Page 278 - Tribology in Machine Design

Rolling-contact bearings 263

nozzle size. Jet velocity in turn has a significant effect on the bearing
temperature. With proper bearing and cage design, placement of nozzles
and jet velocities, jet lubrication can be successfully used for small bore
6
ball-bearings with speeds of up to 3.0 x 10 DN. Likewise for large bore ball-
6
bearings, speeds to 2.5 x 10 DN are attainable.
7.5.5. Lubrication utilizing under-race passages

During the mid 1960s as speeds of the main shaft of turbojet engines were
pushed upwards, a more effective and efficient means of lubricating rolling-
element bearings was developed. Conventional jet lubrication had failed to
adequately cool and lubricate the inner-race contact as the lubricant was
thrown outwards due to centrifugal effects. Increased flow rates only added
to heat generation from the churning of the oil. Figure 7.18 shows the
technique used to direct the lubricant under and centrifically out, through
holes in the inner race, to cool and lubricate the bearing. Some lubricant
may pass completely through and under the bearing for cooling only as
shown in Fig. 7.18. Although not shown in the figure, some radial holes may
be used to supply lubricant to the cage rigid lands. Under-race lubricated
ball-bearings run significantly cooler than identical bearings with jet
lubrication. Applying under-race lubrication to small bore bearings
(<40mm bore) is more difficult because of the limited space available for
the grooves and radial holes, and the means to get the lubricant under the
race. For a given DN value, centrifugal effects are more severe with small
2
bearings since centrifugal forces vary with DN . The heat generated, per
unit of surface area, is also much higher, and the heat removal is more
difficult in smaller bearings. Tapered roller-bearings have been restricted to
lower speed applications relative to ball-bearings and cylindrical roller-
bearings. The speed limitation is primarily due to the cone-rib/roller-end
contact which requires very special and careful lubrication and cooling
consideration at higher speeds. The speed of tapered roller-bearings is
6
limited to that which results in a DN value of approximately 0.5 x 10 DN
1
(a cone-rib tangential velocity of approximately 36ms" ) unless special
attention is given to the design and the lubrication of this very troublesome

Figure 7.18