Servo motors drive the precise positioning behind packaging machines, injection moulders, presses, CNC equipment, printing lines and robots. When one fails, the whole machine usually stops. Most servo motor failures come from a small set of causes: bearings, feedback devices, windings, brakes, heat and contamination. Many give warning signs well before a breakdown. This guide covers the symptoms of a failing servo motor, the most common causes, how to tell a mechanical fault from an electrical one, and how to prevent repeat failures.
How do I know if my servo motor is bad? Common symptoms
| Symptom | Possible causes |
|---|---|
| Grinding, whining or rumbling noise | Worn or contaminated bearings, damaged shaft or gearbox |
| Vibration or rough running | Bearing wear, misalignment, loose coupling, poor drive tuning |
| Motor running hot | Overload, blocked cooling, high duty cycle, winding fault, dragging brake |
| Encoder or feedback alarms on the drive | Faulty encoder or resolver, damaged feedback cable, loose connector, contamination |
| Overcurrent, overload or earth-fault trips | Winding insulation breakdown, moisture ingress, damaged power cable, seized mechanics |
| Following or position errors | Feedback problems, mechanical binding, worn couplings, incorrect tuning |
| Jerky movement or oscillation (hunting) | Encoder faults, loose mechanics, gain settings, damaged cables |
| Loss of torque or holding force | Weakened (demagnetised) rotor magnets, brake problems, winding damage |
| Axis drops or drifts when stopped | Worn or failed holding brake |
Many of these symptoms can also come from the servo drive, the cabling or the machine itself. That is why proper testing matters before a motor is condemned or sent for repair.
The most common causes of servo motor failure
1. Bearing failure
Bearings are the most common mechanical failure point. Excessive radial or axial loads from belt tension, misaligned couplings or heavy overhung loads shorten bearing life. So do contamination, lubricant breakdown and high temperatures. Worn bearings increase friction and heat, cause vibration, and can eventually damage the shaft, the encoder and the windings. Listen for noise, and use vibration checks on critical axes to catch bearing wear early.
2. Feedback device failure (encoder or resolver)
A servo motor is only as accurate as its feedback. Encoders and resolvers report the rotor’s position and speed to the drive. Shock loads, vibration, heat, contamination, water ingress and damaged feedback cables can all cause encoder faults. The results include feedback alarms, erratic movement, position errors or a motor that won’t enable. After an encoder is replaced, it must be aligned to the motor’s magnetic poles. A misaligned encoder costs torque and causes overheating, even if the motor appears to run.
3. Winding and insulation failure
Stator winding insulation breaks down because of overheating, moisture, coolant or oil ingress, voltage spikes from the drive, and age. Insulation faults show up as earth-fault or overcurrent trips, and eventually as shorted turns or a burnt-out winding. Coolant ingress is a particular risk on machine tools and wash-down equipment where seals have worn.
4. Overheating
Servo motors are rated for a specific continuous torque and duty cycle. Running above that rating, blocked cooling fins or fans, high ambient temperatures and repeated rapid acceleration all push the motor past its thermal limits. Heat degrades insulation, dries out bearing grease and, in severe cases, can weaken the rotor’s permanent magnets.
5. Brake failure
Many servo motors have a holding brake that keeps vertical axes or loads in place when the motor is disabled. These brakes are designed to hold, not to stop a moving load. Using them for repeated dynamic stops wears the friction surfaces quickly. Brake faults can let an axis drop, or leave the brake dragging, which causes heat and overload trips.
6. Contamination
Dust, metal particles, moisture, oil and chemicals get into motors through worn shaft seals, damaged connectors and open cable entries. Contamination attacks bearings, encoders and insulation at the same time. In harsh environments, use motors with a suitable IP rating and maintain seals and connector gaskets.
7. Cable and connector damage
Power and feedback cables on moving axes flex constantly. Worn cable chains, tight bend radii, crushed cables and loose connectors cause intermittent faults that are easy to mistake for a motor failure. Always check cables and connectors before replacing a motor.
8. Electrical stress from the supply and drive
Voltage surges, poor earthing and the fast switching of modern drives can stress winding insulation and bearings. Long motor cables without proper screening or filtering make this worse.
9. Mechanical overload and misalignment
Crashes, jams, seized mechanisms and misaligned couplings put shock loads on the shaft, bearings and encoder. A single crash can damage an encoder even if the motor still turns.
Is it a mechanical or an electrical fault?
A few basic checks help narrow it down:
- Turn the shaft by hand with the power safely isolated. Roughness, notching or tight spots point to bearings or mechanical damage. Note that permanent-magnet motors naturally feel slightly “notchy” (cogging).
- Disconnect the load and run the motor on its own. If the problem disappears, the fault is probably in the machine, not the motor.
- Read the drive’s alarm history. Encoder and feedback alarms point to feedback faults, while earth-fault and overcurrent trips suggest winding or cable problems.
- Swap cables or test on a known-good drive where that’s practical.
Proper diagnosis needs specialised equipment, such as insulation resistance testing, back-EMF measurement, and encoder signal and alignment checks. We explain why DIY repairs are risky in Want to Repair a Servo Motor Yourself?
How to prevent servo motor failures
- Keep motors within their rated torque, speed and duty cycle, and review sizing if motors keep running hot.
- Keep cooling paths clean and panel temperatures under control.
- Check coupling alignment and belt tension, and avoid excessive loads on the shaft.
- Inspect and replace shaft seals, connector seals and damaged cables.
- Use holding brakes only for holding, and let the drive decelerate the load.
- Protect motors from coolant, oil and wash-down water.
- Make sure drives, motors and panels are correctly earthed, with proper cable screening.
- Investigate repeated alarms instead of resetting them, and schedule preventative maintenance for critical axes.
Can a servo motor be repaired?
Yes. Most failed servo motors can be repaired, often faster and more cheaply than buying new, especially for imported or obsolete models. A professional servo repair typically covers:
- Full inspection and electrical testing, including back-EMF checks
- Bearing replacement and seal renewal
- Shaft and housing repairs, including machining where needed
- Encoder or resolver testing, replacement and digital alignment
- Brake inspection and repair
- Run testing before the motor goes back to site
Read more about the economics in Repair vs Replace: Why Servo Motor Repairs Save Time and Money.
Frequently asked questions
Why do servo motors fail?
The most common reasons are bearing wear, encoder or feedback faults, winding insulation breakdown, overheating, brake wear, contamination and cable damage. Many of these start with overloading, poor alignment or a harsh environment.
How long does a servo motor last?
It varies widely with load, speed, duty cycle and environment. Bearings and brakes are wearing parts. A well-sized motor in a clean environment with regular maintenance can run for many years, while an overloaded motor in a wet or dusty environment may fail much sooner.
Can an encoder be replaced on a servo motor?
Yes, but it must be aligned correctly to the motor’s rotor position afterwards. Incorrect alignment causes lost torque, overheating and erratic running.
Is the fault in the servo motor or the servo drive?
Both can produce similar alarms. Testing the motor on its own, checking cables and reviewing the drive’s fault history usually shows which one is at fault. Specialist testing confirms it.
Servo motor repairs with a 12-month warranty
Motion Tronic repairs servo motors from all major brands, including Inovance, Lenze, Baumüller, Mitsubishi, Panasonic, Yaskawa, Siemens, Rexroth, Delta and Schneider Electric, in our Durban repair lab. Our team carries out back-EMF checks, bearing refurbishment, housing machining, and encoder checking, replacement and digital alignment. Every repair is run-tested and backed by a 12-month warranty. If you have repeated failures, we will also advise on the cause.
Learn more about our servo motor repairs, or contact us to log a repair.
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