Show Menu

Root Causes of Bearing Failure Cheat Sheet (DRAFT) by [deleted]

This is a draft cheat sheet. It is a work in progress and is not finished yet.

1. Fatigue

Stress over time, which is fatigue failure, is the right reason why bearings should eventually stop rotating. This cause of failure is different than wear. Fatigue failure is related to the stress the bearing is subjected to over time. Fatigue failures in bearings typically originate sub-su­rface and eventually will propagate to the surface, and in time, will appear in what is referred to as a spall

Fatigue failure is, in part, interr­elated to the purity and the quality of the steel. With advances in steel making over the last few decades, micro-­inc­lusion impurities within the bearing steel are minimal. Calcul­ating the fatigue life of a bearing takes into consid­era­tion, load, speed and cycles or time. Bearing engineers use the L10 life of a bearing, along with dimens­ional restri­ctions and the bearing's design, for selecting a bearing. L10 life is a life calcul­ation where 90% of identical bearings are handled, installed, lubricated and run under the same operating and enviro­nmental condit­ions, without metal fatigue. This is the ideal reason for the cause of failure.

2. Handli­ng/­Ins­tal­lation

Improper handling and instal­lation practices often kill the bearing before it has been installed or rotated. Storage in a wet or highly contam­inated enviro­nment is detrim­ental to the bearing's life. Bearings should be stored, flat, dry, clean and in good order. The temper­ature should not vary widely from summer to winter. Humid enviro­nments are not conducive to extended bearing life. Access should be controlled to prevent unnece­ssary opening and movement.

If bearings are stored, or a machine is idle for an extended time, the lubricant within the bearing may leach out and puddle at roller interv­als­-ca­using an acid etching of the surfaces.

Ambient vibration during storage should be minimized, and bearings should never be stored upright on a shelf. False brinelling of the metal surfaces is caused when the roller and ring are in contact with each other and subjected to vibration over time, resulting in the wearing away of the metal.

If the bearing is dropped while it is being handled, a form of mechanical damage referred to as a true brinelling will result. This material displa­cement or dent, where the rollers and races contact each other, may also be caused by force fitting the bearing (see Figure 3), or through the use of an improper mounting tool, such as a hammer.

Striking a steel bearing directly with a hammer is dangerous and could result in the bearing exploding. The human condition that all too often leans towards carele­ssness and urgency is a big part of this failure equation.

If the fit between the shaft and inner ring, or the housing and outer ring, is not within the recomm­ended specif­ica­tions for the applic­ation, fretti­ng-­cor­rosion on the surfaces will be evident. This is a different type of corrosion than that caused by invasive water and oxygen. Fretti­ng-­cor­rosion occurs when there is movement between a bearing ring and shaft or housing because the fit is too loose. Micros­copic steel particles break off due to movement and oxidize.

This will result in the appearance of areas of corrosion on the surfaces of the rings. In extreme cases of inadequate fit, the outer or inner ring may turn or creep, resulting in galling of the surfaces. The remedy for this is to measure all mating components prior to instal­lation and make sure they are within the fit recomm­end­ations. If not, it's time for a new shaft or housing

What to look for in the analysis

1. Save the failed parts and mark them (date, time, direction, location, etc.).
2. Order the analysis. Use a proble­m-s­olving system such as 5-Whys for RCFA.
3. Use a multi-­faceted approach to condition monitoring by incorp­ora­ting, vibration analysis, thermo­graphy, lubricant sampling and ultrasound data collec­tion.
4. Take pictures and document.
5. Analyze the collected data. Watch for trends.
6. Separate and differ­entiate cause, effect and non-au­ditory noise. Determine the root cause of failure.
7. Use a team of employees and industry specia­lists and implement the team's recomm­end­ations.
8. Avoid blaming indivi­duals.
9. Change the culture away from run to failure.
10. Hold regular training sessions; education is important.
11. Follow up.
12. Expand on and commun­icate the successful solution to others.

3. Operat­ional issues

This category includes negative operating conditions other than those related to the enviro­nment. Excessive thrust loads, misali­gnment, extreme vibration or the passage of stray current through the bearing, known as electrical erosion, are a few of the major faults seen while the bearing is in operation. Many of these types of failures are evident when examining the ball or roller path on the raceway surface, especially in the load zone of the rings.

If the bearing is misaligned beyond its capacity, the path of the rollers will appear to skew from one side to the other. Proper alignment is critical to extending the operating life of rotating compon­ents.

Extreme thrust will be evident by observing an off-center roller path. In ball bearings, a dispro­por­tionate thrust load will result in the roller path being offset in the raceway groove and not in the bottom of the arc. Taper roller bearings subjected to misali­gnment will cause edge-l­oading and localized spalling at the ends of the rollers and the mating portion of the raceway.

Excessive vibration in the operating machine due to out-of­-round rotating components will cause the bearing rollers or balls to bounce and skid as they come in and out of the load zone. This wavy or wash-board pattern is referred to as false-­bri­nel­ling. Keep in mind that static false-­bri­nelling may occur while the bearing is idle (see Figure 4). A dynamic false-­bri­nelling pattern is usually more extreme than that which occurs while the bearing is static.

Fluting associated with electrical erosion. Courtesy: Motion Indust­rie­sWith the increased use of variable frequency drives, bearing failures attributed to electrical erosion have become more common. Electrical erosion occurs when there are improper grounding or rotor issues that allow for the passage of current from the race to the rolling elements, creating arc pits in the steel. The first stage is micro-­pits, some of which cannot be seen by the human eye. These soon turn into a pattern referred to as fluting. It usually can be prevented by shunt brushes retrof­itted onto the motor or by installing cerami­c-ball or cerami­c-c­oated outer rings.