Page 245 - Tribology in Machine Design

230 Tribology in machine design

there is seldom enough time for an oil film to form as the shaft starts, stops,
reverses itself and starts again. Some metal-metal contact is inevitable. For
that matter, any equipment that is stopped and started frequently - even if
the rotation is unidirectional will have problems with metallic contact at
low operating speeds. The solution to this problem is offered by a self-
lubricating bearing and actually there are a number of situations where
self-lubricating bearings are run with additional external lubrication. The
self-lubrication is there for start-up, shut-down, and emergencies. Lubri-
cation also increases a bearing's load-carrying capacity. Self-lubricating
bearings do not stick on start-up. For instance, a bronze bushing and babbits
have a start-up coefficient of friction of around 0.3 while a PTFE based
bearing is only 0.05.
Traditionally, the performance of unlubricated bearings is measured in
terms of PV, the product of the bearing's unit loading and the relative
sliding velocity of the bearing and the mating surface. The dimensions of
2
PV— (N/m ) x (m/s) are the dimensions of the energy flux. This is to be
expected as the energy lost to friction and subsequently dissipated as heat
should be proportional to the frictional force multiplied by the sliding
distance. The energy lost per unit time, per unit area of contact, should be
the product of the unit load, coefficient of friction and sliding velocity. As
such, PV should give a good indication of the heat produced in a bearing. If
the bearing dissipates heat at a constant rate, then PV should be the
measure of the pressure and sliding velocity that the bearing can tolerate.
The only complication is that the coefficient of friction is not constant but
changes with speed and the contact pressure. Therefore it is justifiable to
take the PV values with some reserve.
The most important problem for the designer intending to utilize a self-
lubricating bearing is to estimate its service life. Unfortunately there is no
universally accepted, comprehensive design and service life formulae, but
instead each manufacturer of self-lubricating bearings has its own and
usually different method of projecting wear life. This situation is partly
justified by the fact that all calculation methods are based exclusively on
experimental results. For general design purposes, ESDU Item No. 76029 -
'A guide on the design and selection of dry rubbing bearings' can be
recommended. Also, there are a number of so-called 'designers' handbooks'
produced by manufacturers giving detailed information on the selection
and wear-life projection of self-lubricating bearings.

References to Chapter 5 1. D. F. Wilcock and E. R. Booser. Bearing Design and Application. New York:
McGraw-Hill, 1957.
2. P. R. Trumpler. Design of Film Bearings. New York: The Macmillan Co., 1966.
3. F. T. Harwell. Bearing Systems, Principles and Practice. Oxford: Oxford
University Press, 1979.
4. O. Pinkus and B. Sternlicht. Theory of Hydrodynamic Lubrication. New York:
McGraw-Hill, 1961.
5. D. D. Fuller. Theory and Practice of Lubrication for Engineers. New York: Wiley,
1956.