Page 279 - Tribology in Machine Design

264 Tribology in machine design
Figure 7.19
contact. At higher speeds, centrifugal effects starve this critical contact of
lubricant.
In the late 1960s, the technique of under-race lubrication was applied to
tapered roller-bearings, that is, to lubricate and cool the critical cone-
rib/roller-end contact. A tapered roller-bearing with cone-rib and jet
lubrication, is shown schematically in Fig. 7.19. Under-race lubrication is
quite successful in reducing inner-race temperatures. However, at the same
time, outer-race temperatures either remain high or are higher than those
with jet lubrication. The use of outer-race cooling can be used to reduce the
outer-race temperature to a level at or near the inner-race temperature. This
would further add to the speed capability of under-race lubricated bearings
and avoid large differentials in the bearing temperature that could cause
excessive internal clearance. Under-race lubrication has been well de-
veloped for larger bore bearings and is currently being used with many
aircraft turbine engine mainshaft bearings. Because of the added difficulty
of applying it, the use of under-race lubrication with small bore bearings has
been minimal, but the benefits are clear. It appears that the application at
higher speeds of tapered roller-bearings using cone-rib lubrication is
imminent, but the experience to date has been primarily in laboratory test
rigs.
The use of under-race lubrication requires holes through the rotating
inner race. It must be recognized that these holes weaken the inner-race
structure and could contribute to the possibility of inner-race fracture at
extremely high speeds. However, the fracture problem exists even without
the lubrication holes in the inner races.
7.5.6. Mist lubrication
Air-oil mist or aerosol lubrication is a commonly used lubrication method
for rolling-element bearings. This method of lubrication uses a suspension
of fine oil particles in air as a fog or mist to transport oil to the bearing. The
fog is then condensed at the bearing so that the oil particles will wet the
bearing surfaces. Reclassification is extremely important, since the small oil
particles in the fog do not readily wet the bearing surfaces. The reclassifier
generally is a nozzle that accelerates the fog, forming larger oil particles that
more readily wet the bearing surfaces.
Air-oil mist lubrication is non-recirculating; the oil is passed through the
bearing once and then discarded. Very low oil-flow rates are sufficient for
the lubrication of rolling-element bearings, exclusive of the cooling
function. This type of lubrication has been used in industrial machinery for
over fifty years. It is used very effectively in high-speed, high-precision
machine tool spindles. A recent application of an air-oil mist lubrication
system is in an emergency lubrication system for the mainshaft bearings in
helicopter turbine engines. Air-oil mist lubrication systems are commer-
cially available and can be tailored to supply lubricant from a central source
for a large number of bearings.