Rating formulas

This standard provides a method by which different gear designs can be theoretically rated and compared. It is not intended to assure the performance of assembled gear drive systems.

These fundamental rating formulas are applicable for rating the pitting resistance and bending strength of internal and external spur and helical involute gear teeth operating on parallel axes. The formulas evaluate gear tooth capacity as influenced by the major factors which affect gear tooth pitting and gear tooth fracture at the fillet radius.

The knowledge and judgment required to evaluate the various rating factors come from years of accumulated experience in designing, manufacturing, and operating gear units. Empirical factors given in this standard are general in nature. AGMA application standards may use other empirical factors that are more closely suited to the particular field of application. This standard is intended for use by the experienced gear designer, capable of selecting reasonable values for the factors. It is not intended for use by the engineering public at large.

Exceptions

The formulas of this standard are not applicable to other types of gear tooth deterioration such as plastic yielding, wear, case crushing and welding. They are also not applicable when vibratory conditions exceed the limits specified for the normal operation of the gears (see ANSI/AGMA 6000-A88, Specification for Measurement of Lateral Vibration on Gear Units).

The formulas of this standard are not applicable when any of the following conditions exist:

- Damaged gear teeth.

- Spur gears with transverse contact ratio, εa, less than 1.0.

- Spur or helical gears with transverse contact ratio, εa, greater than 2.0.

- Interference exists between tips of teeth and root fillets.

- Teeth are pointed.

- Backlash is zero.

- Undercut exists in an area above the theoretical start of active profile. The effect of this undercut is to move the highest point of single tooth contact, negating the assumption of this calculation method. However, the reduction in tooth root thickness due to protuberance below the active profile is handled correctly by this method.

- The root profiles are stepped or irregular. The YJ factor calculation uses the stress correction factors developed by Dolan and Broghamer. These factors may not be valid for root forms which are not smooth curves. For root profiles which are stepped or irregular, other stress correction factors may be more appropriate.

- Where root fillets of the gear teeth are produced by a process other than generating.

- The helix angle at the standard (reference) diameter(Footnote *) is greater than 50 degrees.

Scuffing criteria are not included in this standard. A method to evaluate scuffing risk can be found in AGMA 925-A03. This information is provided for evaluation by users of this standard, with the intent to include a scuffing evaluation method in a future version of this standard.

Design considerations to prevent fractures emanating from stress risers on the tooth profile, tip chipping, and failures of the gear blank through the web or hub should be analyzed by general machine design methods.

Footnote * - Refer to ANSI/AGMA 1012-F90 for further discussion of standard (reference) diameters.