Bearing Failure Analysis
Operating Conditions Typically Hold the Key
Rolling element bearings are highly reliable components, and the vast majority of bearings will outlive the equipment on which they are installed. However, while bearings account for a relatively small percentage of all equipment breakdowns, they do fail occasionally. And when they fail, it is usually a critical event, resulting in costly repair and downtime.
When bearings do fail, only a small number are caused by material fatigue. Fatigue serves more as a theoretical limit to bearing life than a real cause of bearing failure. When bearings fail by fatigue, it indicates they have served a full life span.
Most bearing failures are caused by some other condition – usually one that can be prevented. This article examines the common causes of bearing failure, with emphasis on operating conditions that can lead to early bearing failure and recommendations on how to avoid failure in the future.
Load Related Failures
The most common bearing rating factors are speed and load. Of the two, load has by far the greater effect on bearing life. For example, speed and life are inversely proportional. Doubling the speed of a ball bearing halves its life while reducing speed by one-half doubles its life. However, doubling the load on a ball bearing reduces its life by a factor of 8 to 10. The detrimental effects of load on life are even more dramatic with roller bearings.
Excessive Loads usually cause premature fatigue failure of the contact surfaces due to repeated loading of the rolling elements on the races. Fatigue usually appears as raceway peeling and spalling, which resembles potholes in a road surface. The speed of fatigue failure depends on the material, on the level of applied load and on how rapidly the load cycles.
Careful examination may be required to identify fatigue damage. While fatigue appears as spalled-off chunks of raceway, it does not start on the raceway surface. Contact between rolling elements and raceway creates Hertzian stresses that produce shear stresses, which reach a maximum at some depth below the raceway surface. This subsurface shear creates raceway fatigue that starts as tiny cracks beneath the surface. The cracks eventually grow and join other cracks until raceway material breaks off. Because fatigue initiates below the surface, a bearing can be fatigued and look undamaged.
Solution: Reduce bearing load or redesign the equipment using a bearing with greater load capacity.
Besides excessive loads, early fatigue failure can also be caused by tight shaft and/or housing fits, improper preloading and brinelling.
Bearing Fitting practices are often overlooked or ignored. Conditions such as poor contact on the bearing seat or shaft and out-of-square housing shoulders can misalign the bearing ring. In addition, the outer ring of a bearing conforms to the shape of the housing bore, so an out-of-round bore pinches the ring radially. This distortion creates two or more load zones, causing additional stress, heat generation and short life. Special attention should be paid to shaft condition. Dirt or burrs on the shaft can create stress risers that cause local overloads, leading to early failure.
Tight fits can be identified by a heavy ball wear path in the bottom of the raceway around the entire circumference of the inner and outer rings. Where interference fits exceed the radial clearance at operating temperature, the balls will be loaded excessively, resulting in a rapid temperature rise accompanied by high torque. Continued operation can lead to rapid wear and fatigue, even catastrophic failure.
Solution: Determine the proper shaft and housing tolerances for specific types of bearings under various conditions. These values are published in manufacturer’s bearing fit tables. Finally, make sure the shaft is clean and free of burrs.
True Brinelling occurs when loads exceed the elastic limit of the ring material. True brinell marks show as indentations in the raceways that increase bearing vibration and noise, leading to premature bearing failure. Any static overload or severe impact can cause true brinelling.
Solution: Eliminate the overload or impact conditions.
Preloading removes a bearing’s internal clearance so it runs stiffer and truer. For example, spindles mounted on preloaded bearings rotate more accurately with little runout. However, the lack of internal clearances limits a bearing’s speed capability; therefore, as operating speeds increase, bearing preload must be reduced.
Solution: As a rule, bearings should not be preloaded unless it is the only way to provide the needed stiffness.
Reverse Loading is a problem with angular contact bearings. These bearings can carry heavy radial load and thrust load in only one direction. When the bearing is loaded in the opposite direction, the low shoulder on that side of the outer ring truncates the elliptical contact area on the outer ring. The result is excessive stress and increased temperature, followed by increased vibration and early failure.
Solution: Simply install the bearing correctly.
The maximum permitted temperature in rolling element bearings depends on the heat treatment of the material. Temperatures above this limit will permanently deform the bearing steel. Gross overheating of the bearing above the heat stabilization temperature can anneal the bearing parts. This can lead to rapid softening of the bearing steel, thus reducing load carrying capacity, and subsequent failure.
Heat softened bearings can be recognized by discoloration of the rings, rolling elements and retainers, as well as a plastically deformed appearance. In extreme cases, rings and rolling elements will deform. However, temperature imbalance failures often are so catastrophic that little is left of the bearing to identify the root cause of failure.
