layer on steel backing
Characteristics
Fatigue damage is readily distinguished from other failure mechanisms. It manifests itself by crazy cracking on the surface; the cracks propagate through the white metal to the tin lining on the bearing shell where they continue until joining up with another crack and form a loose piece that detaches from the backing. These loose pieces have an aspect ration of about 5:1 (Fig. ‘a’). The full fatigue strength of the white metal lining depends on a sound metallurgical bond between the backing material and the white metal that is obtained by tinning the backing before pouring on the molten white metal. If a sound metallurgical bond is not created, the fatigue strength is markedly reduced, making the bearing susceptible to premature fatigue failure. Normal and premature fatigue can be distinguished by examining the bottom of the fatigue pit; this should show a coating of tin on the backing, not steel or a rusty surface (Fig. ‘b’).
Fatigue failure is normally confined to journal bearings and rarely occurs with thrust bearings. The loose pieces of white metal can remain in place, held in by the closing fitting journal, or disappear by getting trapped so that they rub and wipe’.
Possible Causes
Fatigue in journal bearings can result from inadequate design, but is usually the result of excessive alternating loading.
Actions
Where fatigue is the result of excessive load, the only remedy is to change to a more fatigue resistant bearing material. Where it is a consequence of vibration, the vibration level has to be reduced.
4. Cavitation Erosion
This is a particular form of fatigue caused by rapid fluctuation of pressure in the bearing oil film.
When the pressure is low, bubbles of vapour or dissolved gas are formed and then collapse as they go into a high pressure region.
Vaporous cavitation, where the bubble collapse is much more violent, results in shock waves in the lubricant film that cause fatigue failure in the white metal surface. This differs from normal fatigue in that small pits are formed rather than loose pieces. Cavitation damage occurs where there are reciprocating loads, either as part of the normal loading cycle or because of high-frequency
vibration. It can also occur in bearings where there are sharp discontinuities in the thickness of the bearing oil film
Gaseous cavitation, in which the bubbles are of gas from solution in the lubricant, is much less energetic than vaporous cavitation as bubble dispersion depends on re-absorption of the gas by diffusion rather than instantaneous collapse. Gaseous cavitation can still, however, cause damage to soft white metal bearings.
4.1 Cavitation Erosion Damage in Reciprocating Engine Bearing
Cavitation erosion damage to engine bearing caused by rapid approach of shaft to bearing, collapsing vapour bubbles in the oil film.
(Photograph: Glacier Metal Co.)
Main Characteristics
Cavitation damage results in roughening of the bearing lands. It takes the form of arrows with the arrow head pointing against the direction of
motion.
Cause
Rapid fluctuations of pressure in the oil film in reciprocating machine causing the formation of bubbles as the shaft moves away from the bearing followed by rapid collapse of the bubbles as the load is applied and the shaft approaches the bearing.
Note The damage is in the middle of the bearing lands, away from the oil
groove and the oil feed holes.
Possible Confusion with Other Types of Damage
Cavitation damage is very similar to the damage resulting from dirt erosion (see Section 2.4: Erosion damage of white metal journal bearing). In the latter case, however, the arrow head starts at the oil feed hole
Comment
This is a case of vaporous cavitation. For an example where the less violent gaseous cavitation was involved, see Section 4.3: Cavitation Damage Caused by Bearing Instability below and Section 6 "Copper deposit on thrust bearing".
4.2 Cavitation Erosion of White Metal Thrust Collar
Photograph ‘a’
Cavitation erosion of white metal thrust face
Photograph ‘b’
Steel thrust collar with lubricant feed grooves on end of gear shaft
Main
Characteristics Photograph ‘a’ shows typical roughening of white metal caused by erosion.
Cause Photograph ‘b’ shows the thrust collar associated with the cavitation damage shown in Photograph ‘a’. This is an unusual arrangement with the oil grooves machined in the hard steel thrust collar, the reverse of the more normal arrangement with the grooves in the soft white metal.
Comment The problem was caused by the sharp fall in pressure at the beginning of the grooves in the steel collar, followed by bubble collapse as the oil re-entered the high pressure region downstream of the groove. The problem was solved by changing to the more normal arrangement with the grooves in the white metal face.
4.3 Cavitation Damage Caused by Bearing Instability
Photograph 'a'
Cavitation erosion damage to white metal journal bearing in high-speed refrigeration compressor subject to instability
Photograph ‘b’
Part of Photograph 'a' at higher magnification
Main
Characteristics
This is another aspect of cavitation erosion damage, in this case the bearing of a high-speed (25,000 rev/min) refrigeration compressor that was subject to a synchronous vibration. The higher magnification Photograph ‘b’ shows erosion damage resembling the effect of wave action on a sandy shore.
Cause The refrigerant was Refrigerant R12 that was in contact with the lubricating oil.
Because of the vibration, the refrigerant came out of solution in the oil as the shaft moved away from the bearing, only to go back into solution as the pressure
increased as the shaft approached the bearing.
Note This was a case of gaseous cavitation, giving a less energetic bubble collapse, but sufficiently energetic to cause cavitation damage to the bearings.
Comment For another example where gaseous cavitation was involved, see Section 6
"Copper Deposit on Thrust bearing".