Page 277 - Tribology in Machine Design

262 Tribology in machine design

tapered roller-bearings and spherical roller-bearings are generally limited
6
6
to less than 0.2 x 10 DN and 0.1 x 10 DN respectively. These limits are
basically those stated in bearing manufacturers' catalogues.
The selection of a type or a classification of grease (by both consistency
and type of thickener) is based on the temperatures, speeds and pressures to
which the bearings are to be exposed. For most applications, the rolling
element bearing manufacturer can recommend the type of grease, and in
some cases can supply bearings prelubricated with the recommended
grease. Although in many cases, a piece of equipment with grease lubricated
ball- or roller-bearings may be described as sealed for life, or lubricated for
life, it should not be assumed that grease lubricated bearings have infinite
grease life. It may only imply that that piece of equipment has a useful life,
less than that of the grease lubricated bearing. On the contrary, grease in an
operating bearing has a finite life which may be less than the calculated
fatigue life of the bearing. Grease life is limited by evaporation, degradation,
and leakage of the fluid from the grease. To eliminate failure of the bearing
due to inadequate lubrication or a lack of grease, periodic relubrication
should take place. The period of relubrication is generally based on
experience with known or similar system. An equation estimating grease life
in ball-bearings in electric motors, is based on the compilation of life tests
on many sizes of bearings. Factors in the equation usually account for the
type of grease, size of bearing, temperature, speed and load. For more
information on grease life estimation the reader is referred to ESDU -78032.

7.5.4. Jet lubrication
For rolling-element bearing applications, where speeds are too high for
grease or simple splash lubrication, jet lubrication is frequently used to
lubricate and control bearing temperature by removing generated heat. In
jet lubrication, the placement of the nozzles, the number of nozzles, jet
velocity, lubricant flow rates, and the removal of lubricant from the bearing
and immediate vicinity are all very important for satisfactory operation.
Even the internal bearing design is a factor to be considered. Thus, it is
obvious that some care must be taken in designing a jet-lubricated bearing
system. The proper placement of jets should take advantage of any natural
pumping ability of the bearing. This is illustrated in Fig. 7.17.
Centrifugal forces aid in moving the oil through the bearing to cool and
lubricate the elements. Directing jets into the radial gaps between the rings
and the cage is beneficial. The design of the cage and the lubrication of its
surfaces sliding on the rings greatly effects the high-speed performance of
jet-lubricated bearings. The cage is usually the first element to fail in a high-
speed bearing with improper lubrication. With jet lubrication outer-ring
riding cages give lower bearing temperatures and allow higher speed
capability than inner-ring riding cages. It is expected that with outer-ring
riding cages, where the larger radial gap is between the inner ring and the
cage, better penetration and thus better cooling of the bearing is obtained.
Figure 7.17 Lubricant jet velocity is, of course, dependent on the flow rate and the