Rolling bearings are precision machine elements found in a wide variety of applications. They are typically reliable even under the toughest conditions. Under normal operating conditions, bearings have a substantial service life, which is expressed as either a period of time or as the total number of rotations before the rolling elements or inner and outer ring fatigue or fail. Less than 1 percent (0.35 percent specifically-see Figure 1) of rolling bearings do not reach their expected life. (Source: FAG Bearing Antriebtechnik 18 from 1979.)
Premature Bearing Failure
When a bearing does fail prematurely, it is usually due to causes that could have been avoided. For this reason, the possibility of reaching conclusions about the cause of a defect by means of studying its appearance is useful. It is most important to correct the causes and prevent future failures and the costs that follow.
Most bearing failures such as flaking and pitting, spalling, unusual wear patterns, rust and corrosion, creeping, skewing and others are usually attributed to a relatively small group of causes that are often interrelated and correctable. These causes include lubrication, mounting, operational stress and bearing selection and environmental influence.
|Figure 1. Causes of failure in rolling bearings
Proper/Improper Lubrication and "Grease Service Life"
The purpose of lubricating the bearing is to cover the rolling and sliding contact surfaces with a thin oil film to avoid direct metal to metal contact. When done effectively it:
1. Reduces friction and abrasion
2. Transports heat generated by friction
3. Prolongs service life
4. Prevents rust and corrosion
5. Excludes foreign objects and contamination from rolling elements
Grease is generally used for lubricating bearings because it is easy to handle and simplifies the sealing system, while oil lubrication is generally suitable for high speed or high temperature operations.
Generally lubrication failures occur due to:
1. The wrong lubricant type
2. Too little grease/oil
3. Too much grease/oil
4. Mixing of grease/oil
5. Contamination of the grease/oil by objects or water
In addition to considering normal bearing service life, normal grease service life is important to consider since together they maximize bearing life. Grease service life is the time during which proper bearing function is sustained by a particular quantity and category of grease. This is especially critical in pump, compressor, motor and super precision applications.
Mounting and Installation of Bearings
It is critical in the mounting and installation process to pay strict attention to:
1. Use of proper tools and ovens/induction heaters. Use a sleeve to impact the entire inner ring face of the ring that is being press fit.
2. Verify the shaft and housing tolerances. If the fit is too tight, too much preload is created. If the fit is too loose, too little resulting preload may allow the shaft to rotate or creep in the bearing. Check for proper diameters, roundness and chamfer radius.
3. Avoid misalignment or shaft deflection. This is especially critical in mounting bearings that have separable components such as cylindrical roller bearings where successful load bearing and optimal life are established or diminished at installation.
4. Be aware of radial internal clearance (see Figure 2). It is critical to maintain the proper RIC established in the original design. The standard scale in order of ascending clearance is C2, C0, C3, C4, C5. The proper clearance for the application is critical in that it allows for the challenges of:
- Lubrication. A proper film of lubricant must be established between the rolling elements. Reducing internal clearance and impeding lubricant flow can lead to premature failure.
- Shaft fit. A reduction in the radial internal clearance is inevitable when the bearing is press fit.
- Heat. In normal bearing operation, heat is produced that creates thermal expansion of the inner and outer rings. This can reduce the internal clearance, which will reduce the optimal bearing life.
|Figure 2. Radial internal clearance.
Operational Stress and Bearing Selection
It is generally the exception to find a bearing that has been improperly designed into an application. However, factors within the larger application may change.
If loads become too high, overloading and early fatigue may follow. If they are too low, skidding and improper loading of the rolling elements occur. Early failure will follow in each situation. Similar issues arise with improper internal clearance.
The first sign of these issues will be unusual noises and/or increased temperatures.