Gear Backlash

Backlash is the small gap between mating tooth flanks when a gear pair is at rest — the amount one gear can rotate before its teeth actually contact the other gear's teeth. Zero backlash sounds ideal but isn't practical: manufacturing tolerance, thermal expansion, and deflection under load all need some clearance, or the gears will bind.

Backlash is introduced by thinning each tooth slightly at the pitch circle rather than by widening the tooth gap arbitrarily — the tooth stays centered on its nominal position, just narrower. In the involute profile generator behind this site, that shows up as a small angular offset applied to both flanks:

backlash angle = backlash / (4 × pitch radius)

applied symmetrically so the tooth thins by the full backlash amount at the pitch circle while the flank curve itself is unchanged — just rotated slightly inward. The catalog's fixed gears use a conservative default backlash suited to general-purpose fit; the generator exposes backlash as a direct parameter if you need a tighter or looser fit for a specific application.

How much is enough?

• Precision/instrument gearing — minimal backlash, tight tolerances, often metal-cut.
• 3D-printed mechanisms — more backlash than metal gearing, since FDM dimensional accuracy is coarser; see Designing Gears for 3D Printing for specific allowances.
• Laser-cut/CNC gears — backlash needs also account for kerf or cutter-diameter offset, not just the nominal tooth thinning; see Laser Cutting Gears and CNC Machining Gears.

Too much backlash causes play/lash and impact loading on direction reversals; too little causes binding, especially once dirt, wear debris, or thermal expansion take up the margin. When in doubt, err slightly loose — a gear that's slightly loose still works; one that binds can strip teeth or stall a motor.