Why bearings fail 6 primary factors
A rolling bearing is a product of preci- sion manufacturing with clean, Subsurface-initiated fatigue machined surfaces to give accurate 1. Fatigue flawless movements. The bearings com- Surface initiated fatigue ponents have been made to precise dimensions, often to a fraction of a mil- limeter and these dimensions have been checked numerous times during the Abrasive wear manufacturing process. 2. Wear However, after a period of time, the Adhesive wear appearance and performance of the bearing can change in service due to less-than-ideal operating conditions. Moisture corrosion 3. Corrosion These conditions can result in bearing Fretting corrosion damage, reduced service life and some- Frictional corrosion False brinelling times premature bearing failure. To prevent bearing failure, it is impor- tant to understand the most common Excessive voltage factors affecting poor performance and 4. Electrical errosion how to avoid them. Current leakage
The six primary bearing Overload failure modes 5. Plastic deformation Indention from debris There is a great deal of interest in the Indention from handling factors affecting bearing failures where ISO 15243 groups rolling bearing failure modes into six categories. This standard Forced fracture recognizes six forms of primary damage 6. Fracture and cracking or initial failure modes corresponding to Fatigue fracture damage after manufacture. Thermal fracture
ISO 15243: Bearing damage classification – shows 6 primary failure modes and their sub-modes. 1. Fatigue • Follow the bearing manufacturer’s How to avoid it? replenishment/overhaul intervals. In railway axle bearings, adhesive wear Fatigue is a change in the bearing mate- • Be sure that adequate sealing is used. is quite rare. When it does occur, it is rial grain structure caused by the usually due to poor lubrication. repeated cycles of stress in normal ser- • Make sure that the appropriate vice. Evidence of fatigue is usually visible grease is being used for the bearing. when the rolling surfaces fracture, usu- 2. Wear ally known as flaking. • Make sure the sufficient amount of Abrasive wear grease is used. There are two types of fatigue – subsur- face-initiated fatigue and surface-initi- Wear (abrasive wear) is the gradual • Follow the manufacturer’s replenish- ated fatigue. removal of material from the rolling ment/overhaul intervals. contact surfaces during service and is • Be sure that adequate sealing is used. primarily caused by an issue affecting Subsurface-initiated fatigue the lubrication of the bearing such as This fatigue starts underneath the race- contamination of the lubricant with dirt. way surface. Micro cracks can develop With continued use, the bearing sur- 3. Corrosion when structural changes occur in the faces wear and internal clearance is material. When these cracks reach the altered affecting the accurate smooth Moisture corrosion surface, material breaks loose and running of the bearing. Water or corrosive agents inside of the spalls occur. Under normal operating How to avoid it? conditions, pure subsurface fatigue does bearing will result in the formation of • Check seals for effectiveness in stop- not occur frequently. Usually, it happens surface corrosion known as rust. If the ping possible ingress of particles. only after a very long running time. quantity of water or corrosive agents is • Check the grease type. large, the lubricant cannot provide ade- quate protection for the bearing sur- Surface-initiated fatigue • Analyze grease for foreign particles and their possible origin. faces which then soon leads to deep- This fatigue is on the other hand much seated rust. more common. It’s generally caused by How to avoid it? inadequate lubrication. If the lubricant Adhesive wear (also known Check the seal conditions and make supply or lubricant selection is wrong, or as smearing) sure to use appropriate grease. if the lubricant is contaminated, the contact surfaces will no longer be sepa- Adhesive wear, occurs between two roll- rated by an appropriate lubricant film. ing surfaces where material is trans- Frictional corrosion Areas that are rough or uneven can ferred between two contacting surfaces shear over each other and break off. The inside the bearing. Heavy loads can Frictional corrosion occurs in two forms: surface becomes plastically deformed contribute to this form of damage, fretting and vibration corrosion. and sometimes smoothened. Micro especially in bearing designs with thrust spalls occur and, in turn, grow to larger faces such as taper roller bearings. The spalls. roller thrust face (larger diameter roller Fretting corrosion How to avoid it? end) in tapered roller bearings and the corresponding thrust face on the inner Fretting corrosion occurs when there is Lubrication condition is key. ring become smeared with a character- a microscopic relative movement • Make sure that the appropriate istic torn finish. between a bearing rings and shaft or grease is being used for the bearing. housing because a clearance fit is pres- ent. The relative movement causes • Be sure that a sufficient amount of small particles from the surface finish to lubricant is used. become rubbed of each surface. These
