Robot joint backlash: how to measure it and what to do

A robot leaving double weld marks, showing a jump at a point when you reverse an axis direction, or whose TCP drifts a few tenths only on certain paths usually has mechanical backlash in a joint, not a program problem. Backlash (play) is the free movement between the input and output of the transmission before the load responds. A new robot has minimal backlash; with hours and wear it grows until it shows up on the part.

The most expensive mistake made on the shop floor is confusing backlash with calibration loss and re-teaching the TCP to «compensate». It doesn't compensate: backlash is variable —it depends on travel direction and load— and each re-teach hides the symptom for a few weeks while the gearbox keeps opening up. When vibration or noise finally shows, it's already an emergency gearbox replacement.

Telling backlash apart from calibration loss

Both cause point deviation but behave differently:

• Calibration loss: the error is constant and repeatable. The robot always reaches the same wrong spot, whatever direction it comes from. Fixed with mastering.
• Backlash: the error changes with the approach direction. Teach a point approaching from the right, then from the left, and it lands in two different places. That «jump» on reversal is the signature of backlash.

Before touching anything, check repeatability: send the robot to the same point 10 times from the same direction. If it repeats well, it's not static-position backlash; the problem shows up in dynamics or on reversal. If it doesn't repeat even from the same direction, suspect an encoder or a loose mount.

Causes by frequency

1. Gearbox wear (harmonic drive or RV/cycloidal). The #1 cause in high-hour robots. Angular play grows gradually and hits the highest-torque axes first (axes 1, 2 and 3).
2. Loose fastening hardware: motor mount, gearbox flange or wrist bolts. Gives sudden, not gradual, play and sometimes a dry metallic knock.
3. Worn axis bearings: add radial play noticeable mainly with the arm extended and loaded.
4. Motor-gearbox coupling: key or bushing with play. Less common but gives symptoms identical to the gearbox.
5. Lost preload in the transmission: on some designs it's adjustable and is lost with use.

How to measure backlash axis by axis

You need a dial indicator (0.01 mm) on a magnetic base. The procedure, with the robot in manual mode at reduced speed:

1. Place the magnetic base on a fixed part of the structure and the indicator tip against the arm of the axis you want to measure, as far as possible from the pivot centre to amplify the reading.
2. Energise the axis (motors ON, robot stationary). Zero the indicator.
3. Manually push the arm in one direction of rotation with moderate, steady force (don't force it) and note the reading. Release and push the other way. The difference between both ends is the total play referred to that radius.
4. Convert to degrees if you like: divide the measured displacement by the radius to get angular play. Compare against the robot's own history, not an absolute figure.
5. Repeat axis by axis. Log each value to build a baseline and spot the trend at future inspections.

What matters is not the one-off value but the trend: an axis that has doubled its play in six months warns you the gearbox is entering the replacement zone before it fails in production.

Tips to locate the source within the axis

With the indicator still mounted, move by hand and listen/feel:

• If you feel a dry «clack» before the smooth movement → fasteners or coupling with play. Check torques.
• If the movement is progressive and springy → typical of the gearbox (the harmonic drive flexspline «gives» before driving).
• If the play changes a lot as you vary arm extension → suspect a bearing.

Warning signs that force a stop

• Metallic noise or knocking synced with a specific axis's movement.
• Growing vibration that also drives up that axis motor's temperature.
• Metal particles appearing in the gearbox oil when you drain it.
• The robot no longer holds repeatability even after re-teaching the TCP.

If noise + vibration + particles coincide, stop producing: the gearbox can fail completely and take the encoder or motor with it, multiplying the cost. At that point go for corrective diagnosis and plan the gearbox replacement before it becomes an emergency stop.

Common shop-floor mistakes

• Repeatedly re-teaching the TCP to «fix» a jump that is backlash. It only masks and delays the real repair.
• Measuring play with motors off: brakes and inertia distort the reading. Measure with motors ON.
• Tightening hardware «by feel» without a torque figure: you can deform flanges or overshoot. Use the manufacturer's torque.
• Replacing the gearbox without checking bearings and coupling on the same axis: if the play came from elsewhere, the problem returns.

When to call a specialist

Measuring play and checking torques is within reach of a maintenance team with a dial indicator. Replacing a gearbox or axis bearings —arm removal, preload management, remastering and oil filling— is another story: it needs tooling, torque control and post-repair recalibration. If the play grows and already affects the part, lean on a preventive plan with backlash tracking and have the gearbox spare or exchange unit sourced so you don't depend on factory lead times.