Bearing Secrets Every Engineer Should Know

Most engineers assume there is very little that can go wrong with rolling bearings. The stark reality, however, is that there are a myriad of potential failure modes that can occur. To list but a few, these include:

• Flaking, peeling, scoring, smearing
• Fracture, cracks, cage damage, denting
• Pitting, wear, fretting, false brinelling
• Creep, seizure, electrical corrosion, rust
• Mounting flaws, discoloration

When a rolling bearing is damaged during operation, the entire machine or equipment may seize or malfunction. From this we can conclude that since some bearings fail prematurely or unexpectedly cause problems, it is important to be able to diagnose and anticipate failure beforehand so that preventive measures can be adopted.

Mounting advice

Correct mounting influences the bearing’s running accuracy, life and performance. As a result, the following steps are recommended: 

• Clean the bearing and surrounding parts
• Check the dimensions and finish conditions of related parts
• Follow the specified mounting procedure
• Check that the bearing is mounted correctly
• Supply with the correct type and quantity of lubricant

Since most bearings rotate with the shaft, the bearing mounting method is generally an interference (tight) fit for the inner ring and shaft, while giving a clearance (loose) fit for the outer ring and housing.

Bearing performance factors 

Among the key bearing performance factors during operation are bearing noise, vibration, temperature, and lubricant state. 

• Noise: During operation, sound detection instruments can be used to investigate the volume and characteristics of the bearing rotation noise
• Vibration: A frequency spectrum analyser can be used to measure the magnitude of vibration and distribution of frequencies, with test results determining the likely cause
• Temperature: The bearing steady state temperature depends on load, rotational speed and heat transfer properties of the machine. Insufficient or excessive lubrication and improper mounting might cause the bearing temperature to rise rapidly
• Lubricant state: A heavy film thickness prolongs fatigue life, but it is shortened if the viscosity of the oil is too low, so that the film thickness is insufficient 

The right lubrication

The lubrication (grease or oil) must match the application conditions and purpose in order to attain best bearing performance. For instance, oil lubrication methods are more suitable for higher speed applications and where it is necessary to dissipate heat to the exterior.

Users will also have to decide regarding different application methods, as well as maintain the replenishment and replacement of lubricant. Even if high quality lubricant is used, there is a deterioration in properties with time.

Stop and replace

If your routine inspection discovers any of the following kinds of damage, then the bearing must be replaced:

• Cracks or chipping in the cage, rolling elements or raceway ring
• Flaking in the rolling elements or raceway ring
• Notable scoring on the rolling elements, rib face (collar) or raceway surface
• Notable wear on the cage or loose rivets
• Flaws or corrosion on the rolling elements or raceway surface
• Notable dents on the rolling elements or raceway surface
• Notable creep of the outer ring external surface or inner ring bore
• Discoloration due to heating
• Serious damage on the shield or seal of grease packed bearings

Bearing damage and countermeasures

If all conditions are known for the times both before and after failure – including the application, operating conditions and environment – then a countermeasure can be determined by studying the nature of the failure and its probable causes. A successful countermeasure will reduce similar failures or prevent them from reoccurring.

Any engineers interested in learning more about the failure diagnosis and maintenance of bearings should download the NSK Bearing Doctor guide below.