Which plain bearing failures are most common and what causes them?

asked 19 Jun '17, 05:42

KnowledgeBase's gravatar image

KnowledgeBase
66254298387
accept rate: 0%


Two most common types of failures are explained below:

Abrasive and Erosive Wear

Characteristics


Wear caused by hard particles which larger than the oil film thickness in the bearing produce continuous scores extending to a edge of the bearing surface (oil groove, trailing edge of thrust pad) or ending in an embedded foreign particle.
Erosive wear is a fatigue process forcing small scale degradation of material that results in a roughening of the surface. This can resemble cavitation erosion and electric erosion/spark erosion, but the those occurs in the centre of bearing and lands away from oil feed holes and the latter results in small rounded shiny melt pits that are quite different in

Possible Causes


Two causes of wear damage: abrasive wear caused by the cutting action of hard particles that bridge the hydrodynamic oil film; erosive wear, the small-scale removal of material by fatigue resulting from the mechanical action of small particles repeatedly striking the surface.

Actions

Where damage is caused by solid particles in the lubricant, the only remedy is improved filtration of the oil. Duplex filters give better protection, but single filters with over-pressure relief can release particles into the lubricant when the filter becomes blocked and there is a risk of particle release when filter changes are made. Magnetic plugs should be fitted to reduce damage by ferrous wear particles. 
Analysis of filter debris may be useful in identifying the source of solid contaminants, with iron printing using absorbent paper soaked in potassium ferrocyanide a useful technique for the identification of ferrous particles embedded in soft bearing materials.

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.

Characteristics


Cavitation erosion gives a fine scale roughened texture to the damaged surface. This could possibly be confused with wear by fine particles or even electrical erosion damage, but can readily be distinguished by closer examination (electrical damage, for example, shows rounded shiny pits from which molten metal has been removed) and the circumstances in which the damage occurred.

Possible Causes

Cavitation damage is associated with high-speed reciprocating machines, or machines in which the bearings are subject to high frequency vibration. Gaseous cavitation should be suspected in machines handling lubricating oils with gas in solution, most particularly refrigeration compressors in which the refrigerant is in contact with the lubricating oil.

Actions

In many cases a modification of the bearing design can prevent bubble formation in a sensitive part of the bearing. Where cavitation is associated with vibration, it is a secondary effect and the cure is to remove the cause of the vibration.

permanent link

answered 19 Jun '17, 05:58

SeamenExchangeExpert's gravatar image

SeamenExchan...
299111823
accept rate: 4%

add your answer

MarineProHelp 2018.