Troubleshooting Rolling Element Bearings
Troubleshooting rolling element bearing problems and determining their root cause of failure is often difficult, because many failure types look very similar. This is because bearing failures are almo...
April 1, 2006 | By Lloyd (Tex) Leugner
Troubleshooting rolling element bearing problems and determining their root cause of failure is often difficult, because many failure types look very similar. This is because bearing failures are almost always precipitated by spalling or flaking conditions of the bearing component surfaces.
Spalling occurs when a bearing has reached its fatigue life limit but also when premature failures occur. For this reason, it is important for the troubleshooter to be aware of and able to recognize all of the common failures of rolling element bearings.
This ability to correctly troubleshoot and recognize the root cause of bearing problems will lead the analyst to the right conclusions with regard to the bearing failure.
How many times have you heard the comment, even by knowledgeable and well-meaning engineers and technicians, “This bearing failed prematurely because it was defective.” Manufacturing defects in rolling element bearings make up a tiny fraction of the millions of bearings in use today around the world and this small defect rate is being reduced continually by improvements in manufacturing techniques and bearing materials.
Bearing manufacturers use ultrasonic inspection devices to detect surface and subsurface bearing material defects, eliminating poor quality products during the production process. Eddy current testing is used to evaluate surface hardness and detect cracks to ensure 100% product conformance to bearing specifications.
Only a very small fraction of all the bearings in use fail because they have reached their material fatigue limit. The vast majority of bearings outlive the machinery or component in which they are installed. Therefore, the first question that must be answered is, “What constitutes bearing fatigue life limits?”
What is a bearing’s life expectancy?
Rolling element bearing life expectancy is directly related to the number of revolutions performed by the bearing, the magnitude of the load, and the lubrication and cleanliness of the lubricant (assuming correct initial bearing selection and installation).
Fatigue is the result of shear stresses, referred to as elastic deformation, cyclically appearing immediately below the load-carrying surface, as the rollers or balls pass over the raceway. After many revolutions, these stresses between the rolling element and raceway surfaces will cause subsurface cracks to appear that will gradually extend to the surface of either the rolling element, raceway or both. These cracks may cause surface fragments of bearing material to break away. This condition is referred to as flaking or spalling. The spalling continues until the bearing is no longer serviceable and it has reached its life limit (see Fig. 1).
It should come as no surprise to experienced equipment troubleshooters, that assuming proper design and application, rolling element bearings will fail sooner or later due to their natural material fatigue life limit, but all bearings will fail prematurely from abuse or neglect.
According to many bearing experts, the following statistics apply to rolling element bearings failures, no matter in what type of rotating equipment they are installed (electric motors, pumps, fans, gear drives, etc.):
* 10% reach their natural fatigue life expectancy
* 20% fail prematurely due to inadequate lubrication
* 20% fail prematurely due to contaminated lubricant, either oil or grease
* 30% fail prematurely due to improper selection or faulty installation
* 20% fail prematurely due to mechanical vibration, excessive temperatures, electrical discharge caused by static electricity or current flow, or by operating conditions which allow overloading and/or overspeeding.
These bearing life percentages may vary from industry to industry, depending on operating conditions, maintenance practices and an industry’s operational culture. For example, in the pulp and paper industry, poor lubrication or contaminated lubricants are the main causes of failure.
Bearing manufacturers will provide their customers with bearing life expectancy ratings, defined as the number of revolutions or number of operating hours at a given constant speed that a bearing is capable of, before the first sign of fatigue spalling occurs on one of the rings or rolling elements.
This is often called the ‘basic life rating’ or the L10 bearing life in millions of revolutions, which is the life that 90% of a sufficiently large group of apparently identical bearings can be expected to attain or exceed under identical operating conditions.
The challenge for troubleshooters is to learn to recognize the difference between the 10% of bearings that display material fatigue spalling and the remaining 90% of bearings that display premature spalling referred to earlier, because in many instances they look similar to the untrained eye.
The result is that frequently, the troubleshooter will conclude that the bearing failed due to a defect in manufacture or material and the root cause of failure may never be determined.
What causes premature spalling?
The existing literature available from bearing manufacturers, along with equipment failure experts, generally agree that the primary causes of premature (and therefore preventable) spalling of rolling element bearings includes the factors on the following list:
Misalignment of either the bearing itself or the shafts upon which they may be mounted. Misalignment can be traced as the cause of about 50% of the breakdowns of rotating machinery. A 20% load increase caused by misalignment can reduce the calculated bearing life by almost 50%.
Faulty mounting or installation practices including the careless use of excessive or uneven heating of the bearing prior to the interference fitting to a shaft or into a housing. If heat is required to expand an inner ring, the temperature should never exceed 125C (255F).
If induction heaters are used, it is important to remember to demagnetize the bearing prior to installation. (A magnetized bearing will fail very quickly due to its attraction of ferrous metal particles — see Fig. 2).
Sealed, pre-packed bearings, such as those frequently used in electric motors, must never be heated unless approved by the manufacturer. Bearings containing shields should also not be heated.
Clean hands, clean tools and a thoroughly clean work area are absolutely essential when tradesmen and technicians install new bearings. A small piece of dirt or metal chip trapped in a newly installed bearing is an invitation to dealing with another bearing failure.
When pressing bearings on to a shaft or into a housing, adequate presses or hydraulic tools must be used, and hammers and punches must never be used, if premature spalling failure of a new bearing is to be avoided (see Fig. 3).
