Not every robot axis drives the load through a direct gearbox: many models use a timing belt between the servomotor and the gearbox input, especially on the wrist axes (4, 5 and 6) and sometimes on axis 3. When that belt loosens, hardens or loses teeth, you get symptoms that are easily mistaken for miscalibration, gearbox backlash or an encoder fault. Here's how to tell them apart and what to do before dismantling anything expensive.
Symptoms of a troubled drive belt
Ordered by what we see most on the shop floor:
- Sharp squeal when the axis starts cold that fades once the robot has been running a while. Dried-out belt or slipping from low tension.
- Rhythmic marks or "steps" on the part (in cutting, dispensing or welding) that weren't there before. A jumped tooth or uneven wear introduces micro-jumps in position.
- Loss of repeatability on a single axis that re-mastering doesn't fix: it comes back shortly after calibrating.
- Cyclic "tick-tick" noise proportional to axis speed, different from the continuous whine of a bearing.
- Intermittent following-error alarms on that axis when the belt slips under load. If you see them, also check the position following error, because the symptom is identical.
Why timing belts fail
Low tension
The most common cause. With a slack belt, under hard acceleration the teeth "climb" over the pulley (tooth jumping) and skip one or more steps. The robot thinks it's where the motor encoder says, but the gearbox hasn't turned the same amount. Result: a position offset that keeps returning.
Excess tension
The opposite mistake and a very damaging one. An over-tensioned belt hammers the motor shaft bearing and the driven pulley bearing. You can kill a bearing in a few months and end up replacing a motor that didn't need it. More tension is not better.
Elastomer ageing
Heat, oil and hours. A belt in a hot environment or exposed to lubricant splashes hardens, cracks at the tooth root and loses elasticity. On robots past 30,000–40,000 hours it's normal for the original belt to be at the end of its life even if it "looks" fine.
Oil or coolant contamination
Oil attacks the compound of many belts and reduces grip. If you have a nearby leak, the belt slips even with correct tension. Fix the source before replacing it or you'll be back to square one.
How to check the belt in order
With the robot stopped and locked out (power cut and locked), follow this order:
- Visual inspection. Remove the transmission access cover for that axis. Look for rounded teeth, cracks at the tooth root, frayed cords, accumulated black rubber dust (a clear wear sign) and shine/oil on the belt.
- Rough manual tension check. Press the belt in the free span between pulleys. It should give a little and spring back firmly. If it sinks easily or "hangs", it's slack. If it's rigid like a bar, it's over-tensioned. For exact values, always consult the manufacturer's manual or use a frequency tension meter; don't improvise a reference figure.
- Manual axis rotation. Move the axis by hand (depending on the model, after releasing the brake with proper precautions). A "click" or irregular resistance points to a damaged tooth or dirt on the pulley.
- Repeatability check. Teach a point and have it repeat several times from different approaches. If that axis deviates and the others don't, the suspicion narrows.
- Check the pulley. Often the belt isn't the only culprit: worn or notched pulley teeth will destroy the new belt in weeks. Replace both if the pulley shows wear.
Common shop-floor mistakes
- Re-tensioning "by eye" every time it squeals. You keep adding tension until the noise stops and end up killing a bearing. The squeal may be ageing, not lack of tension.
- Calibrating over and over without looking at the mechanics. If the axis loses repeatability from a jumping belt, mastering masks the symptom for a few hours and it returns.
- Replacing the belt and leaving a worn pulley. Wasted work.
- Not investigating the nearby oil leak. The new belt will slip just the same.
- Not re-mastering after the change. Loosening the transmission loses the axis reference; you must redo that axis's zero afterwards.
Repair, tension or replace?
Rule of thumb: if the belt is simply slack, correct tension (per the manual) recovers the axis. If it shows cracks, rounded teeth, oil contamination or has already jumped once, replace it; there's no middle ground, because a belt that has jumped teeth is already fatigued. Take the chance to inspect pulley and motor bearing in the same intervention.
Belt replacement belongs in a well-run preventive maintenance plan: including it in hour-based inspections avoids the downtime scenario of a jumping axis mid-production. If you need the right belt for your model or a corrective maintenance intervention, we work with spares for ABB, KUKA and FANUC.
When to call a specialist
- If the belt is on a wrist axis with a hard-to-reach integrated gearbox and you don't have the manufacturer's tensioning procedure.
- If after replacing it and re-mastering the axis still loses repeatability (possible gearbox or pulley backlash).
- If overcurrent or following-error alarms appear on that axis despite the new belt.
- If you can't guarantee correct tension with a tension meter: a badly tensioned belt ruins bearings or jumps again.
A belt is cheap; the motor or gearbox it ruins through bad tensioning is not. When in doubt, get a diagnosis before you tighten.