Gear Wear, Adhesion and Design Guidelines

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New gears contain surface imperfections or roughness that are inherent to the manufacturing process. During the initial run-in period, these imperfections are reduced through wear. Smoothing of the gear surfaces is to be expected . Mild wear will occur even when adequate lubrication is provided, but this wear is limited to the oxide layer of the gear teeth. Mild wear is beneficial because it increases the contact areas and equalizes the load pressures on gear tooth surfaces. Furthermore, the smooth gear surfaces increase the film thickness and improve lubrication.

The amount of wear that is acceptable depends on the expected life, noise, and vibration of the gear units. Excessive wear is characterized by loss of tooth profile, which results in high loading, and loss of tooth thickness, which may cause bending fatigue.

Wear cannot be completely eliminated. Speed, lubricant viscosity, and temperature impose practical limits on gear operating conditions. Gears that are highly loaded, operate at slow speeds, i.e., less than 30 m/min (100 ft/min), and rely on boundary lubrication are particularly subject to excessive wear. Slow-speed adhesive wear is highly dependent upon lubricant viscosity. Higher lubricant viscosities provide significant wear reduction, but viscosities must be carefully selected to prevent overheating.

The following guidelines should be observed to minimize the onset of adhesive wear in gear units:

• Gear teeth should have smooth surfaces.

• If possible, the run-in period for new gear units should be restricted to one-half load for the first hours of operation.

• Use the highest speeds possible. High-load, slow-speed gears are boundary lubricated and are especially prone to excessive wear. For these applications, nitrided gears should be specified.

• Avoid using lubricants with sulfur-phosphorus additives for very slow-speed gears (less than 3 m/min, or 10 ft/min).

• Use the required quantity of cool, clean, and dry lubricant at the highest viscosity permissible.