Impact or oscillatory damage; how many of us think about these when we are storing, handling and installing bearings? How often do we consider it during the repair or alignment of machines/equipment with rolling element bearings?
- dropped a bearing (oops!)?
- impacted an interference fit fitment, such as a coupling hub etc., that is seized onto a shaft (hammer time oops!)?
- beat a bearing onto a shaft or into a housing?
- hit the machine with the big hammer to move it (no jacking bolts oops!),
- let that spare machine sit on the warehouse shelf or in the field for months or even years with no rotation (oops!)?
And the list could go on and on…we’ve all probably been guilty of at least one or more of these infractions at some point. “What’s the big deal anyway?” Well, quite often we talk about bearing failures as if they are simply normal and should be expected. While there are some normal/end of service life/fatigue type failures in the bearing world, the overwhelming majority (95%) are premature and therefore warrant a closer look.
In fact, two very common bearing damage types are responsible for many of these failures and are easily preventable. They can at least be minimized by simply doing “the right things”.
The damage types are True and False Brinelling.
True Brinelling is indentation damage caused by impact. Metal is displaced by the rolling element striking the raceway surface. Of course, when the bearing is placed in service, the damaged area acts as a stress riser and the impacted areas will spall/flake prematurely.
One of the most common ways that a bearing suffers true Brinelling is from impacts during installation. The solution here is to ensure that all bearing fits are properly maintained.
Proper fits eliminate the need for hammers.
Use of bearing heaters is strongly recommended no matter how small the bearing.
False Brinelling is rubbing action leading to metal to metal contact, between the rolling element and the raceway surface. This is caused by vibration or an oscillatory source acting on the bearing or machine while it is stationary. During operation the bearing surfaces are protected by the oil film. When stationary the film cannot regenerate itself. Once the film has been removed metal to metal damage occurs.
Here the vibratory action between the rolling element and the raceway surface creates axial lines. If the bearing remains in that position long enough, it will create hollow spots. Again when placed in service the damaged areas act as stress risers and fatigue failures ensue.
Whenever a bearing fails prematurely, the first question asked should be “what caused it to fail and how can we not have this same failure reoccur?” In the case of Brinelling, it is typically avoidable. At the very least the risk can be minimized greatly.
To prevent True Brinelling:
- Minimize impacts to bearing or machine during transportation
- Use precision installation techniques (measure shaft and housing fits precisely)
- Use correct installation tools
- Practice handling bearings with care
To prevent False Brinelling:
- Avoid vibration while bearing/machine is stationary
- Secure shafts on machines during transport
- Periodically rotate stored spares
- Periodically rotate field spares in/out of service
In summary, most Brinelling failures are preventable with just a little extra care and attention. Take the time to observe bearing failures to understand root causes that can be eliminated or at least reduced greatly. Reliable Manufacturing principles ensure that we examine, learn, grow and move forward with confidence.