Question # 76

How are the bearings of diesel engines lubricated? What do you understand by the terms fluid-film lubrication and boundary lubrication?

How are the bearings of diesel engines lubricated? What do you understand by the terms fluid-film lubrication and boundary lubrication?

By: Dimitros
on Oct. 1, 2016, 6:50 a.m.

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Answer(s):

Diesel engine bearings are lubricated by fluid Aims. The journal is always smaller than its surrounding bearing. When the shaft is static it will make contact with the bearing and this contact will be a line. On each side of this line the normal distance between the shaft and the bearing will increase gradually and will in effect be a curved wedge. When the shaft revolves in the presence of an adequate liquid supply (lubricating oil), the oil is pulled into the wedge and pressure is set up. If these liquid pressures were to be plotted on lines drawn -radially from the centre of the bearing.it would be seen that the plot of these pressures would form a bulge something like a cam profile. The pressure of liquid in the wedge-shaped space sets the shaft over to one side and lifts the shaft away from the bearing so that it is supported on an oil film. The position where the oil film thickness is least will be a small distance away from the static contact line in the direction of shaft rotation. For pressures to be built up to a value high enough to separate the shaft from the bearing, the oil must have sufficient viscosity and the speed of the shaft must be above a certain value. This form of lubrication is referred to as fluid film or hydrodynamic.
Boundary lubrication occurs when the rotational shaft speed falls and the oil wedge is lost. Metal to metal contact then occurs. To prevent metallic contact under boundary conditions greases may be used or additives may be added to the oils. The bearings of a diesel engine do not work under boundary conditions. Very highly loaded crosshead bearings in two-stroke engines may approach boundary conditions.
Diesel engine bearings are lubricated by oil films built up under the conditions described. The bearings are supplied with large amounts of oil which are used to maintain the oil Aim and remove the heat generated. Removal of the heat generated keeps the working parts at temperatures that will not reduce the oil viscosity to values low enough to allow breakdown of the oil Aim.
By: Dimitros
on Oct. 1, 2016, 6:51 a.m.

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