the drive system should be verified by testing belts produced over their full length tolerance range on drive systems representing the full potential center-distance variation.

SECTION 13 IDLER USAGE

Idlers in synchronous belt drives are commonly used to take up belt slack, apply installation tension or to clear obstructions within a system. While idlers cause additional belt bending, resulting in fatigue, this effect is generally not significant as long as proper design procedures are followed. Synchronous belts elongate very little over time, making them relatively maintenance free. All idlers should be capable of being locked down after being adjusted and should require little additional attention. Specific guidelines and recommendations are given below.

13.1 Inside/Outside

Inside idlers are generally preferred over backside idlers from a belt fatigue standpoint. Both are commonly used with good success. Inside idlers should be pulleys, but can be flat, if the O.D.is equivalent to the pitch diameter of a 40-groove pulley. Backside idlers should be flat and uncrowned.

13.2 Tight Side/Slack Side

Idlers should be placed on the slack (or nonload-carrying) side, if possible. Their effect on belt fatigue is less on the slack side than on the tight (or load-carrying) side. If spring-loaded idlers are used, they should never be placed on the tight side (see Spring-Loaded Idlers). Also, note that drive direction reversals cause the tight and slack spans to reverse, potentially placing the idler on the tight side.

13.3 Idler Placement

In synchronous belt drives, idlers can be placed nearly anywhere they are needed. Synchronous drives are much less sensitive to idler placement and belt wrap angles than V-belt  drives. The designer should make sure that at least 6 belt teeth are in mesh on load-carrying pulleys. For every tooth in mesh less than this (with a minimum of 2), 20% of the belt torque rating must be subtracted. In order to minimize the potential for belt ratcheting, each loaded pulley in the system should also have a wrap angle of at least 60°. If a loaded pulley has less than 6 teeth in mesh and 60° of wrap, idlers can often be used to improve this condition. Non-loaded idler pulleys do not have tooth meshing or wrap angle restriction.

13.4 Spring-Loaded Idlers

Using a spring to apply a predetermined force against a tensioning idler to obtain proper belt installation tension is acceptable as long as the idler can be locked down after belt installation.

Dynamic spring-loaded idlers are generally not recommended for synchronous belt drives. If used, spring-loaded belt idlers should never be used on the tight (or load-carrying) side. Tight side tensions vary with the magnitude and type of load carried by the system. High tight side tensions can overcome the idler spring force allowing the belt to ratchet. In order to prevent this from occurring, an excessively high spring force is required. This high spring force can result in highs haft/bearing loads and accelerated belt wear.

If dynamic spring-loaded idlers are to be used, they should be used on the slack (or nonload-carrying) side of the drive. Potential drive loading variations in the system will have the least possible impact on idler movement due to spring compression with the idler placed in this way. Be sure to note that the tight and slack spans shift as the direction of drive rotation reverses. This could place the spring-loaded idler on the tight side. In some cases, drive vibration and harmonic problems may also be encountered with the use of spring-loaded idlers.

13.5 Size Recommendations

Inside idler pulleys can be used in the minimum recommended size for each particular belt pitch. Inside flat idlers can be used on the tooth side of synchronous belts as long as they are of a diameter equivalent to the pitch diameter of a 40-groove pulley in the same pitch. Drives within side flat idlers should be tested, as noise and belt wear may occur. Flat backside idlers should be used with diameters at least 30% larger than the minimum recommended inside pulley size. Table 21 summarizes our idler size recommendations.

13.6 Specifying Shaft Locations In Multipoint Drive Layouts

When collecting geometrical layout data for multiple pulley drive layouts, it is important to use a standard approach that is readily understood and usable for drive design calculations. This is of particular importance when the data will be provided to our Application Engineering Department for analysis.

2-Point Drive

When working with a simple 2-point drive (driver/driven only) it is sufficient to specify the desired distance between shaft centers for belt length calculations.

3-Point Drive

When working with a 3-point drive (driver/driven/idler), X-Y coordinates are desirable. It is sufficient, however, to specify desired center distances between each of the three shaft centers to form a triangle. In either case, pulley/idler movement details for belt tensioning and take up are also necessary.

Multi-Point Drive

When working with a drive system having moret han 3 shafts, the geometrical layout data must be collected in terms of X-Y coordinates for analysis. For those unfamiliar with X-Y coordinates, the X-YCartesian coordinate system is commonly used in mathematical and engineering calculations and utilizes a horizontal and vertical axis as illustrated in Figure 27.

