Electric motor bearings are predominantly of the rolling type, though sliding bearings are also encountered in specific applications. When the operational temperature of a rolling bearing exceeds approximately 90 °C, it is classified as an overheating condition. Excessive heat triggers a cascade of complications: lubricating grease may degrade or leak, leading to diminished lubrication and accelerated wear; prolonged exposure to elevated temperatures can also alter the metallurgical properties of the bearing, reducing its load capacity and operational lifespan.
Common causes include poor lubrication, improper installation, overloading, and bearing damage. Accurate fault tracing and corrective maintenance are critical to restoring reliable performance.

Inadequate Lubrication

When overheating is detected, remove the bearing covers on both ends of the motor and inspect the grease visually. Prolonged service often results in grease contamination, ingress of foreign particles, or desiccation. Any of these conditions compromises lubrication and promotes excessive frictional heating. Select a high-quality grease of the correct specification and replace the degraded lubricant entirely to ensure optimal performance.

Excessive or Insufficient Grease

Grease volume has a direct influence on bearing temperature. Over-packing forces the rolling elements to churn through the lubricant, generating unnecessary frictional resistance and heat. Conversely, insufficient grease allows metal-to-metal contact, resulting in abrasive wear and rapid temperature rise.
For motors running below 1500 r/min, the bearing cavity should be filled to approximately two-thirds of its volume. For speeds at or above 1500 r/min, half of the bearing cavity is generally sufficient to maintain lubrication without inducing drag.

Improper Bearing Installation

Incorrect installation leads to misalignment and defective fits between the bearing, shaft, and end cover. Field inspections often rely on assessing radial clearance and the mating fit of the inner and outer races. Both excessive and insufficient clearances create abnormal load distribution, producing heat during operation.

Radial Clearance Assessment

Rotate the bearing’s outer race with force and listen carefully. Persistent noise or visible wobbling during inertial movement indicates excessive clearance. Another method is to grip the outer ring and attempt axial rocking; significant movement confirms the need to replace the bearing.

Inner Race Fit (Static Fit)

For bearings mounted on shafts, clean off lubricant before checking the fit. If the bearing resists free rotation, the inner race may be excessively tight, reducing radial clearance and increasing friction. Machining the shaft to the specified diameter restores the correct fit.
If, when pulling axially, the inner race moves relative to the shaft, the fit is too loose, leading to relative rotation under load and heating. Chrome-plating or sleeving the shaft to restore the tolerance is recommended. In emergency scenarios, creating stippled dimples on the shaft surface can temporarily enhance the grip.

Outer Race Fit (Transition Fit)

Apply a thin coat of grease to the bearing’s outer race and attempt to seat it in the end cover bore. A correct fit should allow firm seating with light tapping using a wooden mallet. Excessive force indicates an overly tight fit, causing axial skewing and off-track rolling. Machining the bore to specification corrects this.
If the bearing slips in by hand or falls out under its own weight, the fit is too loose. This leads to relative rotation between the outer race and the bore during operation, generating heat. Sleeving the bore restores the correct dimension; in urgent cases, stippling the bore surface can provide a temporary solution.

Other Mechanical Factors

Coupling misalignment, overly tightened drive belts, or excessive vibration during operation can all induce abnormal loading on the bearings, resulting in heat build-up. Align the coupling so both shafts share the same centerline, and adjust belt tension to the lowest value that maintains torque transmission without slippage. Eliminating vibration stabilizes bearing operation and prolongs service life.

By methodically analyzing lubrication condition, grease volume, installation accuracy, and external mechanical influences, the root causes of bearing overheating can be identified and rectified. Proper maintenance ensures the bearing operates within safe thermal limits, safeguarding the motor’s performance and longevity.