Robot wrist wear (axes 4, 5, 6): symptoms and what to do

The wrist —axes 4, 5 and 6— is the part of the robot that fails first and the last one people inspect. It racks up more revolutions per hour than the major axes, works in positions where lubrication reaches worst, and usually carries an off-centre load from the tool. When it starts to fail it doesn't throw a clear alarm: it degrades part quality first. This article helps you recognise that wear, tell it apart from a major-axis fault, and decide when to act.

Typical symptoms of a worn wrist

The usual pattern shows up in the product, not on the controller:

• Double-application mark or irregular bead in dispensing/sealing, only in certain tool orientations.
• Intermittent quality rejects that appear when axis 5 or 6 works near its limits.
• Metallic noise or hum localised in the wrist assembly when moving 4, 5 or 6, changing with load.
• TCP drift: the tip no longer repeats the same, especially on axis 6 rotations.
• Elevated temperature on the wrist housing after a while cycling, versus a cold start.

The key is that these symptoms are directional: they appear in some orientations and not others. A major-axis problem (1, 2, 3) affects the robot's whole motion; a wrist problem shows up at the end of the arm.

Causes ranked by frequency

1. Wrist gearbox wear from poor lubrication. On many models axes 4/5/6 share a compact gear train that degrades if the oil/grease is old or below level. This is cause number one.
2. Off-centre load or wrong TCP declared. A tool heavier or more offset than expected continuously punishes the wrist bearings. Very common after swapping grippers without recalculating the payload.
3. Worn wrist bearings from service hours (play that grows progressively).
4. Contaminant ingress (abrasive dust, chips, wash-down water) through aged seals.
5. Aftermath of a collision not thoroughly inspected: the wrist is the first thing to hit.

What to check and in what order

Work from cheapest and quickest to most invasive. Don't dismantle anything before these checks:

1. Review the declared payload. Confirm weight, centre of gravity and moments of the current tool. If you changed grippers and didn't recalculate, fix that first; it may be the whole problem.
2. Visual inspection of seals and leaks. Look for oil or grease pushed out around the wrist. A leak points to a damaged seal and compromised lubrication.
3. Mechanical play test. With the robot stopped and safe, move the tool by hand and try to detect angular play in axes 4, 5 and 6. Compare with a twin robot if you have one. Noticeable play that wasn't there before is a firm sign.
4. Listen and feel with the robot in slow motion. Move each wrist axis separately at low speed. Locate the noise: is it 4, 5 or 6? Does it change with load? Note the exact orientation where it appears.
5. Measure temperature after 20-30 min of cycling. Compare the wrist housing with the cold start. Localised overheating indicates abnormal friction.
6. Verify TCP repeatability. Teach a reference point and check drift across several tool orientations. Uniform miscalibration is fixed with mastering; wear produces an error that varies with orientation.
7. Check lubrication traceability. When was the wrist grease/oil last changed? If it exceeds the manufacturer's interval (indicative: every 10,000-12,000 h or per hours and temperature), that's your first suspect.

Common mistakes made on the shop floor

• Running mastering to "fix" lost repeatability. If the problem is mechanical wear, mastering masks the symptom for a few weeks and the play returns. Tell them apart before touching calibration; it's explained in mastering and axis calibration.
• Swapping the gripper without updating the payload. A wrongly declared TCP and payload accelerate wrist wear and overload servos.
• Ignoring a small leak. Low grease in the wrist means accelerated gear-train wear; it's not "cosmetic".
• Pushing cycle speed after a product change without checking the wrist can handle the new acceleration profiles.

When to stop and call a specialist

Stop the robot and escalate if you find:

• Clear and growing mechanical play in any wrist axis.
• Loud metallic noise or vibration getting worse cycle by cycle.
• Localised overheating with lubrication up to date.
• Quality rejects that aren't fixed by recalibration or payload adjustment.

Early-stage wrist wear can be slowed with a lubricant change and payload correction. Advanced wear requires replacing the gearbox assembly or the whole wrist, a job that needs disassembly, re-mastering and repeatability verification. If in doubt, a timely diagnosis is cheaper than forcing the wrist to failure and dragging the arm down. Check our corrective maintenance service or, better, get ahead with a preventive maintenance plan that includes the wrist in the lubrication route.