Why bearings fail and what to do about it November 2, 2020

Motor bearings are responsible for 51% of all motor failures. (The U.S. Department of Energy puts the number even higher, at 67%.) Find out why bearings are important, what causes bearing failure and get a checklist so you know what to do about it.

Why are bearings important in your motor?

Bearings are critical to the efficient functioning and longevity of an electric motor. Rolling element bearings support and situate the rotor, ensure a small and consistent air gap between the rotor and stator, and transfer load from the shaft to the frame. Without bearings, the motor loses efficiency and power and has increased friction and vibration. Not surprisingly, when bearings fail, there is increased drag, more heat and reduced efficiency.

Causes of bearing failure

  • Improper lubrication
  • Misalignment
  • Incorrect installation and fit
  • Electrical damage
  • Overloading
  • Overheating
  • Contamination

Improper lubrication. According to an article in Reliable Plant, 80% of bearing failures are the result of lubrication failure. That’s a lot of responsibility for a little bit of grease. Lube your bearings properly with the right type of lubricant and keep an eye on motor temperature, as high temps degrade lubricant.

Misalignment. This one can be sneaky, since a motor and load that’s aligned when powered off may be misaligned when normal operation increases the temperature. Watch out for soft foot. (No, soft foot isn’t some weird old timey dance step your grandparents did. It’s improper contact between the machine casing and the base plate, and it’s a major cause of alignment issues.)

Incorrect installation. There are lots of ways to f-up bearing fit, including mounting shaft bearings by applying pressure to the outer race, mounting bearings into a housing by applying pressure to the inner ring, loose fits, tight fits, housings that are out-of-round and bearing seats with a poor finish.

Electrical damage. The passage of electrical current from one ring to the other through the ball, even at low currents, can lead to electrically induced bearing damage over time. The high current of an arcing event causes flaking of the surface material and tempering or melting and re-hardening of the raceway, which forms craters in the raceway and damage to the rolling elements. Even if arcing leaves the bearing intact, the high temperatures wreck the lubricant, which damages the bearing.

Overloading. Whenever a bearing operates beyond rated capacity for temperature, speed or load, overloading occurs. This could be due to production demands, operator error or lower manufacturing standards. Don’t take too much of a load off, however. Bearings need a minimum load to function properly, particularly cylindrical roller bearings which are designed for heavier loads. The exception is preloaded bearings.

Overheating. Bearings have different clearances to allow for thermal expansion during operation, so selecting the correct bearing for the application is important.

Contamination. Watch out for moisture (either from humid environments or temperature swings that cause condensation), poor quality lubricant, acid, dust and dirt. They all cause corrosion.

Look for patterns

Bearing failure is rarely simple. What looks like the cause on first examination may not be what started it all, which is why a root cause analysis is an important way to prevent problems from occurring again. (Our essential guide to motor failure offers some quick tips on doing a root cause analysis.)

Luckily, the culprit leaves evidence behind. Each of the events that leads to bearing failure has a unique damage pattern, and a trained investigator can use clues such as denting, cracking, metal fragments, residues, grooves, marks and wear paths and hints such as noise, colour, shape, orientation, temperature and vibration to identify the perpetrator.

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Bearing failure checklist

Brown or blue rolling elements, followed by excessive wear of bearing components Lubrication problem Grease the motor bearings according to the manufacturer’s specs—no more and no less. Don’t mix lubricants, as some are incompatible. Don’t let your motor get too hot, as this destroys lubricants. Follow these greasing tips. And don’t lubricate sealed bearings.
Vibration from damaged rolling elements and raceways, rust Improper storage or handling Store bearings in a climate-controlled environment. Keep them covered at all times in their anti-corrosive wrapping prior to installation. Minimize the amount of time bearings will be stored.
Denting, wear, cracked rings, high operating temperatures Improper mounting (true brinelling) Mount your bearings properly, according to OEM recommendations. In most cases, that means mounting bearings with a press fit on the rotating ring.
Wear path that’s not parallel to the raceway edges of the non-rotating ring Misalignment Use precision-grade locknuts. Be on the lookout for bent shafts and out-of-square shaft shoulders, spacers and clamping nuts. Check alignment every 2000 hours of operation. Cross-torque the holding bolts of your motor.
Elliptical wear marks at each ball position False brinelling Eliminate unnecessary vibration when the motor is idle. Use anti-wear lubricants.
Increased vibration, brownish-red marks on bearing components Corrosion Use stainless steel bearings or integrally sealed bearings. In particularly hostile environments, use external seals.
Brown marks running parallel to the axis on the raceway (aka fluting) Electrical damage Properly ground the motor or use insulated bearings or hybrid bearings.
Increased vibration and noise, cracks in running surfaces, metal flakes on inner ring, outer ring and balls Fatigue (aka spalling) Install overload controls or use a different bearing if the cause is overload. If the cause is tight fit, install according to manufacturer’s instructions.
Grease bleeds (aka purges), dry “soapy” residue, discoloration of bearing components (colours range from gold to blue), malformation Overheating Add extra cooling, thermal controls and heat paths. If the reason for overheating is overloading, install overload controls or select a bearing that can handle the load.
Overheating, a crack on the inner ring or significant ball wear path at the bottom of the raceway Tight fit Install the bearings according to the manufacturer’s instructions for proper fit.
Wear around the circumference, presence of fine, brown metal particles Loose fit Follow the instructions for bearing installation and check for proper fit.
Grooved wear band similar to tight fight Reverse load If you’re using angular contact bearings, install them so the thrust is on the wide face of the outer ring and the opposite face of the inner ring.
Spalled area in ball path Excessive load Reduce the load or switch out the bearings for ones with greater capacity.
Denting of balls and raceways Contamination Clean up your act. Keep work areas clean, filter your lubricant and protect bearings at all times from dirt, dust and other contaminants. That means keeping your tools and hands clean, too. Also choose the right bearing seal for the application.

If you’re thinking there’s a lot that can go wrong with motor bearings, you’d be right. Fortunately, there’s a single action you can take to extend the life of your bearings and, as a result, your electric motors: put a good preventive maintenance program in place.

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