The arm drops a few millimetres when power is cut or the e-stop is pressed. The operator notices it, mentions it during shift handover, and nobody worries because "the robot still runs fine". That's the number one symptom of a degraded axis brake, and it's the one most poorly handled on the shop floor because it doesn't trigger an alarm until the fault is already serious.
What the axis brake actually does, and why it fails silently
Every motor on a loaded axis (typically axis 2 and axis 3, and on some robots the wrist too) carries a spring-applied electromagnetic brake that closes when power is removed and releases when energised. Its job isn't to stop motion: it's to hold position when the motor is de-energised, whether from a stop, a power fault, or shutting down the controller. If the brake loses holding torque, the arm gives way to the weight of the tool and the link itself as soon as current is cut.
The problem is that a brake that has lost 20-30% of its torque still "holds" during most normal stops. It only shows up under heavy loads, in unfavourable positions (arm extended), or after many thermal cycles. That's why the failure is usually caught late, once there's a visible drop or the robot starts throwing a brake alarm on start-up.
Most frequent causes, ranked
- Mechanical wear of the brake disc/friction pad. The most common cause in robots with many service hours and frequent stop cycles (lines with lots of micro-stops). Friction material wears away and holding torque drops progressively.
- Contamination from gearbox oil or grease. Once the gearbox seal starts leaking, grease migrates into the brake and drastically reduces the friction coefficient. Very common on axes 2 and 3 with aging gearboxes: the brake "slips" even though it's mechanically fine.
- Electronic fault in the brake driver (inside the amplifier or controller). The brake doesn't open or close correctly because the control circuit has failed, not because the physical brake is damaged. It's detected by erratic behaviour: it releases fine sometimes and not others.
- Damaged brake cable or connector. Fatigue cracking in the axis cable, a corroded connector or a poor contact. Closely related to what we covered on cable and connector inspection.
- Broken or fatigued brake spring. Less common, but shows up in very old robots or brakes that have exceeded their rated cycle life.
How to check it: the brake test, step by step
All three manufacturers offer a brake test routine built into the controller software. It's not optional in a serious maintenance plan: it's the only objective way to measure actual holding torque, not just "see if it holds".
- Check the alarm history. Look for brake-related alarms, overcurrent during braking, or position deviation after a stop. Often the controller already flagged it before the operator noticed anything.
- Visually inspect the brake wiring in the arm and the cabinet, looking for cracked insulation, moisture, or connectors showing arc damage.
- Check for gearbox grease leaking near the motor or the brake itself. If there's grease where there shouldn't be, the problem isn't just the brake: check gearbox condition too (see our article on gearbox replacement).
- Run the manufacturer's brake test routine with the robot in the conditions specified by the manual (usually no external TCP load and the axis in a specific position). The system measures the time/travel of controlled drop and compares it to the reference value.
- Repeat the test in several axis positions if the result is borderline: a brake with uneven wear can pass in one posture and fail in another.
- If the test fails or is on the edge, don't resume the production cycle without mechanically blocking the axis (wedge or physical support) until you decide on replacement.
Common mistakes on the shop floor
- Confusing a brake drop with gearbox backlash: the symptoms look similar (the arm "moves on its own" a few millimetres) but the cause and repair are completely different. If in doubt, compare it against what we cover on abnormal vibrations.
- Tweaking the program to "compensate" the position drop instead of fixing the brake. It's a patch that hides the problem and raises the risk of a sudden arm drop.
- Not re-running the brake test after a gearbox repair. If there was grease leakage, the brake needs to be verified even if the original fault was a different component.
- Waiting for the hard alarm to act. The periodic brake test exists precisely to catch progressive wear, not just total failure.
When to stop the line and call a specialist
Stop the robot immediately if there's a visible arm drop on power-off, if the brake test fails in two different positions, or if any brake alarm appears together with a metallic noise from the axis. A brake that fails completely with a load at the TCP is a genuine physical hazard, not just a production problem. If you don't have staff trained on the manufacturer's specific routine or the brake replacement kit, it's time to request a corrective diagnosis before attempting to restart.
As a general guideline, for robots with medium-heavy loads on axis 2/3 and intensive cycles, include the brake test within preventive maintenance checks every 4,000-8,000 service hours, or at least once a year, adjusting based on each cell's alarm history. For replacing the brake assembly, weigh whether an original part or a component from the spare parts and exchange service makes more sense given lead time and line criticality.