Rating formulas

This standard provides a method by which different gear designs can be compared.

The formulas in this standard are intended to establish a uniformly acceptable method for calculating the pitting resistance and bending strength capacity of generated straight bevel, zerol bevel and spiral bevel gear teeth; curved and skewed tooth. They apply equally to tapered depth and uniform depth teeth.

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 rating formulas in this standard are not applicable to other types of gear tooth deterioration such as scuffing, wear, plastic yielding, scoring, case crushing and welding and are not applicable when vibratory conditions exceed the limits specified for the normal operation of the gears (see ANSI/AGMA 6000-B96, Specification for Measurement of Lateral vibration on Gear Units).

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

- bevel gears with offset, such as hypoids;

- straight and zerol bevel gears with transverse contact ratios, m_p (e_a) less than 1.0;

- bevel gears with modified contact ratios, m_o (e_o) less than 1.0;

- bevel gears which have a poor contact pattern;

- interference exists between tips of teeth and root fillets;

- teeth are pointed;

- backlash is zero;

- bevel teeth finished by forging, casting or sintering.

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.

Scuffing

Formulas for scuffing resistance on bevel gear teeth are not included in this standard. At the present time, there is insufficient agreement concerning the method for designing bevel gears to resist scuffing failure.

Wear

Very little attention and concern have been devoted to the study of gear tooth wear. This subject primarily concerns gear teeth with low surface hardness or gears with improper lubrication. No attempt has been made to cover gear tooth wear in this standard.

Plastic yielding

This standard does not extend to stress levels above those permissible for 10³ cycles, since stresses in this range may exceed the elastic limit of the gear tooth in bending or in surface compressive stress. Depending on the material and the load imposed, a single load cycle exceeding the stress level for 10³ life cycles (see clause 16) can result in plastic yielding of the gear tooth.