Machine Design

Special Gear Requirements

Backlash: The AGMA standards recommend backlash ranges to provide proper running clearances for mating gears. An overly tight mesh may produce too much noise, overheating, and structural overload. However, some applications require zero backlash.

Quiet gears: To make gears as quiet as possible, specify the finest pitch allowable for load conditions. In some instances, pitch is coarsened to change mesh frequency to produce a more pleasant, lower-pitch sound. For quiet gears use a low pressure angle. Use a modified profile to include root and tip relief. Allow enough backlash. Use high-quality numbers. Specify a surface finish of 20 ∝in. or better. Balance the gear set. Use a nonintegral ratio so the same teeth do not repeatedly engage if both gear and pinion are hardened steel. If the gear is made of a softer material, an integral ratio allows the gear to cold work and conform to the pinion, thereby promoting quiet operation. Make sure critical speeds are at least 20% apart from operating speeds or speed multiples and from frequency of tooth mesh.

Tooth spacing: Some gears require high accuracy in the circular spacing of teeth. Thus, specification of pitch may be required in addition to an accuracy class specification.

Matched-set gearing: In applications requiring extremely high accuracy, it may be necessary to match pinion and gear profiles and leads so a mismatch does not exceed the tolerance on profile and lead for the intended application. For a more complete discussion of single-speed gear drives, read Gearboxes in the Mechanical Fixed-Speed Drives section. For multiple-speed transmissions, read the Mechanical Adjustable-Speed Drives section.

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