Gear tooth contact patterns

The way in which mating gear teeth contact indicates how well they are aligned, Figure 1. If practical, record tooth contact patterns under either loaded or unloaded conditions. For no-load tests, paint the teeth of one gear with marking compound. Then, roll the teeth through mesh so the compound transfers the contact pattern to the unpainted gear. Lift the pattern from the gear with scotch tape and mount it on paper to form a permanent record.

For loaded tests, paint several teeth on one or both gears with machinist’s layout lacquer. Run the gears under load for a sufficient time to wear off the lacquer and establish the contact patterns. Photograph the patterns to obtain a permanent record.

Describe all important observations in writing, using sketches and photographs where needed. Identify and mark each component (including gear teeth and bearing rollers), so it is clearly identified in the written description, sketches, and photographs. It is especially important to mark all bearings, including inboard and outboard sides, so their location and position in the gearbox can be determined later.

Describe components in a consistent way. For example, always start with the same part of a bearing and progress through the parts in the same sequence. This helps to avoid overlooking any evidence.

Concentrate on collecting evidence, not on determining the cause of failure. Regardless of how obvious the cause may appear, do not form conclusions until all the evidence is considered.

Gear geometry. The load capacity of the gearset will need to be calculated later. For this purpose, obtain the following geometry data, either from the gears and gear housing or their drawings:

• Number of teeth.

• Outside diameter.

• Face width.

• Gear housing center distance for each gearset.

• Whole depth of teeth.

• Tooth thickness (both span and top land measurement).