A reduction gearbox for a packaging machine was spec’d with 0.05 mm backlash at the output. The gearbox vendor built it to spec — matched gears, ground teeth, preloaded bearings. After installation, the gearbox ran hot. 85°C oil temperature after an hour of running. The customer thought it was undersized. It wasn’t. The backlash was too tight. The gears were trying to roll against each other.
Why “minimum backlash” is the wrong target
Gears need backlash for three reasons. First, thermal expansion — the gear and shaft heat up when running. Without clearance, the teeth bind. Second, lubricant film — the oil needs a path to reach the contact surfaces. If backlash is zero, the oil can’t get in, and the gears run dry. Third, manufacturing variation — even ground gears have a tooth thickness variation of ±0.01 mm. At 0.05 mm total backlash, a slightly thick tooth on one gear binds against a slightly thin gap on the other.
The vendor built to 0.05 mm backlash. At room temperature. When the gears warm up to 60°C, the pitch diameter grows by about 0.02 mm per 100 mm. The backlash collapses to 0.03 mm. The oil film can’t form. The gears run at 85°C. The customer burns through gear oil every three months.
The backlash that works
For a 100 mm pitch diameter steel gear on steel, 0.10 to 0.15 mm backlash at the mesh is the practical minimum. That’s the number that allows thermal growth, oil film formation, and normal manufacturing variation. For a 200 mm gear, 0.15 to 0.25 mm. These aren’t “loose” — they’re the backlash values that standard industrial gearboxes run. A new gearbox from a reputable manufacturer will have about this range out of the box. If you’re building your own gear drive, aim for 0.1 mm per 100 mm of pitch diameter.
The customer loosened the backlash on their gearbox. They adjusted the center distance by 0.05 mm (the gearbox had eccentric hubs). The backlash went from 0.05 to 0.12 mm. The oil temperature dropped from 85°C to 58°C. The gear noise dropped by 5 dB. The positioning accuracy didn’t suffer — the packaging machine didn’t need 0.05 mm repeatability, it needed 0.5 mm. The spec of 0.05 mm was overkill and actively harmful.
When you actually need tight backlash
Tight backlash (0.02 to 0.05 mm) is required in two cases. First, reversing motion where the load direction changes every cycle. A CNC rotary axis that moves back and forth needs minimal backlash to avoid lost motion. Second, precision indexing where the position must be repeatable within 0.01 degrees. For these applications, use a preloaded gear (double gear with spring) or a worm gear. Don’t try to get low backlash from a single spur mesh by squeezing center distance. That just makes the gears run hot.
For a packaging machine conveyor drive, a pump drive, or a fan drive, you don’t need tight backlash. The load runs in one direction. The positioning accuracy is ±1 mm at best. Spec 0.10 mm backlash and move on. The gearbox will run cool and quiet.
Measuring backlash
Backlash is measured at the pitch circle. Lock the output shaft. Rotate the input shaft back and forth until you feel the play. Measure the rotation at the input. Convert to linear movement at the pitch circle. Don’t measure at the teeth directly — that’s where the play is most visible but it’s not the engineering number. Use the input rotation method. A dial indicator on the input shaft coupled with a torque arm gives a repeatable measurement.
0.1 mm per 100 mm pitch diameter is the sweet spot for spur gears. Tighter makes them run hot and bind. Looser makes them noisy but harmless. If you need reversing accuracy, use a preloaded gear — don’t squeeze center distance on a single mesh. The hot-running gearbox wasn’t underpowered, it was over-tightened.
