Gearbox Backlash: What It Is, What Is Normal, and How to Measure It
Gearbox Backlash: What It Is, What Is Normal, and How to Measure It
Backlash is the small amount of free movement between meshing gear teeth. Every gearbox has some, by design. It is only a problem when it is excessive, uneven, or misunderstood. This article explains what the numbers mean, how to measure backlash on site, and how to tell wear from a specification issue.
Backlash in one paragraph
When two gears mesh, the tooth thickness is slightly less than the space between the mating teeth. That gap is backlash. It exists because gears cannot be manufactured or mounted perfectly, because they expand when hot, and because a film of oil must fit between the flanks. A gearbox made with zero backlash would seize as soon as it warmed up.
Typical backlash values
Backlash is measured at the pitch circle and quoted in millimetres or as an angular value. As a rough guide for industrial reducers:
| Gearbox size | Normal backlash (new) | Investigate beyond |
|---|---|---|
| Small (worm units, up to 60 mm centre) | 0.05 – 0.15 mm | 0.3 mm |
| Medium (helical units, 60–160 mm centre) | 0.10 – 0.30 mm | 0.6 mm |
| Large (industrial units, above 160 mm) | 0.20 – 0.60 mm | 1.0 mm |
Precision planetary and servo reducers are a different category entirely: backlash of 1–3 arc-minutes (essentially zero for our purposes) is normal there, and they are built with shimmed or preloaded bearings to achieve it.
Why backlash increases over time
- Normal tooth wear — the flanks wear away, so the gap grows. Slow and expected.
- Bearing wear or loss of preload — the shaft moves off position, and backlash changes with load direction.
- Wrong oil or neglected oil — accelerates flank wear dramatically.
- Overload events — a jam can bend a shaft or deform a gear, producing sudden, uneven backlash.
- Mounting errors — distortion of the housing when bolts are over-torqued on an uneven base.
The key diagnostic: backlash that is uniform around the full revolution is wear. Backlash that varies depending on where the gear stops indicates a bent shaft, an eccentric gear or a damaged tooth — that needs an inspection, not an oil change.
How to measure backlash on site
You need a dial indicator and a way to hold one shaft still.
- Lock the input shaft (or the output, whichever is easier to hold).
- Mount the dial indicator on the housing so its probe touches a point on the other shaft's gear tooth flank, at the pitch circle if possible.
- Rock the free shaft gently by hand through its full free travel.
- Read the total movement — that is the backlash for that position.
- Repeat at four positions, 90° apart, on the same gear.
- Compare with the nameplate or the manual value.
Do this cold, and then again after the unit has run to normal temperature. Backlash falls as the unit heats up because the gears expand; a unit that is tight when hot is a real risk.
When backlash actually matters
- Positioning axes — any drive that must stop at an exact angle needs low backlash. This is the one case where it is a hard specification.
- Reversing drives — repeated direction changes make backlash audible and can shock-load the gears. Consider a low-backlash design or a different gear type.
- Synchronised lines — where two or more drives must stay in step, unequal backlash causes drift.
- High-speed drives — backlash contributes to noise and dynamic loading.
For conveyors, mixers, pumps and most continuous-duty machinery, normal backlash is irrelevant to performance and should not be a selection criterion.
Adjustment and repair
Some designs allow backlash to be adjusted by shimming or by repositioning the bearing carriers. Others — most modern hardened-and-ground gearboxes — do not. The practical guidance:
- If backlash is within the wear limit and uniform, keep running and monitor.
- If backlash exceeds the limit but the unit is otherwise sound, plan a rebuild at the next scheduled stop.
- If backlash is uneven, or accompanied by noise, heat or metal in the oil, stop the unit and inspect.
WGT supplies replacement gear sets, bearings and output shafts for our own gearboxes, and will advise on repair versus replacement. Send the nameplate and backlash readings through the contact page and our engineering team will confirm the wear limit for your specific unit.
Frequently asked questions
How much backlash is too much?
It depends on frame size, but as a rule of thumb anything above roughly twice the new-unit value warrants investigation — and any sudden change warrants an immediate stop.
Does more backlash mean the gearbox is worn out?
Not necessarily. It means the tooth flanks have worn. Whether that is acceptable depends on the application and the remaining load capacity.
Can backlash be reduced on a standard gearbox?
Only if the design provides adjustment. Most hardened-and-ground industrial gearboxes do not, and attempts to shim them usually cause more harm than good.
