MECHANICAL GEARS ACCURACY REQUIREMENTS BASICS AND TUTORIALS


Control of gear accuracy may be necessary for many reasons. Proper operation, particularly at higher speeds, and low dynamic loads are probably the most important.

Depending on the application, specifications on sound level, transmission smoothness, and accuracy of motion are often controlling.

AGMA 2000-A88, Gear Classification and Inspection Handbook—Tolerances and Measuring Methods for Unassembled Spur and Helical Gears;6 ANSI/AGMA 2009-A98, Bevel Gear Classification,Tolerances, and Measuring Methods;7 and ANSI/AGMA 2011-A98, Cylindrical Wormgearing Tolerance and Inspection Methods,8 contain classification systems covering tolerances for common types of gears.

In these systems, tolerances are tabulated for a series of accuracy grades that range from 3 to 15.When selecting an accuracy grade number, care must be taken as the newer international convention standards use lower numbers to represent more precise tolerances.

In older AGMA standards, the higher numbers represent more precise tolerances.The accuracy grade standards cover the gears themselves and do not generally apply to gears in enclosed drives.

By far the majority of applications can be satisfied by gears in the midrange, around grade 8. Rarely does an application require a gear in the most precise grade classification.The selection of the proper accuracy grade for a specific application requires good judgment and experience.

Care should be used in making a final selection to meet the specific conditions, as more precise gears are more expensive to manufacture.

Precise Motion Transmission.
Many applications use gears as a means for precise transmission of motion. Usually, more precise accuracy grades are required for these gears. Examples of such applications are navigational tracking, telescopes, printing presses, and index devices for machine tools.

High-Speed Considerations.
Gear inaccuracies become more critical as operating speed increases.The most obvious problem may be high noise level. In addition, the dynamic loads on the teeth caused by inaccuracies may be a substantial part of the total transmitted load.

Accuracy Versus Cost. 
The cost of manufacturing and inspection escalates rapidly with increasing accuracy.Therefore, the user should use care to avoid specifying higher grades than required for the application.Table 5.5 compares relative first cost to gear type.


Mounting of Gears
Close attention must be given to the mounting of gears and the maintenance of alignment under operating conditions. Obviously, the accuracy and stiffness of mounting must be commensurate with the precision of the gears themselves to obtain optimum results.

Some types of gears require close axial positioning of either or both members of a pair to obtain proper operation. Examples are bevel gears (both), cylindrical wormgears (gear only), and double-enveloping wormgears (both).

The gearshaft positioning must be supported by bearings and bearing mounts of suitable capacity to accommodate all radial and thrust loads involved.

Additional Considerations
When specifying the quality of a given gear, there are additional or special considerations that should be reviewed.

● Backlash allowances in tooth thickness
● Matching pinion and gear mating modifications
● Replacement gearing

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