The axes cross at the zero point, or origin. Along the horizontal, or "X" axis, all values to the right of the zero point are positive, and all values to the left of the zero point are negative. Along the vertical, or "Y" axis, all values above the zero point are positive, and all values below the zero point are negative. This is also illustrated in Figure 27.

When identifying a shaft center location, each X-Y coordinate is specified with a measurementin the "X" as well as the "Y" direction. This requires a horizontal and vertical measurement for each shaft center in order to establish a complete coordinate. Either English or Metric units of measurement may be used.

A complete coordinate is specified as follows:

(X, Y)

where: X = measurement along X-axis (horizontal)
Y = measurement along Y-axis (vertical)

In specifying X and Y coordinates for each shaft center, the origin (zero point) must first be chosen as a reference. The driver shaft most often serves this purpose, but any shaft center can be used. Measurements for all remaining shaft centers must be taken from this origin or reference point. The origin is specified as (0, 0).

An example layout of a 5-point drive system is illustrated in Figure 28. Here, each of the five shaft centers are located and identified on the X-Y coordinate grid. When specifying parameters for the movable or adjustable shaft (for belt installation and tensioning), the following approaches are generally used:

Fixed Location: Specify the nominal shaft location coordinate with a movement direction.

Slotted Location: Specify a location coordinate for the beginning of the slot, and a location coordinate for the end of the slot along its path of linear movement.

Pivoted Location: Specify the initial shaft location coordinate along with a pivot point location coordinate and the pivot radius. Performing belt length and idler movement/positioning calculations by hand can be quite difficult and time consuming. With a complete geometrical drive description, we can make the drive design and layout process quite simple for you.

SECTION 14 BELT PULL AND BEARING LOADS

Synchronous belt drives are capable of exerting lower shaft loads than V-belt drives in some circumstances. If pre-tensioned according to SDP/SI recommendations for a fully loaded steady state condition, synchronous and V-belt drives will generate comparable shaft loads. If the actual torque loads are reduced and the level of pre-tension remains the same, they will continue to exert comparable shaft loads. In some cases, synchronous belts can be pre-tensioned for less than full loads, under nonsteady state conditions, with reasonable results.    

Reduced pre-tensioning in synchronous belts can be warranted in a system that operates with uniform loads most of the time, but generates peak loads on an intermittent basis. While V-belt drives require pre-tensioning based upon peak loads to prevent slippage, synchronous drive pre-tensioning can be based upon lower average loads rather than intermittent peak loads, as long as the belt does not ratchet under the peak loads. When the higher peak loads are carried by the synchronous drive, the belt will self-generate tension as needed to carry the load. The process of self-tensioning results in the belt teeth riding out of the pulley grooves as the belt enters the driven pulley on the slack side, resulting in increased belt tooth and pulley wear. As long as peak loads occur intermittently and belts do not ratchet, reduced installation tension will result in reduced average belt pull without serious detrimental effects. Synchronous belts generally require less pretension than V-belts for the same load. They do not require additional installation tension for belt wrap less than 180 degrees on loaded pulleys as V-belt drives do. In most cases, these factors contribute to lower static and dynamic shaft loads in synchronous belt drives.

Designers often wish to calculate how much force a belt drive will exert on the shafting/bearings/framework in order to properly design their system. It is difficult to make accurate belt pull calculations because factors such as torque load variation, installation tension and pulley runout all have a significant influence. Estimations, however, can be made as follows:

14.1 Motion Transfer Drives

Motion transfer drives, by definition, do not carry a significant torque load. As a result, the belt pull is dependent only on the installation tension. Because installation tensions are provided on a per span basis, the total belt pull can be calculated by vector addition.

14.2 Power Transmission Drives

Torque load and installation tension both influence the belt pull in power transmission drives. The level of installation tension influences the dynamic tension ratio of the belt spans. The tension ratio is defined as the tight side (or load carrying) tension TT divided by the slack side (or non-load carrying) tension TS. Synchronous belt drives are generally pre-tensioned to operate dynamically at a 5:1 tension ratio in order to provide the best possible performance. After running for a short time, this ratio is known to increase somewhat as the belt runs in and seats with the pulleys, reducing tension. Equations (14-1) and (14-2) can be used to calculate the estimated TT and TS tensions assuming a 5:1 tension ratio. TT and TS tensions can then be summed into a single vector force and direction.

