An axis that hums, whines or rattles continuously and runs hotter than the rest usually points to two very different culprits: the servomotor's own bearing or the gearbox. The bill and the downtime differ a lot depending on which one it is, and on the shop floor they get confused constantly because noise travels through the structure and seems to come from somewhere else. Here's how to separate them before ordering a part.
Why a motor bearing sounds different from a gearbox
A servomotor bearing turns at motor speed (thousands of rpm), so it produces a sharp, constant, high-frequency noise: a whine or hum that rises and falls with axis speed. The gearbox turns at the output at far fewer revolutions, so its wear gives low, cyclic noises, with knocking or a "grinding" sound that appears mainly under load. If the sound clearly changes as you vary motor speed but doesn't depend much on load, suspect the motor bearing.
Signs pointing to the motor bearing
- Sharp, continuous whine or hum, not cyclic.
- That axis's motor runs hotter than the others at rest with the brake applied and also while moving.
- The noise appears even at low speed and with no appreciable load.
- Sometimes a slight high-frequency vibration you can feel with your hand on the motor housing.
Signs pointing to the gearbox
- Knocking or low "grinding" noise that rises with load or torque.
- Mechanical play when moving the axis by hand (robot safe, de-energised).
- Grease/oil leak or bead near the joint.
- Position error or vibration that worsens on effort-heavy paths.
If you find play or leaks, the diagnosis goes another way: check our guide on joint backlash before touching the motor.
What to check and in what order
- Locate the real axis. With the robot in manual mode and at low speed, move one axis at a time and listen. Transmitted noise deceives: confirm by touching the motor housing with your hand (or a screwdriver as a stethoscope) while the axis moves.
- Measure temperature. With an infrared thermometer, compare that motor's housing with the others after a normal production cycle. A clear, sustained difference versus equivalent axes signals abnormal friction. Don't focus on an absolute value: focus on the difference between axes.
- Test without load. Run a slow move of the axis with no tool or part. If the sharp noise persists unchanged, the motor becomes the stronger suspect over the gearbox.
- Test with speed. Raise and lower the axis speed. Noise that scales with motor rpm → motor bearing. Cyclic noise appearing under torque → gearbox.
- Check servo consumption. On ABB, KUKA or FANUC you can read axis torque or current. A seized motor bearing raises consumption steadily, not just at load peaks. Log the value and compare it with a healthy axis.
- Check vibration by hand. Fine, high-frequency vibration on the motor housing points to the bearing; low, slow vibration at the joint points to the gearbox.
Common shop-floor mistakes
- Replacing the gearbox for a motor noise. The most expensive mistake. The noise "seems" to come from the joint through structural transmission.
- Ignoring differential temperature. A motor hotter than its neighbours has been warning for weeks before seizing.
- Greasing the gearbox "just in case". It doesn't fix a motor bearing and masks the diagnosis.
- Waiting for it to "fully stop". A seizing motor bearing can cause overcurrent, servo trip and, worst case, winding damage.
- Not recording the sound. Record audio and note speed and load. On the second warning you'll have something to compare with.
When to replace only the bearing and when the whole motor
On many robot servomotors the bearing is integrated with the encoder and brake in an assembly that isn't always practical to open on-site without a bench and alignment tooling. If the bearing is at an early stage (noise but no play or severe overheating), a specialist workshop can replace it and recondition the motor. If there are seizing marks, repeated overheating or the encoder has started throwing errors, the sensible option is an exchange motor to minimise downtime and verify the original on a bench.
When to stop and call a specialist
- An axis motor runs clearly hotter than the others and the noise worsens day by day.
- Overcurrent, overtemperature or encoder alarms appear on that same axis.
- The axis servo consumption rises steadily versus a healthy axis.
- You notice high-frequency vibration and a burnt smell around the motor.
A motor bearing that locks with the robot running can drag the axis into a trip mid-path. If you're unsure between motor and gearbox, a diagnosis with torque, temperature and vibration analysis confirms it without dismantling blindly. And if the fleet is old, plan preventive maintenance to catch these noises before they become an emergency stop.