How do servo motors break down? Nine problems that cannot be avoided

Keeping servo motors running in optimal condition is vital for any industrial business, especially those in industries such as robotics or CNC machine tools, as downtime caused by servo motor failure can be very expensive, both financially and in terms of time.

Bearing failure

As the main wear part in servo motors, more than half of servo motor failures are usually attributed to bearing problems. The specific manifestations are varied, ranging from vibration and abnormal noise when the motor rotates to the serious one that causes the motor shaft to get stuck.

It is worth noting that if the bearing failure is not handled in time, it will usually cause secondary damage. For example, the debris of bearing corrosion will fly into the brake or motor encoder, causing more serious losses.

Factors that affect the life of motor bearings include: axial load, radial load, motor speed, operating temperature and bearing rated parameters. There are many reasons for bearing failure, the most common of which include:

Improper mechanical loading (e.g. overload, radial misalignment, axial thrust, belt tension issues)

Excessive vibration and shock

Overspeed

Shaft current

Overheating (causing loss of lubrication)

Wet or liquid

Contamination (e.g., use of incompatible grease, water condensation, dust/dirt contamination)

Treatment method:

When using a servo motor, do not run it over the rated load for a long time.

For applications with shaft current, add conductive brushes or use motors with insulated bearings.

Performing Preventive Maintenance on Servo Motors

2

Shaft seal wear

Possible causes

Accidental Damage

Normal wear and tear

Treatment method:

Preventive Maintenance

Depending on usage, it is recommended to replace it every 3 months, and no longer than 12 months.

3

Stator and winding problems

When a winding fails, part of the motor shorts out, causing internal burns in the motor.

Possible causes

Overload

Overpressure

Phase loss

Wrong wiring

Improper drive parameter settings

Ambient temperature is too high

Cooling device failure

Physical damage

Treatment method:

When using a servo motor, do not run it over the rated load for a long time.

Monitor current and current accumulation over time

Monitoring winding temperature

4

Rotor and shaft problems

Fault phenomenon

Shaft fracture or deformation

Magnetic disc falls off

Possible causes

Excessive vibration (e.g., radial misalignment, axial thrust, highly variable duty cycle)

Too many starts or reversals, or too short intervals between starts/reversals

Overheating (e.g., high ambient temperature, overload, or rotor locked operation)

Unexpected collision

Countermeasures:

Run at rated load

Avoid accidental collisions

5

Motor feedback device (resolver, encoder, etc.) problems

Fault phenomenon

Zero position (turns) lost

Resolver or encoder wear

Broken glass disc

Encoder electrical fault

Possible causes

For encoders that use a backup battery, the zero position may be lost as the battery runs out during use. The battery status should be checked first.

As a secondary damage to the aforementioned motor bearing failure, motor bearing problems can also cause mechanical wear of the encoder or resolver.

Long-term motor shaft current may not only act on the bearings of the motor itself, but also harm the bearings built into the encoder, causing burning and damage to the encoder bearings.

The impact and vibration during the transportation or installation of the motor can easily cause the glass disc of the optical encoder to break. Especially when adding keys, pulleys or couplings to the motor shaft, the motor shaft must not be struck.

In addition to incorrect encoder wiring, electromagnetic interference caused by improper wiring is also one of the main causes of encoder electrical failure.

Countermeasures:

Depending on the specific application environment, the battery life is usually one year or several years. Regularly replacing the battery can reduce the risk of such accidents. Or, a more permanent solution is to switch to a mechanical multi-turn absolute encoder.

The motor must be reliably grounded when installed. If there is shaft current, it is necessary to consider using insulated bearings and insulated encoders or installing a motor shaft grounding device.

During the installation of the motor, for example, when adding a pulley or coupling, if knocking is unavoidable, you can consider removing the encoder first and saving it, and then installing the encoder after all mechanical installations are completed. In this case, the encoder phase angle needs to be readjusted in the servo drive.

Another way to prevent code disk failure is to use metal code disk encoders, which have become popular in recent years. Compared with glass code disks, metal code disks have much better vibration and impact resistance, and can be comparable to glass code disks in resolution and accuracy.

6

Braking device (holding brake) problem

The motor brake is used to brake the motor shaft to prevent rotation when the power is turned off; when the brake is energized, the brake is in a released state.

