A Complete Guide to Gears

Spur Gears

Spur Gears have teeth parallel to their axis and a cylindrical shape. It is also the easiest of the gears to manufacture and has the greatest number of applications.

Helical Gears

Helical Gears are cylindrically shaped gear with helicoid teeth. An advantage of helical gears are they can bear more load than spur gears and they run quieter. A disadvantage is the axial thrust force the helix form causes. Helical gears are widely used in industry.

Helical gears have a higher tooth strength due to the elongated helical wraparound tooth base support. There is also greater tooth overlap, creating a larger contact ratio, this is what allows them to carry more weight. 

Parallel helical gears, which transfer power between parallel shafts, and crossed helical gears, which transfer power between non-parallel shafts, are the two main types of helical gears.

Double Helical Gears

This is a gear with both left-hand and right-hand helical teeth. The double helical form balances the inherent thrust forces.

Screw Gears (Crossed Helical Gear)

Two helical gears of opposite helix angles will mesh if their axes are crossed. As separate gear components, they are merely conventional helical gears. Installation on crossed axes converts them to screw gears. They offer a simple means of gearing skew axes at any angle. Because they have point contact, their load carrying capacity is very limited.

Internal Gears

Internal gears are cylindrical shaped gears with their teeth inside the circular ring. They can mesh with a spur gear and are often used in planetary gear systems.

The way internal gears mesh is dependent on specific dimensions including gear size (pitch diameter) and the angle of the teeth on the gear;  proper calculations are required to ensure an accurate fit. 

When utilizing internal gears, there is potential for excessive noise, premature tooth wear, and vibration in the gear train. The three different types of interference causing these issues include: 

• Involute Interference – often due to the use of a pinion with too few teeth where one gear tooth's non-involute portion comes into contact with the other gear tooth's involute portion.
• Trochoid Interference - occurs when there are more contact points due to the tooth profile's deviation from the ideal involute curve.
• Trimming Interference - When trying to pull the meshing gears apart, radial interference occurs, making it impossible to separate them properly.

Properly cut and applied internal gears excel in systems needing high reduction ratios (slowing things down in a single step). They work great in compact situations such as for robotics or heavy machinery. 

Spur Racks

This is a linear shaped gear which can mesh with a spur gear with any number of teeth. The spur rack is a portion of a spur gear with an infinite radius.

Helical Racks

This is a linear shaped gear which meshes with a helical gear. Again, it can be regarded as a portion of a helical gear with infinite radius.

Viewed in the normal direction, the meshing of a helical rack and gear is the same as a spur gear and rack. However, you can also perform calculations related to helical racks in the radial system (such as when it’s perpendicular to a gear’s axis). 

Face Gears

This is a pseudo-bevel gear that is limited to 90° intersecting axes. The face gear is a circular disc with a ring of teeth cut in its side face, hence the name face gear. Tooth elements are tapered towards its center. The mate is an ordinary spur gear. It offers no advantages over the standard bevel gear, except that it can be fabricated on an ordinary shaper gear generating machine.

Straight Bevel Gears

This is a gear in which the teeth have tapered conical elements that have the same direction as the pitch cone base line (generatrix). The straight bevel gear is both the simplest to produce and the most widely applied in the bevel gear family.

Spiral Bevel Gears

This is a bevel gear with a helical angle of spiral teeth. It is much more complex to manufacture but offers a higher strength and lower noise than other bevel gear pairs.

Worm And Worm Gears

Worm set is the name for a meshed worm and worm gear. The worm resembles a screw thread; and the mating worm gear a helical gear, except that it is made to envelope the worm as seen along the worm’s axis. The outstanding feature is that the worm offers a very large gear ratio in a single mesh. However, transmission efficiency is very poor due to a great amount of sliding as the worm tooth engages with its mating worm gear tooth and forces rotation by pushing and sliding. With proper choices of materials and lubrication, wear can be contained, and noise is reduced.

Double Enveloping Worm Gears

This worm set uses a special worm shape in that it partially envelops the worm gear as viewed in the direction of the worm’s gear axis. Its big advantage over the standard worm is a much higher load capacity. However, the worm gear is very complicated to design and produce.

Zerol Gears

Zerol gears are a special case of spiral bevel gear. It is a spiral bevel with zero degrees of spiral angle tooth advance. It has the characteristics of both the straight and spiral bevel gears. The forces acting upon the tooth are the same as for a straight bevel gear.

Hypoid Gears

This is a deviation from a bevel gear that originated as a special development for the automobile industry. Hypoid gears permit the drive to the rear axle to be nonintersecting and thus allow the auto body to be lowered. It looks very much like the spiral bevel gear. However, it is complicated to design and is the most difficult to produce on a bevel gear generator.