Backlash (Gear)
Backlash j is the clearance between the working and non-working tooth flanks of two meshing gears. It is necessary for lubrication, accommodation of thermal expansion, and adherence to manufacturing tolerances. Excessive backlash leads to reversal play and noise; insufficient backlash can cause jamming.
Causes and Necessity
Backlash arises from the sum of manufacturing tolerances (tooth thickness, center distance) and is unavoidably by design. A minimum clearance must exist for lubrication of the gear mesh so that the lubricant can penetrate between the flanks. Thermal expansion at operating temperature increases the gear diameters; minimum backlash prevents jamming. The gear accuracy class (DIN 3961/3962, ISO 1328) defines the permissible backlash through tooth thickness and center-distance tolerances.
Backlash: Effects of Too Large / Too Small
| Condition | Consequence | Measure |
|---|---|---|
| Excessive backlash | Reversal play, impacts, noise | Reduce center distance, tighter tolerance class |
| Insufficient backlash | Jamming, overload, scuffing | Increase center distance, reduce tooth thickness |
| Standard backlash | Smooth running, reliable lubrication | Maintain tolerance class |
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