Applicability

This standard provides a method to determine the power rating of gear sets for cylindrical grinding mills, kilns, coolers, and dryers. The formulas are applicable to steel and spheroidal graphitic iron gears with machined spur, single helical, double helical, or herringbone gear teeth commonly used for this purpose. Calculations determine the allowable rating for pitting resistance and bending strength of external spur and helical involute gear teeth.

Rating formulas

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

These rating formulas are applicable for rating the pitting resistance and bending strength of external spur and helical involute gear teeth operating on parallel axes with adjustable center distances. 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.

This standard is intended for use by experienced gear designers, capable of selecting reasonable values for the rating factors. It is not intended for use by the engineering public at large.

Values for factors assigned in other standards are not applicable to this standard nor are the values assigned in this standard applicable to other standards. Mixing values from other standards with those from this standard could lead to erroneous ratings.

The gear designer or manufacturer is not responsible for the total system unless such a requirement is clearly identified in the contractual agreement.

It is imperative that the system designer be satisfied that the system of connected rotating parts is compatible, free from critical speeds and from torsional or other vibrations within the specified speed range, no matter how induced.

Where empirical values for rating factors are given by curves, curve-fitting equations are provided to facilitate computer programming. The constants and coefficients used in curve fitting often have significant digits in excess of those inferred by the reliability of the empirical data. Experimental data from actual gear unit measurements are seldom repeatable within a plus or minus 10 percent band. Calculated gear ratings are intended to be conservative, but the scatter in actual results may exceed 20 percent.

CAUTION: Compliance with this standard does not constitute a warranty of the rating of the gear set under installed field service conditions.

Limitations

- Rating procedures are limited to open or semi-- enclosed gearing where the gear reaction forces are transmitted through a structure which provides independent bearing support for the gear and pinion. Open gears operate without any enclosure. Semi--enclosed gears operate with a guard that provides some degree of protection against contamination from dust or dirt and retains lubricant.

- Enclosed gear drives or speed reducers are expressly excluded from this standard.

- When multiple pinions are used, the number of contacts per revolution, q, shall be the same as the number of pinions.

- Unless otherwise specified by contractual agreement, the connected motor nameplate power including motor service factor shall be used to determine service factors as defined later within this standard. When not provided by the purchaser,motor service factor equal to 1.0 shall be used.

- This standard does not include gearing which has been surface hardened by nitriding or flame hardening. This gear rating practice is limited to through hardened steel and spheroidal graphitic iron gears operating with through hardened, carburized, or induction hardened steel pinions.

- Axial contact ratio of helical gear sets, mF, shall be equal to or greater than 1.0.

- Information on alignment and drive characteristics is given in annexes A and B.

- Formulas do not apply to external loads such as dropped charges, electrical short circuits and earthquakes.

- Spheroidal graphitic iron data presented in the body of the standard does not apply to austempered spheroidal graphitic iron (ADI). Rating of ADI gearing is not covered by this standard. ADI is discussed further in annex J.

- This gear rating practice is limited to maximum operating speeds of 2000 feet per minute gear pitch line velocity.

- This gear rating practice is limited to gears with normal diametral pitch of 3.0 or coarser.

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--B96.

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

- Damaged gear teeth;

- Spur gears with transverse contact ratio,mp, less than 1.0;

- Spur or helical gears with transverse contact ratio, mp, greater than 2.0;

- Interference exists between tips of teeth and root fillets;

- Teeth are pointed as defined by this standard, see clause 7;

- Backlash is zero;

- Undercut exists in an area above the theoretical start of active profile;

- The root profiles are stepped or irregular, or deviate from the generated form. The J factor calculation uses the stress concentration factors developed by Dolan and Broghamer [1]. 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;

- The helix angle at the standard (reference) pitch diameter is greater than 20 degrees for single helical and 35 degrees for double helical.

Fractures emanating from stress risers on the tooth profile, tip chipping, and failures of the gear blank through the web or rim should be analyzed by general machine design methods.