Page 21 - Tribology in Machine Design

8 Tribology in machine design

control is by far the most important purpose of rolling contact lubrication.
It is almost universally true that lubrication, capable of providing failure-
free operation of a rolling contact, will also confine the friction forces within
tolerable limits.
Considering failure control as the primary goal of rolling contact
lubrication, a review of contact lubrication technology can be based on the
interrelationship between the lubrication and the failure which renders the
contact inoperative. Fortunately for the interpretive value of this treatment,
considerable advances have recently been made in the analysis and
understanding of several of the most important rolling contact failure
mechanisms. The time is approaching when, at least for failures detected in
their early stages, it will be possible to analyse a failed rolling contact and
describe, in retrospect, the lubrication and contact material behaviour
which led to or aggravated the failure. These methods of failure analysis
permit the engineer to introduce remedial design modifications to this
machinery and, specifically, to improve lubrication so as to control
premature or avoidable rolling contact failures.
From this point of view, close correlation between lubrication theory and
the failure mechanism is also an attractive goal because it can serve to verify
lubrication concepts at the level where they matter in practical terms.

1.2.3. Piston, piston rings and cylinder liners
One of the most common machine elements is the piston within a cylinder
which normally forms part of an engine, although similar arrangements are
also found in pumps, hydraulic motors, gas compressors and vacuum
exhausters. The prime function of a piston assembly is to act as a seal and to
counterbalance the action of fluid forces acting on the head of the piston. In
the majority of cases the sealing action is achieved by the use of piston rings,
although these are sometimes omitted in fast running hydraulic machinery
finished to a high degree of precision.
Pistons are normally lubricated although in some cases, notably in the
chemical industry, specially formulated piston rings are provided to
function without lubrication. Materials based on polymers, having intrinsic
self-lubricating properties, are frequently used. In the case of fluid
lubrication, it is known that the lubrication is of a hydrodynamic nature
and, therefore, the viscosity of the lubricant is critical from the point of view
of developing the lubricating film and of carrying out its main function,
which is to act as a sealing element. Failure of the piston system to function
properly is manifested by the occurrence of blow-by and eventual loss of
compression. In many cases design must be a compromise, because a very
effective lubrication of the piston assembly (i.e. thick oil film, low friction
and no blow-by) could lead to high oil consumption in an internal
combustion engine. On the other hand, most ofthe wear takes place in the
vicinity of the top-dead-centre where the combination of pressure, velocity
and temperature are least favourable to the operation of a hydrodynamic
film. Conditions in the cylinder of an internal combustion engine can be