One unusual consequence of overheating is “hollow ball.” Because heat cannot be conducted away from balls as quickly as it can from rings, the center of the balls can become hot enough to melt the material there. Centrifugal forces, then, can cause heat-softened material to flow away from the center to the cooler outer surface. This produces hollow balls that can explode if the heat differential is high enough.
Solution: In general, bearing operating temperatures are governed by the operating viscosity of the lubricant. The minimum required viscosity for ball bearings is 13 cSt, for roller bearings 21 cSt, and for thrust bearings 32 cSt at the bearings’ operating temperature.
Vibration of a bearing in a static state can force lubricant from between the rolling elements and their raceways, even when there is no static overload. This can tear submicroscopic particles away from the high points of contact. The particles are so small – down to one ten-millionth of an inch – that they immediately oxidize in air. The iron oxide particles act as a lapping compound and tear away additional particles, spreading the damage and causing rough, noisy operation.
This type of failure is called false brinelling or frictional corrosion because it creates elliptical indentations running axially or circumferentially on the races. Besides the indentations, the other telltale sign of false brinelling is a reddish oxide in the lubricant.
False brinelling can ruin a bearing before it has had a chance to run. Equipment shipped by truck or train is exposed to constant vibration that can cause the bearing elements to settle through the lubricant, creating metal-to-metal contact and the prime opportunity for false brinelling. Stand-by equipment mounted on the same base as running machinery is also susceptible to false brinelling. Even stored bearings could false brinnel if they are stored on end and subjected to vibration for long periods.
One important effect of false brinelling is its contribution to fatigue failure. Fatigue cracks can originate in or at the edges of a fretted area.
The following factors influence the rate at which false brinelling occurs:
- Slip – False brinelling cannot occur unless relative motion is sufficient to produce slip between the surfaces.
- Frequency – Frictional wear rates increase at lower frequencies and become almost constant as frequency increases.
- Normal Load – Frictional wear generally increases with applied load.
- Duration – False brinelling increases almost linearly with the number of cycles.
- Temperature – Generally, false brinelling tends to increase with decreasing temperatures.
- Atmosphere – False brinelling is more severe in an air or oxygen atmosphere than in an inert atmosphere.
- Surface Finish – False brinelling is generally more serious when the surfaces are smooth because a smooth finish has smaller and fewer lubricant pockets.
- Lubricant – Lubricants that restrict the access of oxygen reduce frictional wear.
- Hardness – Generally increased hardness reduces frictional wear.
Solution: The primary way to prevent false brinelling is to eliminate the source of vibration. In addition, steps can be taken to remove the relative displacement between parts by:
- Decreasing internal clearances if it does not adversely affect bearing operation
- Locking bearings with a light thrust load (not double-row bearings such as SRBs, SABB, etc.)
- Keeping all surfaces lubricated by periodically rotating stationary equipment
- Pumping grease into the bearing while rotating it if this is a grease application
- Reducing lubricant viscosity so that the lubricant can wet and separate contact surfaces better
Contamination is one of the leading causes of bearing failure. Despite their sturdy mechanical appearance, rolling element bearings are actually precision components with internal tolerances on the order of millionths of an inch – more precise than an expensive wristwatch.
Symptoms of contamination are particle denting of the rolling elements and raceways, resulting in high vibration and abrasive wear. The effect of contamination on bearing life depends on bearing type and size, relative lubricant film thickness and the size, hardness and distribution of solid contaminant particles. Although foreign matter can enter the bearing during mounting, the most direct and sustained area of entry is the housing seals and lubricant.
Bearing manufacturers realize the damaging effect of dirt and take extreme precautions to assemble and deliver clean bearings. Some even assemble their bearings in air-conditioned clean rooms.
Solution: Clean work areas, tools, fixtures and hands to help reduce contamination failures. Also, keep grinding operations away from bearing assembly areas, and keep bearings in their original packaging until they are to be installed. Bearings are wrapped in special, neutral, acid-free paper. They should always be rewrapped in such paper for storage, and they should never be stored unwrapped or wrapped in plastic. The plastic traps moisture, causing rust.
When personnel handle clean bearings, particularly the rolling surfaces of separable bearings, they should wear surgical gloves. This prevents skin acids from leaving a deposit that stains bearing surfaces, leading to etching and corrosion. If gloves are not available, hands should be clean and dry.
Rolling element bearings should always be lubricated with clean grease or oil, and the lubricant should be kept clean during operation. Seals should be inspected periodically to ensure that they are not damaged and permitting contamination to enter the bearings.