If both direction and magnitude of belt pull are required, the vector sum of TT and TS can be found by graphical vector addition as shown in Figure  29. TT and TS vectors are drawn parallel to the tight and slack sides at a convenient scale. The magnitude and direction of the resultant vector, or belt pull, can then be measured graphically. The same procedures can be used for finding belt pull on the driven shaft. This method can also be used for drives using three or more pulleys or idlers. For two pulley drives, belt pull on the driver and driven shafts is equal but opposite in direction. For drives using idlers, both magnitude and direction may be different. If only the magnitude of the belt pull is needed in a two pulley drive, use the following procedure:

1. Add TT and TS

2. Using the value of (D – d )/C for the drive, find the vector sum correction factor using Figure 30. Or, use the known arc of contact on the small pulley, where: D = large diameter, d = small diameter, C = center distance

3. Multiply the sum of TT and TS by the vector sum correction factor to find the vector sum, or belt pull.

For drives using idlers, either use the graphical method or contact our Application Engineering Department for assistance.

14.3 Registration Drives

Synchronous belt drives used for purposes of accurate registration or synchronization generally require the use of higher than normal installation tensions (see section on Belt Tensioning). These drives will operate with higher belt pulls than normal power transmission drives. Belt pull values for these types of applications should be verified experimentally, but can be estimated by adding the installation tension in each belt span vectorially.

14.4 Bearing Load Calculations

In order to find actual bearing loads, it is necessary to know the weights of machine components and the value of all other forces contributing to the load. However, sometimes it helps to know the bearing load contributed by the belt drive alone. The resulting bearing load due to belt pull can be calculated if both bearing spacing with respect to the pulley center and the belt pull are known. For approximate bearing load calculations, machine designers use belt pull and ignore pulley weight forces. If more accurate bearing load calculations are needed, or if the pulley is unusually heavy, the actual shaft load (including pulley weight) should be used.

SECTION 15 HANDLING AND STORAGE

The following has been condensed from RMA Bulletin No. IP-3-4: "Storage of Power Transmission Belts":

Recommendations for proper belt storage is of interest to designers as well as to users. Under favorable storage conditions, high-quality belts maintain their performance capabilities and manufactured dimensions. Good storage conditions and practices will result in the best value from belt products.

Power transmission belts should ideally be stored in a cool and dry environment. Excess weight against belts resulting in distortion should be avoided. Avoid storing belts in environments that may allow exposure to sunlight, moisture, excessive heat, ozone, or where evaporating solvents or other chemicals are present. Belts have been found to be capable of withstanding storage, without changing significantly, for as long as 8 years at temperatures less than 85° F (30° C) and relative humidity below 70 percent without direct contact with sunlight.

Proper handling of synchronous belts is also important in preventing damage that could reduce their performance capabilities. Synchronous belts should never be crimped or tightly bent. Belts should not be bent tighter than the minimum recommended pulley size specified for each belt section, or pitch. Belt backside bending should be limited to the values specified in Table 21 for a minimum diameter backside idler.

SECTION 16 STANDARDS APPLICABLE TO BELTS

Different belt tooth configurations are shown in Figure 19 and their characteristics are describedin Table 3, both on page T-15. Since synchronous belts are manufactured by several manufacturers, each has established individual standards. Subsequently, the following general standards have been published:

1. Specifications by the Rubber Manufacturers Association for Drives using Synchronous Belts.

2. Synchronous Belt Drives – specification by the International Organization for Standardization.

Based on these, as well as standards developed by belt manufacturers, the following information is presented in this handbook:

Recommended Tension for Length Measurement ................................... Table 22

Belt Width Tolerances ....................................................................................Table 23

Pitch Length Tolerances ............................................................................... Table 24

Center Distance Tolerances  ........................................................................ Table 25

Overall Belt Thickness dimensions ............................................................. Table 26

Overall Belt Thickness Tolerances ............................................................... Table 27

Length Measurement

The pitch length of a synchronous belt is determined by placing the belt on a measuring fixture comprising two pulleys of equal diameter, applying tension and measuring the center distance between the two pulleys. One of the pulleys is fixed in position, while the other is movable along a graduated scale.

The fixture is shown schematically in Figure  33. Any pair of equal-diameter pulleys of the proper pitch and manufactured to specifications may be used for measuring. The measuring tension is given in Table 22.

