Page 244 - Tribology in Machine Design

Sliding-element bearings 229

practice. Inaccessible bearings on all types of equipment are ready
candidates for self-lubrication, as are pieces of equipment in remote places.
By replacing metal bearings with self-lubricating ones, substantial savings
could be made. Similarly, self-lubrication can improve the service life of
equipment bound to be neglected, as for instance, consumer appliances.
Seldom used but critical bearings are also prominent candidates; the
pivots on an elevator emergency brake might remain motionless for years,
but if called upon, the joint must move easily. There is no reason to prevent
the designer from using a self-lubricating bearing as a hydrodynamically
lubricated one. There is little point in this, however, if the bearing will be
properly and continuously lubricated, but if there is a chance that the oil
flow could stop, a self-lubricating bearing could prevent serious damage
and the need for protracted shut-down and repair.
A significant improvement in bearing performance may often be
obtained by conventional liquid lubrication, and like any well-made
journal bearing, the oil lubricated self-lubricating bearing should last
almost forever. There are, however, a number of subtle interfacial
phenomena which are sometimes noted and some of which are deleterious
to good operation. A particular type of problem arises when the fluid
migrates to a significant depth into the matrix of the polymer and causes a
premature failure. This problem of premature failure is especially acute at
intense levels of energy dissipation within the contact area.
Another reason for selecting self-lubricating bearings is the necessity to
cope with hostile environments. Self-lubricating bearings retain their load-
carrying capacity at high temperatures. They can operate where rolling-
element bearings fail due to fatigue, and where conventional lubricants
oxidize rapidly. Furthermore, many self-lubricating polymers resist corro-
sion very well.
An important issue related to the operation of machines is the protection
of the environment from contamination. Sliding bearings do not make as
much noise as rolling-contact bearings, and the plastic liners can act as
dampers absorbing some vibration energy. At the same time, many self-
lubricating bearings are completely oil free, so that they cannot con-
taminate their surroundings with a hydrocarbon mist - a point especially
important to designers of medical equipment, food processing equipment
and business machines. However, it should be pointed out that some self-
lubricating materials, like the various lead-filled polymers, may emit
contaminants of their own. It is known that fatigue limits the service life of
rolling-contact bearings, while wear constitutes the main limitation to the
life of self-lubricating bearings. So it is not surprising that dry bearings
should perform much better in applications that defeat rolling-element
bearings. Oscillating motions of the order of a few degrees, for example,
greatly accelerate needle bearing fatigue. The rolling elements do not
circulate in and out of the load zone but instead, a single roller or a couple of
rollers will rock in and out of the zone always under load. Under these
conditions rolling elements undergo accelerated fatigue and fail quickly.
Oscillating motions pose even bigger problems for hydrodynamic bearings;