The clearance between the non-working flanks of mating teeth, measured along the pitch circle, along the line of action, or as rotation of one gear with the other held. Some backlash is needed for an oil film, thermal growth and manufacturing tolerances; too little and the gears bind and overheat, too much and they clunk on load reversal and lose positioning accuracy. It is set by tooth thickness and center distance.
Backlash checks at five thou on the indicator, rock it at four places around the gear to be sure it's not tight somewhere.
Always check backlash at several positions around the gear; runout can make it tight at one spot and generous everywhere else.
Also heard as
- lash
- circumferential backlash
- normal backlash
Related terms
Lash
Shop clipping of backlash, the free play between mating gear teeth, as in set the lash or check lash.
Slop
Shop word for too much play in a gear train or drive, felt as free movement before anything turns.
Tooth thickness
How thick a gear tooth is along the pitch circle, the dimension that is thinned to create backlash.
Center distance
The distance between the axes of two meshing gear shafts, set by the housing bores.