In measuring the length of a synchronous belt, the belt should be rotated at least two revolutions to seat it properly and to divide the tension equally between the two spans. The pitch length is calculated by adding the pitchcircumference of one pulley to twice the center distance:

where C is the Center Distance expressed in same units as the Pitch.

SECTION 17 STANDARDS APPLICABLE TO PULLEYS AND FLANGES

Pulleys are components manufactured to close tolerances in order to achieve best performanceand long belt life. They are available in finished form or as bar stock which can be used for in-house manufacture of prototypes or smaller quantities. For an uninitiated observer, a pulley may appear simply as a component with some trapezoidal or curvilinear grooves. In fact, the efficiency and integrity of a belt drive is closely attributed to the quality of pulleys involved. The  pulleys, therefore, should be supplied by qualified and licensed suppliers. In case of HTD and GT drives, the suppliers must be licensed by the Gates Rubber Company. Stock Drive Products is one of such licensed full line suppliers.

To achieve the reproduction of the correct pulley profile, licensed hobs are used. The following inspection and design aids are used as well:

Master Profile: A scaled line drawing of the ideal groove profile with tolerance bands plotted on dimensionally stable translucent material. Suitable for groove inspection purposes on an optical comparator.

Dimensional Profile Drawing: A line drawing of the ideal groove profile with all arcs and radii defined. Suitable for mold design. Digitized Points: A series of X and Y coordinates defining the ideal groove profile. Availablein printed form or on a floppy disk. Suitable for mold design.

Tolerancing/Inspection Procedure: A typical pulley groove tolerance band is illustrated in Figure  34. Groove inspection must be made on an optical comparator at a   specified magnification. The actual pulley groove profile must fit within the specified tolerance bands without any sharp transition or undercuts.

17.1 Pulley Tolerances

Stock Drive Products has accepted, as a minimum requirement, the Engineering Standards recommended by the Mechanical Power Transmission Association. The   Rubber Manufacturers Association, Inc. (RMA), the RubberAssociation of Canada and the Gates Rubber Company standards are approved by the Technical Committee of the above associations. These standards are in substantial compliance with standards   developed by the International Organization for Standardization (ISO).

Requirements of some belt manufacturers exceed those of RMA and ISO. Whenever practicable, Stock Drive Products adheres to those specifications which are more stringent. The following tables contain the applicable tolerances:

The following definitions are being used when considering quality of pulleys:

Eccentricity: The  allowable amount of radial run out from the pulley bore to the O.D. is shown in Table 29.

Helix Angle: Grooves should be parallel to the axis of the bore within 0.001" per inch (0.025mm per 25.4 mm) of pulley groove face width.

Draft: The maximum permissible draft on the groove form is 0.001" per inch (0.025 mm per25.4 mm) of face width and must not exceed the O.D. tolerance.

Parallelism: The bore of the pulley is to be perpendicular to the vertical faces of the pulley within 0.001" per inch (0.025 mm per 25.4 mm) of diameter with a maximum of 0.020" (0.51 mm) total indicator reading.

Pitch Accuracy: Adequate pitch to pitch accuracy is generally more difficult to achieve with molded pulleys than with machined pulleys. Recommended tolerances are listed  in Table 31.

Balancing: Balancing is often not required on machined metal pulleys. All pulleys should be statically balanced to 1/8 oz (3.5 grams) in all sizes. Drives exceeding 6500 ft/min (33m/s) may require special materials, and should be dynamically balanced to 1/4 oz×in (1.78N×mm).

Production pulleys should be made as closely to these tolerances as possible in order to maximize drive performance.

In addition to the Tables 29, 30 and 31 which define the tolerances related to pulleys manufactured by SDP/SI, Tables 32 through 35 are given for reference only, as published by ISO (International Organization for Standardization) and RMA (Rubber Manufacturers Association).

17.2 Pulley Materials

There is a wide variety of materials and manufacturing processes available for the production of  synchronous belt pulleys. In selecting an appropriate material and production process, the designer should consider dimensional accuracy, material strength, durability and production quantity. Some broad guidelines and recommendations are as follows:

1. Machining

Excellent dimensional accuracy. Economical for low to moderate production quantities.

Typical materials:
Steel – Excellent Wear Resistance.
Aluminum – Good Wear Resistance;  pulleys for power transmission drives should be hard anodized.

2. Powdered Metal and Die Casting

Good dimensional accuracy. Economical for moderate to high production quantities.

