Backlash – the intentional clearance between meshing gear teeth – is critical for thermal expansion, lubrication, and manufacturing tolerances. Too little backlash causes jamming, overheating, and tooth breakage; too much results in position error, rattle, and impact loading. Determining the correct value involves several steps.
First, consider operating temperature – if your gearbox runs at 80°C with aluminum housing and steel gears (CTE mismatch ~12 ppm/°C vs 11.5 ppm/°C), calculate thermal contraction: For a 100 mm center distance, ΔC = (α1-α2)CDΔT ≈ 0.0005*100*60 = 0.03 mm. Add this to minimum backlash.
Second, speed and precision – robotic joints need near-zero backlash (0.01–0.03 mm) via anti-backlash gears or dual-path drives; conveyor drives can tolerate 0.1–0.25 mm.
Third, lubrication type – grease needs 20–30% more backlash than oil baths to allow film formation.
Fourth, AGMA standards suggest: For general industrial (AGMA 9–10), normal backlash = 0.04 ÷ module (metric) or 0.005 ÷ diametral pitch (inch). For high-speed (over 25 m/s), increase by 50% to prevent tooth interference at resonance.
Fifth, tooth thickness tolerance – gears cut to the “deep” side of tolerance (smaller teeth) increase effective backlash. You specify backlash by requesting a center distance offset or specifying tooth thickness reduction per DIN 3967 or AGMA 2002.
Practical method: For medium carbon steel gears in a cast iron housing (room temperature), start with 0.05–0.10 mm for module 2–4, or 0.10–0.20 mm for module 6–10. Then validate with a test assembly – if you feel no play but rotate freely after running hot, it’s correct. For servo systems, use a “spring-loaded” split gear or zero-backlash coupling instead of relying on fixed backlash. Always specify backlash before manufacturing – after gears are cut, you can only increase backlash (by grinding or lapping), never decrease. Document your requirement as “backlash measured at standard center distance, per AGMA 2000-A88” or ISO 1328-1:2013.