Common Causes of Bearing Failures
There are many causes of bearing damage. It is not always easy to determine the exact cause, but most bearing failures can be attributed to one or more of the following major causes:
Foreign Matter: One of the most common sources of trouble in bearings is wear and pitting caused by foreign particles. This could be in the form of dirt, abrasive grit, lint, dust, steel chips, etc.
Improper Mounting: Bearings should be mounted with a press fit on the rotating ring. Generally, the shaft rotates and the inner ring is mounted with a press or interference fit.
- Mounting bearings on shafts by applying blows or pressure to the outer race will usually cause denting (true brinell)
- Loose shaft fit – rotation of the shaft within the inner ring can produce heat and small loose particles of metal that will eventually get into the bearing, causing wear
- Loose housing fit – damage similar to loose shaft fit
- Excessive tight fits – (shaft and housing) can cause rings to crack. Usually causes excessive internal preload because of the removal of internal clearance. Causes high operating temperature and premature failure
- Out of round housings – usually found in split housings where careful machining is necessary to obtain round housing. Causes localized overloading with abnormal wear on surfaces and retainer pockets. Early fatigue occurs in these areas
- Poor finish on the bearing seat – a coarse finish on the bearing seat will soon break down causing a loose fit condition, previously described
Bearing Misalignment: A frequent source of trouble resulting in overheating and separator failure. Common causes are bent shafts, out-of-square shaft shoulders, out-of-square spacers, and out-of-square clamping nuts. Inspection of the raceways will show the ball track veering from one side to the other.
Vibration Brinell (False Brinell): Caused by the rapid movement of the balls in the raceway while the equipment is idle. Rolling elements quickly remove lubrication and, because there is not enough rotation of the bearing, fresh lubricant is not moved back into the spot. This means the bearing is sitting in one spot, devoid of lubrication, and the movement of the rolling elements wears away the metal. The indentations run axially across the races.
Electrical Damage (Fluting): When electric currents pass through a bearing, there is arcing and burning at the points between the races and the rolling elements where the current jumps the air gap.
Pitting or cratering of a bearing is caused by relatively large charges of electricity.
A line of small burns along the line of contact of the rolling elements is caused by a low current constantly passing through the bearing. This fluting or grooving is formed on all parts as the current continues to pass through the bearing, and the contact points change as the bearing rotates. The steel melts in the affected zone. Electrical damage will cause early spalling and results in a noisy bearing which will have to be replaced.
Improper Bearing Lubrication: Lack of or improper lubrication generally causes overheating or excessive wear in the bearing. These conditions can result from insufficient lubrication, improper lubricants, complete absence of lubrication, or insufficient lubrication due to loss through leakage. Also to be considered is the breakdown of lubricants either by oxidation or exposure to atmospheric conditions.
Bearing Fatigue: Fatigue means the fatiguing of the metal in the components of the bearing. It is a result of stress reversals produced when rotating members create flexing of the metal. Fatigue develops due to the magnitude of the load and the number of times it is repeated. Actually, the rolling elements create a wave of metal in front of them as they roll. Thus, the metal in the components is alternately put in tension and then compression. This action eventually results in flaking of the metal.
Bearing Corrosion: The finely finished surfaces of ball and roller bearings are readily subject to corrosion by water, acids, and other agents. Corrosion is basically abrasive in nature and will account for excessive or abnormal wear in bearings. Common causes of corrosion include moisture, acid action, poor or broken down greases, poor wrappings, and condensation resulting from excessive temperature reversals.
Defective Sealing: This enables foreign material and contaminants to enter the bearing, causing wear.
High Temperatures: High temperatures frequently cause premature bearing failure, the nature of the failure being predicated on the temperature to which the bearing is raised and the grease with which it is lubricated. Mild temperature elevations may cause grease to bleed which reduces the efficiency of the lubricant. Under increasingly elevated temperature conditions, oxidation causes loss of lubricating elements and the formation of carbon. The carbon thus formed may lock or jam the bearing. High temperatures may also reduce the hardness of the metal causing early failure. High temperatures can cause loss of internal clearance and preloading results. Many bearing failures can be traced to dirt. Cleanliness is always a must.
Storage: Dampness (rust) - store bearings in a dry room.
Unwrapping: Not sure of bearing numbers, tearing wrappings and boxes - lets in dirt, corrosion, etc.
Always take bearing to the place of installation before unwrapping. This keeps the bearing clean.
Make sure parts are clean.
If the bearing and parts are to be left out, cover to keep them clean.