Typical materials:
Sintered Iron – Excellent Wear Resistance.
Sintered Aluminum – Good Wear Resistance;  Light Weight and Corrosion Resistant.
Zinc Die Cast – Good Wear Resistance.

3. Plastic Molding

Good dimensional accuracy. Economical for high production quantities. Best suited for light to moderate torque loads. Fiber loading improves overall material strength and dimensional stability. However, increased belt wear can result from the presence of sharp abrasive fiber ends on the finished surface.

Assistance for total drive system design is available. Please contact our Application Engineering Department.

17.3 Flange Design And Face Width Guidelines

Figure 35 illustrates the expressions used in flange and pulley design. Tables 36 and 37 pertain to flange dimensions and pulley face widths respectively.

17.4 Guidelines For GT2 Flange Design

In some instances, special pulleys are used whch are made from pulley stock. The following guidelines are given to establish the design parameters for flanges which would fit these special pulleys. If possible, standard available flanges should be used to avoid tooling charges associated with production of special sized flanges.

Steps:

  1. Determine pulley size and finished O.D. (See Tables 12 through 14 on pages T-32 thru T-37).
  2. Determine root diameter (Root Diameter = Finished O.D. – 2 x Nominal Groove Depth). See Figure 19, page T-15.
  3. Determine maximum flange seat diameter. (Maximum Flange Seat Diameter = Root Diameter – Pitch Factor).
  4. Select flange with inside diameter less than maximum flange seat diameter (see available flange sizes in the product section).
  5. Determine flange seat diameter (Flange Seat Diameter = Flange I.D. +.000" –.003")
  6. Determine flange seat width (Flange Seat Width = Flange Gauge + .020" ±.005";  seeavailable flange sizes).
  7. Flanges can be rolled, staked or punched on.

SECTION 18 DOUBLE-SIDED BELT TOLERANCES

This type of belt was introduced briefly in Section 5.1, page T-10. As previously described, this type of belt has teeth on both sides to provide synchronization from both driving surfaces. This special feature makes possible unique drive designs, such as multipoint drives, rotation reversal with one belt, serpentine drives, etc. It may also provide solutions to other difficult design problems.

Double-Sided Belts are similar in construction to regular synchronous belts, including  nylon-faced teeth on both sides. This construction uses essentially the same design parameters as standard synchronous belts. Their torque ratings are the same as conventional PowerGrip Belts ofidentical pitch and width.

Double-Sided Belts are available in MXL, XL, L, 3 and 5 mm HTD and T5 and T10 pitches from stock. See "Timing Belts and Pulleys Locator Charts" in the product section.

Double-Sided Construction

Tensile members of the PowerGrip Double-Sided Belt are helically-wound fiberglass cords providing the same load-carrying capacity as singlesided PowerGrip belts. The body is Neoprene rubber providing oil and weather resistance and protection for the fiberglass cords. Both sides ofthe belt have a specially treated nylon tooth facing that provides a tough wear-resistant surface with minimal friction.

Double-Sided Tolerances

Since Double-Sided Belts are manufactured and cut to the required width by the same method as standard PowerGrip belts, the same manufacturing tolerances apply, except for the thickness and center distance tolerances listed in Tables 38 and 39. Overall thickness, opposing teeth symmetry and pitch line symmetry are closely controlled during Double-Sided Belt manufacture.

Specifying Double-Sided Belts

The available Double-Sided Belts and other double-sided belts from stock can be found from the Timing Belt Locator Chart, on page 1-2 of the product section.

Double-Sided Drive Selection

Double-Sided Belts can transmit 100% of their maximum rated load from either side of the belt or in combination where the sum of the loads exerted on both sides does not exceed the maximum rating of the belt. For example, a Double-Sided Belt rated at 6  lb×in could be used with 50% of the maximum rated on one side, and 50% on the other; or 90% on one side, and 10% on the other.

Drive selection procedures for drives using Double-Sided Belts are much the same as for drives using conventional belting. Refer to the appropriate product and engineering sections in this catalog for drive torque ratings, engineering information, pulley details, belt tension recommendations,etc. Some manufacturers, however, are producing double-sided belts which have nylon faced teeth on one side only. For those belts, the limitations given in Section 5.1, page T-10 apply.

SECTION 19 LONG LENGTH TIMING BELT STOCK SPECIFICATIONS

Brief mention of this type of belt was given in Section 5.2, page T-10. As previously indicated, long length belt stock is produced in spiral form. Spiral cut belting is produced from a belt sleeve by moving the slitter laterally while the belt sleeve is rotating.