Defective bearing seats on shafts and in housings. Factors that produce defective seats include shaft seats and housing bores that are over- or under-size, tapered or oval. Oval or out-of-round housings or undersize shafts can cause a condition called fretting corrosion, where the bearing ring will actually move on its seat during operation. An over-sized shaft can cause a bearing’s inner ring to crack during the cooling period, after installation. An under-sized or oval housing can also cause the bearing outer ring to become pinched, causing premature failure (see Fig. 4).
Improper shaft or housing fits. The degree of tightness or looseness with which a bearing is mounted on shafts or in housings is governed by the load and speed to which the bearing will be subjected. If a bearing ring rotates with the load, an interference fit is required.
For example, in an automotive front-wheel bearing, the outer ring or cup rotates with the wheel and therefore has an interference fit with the wheel hub. On the other hand, the inner
rings rotate relative to the load in a gear reducer or electric motor and are therefore mounted on the shaft with an interference fit. A ‘too loose’ interference fit may cause a condition called creep, resulting in scoring of the inner ring. If the lubricant can penetrate the loose fit, the bore, as well as the shaft seat, will appear polished.
In contrast, an excessive interference fit may cause the bearing ring to crack. The resulting creep in the first condition and the cracked inner ring in the second condition will generate heat and wear particulate, both of which can promote premature spalling and early bearing failure.
Either of these conditions may cause a far more serious problem, such as a bearing seizure, resulting in a catastrophic machine failure.
It is very important to remember that the degree of fit is governed by the principle that heavier loads require greater interference. The presence of shock or continuous vibration also requires a higher interference fit of the ring that rotates with the load.
These concepts related to bearing fits should make it clear that any plant or facility that arbitrarily increases loads or speeds on industrial equipment must be prepared to expect premature bearing failures.
Ineffective sealing. The use of incorrect seal materials that are incompatible with the process fluids or the lubricant used, improper seal installation or improper operation or maintenance of mechanical seals, or the use of seals that cannot effectively operate under the existing temperature or contamination conditions, are just a few of the considerations which must be reviewed when troubleshooting bearings for premature spalling (see Figs. 5, 6 & 7).
Incorrect initial bearing selection. All rolling element bearings must have some internal clearance between components in order to compensate for slight variances in housing and shaft fits and to allow for thermal expansion due to normal operating temperatures.
Reduced levels of internal clearance caused by improper initial bearing selection (or incorrect selection of replacement bearings), excessive operating temperature or out-of-round housings that place excessive loads on bearing components, will all increase bearing loads, causing premature failure that frequently is accepted as a fatigue spalling condition.
Here are some internal radial clearance classifications for spherical roller bearings:
* C1 has the least internal clearance, approximately 4-12 ten thousandths of an inch.
* C2 — clearance of 12-20 ten thousandths of an inch
* C0 — clearance of approximately 21-29 ten thousandths of an inch
* C3 –clearance of approximately 30-43 ten thousandths of an inch
* C4 — clearance of 44-57 ten thousandths of an inch
* C5 — has the most clearance, approximately 57-70 ten thousandths of an inch.
It is a serious mistake to simply select a C3 classification if it should be a C5.
Unacceptable operating conditions. The operating conditions that will cause premature bearing failure include excessive vibration, overloading, overspeeding, high temperatures and electrical discharge.
If a typical bearing load is doubled, the bearing life may be reduced by up to 90%. Doubling the rated speed will reduce bearing life by about 50%. These are principles that must be kept in mind when production increases are demanded without increasing equipment capacity (see Fig. 8).
Electrical discharge is becoming a serious problem in some equipment. V-belt drive systems build up high levels of static electricity during operation and this current can dissipate through the bearings to ground, causing pits or fluting to form on the bearing.
Stray magnetic fields in electric motors, both AC and DC, can generate currents that will pass through bearings. To eliminate these potential problems, grounding brushes should be used to ground motor shafts and V-belts.
Silicone greases contain electric insulation properties and these greases might be considered for some applications.
In many of today’s machines, insulated bearings are used to eliminate the problem of electrical discharge causing pitting or fluting of the bearing surface (see Fig. 9).
Vibration in a bearing while stationary can cause damage called false brinelling. The damage may be either brightly polished depressions or the characteristic reddish stain common to fretting. The marks left by false brinelling will be equal to the distance between the rolling elements, just as it is in cases of true brinelling, so these two conditions are often difficult to distinguish.
Operating bearings at higher temperatures than those recommended by the manufacturer will dramatically shorten the life of bearings, no matter what type, quality or amount of lubricant used. To illustrate the importance of this point; consider the fact that a good quality, well-refined mineral oil will begin to oxidize at 71C (160F). The same result will occur in greases where such oils are used as the lubricating agent.
What this illustrates is that excessive temperatures — that is, temperatures continually exceeding 71C — will have a detrimental effect on both the bearing and the lubricant used. In fact, mineral oils have a high temperature limit of around 300C (550F), at which point the oil decomposes to a soot- or tar-like substance (see Fig. 10).
Improper or inadequate lubrication. As already illustrated, about 70% of bearing failures occur for reasons other than their lubrication quality or quantity, yet users of industrial equipment will very often blame the lubricant used when a bearing failure occurs.
Contributor Lloyd (Tex) Leugner is the principal of Maintenance Technology International Inc. of Cochrane, Alta., a company that specializes in the resolution of maintenance problems and provides training for industry. He can be reached at 403-932-7620 or texleug@shaw.ca.
References: The Practical Handbook of Machinery Lubrication, 3rd Edition, L. Leugner; SKF Bearing Maintenance Handbook, The SKF Manufacturing Group; Care and Maintenance of Bearings, The NTN Bearing Corporation; Failure Atlas for Hertz Contact Machine Elements, 2nd Edition, T.E. Tallian.