Corrosion is the destructive chemical or electromechanical reaction of a material with its environment. Many forms of corrosion can lead to the failure of metal parts or render them susceptible to other forms of mechanical failure. Therefore, several factors must be considered to determine whether corrosion caused or contributed to the failure. Analyzing the effects of corrosion can be complex; however, a simple visual examination or study of events leading to the failure provides adequate information about the failure.
Rust forms if water or corrosive agents reach the inside of the bearing in such quantities that the lubricant cannot protect the steel surfaces. A thin protective oxide film forms on clean steel surfaces exposed to air. However, this film is not impenetrable, and if water or corrosive elements contact the steel surfaces, patches of etching will form. This process will soon lead to deep-seated rust.
Deep-seated rust can initiate flaking and cracks. The appearance of grayish black streaks across the raceways, coinciding with rolling element spacing, is the first sign of corrosion. At later stages, pitting of raceways and other surfaces appears. Corrosion also appears as red/brown discoloration on rolling elements, raceways and retainers. In extreme cases, corrosion can initiate early fatigue failure.
Acid liquids corrode the steel quickly while alkaline solutions are less dangerous. Salts present in fresh water form an electrolyte that allows galvanic corrosion, known as water etching. Salt water is even more dangerous to bearings.
Solution: The first step is to examine the type of corrosion, corrosion rate and environmental influences on the corrosion process. Factors affecting corrosion are not always constant; as these factors change, corrosion rates change. The most common step to prevent bearing corrosion is to simply divert corrosive fluids away from bearing areas. Also, integrally sealed bearings are available that resist the incursion of corrosive materials.
In certain applications, electric current can pass through a bearing. In other instances, an electric fault can occur with current grounding through the bearing. Either occurrence can create arcing and sparking across the working surfaces, resulting in burn and erosion damage that will destroy the bearing. Apart from faulty electric motors, other common sources of spark erosion are an incorrectly grounded electric arc welder and static electricity.
Point arcing produces small melted holes in the inner surfaces of the rings. More common is continuous parking, which creates a regular fluting effect burned all over the bearing raceway surfaces and rolling elements. Static electricity can induce a current flow through the bearing. When current breaks at the contact surfaces between rolling elements and raceways, arcing results, producing a localized high temperature and consequent damage. Overall damage is proportional to the number and size of individual damage points. In severe cases, the rolling elements can be welded to the raceways.
Another type of electrical damage occurs when current passes through a bearing for prolonged periods and the number of pits accumulates astronomically. Flutes can develop considerable depth, producing noise and vibration during operation and eventual fatigue from local overstressing. Flute formation can be related to initial synchronization of shocks or vibrations with the current breaking. Once fluting has started, it is probably self-perpetuating.
Individual electric marks, pits and fluting have been produced in bearings running in the laboratory. Both alternating and direct current can cause this damage. Amperage rather than voltage is the governing factor determining the amount of damage. When a bearing is under radial load, greater internal looseness appears to result in greater electric damage or the same current level.
Solution: For large electric motors, one solution is to install an insulated (ceramic) layer to the outside of the bearing. Otherwise, install improved grounding to bypass the bearing.
Although load has more effect on bearing life than speed, excessive speed can shorten bearing life. The usual effect of high speeds is overheating caused by churning in the oil or grease. Churning increases friction and torque characteristics, which cause the bearing and lubricant to operate at a higher temperature. Overheating, then, leads to lubricant failure and some of the problems described under High Temperatures.
Another result of high speed is broken cages and retainers. High speeds increase inertial forces within the bearing. These forces, combined with inadequate lubrication and sudden stopping or starting, can produce high forces between rolling elements and the retainer. Repeated forces skew and eventually crack the cage or retainer.
Solution: Observe the speed ratings of the bearing. Also, ensure that the bearing is properly lubricated: Over lubricating increases churning; under lubricating does not allow for adequate separation of bearing parts.
When Fatigue Is a Good Thing
Every mechanical component, including rolling element bearings, has a fatigue life. When a bearing reaches its normal fatigue life, it is an indication that the bearing has been properly designed and served its full lifespan. Material fatigue is the way bearings are supposed to fail; therefore, fatigue represents a theoretical limit to bearing life rather than a real cause of bearing failure.
Material fatigue failure – normally called spalling – is a fracture of the running surfaces and subsequent removal of small particles of material. Spalling can occur on the inner or outer rings or rolling elements. This type of failure is progressive and, once started, will spread as a result of further operation. It will always be accompanied by a marked increase in vibration.
In theory, the bearing should outlive the equipment on which it is installed. If a bearing fails by normal fatigue before the equipment reaches the end of its life, the installation should be redesigned to use a bearing with a longer calculated fatigue life.
Best Practices for Storage, Handling and Installation
Lubricant Failure = Bearing Failure
Photos courtesy of The Timken Company.