The resulting belting does not have continuous tensile cords, and the teeth are not perfectly perpendicular to the longitudinal axis of the belt. As a result, wider belts may cause performance problems (shown with * in Table 40). As long as the belt width is narrow, these properties have been found to contribute little if any detrimental effects to belt performance. The maximum belt width available using this process is 1" (25 mm). Tensile modulus and strength are equivalent to conventional endless and long length belting.

This innovative product is available in all types of belting in all pitches. Reciprocating carriage drives requiring the use of higher performance  curvilinear tooth belt products, in long length form, can now be easily handled.

This type of belt is called belt stock, and its availability from stock is indicated on the Timing Belt Locator Chart, on page 1-2, at the beginning of the belt product section.

Drive Selection with Neoprene Long-Length Belting

Drive selection procedures for drives using Long-Length Belting are much the same as   for drives using conventional endless belting. Refer to the appropriateproduct and engineering sections in this catalog for drive torque ratings, engineering information, pulley details, belt tension recommendations etc. Table 40 includes rated belt working tension data, for those applications for which it could be helpful, as well as maximum length available in each pitch. For drive design selection assistance with belt stock, contact our Application Engineering Department.

This innovative product is available in all types of belting in all pitches. Reciprocating carriage drives requiring the use of higher performance  curvilinear tooth belt products, in long length form, can now be easily handled.

This type of belt is called belt stock, and its availability from stock is indicated on the Timing Belt Locator Chart, on page 1-2, at the beginning of the belt product section.

Drive Selection with Neoprene Long-Length Belting

Drive selection procedures for drives using Long-Length Belting are much the same as   for drives using conventional endless belting. Refer to the appropriateproduct and engineering sections in this catalog for drive torque ratings, engineering information, pulley details, belt tension recommendations etc. Table 40 includes rated belt working tension data, for those applications for which it could be helpful, as well as maximum length available in each pitch. For drive design selection assistance with belt stock, contact our Application Engineering Department.

SECTION 21 DRIVE RATIO TABLES

In the design of belt drives, we usually know the speed ratio (transmission ratio) and we need to determine pulley sizes, center distance and belt length. These quantities are shown in Figure 39, for an open (uncrossed) belt. The Drive Ratio Tables (Table 41, starting on page T-74) are designed to facilitate the determination of these quantities. They list the following information:

N1/N2 = the transmission ratio obtained when the larger pulley (N1 teeth) is the input andsmaller pulley (N2 teeth) is the output. Given to 3 decimal places.
N2/N1= the transmission ratio obtained when the larger pulley (N1 teeth) is the output and the smaller pulley (N2 teeth) is the input. Given to 3 decimal places. (Note that N1/N2 is the reciprocal of N2/N1)
N1 = number of teeth on larger pulley.
N2 = number of teeth on smaller pulley.
N1 – N2 = difference between number of teeth on larger and smaller pulleys. This number isuseful in center-distance determination.
C MIN = The minimum center distance between pulleys for a belt of unit pitch. If the pitch is denoted by p, the actual minimum center distance is a product of C MIN and p. The minimum center distance is determined from the condition that at the minimum center distance, the pitch circles of the pulleys can be assumed to touch. This will generally give a satisfactory approximation to the practical minimum center distance. The table is based on the equation:

At the beginning of the table, a list of standard pulley sizes is shown. The smallest pulley has 10 teeth and the largest, 156 teeth.  A standard size will be the most economical. If a nonstandard size is needed, however, please contact Stock Drive Products for assistance.

The use of the tables is best illustrated by means of examples.

Example 1: For a transmission ratio of 1.067, find the number of teeth of the pulleys and the minimum center distance for a belt of 5 mm pitch.

When the transmission ratio is greater than unity, the larger pulley is the input and the smaller pulley is the output. That is to say, the transmission ratio is equal to N1/N2. The table is organized in order of increasing values of N1/N2 and decreasing values of N2/N1. Referring to the table at this value of N1/N2, we find the following entries:

Hence, there are 2 different pulley combinations for the given transmission ratio of 1.067. For each of these, the minimum center distance is 5 x (C MIN) in mm. If the smaller pulley were driving, the transmission ratio would have been 0.938. The quantity (N1 – N2) is needed in center-distance calculations, as described in the next section.

