Cavitation damage in plain bearings

Cavitation damage in plain bearings

A recent development is the support bearing for the analysing system for the Nuclear Structure Facility which is being built by the Science Research C...

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A recent development is the support bearing for the analysing system for the Nuclear Structure Facility which is being built by the Science Research Council's Daresbury Laboratory. The analysing system weighs 50 tonnes and includes a 28 tonne 90 degree magnet which analyses and steers the ion beam from the 30 MV tandem Van der Graaff accelerator. The bearing assembly has to be a precision system to enable the beam to be correctly steered and aligned. The bearings employed for this purpose are simple flat pads with capillary control.

good example of this is the hydrostatic workhead spindle "on the Queens Award winning Wickman-Scrivener Model 345 Internal Grinding Machine. This spindle demonstrates another novel hydrostatic bearing feature which is the application of diaphragm valve control to achieve very high bearing stiffness. For practical purposes this stiffness may be infinite. Hydrostatic bearings have also been increasingly applied to high speed grinding spindles where the advantages may also include high load capacity, high stiffness, moderate power dissipation, and low wear.

There are many other examples in large and small machines employed principally for precision milling, boring, grinding or turning. Some applications employ hydrostatic bearings because of the ability to gain high rotational accuracy. A

Hundreds of papers have b~en published in recent years dealing with analysis and design. Four major reviews published in recent years form a useful guide to the subject 7 ~-74 One of the advantages of hydrostatic bearings for system

Cavitation damage in plain bearings Cavitation erosion is a mechanism of damage whereby solid surfaces in contact with a liquid suffer deformation and material loss as a result of the formation and collapse of vaporous cavities in the liquid. In engine bearings the conditions for cavitation are provided by rapid motion between journal and bearing under the influence of inertia and combustion loads. Damage will only result if cavities form and collapse in sufficiently high concentrations; design trends towards higher rotational speeds and, in some cases, higher rates of change of cylinder pressure rise, increasingly provide the conditions under which this can occur. In the past palliatives such as increased oil pressure, reduced diametral clearance, and the specification of harder, and generally more erosion resistant, bearing materials have been applied on an 'ad hoc' basis with varying degrees of success. However, the requirement for reliable generalised design criteria remains outstanding.

Fig 7 "Suction" erosion damage in upper half main bearing

For convenience the most commonly observed cavitation damage in engine bearings may be categorised as: that associated with flow discontinuities arising from interaction of bearing surface features and crankshaft drillings; and damage attributable to journal/bearing dynamics resulting from combustion and inertia effects. Damage in both categories is more common in diesel than gasoline engines, due to the generally more complex lubrication systems and higher

Fig 8 "Discharge" erosion damage in lower half main bearing combustion Ioadings. The rare examples of cavitation damage in gasoline engines are usually a result of abnormal operation.

Fig 6 Cavitation erosion damage resulting from interaction between a partial groove and a rotating crankshaft drilling

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TRIBOLOGY international February 1978

Fig 6 shows damage resulting from flow discontinuities associated with the passage of the crankshaft drilling over the end of a partial groove in a lower half main bearing, As the flow into the drilling is abruptly stopped, inertia effects give rise