2 particles oxidize when exposed to the 4. Electrical erosion oxygen in the atmosphere turning to a rust colour hence the term fretting cor- Excessive voltage rosion. It usually appears between the outer diameter of the outer ring and the When an electric current passes bearing housing in railway applications. through a steel bearing, damage to the contacting surfaces will occur even at very low levels as the electricity travels How to avoid it? to earth. On the contacting surfaces, a • Use special anti-fretting paste on the process is similar to electric arc welding surfaces. occurs when the bearing surfaces are • Implement bearing units with a poly- momentarily welded together. As the amide spacer between the backing bearing continues to rotate, the weld is ring and the inner ring side face in broken leaving characteristic pits in the case not already used before. bearing rolling surfaces. Permanent damage will have been made to the metal structure leading to subsurface Vibration corrosion flaking with continued running. (known as false brinelling) How to avoid it? Vibration corrosion is actually better • Make sure earth return devices known and easier to understand as (brushes) work properly. wear caused by vibration. When subject • When welding, make sure the earth to vibration whilst stationery, the clear- connection is properly done. ance inside the bearing between com- ponents allows the components to vibrate against each other which in Current leakage extreme cases can create significant erosion of the contacting surfaces. The When current flows continually through term false brinelling comes from the the bearing in service, even at low appearance which looks similar to that intensity, the raceway surfaces become made by the Brinel hardness test where eroded as many thousands of micro- a ball is pressed into a surface to meas- scopic pits or craters are form on the ure its hardness. rolling surfaces. In extreme cases, elec- tric current erosion appears as flutes How to avoid it? burnt into the rolling surfaces and is • Avoid using vibratory equipment also known as washboard damage close to rolling stock at standstill. because of its appearance. Flaking of the surfaces will follow leading to cata- strophic bearing failure. How to avoid it? • Make sure earth return devices (brushes) work properly.
3 5. Plastic 6. Fractures and deformation cracking
Overload Overload is caused by static or shock Forced fracture loads, leading to plastic deformation or As well as shock loads, forced fracture is indentations. Typical causes are incor- commonly caused by trying to install rect mounting techniques (force applied bearings into housings or onto shafts through the rolling elements and rings) with a poor shape or incorrect dimen- or a heavy impact at very low bearing sions. Use of incorrect tooling or assem- rotational speed. bling onto axle journals that have a poor How to avoid it? shape and incorrect size can cause ring • Use the right tools when mounting fracture. bearings. How to avoid it? • Prior to mounting, make sure the journals are the correct size. Indentation from debris • Use the correct tools. Particles of contamination can cause • Never use a hammer on any indentations in the rolling contacting component. surfaces if the bearing can roll over the debris. The particles producing the indentations do not need to be hard. Fatigue fracture Contaminants also absorb the oil in the lubricant which can lead to premature Fatigue fracture occurs when the fatigue bearing failure. strength is exceeded due to cyclic bend- How to avoid it? ing for example if the bearing rings are not adequately supported and subject to • Check seal conditions. bending forces in service. A crack is initi- • Be sure to use appropriate and clean ated which will then propagate until the grease during overhaul. crack grows through the ring. How to avoid it? Indentation by handling • Make sure the bearing seats are correct. Handling rolling bearings correctly dur- ing storage, transportation and assem- bly is critical to keep the components in good condition. Always use the correct tooling to handle bearing components and take care when transporting and storing bearings to avoid contamination and corrosion. Shock loads during mounting or overhaul from inappropri- ate handling techniques can cause damage to the rolling surfaces which may be felt and heard when the bearing rotates.
4 Thermal cracking Brown discolouration Generally, brown discolouration is Thermal cracking can occur in a bearing caused by residue left on the running ring where friction from sliding causes surfaces from ingredients in the lubri- heat. Cracks occur in the contacting sur- cant and can be due to heat too. A thor- faces and can happen when a bearing is ough examination of a discoloured not correctly seated and the adjacent bearing by an expert is necessary to components, such as backing rings and confirm the cause of discolouration to end caps, are free to turn because they avoid scrapping serviceable bearings. are not locked in position. If you want to know more about bearing How to avoid it? failures in general or if you would like to • When mounting a tapered roller diagnose a specific bearing damage to bearing unit (TBU), make sure all find the root cause, SKF application components are locked correctly. engineers are available to support ([email protected]). SKF knowledge is also captured in publication EN 14219 Other damages where descriptions and pictures of each form of failure mode is documented for Blue discolouration a deeper understanding of this subject. A table also assists in finding out what The components within a bearing or factors influence bearing performance bearing unit can become discoloured and failure so that SKF customers can blue and this is usually a sign of heat achieve ever increasing performance damage. Heat discolouration usually from SKF products. occurs all over the bearing even in places where there is no contact between surfaces such as roller ends.
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