Example 2: Given a transmission ratio of 0.680, determine the pulley sizes. Since the transmission ratio is less than one, the smaller pulley is the input and the transmission ratio is given by N2/N1 = 0.680. Looking up this ratio in the table, we find N1 = 25, N2 = 17,N1 – N2 = 8. In this case, only one pulley combination is available.

Example 3: Given a driving pulley of 48 teeth and a driven pulley of 19 teeth, find the minimum center distance for a belt pitch of 3 mm. The transmission ratio is N1/N2 = 48/19 = 2.526. Looking up this ratio in the table, we find C MIN = 10.663. The minimum center distance, therefore, is given by 3 x 10.663 or 31.989 mm.

Example 4: Given a transmission ratio of 2.258, find the pulley sizes. Looking through the table, there is no entry at this value of the transmission ratio. The nearest entries are:

Since the difference between the desired ratio and the nearest available ratios is only about 0.008, it is likely that the 2.250 or 2.273 ratios will be acceptable. If this is not the case, however, the design may require review, or a nonstandard pulley combination may be considered.

SECTION 22 CENTER DISTANCE FORMULAS

22.1 Nomenclature And Basic Equations

Figure 40 illustrates the notation involved. The following nomenclature is used:

C = Center Distance (in)
L = Belt Length (in) = p NB
p = Pitch of Belt (in)
NB = Number of Teeth on belt = L/p
N1 = Number of Teeth (grooves) on larger pulley
N2 = Number of Teeth (grooves) on smaller pulley
f = One half angle of wrap on smaller pulley (radians)
q = p/2 – f = angle between straight portion of belt and line of centers (radians)
R1 = Pitch Radius of larger pulley (in) = (N1) p/2p
R2 = Pitch Radius of smaller pulley (in) = (N2) p/2p
p = 3.14159 (ratio of circumference to diameter of circle)

The basic equation for the determination of center distance is:

These equations can be combined in different ways to yield various equations for the determination of center distance. We have found the formulations, which follow, useful.

22.2 Exact Center Distance Determination – Unequal Pulleys

The exact equation is as follows:

and ø is determined from:

The value, C/p, is called the center distance factor and is tabulated for various combinations of(NB – N1) and (N1 – N2) in Table 42 in the next section.

The value of k varies within the range (1/p, 1/2) depending on the number of teeth on the belt.All angles in Equations (22-4) through (22-5) are in radians.

The procedure for center distance determination is as follows:

  1. Select values of N1, N2 (in accordance with desired transmission ratio) and NB.
  2. Compute Q = (NB – N1)/(N1 – N2).
  3. Compute f by solving Equation (22-5) numerically.
  4. Compute k from Equation (22-4).
  5. Compute C from Equation (22-3).

22.3 Exact Center Distance Determination – Equal Pulleys

For equal pulleys, N1 = N2 and Equation (22-3) becomes:

22.4 Approximate Center Distance Determination

Approximate formulas are used when it is desirable to minimize computation time and whenan approximate determination of center distance suffices.

An alternative to Equation (22-1) for the exact center distance can be shown to be the following:

where S varies between 0 and 0.1416, depending on the angle of wrap of the smaller pulley. The value of S is given very nearly by the expression:

In the approximate formulas for center distance, it is customary to neglect S and thus to obtain following approximation for C:

The error in Equation (22-9) depends on the speed ratio and the center distance. The accuracy is greatest for speed ratios close to unity and for large center distances. The accuracy is least at minimum center distance and high transmission ratios. In many cases, the accuracy of the approximate formula is acceptable. Alternatively, center distance can be obtained to sufficient accuracy using the center distance factor table (See Section 23).

22.5 Number Of Teeth In Mesh (TIM)

It is generally recommended that the number of teeth in mesh be not less than 6. The number, TIM, teeth in mesh is given by:

where l = f/p when f (see Equation (22-5)) is given in radians (see also the derivation given for pTIM in this Handbook).

22.6 Determination of Belt Size For Given Pulleys and Center Distance

Occasionally, the center distance of a given installation is prescribed and the belt length is to be determined. For given pitch, number of teeth on pulleys and center distance, the number of teeth of the belt can be found from the equation:

where the arcsin is given in radians and lies between 0 and p/2. Since NB, in general, will not be a whole number, the nearest whole number less than NB can be used, assuming a slight increase in belt tension is not objectionable. An approximate formula can be used to obtain the belt length:

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