The bearing of a stepper motor is a connecting component between the stationary and rotating parts of the motor. It not only affects the motor's operation but also its lifespan, thus impacting product quality. Therefore, daily use and maintenance should be prioritized.

1. Common Failure Analysis of Needle Roller Bearings

Needle roller bearings have low frictional resistance, high mechanical efficiency, easy lubrication and maintenance, and are standardized, making them widely used in stepper motors. Common failures of needle roller bearings are as follows.

1.1 Overheating of Bearings

During the operation of a stepper motor, touching the small cover or bearing cap should not feel hot. If it does, it indicates that the bearing temperature is too high. The causes of overheating bearings include:

   (1)  The lubricating grease does not meet the requirements or has deteriorated, the viscosity is too high, or there is too much grease. The amount of grease added to the motor should not be too much or too little; generally, it should occupy about 1/3 to 1/2 of the bearing seat. Bearings with low speed and light load can have a little more grease; bearings with high speed and heavy load should have less grease.

   (2)   Improper assembly of the needle roller bearing. A new bearing or re-machining the journal and bearing housing without a back-off groove or improper machining Size of the journal can cause improper bearing installation. When the bearing clearance is too small, excessive shear friction loss in the clearance can cause bearing heating; at the same time, when the clearance is too small, the oil volume will decrease and the heat generated by friction cannot be taken away in time, which will further increase the temperature rise of the bearing. However, excessive clearance will change the dynamic characteristics of the bearing and cause unstable rotor operation. Therefore, it is necessary to select the appropriate bearing clearance for different equipment and operating conditions. Adjust the assembly process, if unqualified, re-assemble after secondary machining on a lathe.

   (3)   Bearing raceway running. There are two types of bearing raceway running: one is the outer race running, that is, there is relative displacement between the outer ring of the bearing and the bearing chamber; the other is the inner race running, that is, there is relative displacement between the inner ring of the bearing and the shaft neck. When the bearing raceway runs, it will generate heat and accompanied by a "squeaking" sound. The treatment method can be mitigated by adhesive, or dismantled, re-machined, and replaced.

  (4) Lack of oil in the needle roller bearing. When the electric motor is running normally, it emits a continuous uniform "humming" sound. If the motor bearing during operation makes a "gurgling" sound and intermittent howling, it indicates that the bearing is lacking lubricating grease. At this time, the bearing should be lubricated with a grease gun through the bearing oil hole; for the rear bearing, add oil after the machine stops.

  (5) There are seven types of needle roller bearing damage:

①      Metal spalling on the raceway surface. The bearing rolling elements and the inner and outer ring raceways are subjected to periodic pulsating loads, resulting in periodically changing contact stresses. When the number of stress cycles reaches a certain value, fatigue spalling occurs on the working surface of the rolling element or the inner and outer ring raceways. If the bearing load is too large, fatigue will be exacerbated. In addition, improper bearing installation, shaft bending, and misalignment between the motor shaft and the mechanical rotating shaft can also cause raceway spalling. Fatigue spalling of the bearing raceway will reduce the running accuracy of the shaft, causing vibration and noise in the mechanism.

Bearing burn.

Plastic deformation

Bearing seat ring cracks

Cage fracture

Metal adhesion of the cage to the rolling element

Severe wear of the seat ring raceway

1.2 Bearing Noise

A slight running sound is acceptable for needle roller bearings during operation. If the sound is too loud or there is abnormal noise or impact, it indicates that the bearing is malfunctioning. Different parts of the needle roller bearing will produce different sounds; during operation, a stepper motor emits a uniform, periodic "clattering" sound within a continuous "humming" sound, indicating that there are scratches, grooves, or rust on the rolling elements and inner and outer raceways; if the bearing makes an irregular "scraping" sound, it indicates that iron filings or small sand grains have entered the bearing; if the bearing makes an intermittent "clacking" sound, it may be caused by a cracked outer ring or cage; if the bearing makes a periodic "squeaking" sound, it indicates that the outer ring is running; if the bearing makes a "gurgling" sound and intermittent howling, it indicates that the bearing is lacking lubricating grease.

When the stepper motor emits the above sounds, it should be stopped for inspection. The manufacturer should be contacted to check whether the front and rear bearings are running smoothly and whether there is any abnormal noise. Then, use a flashlight to check whether the cage, grooves, and inner and outer rings have any cracks, scratches, rust, etc. If there is any abnormal sound or jamming during bearing rotation, the bearing should be considered for replacement.

Bearings are key and easily damaged components of stepper motors. The smooth operation of the stepper motor depends on whether the bearing can operate normally. Bearing itself and various external factors often affect the normal operation of the bearing, making the diagnosis of bearing faults complicated. Combined with maintenance practices, this article analyzes the common faults and treatment methods of stepper motor bearings, uses simple and practical auditory and tactile methods to detect the cloud operation status of the bearings, and proposes corresponding measures to prevent their damage.