BACKLASH

Backlash is the clearance between mating tooth surfaces of a gear pair when assembled at the designed center distance. Backlash at the operating mounting distance is a desirable condition of most gear assemblies as a means of avoiding tooth jamming due to permissible errors in profile, pitch, concentricity, lead and so forth, and also due to thermal expansion of gears running at high speed. A certain amount of clearance between the mating tooth surfaces is also beneficial for the lubrication of the operating gears.

The required minimum backlash of operating gears can be provided by deliberately thinning the teeth during the manufacture of the gears, thus producing gears with teeth thinner at the reference circle than the spacing between the adjacent teeth. That condition of the gear can be inspected by measuring the tooth thickness directly, using one of the methods described earlier.

Because of the permissible errors of gear parameters that affect the backlash of assembled gears, backlash variations are commonly present and may be a specified condition requiring inspection. The most commonly specified backlash tolerance refers to the minimum backlash, measured at the tightest point of mesh between mating gears. Generally backlash is measured by means of an indicator installed with its axis perpendicular to the tooth surface with which the indicator point is in contact. The mating gears are mounted at operating distance and one of the gears, the master pinion when checking a particular work gear, is held solidly and the gear being tested is turned in both directions. The range over which the work gear can be moved is the measure of backlash as the gear teeth in engagement. For determining the minimum backlash of the gear that test might have to be repeated over a complete or, in the extreme case, over several revolutions of the gear and pinion.

For checking the minimum backlash of large gear pairs that are too heavy to be rotated by hand feeler gages may be used; these measure directly the clearance between meshing teeth in various positions around the gear being inspected.