General

Calculation

This information sheet describes a procedure for calculating the load capacity of a pair of powder metallurgy (P/M) gears based on tooth bending strength. Two types of loading are considered: 1) repeated loading overmanycycles;and 2) occasional peak loading. This procedure is to be used on prepared gear designs which meet the customary gear geometry requirements such as adequate backlash, contact ratio greater than 1.0, and adequate top land. An essentially reverse procedure for establishing an initial design from specified applied loads is described in annex D.

Strength properties

Fatigue strength and yield strength properties used in these calculationsmaybe taken from previous test experience, but may also be derived from published data obtained from standard tests of the materials.

Application

This procedure is intended for use as an initial evaluation of a proposed design prior to preparation of test samples. Such test samples might be machined from P/M blanks ormade from P/M tooling based on the proposed design after it passes this initial evaluation. Final acceptance of the proposed design should be based on application testing and not on these calculations. If samples made from tooling fall short in testing, it may be possible to use the same tooling for a design adjusted for greater face width.

Limitations

Gearsmade from allmaterials and by all processes, including P/M gears, may fail in a variety of modes other than by tooth bending. This information sheet does not address design features to resist these other modes of failure, such as excessive wear and other forms of tooth surface deterioration.

CAUTION: The calculated load capacity from this procedure is not to be used for comparison with AGMAratings of wrought metal gears, even though there are many similarities in the two procedures.

Types of gears

This calculation procedure is applied to external spur gears, the type of gear most commonly produced by the P/M process.

Dimensional limitations

This procedure applies to gears whose dimensions conform to those commonly produced by the P/M process for load carrying applications:

- Finest pitch: 0.4 mm module;

- Maximum active face width: 15 × module, with a 65 mmmaximum;

- Minimum number of teeth: 7;

- Maximum outside diameter: 180 mm;

- Pressure angle: 14.5° to 25°.

Gear mesh limitations

Some of the calculations apply only to meshing conditions expressed as a contact ratio greater than one and less than two. This translates into the requirement that there is at least one pair of contacting teeth transmitting load and no more than two pairs.