Fault phenomenon

Abnormal sound

Cannot release

Unable to brake (locking)

Possible causes

The intrusion of foreign matter from faulty bearings is a common cause of abnormal brake noise.

Secondly, the brake may be damaged if the brake is forced to operate in a power-off state due to a fault in the brake drive circuit .

Countermeasures: It is worth noting that as a device to keep the motor stationary, the brake should not be used as a motor deceleration device when the motor is energized, as this will accelerate the wear of the brake.

7

Cooling unit problem

Most small and medium power servo motors are self-cooled. For servo motors with larger power or special applications, air cooling or liquid cooling is also common.

Fault phenomenon

Fan shaking or stalling

Coolant Leakage

Possible causes

The culprit behind most fan failures is dust. This is because dust accumulates on the fan blades over time. The increased load on the blades causes vibrations and subsequent damage.

Most coolant leaks occur at the joints, and seal failure is usually the root of the problem.

Physical damage caused by accidental impact.

Countermeasures

Add filters to fans and replace them regularly

Check cooling device regularly

8

Electrical connection device

This includes the wiring terminal boxes and sockets.

Fault phenomenon and cause

As a non-wearing part, the failure of the connecting device is mostly due to mechanical damage.

Countermeasures: Be more careful when using and try to avoid accidents.

9

Couplings and Pulleys

A torsionally rigid coupling or a reinforced belt is required to connect the motor shaft. After the motor has been running for a period of time, frequent acceleration and deceleration may cause the coupling or belt to become loose or slip, and it should be checked again.

Fault phenomenon and cause

If the shaft is subjected to severe impact during the installation of the coupling and pulley, fatal damage may occur to the motor bearings and/or encoder.

Countermeasures: Therefore, during installation or removal, it is strictly forbidden to use tools to hit the shaft, coupling or pulley. When trying to remove any equipment from the motor shaft, a hydraulic device should be used to push it out from the shaft end.

Other Problem Solving Techniques for Servo Motors

(1) Motor oscillation: oscillation occurs during feeding, and the speed measurement signal is unstable, such as cracks in the encoder; poor contact of the terminal, such as loose screws, etc. When the oscillation occurs at the moment of switching from positive direction to reverse direction, it is generally caused by the reverse clearance of the feed transmission chain or excessive servo drive gain;

(2) Motor creep: mostly occurs during the start-up acceleration phase or low-speed feeding, which is generally caused by factors such as poor lubrication of the feed transmission chain, low servo system gain and excessive external load. In particular, it should be noted that the coupling used to connect the servo motor and the ball screw may be loose or have defects in the coupling itself, such as cracks, which may cause the ball screw and the servo motor to rotate out of sync, causing the feed motion to speed up and slow down;

(3) Motor vibration: When the machine tool runs at high speed, it may generate vibration, which will generate an overcurrent alarm. Machine tool vibration problems are generally speed problems, so you should look for speed loop problems;

(4) Motor torque reduction: When the servo motor changes from rated stall torque to high-speed operation, the torque suddenly decreases. This is caused by heat dissipation damage to the motor windings and heating of the mechanical parts. At high speeds, the motor temperature rises. Therefore, the load of the motor must be verified before using the servo motor correctly.

(5) Motor position error: When the servo shaft moves beyond the position tolerance range (the factory standard setting of KNDSD200 is PA17:400, position tolerance detection range), the servo driver will display a "4" position tolerance alarm. The main reasons are: the tolerance range set by the system is small; the servo system gain setting is improper; the position detection device is contaminated; the cumulative error of the feed transmission chain is too large, etc.

(6) The motor does not rotate: In addition to the pulse + direction signal, the CNC system also has an enable control signal to the servo drive, which is generally a DC + 24 V relay coil voltage. If the servo motor does not rotate, the common diagnostic methods are: check whether the CNC system has pulse signal output; check whether the enable signal is connected; observe the system input/output status on the LCD screen to see if it meets the start conditions of the feed axis; confirm that the brake is on for servo motors with electromagnetic brakes; the drive is faulty; the servo motor is faulty; the coupling between the servo motor and the ball screw fails or the key is disengaged, etc.