Page 276 - Tribology in Machine Design

Rolling-contact bearings 261

PTFE-filled materials are not suitable, but lead and lead-alloy coated cages
can supply satisfactory transfer film lubrication.
In very high temperature applications, lubrication with loose powders or
bonded films has provided some degree of success. Powders such as
molybdenum disulphide, lead monoxide and graphite have been tested up
to 650 °C. However, neither loose powders nor bonded films have seen
much use in high-temperature rolling-element bearing lubrication. Primary
use of bonded films and composites containing solid film lubricants occurs
in plain bearings and bushing in the aerospace industry.

7.5.3. Grease lubrication
Perhaps the most commonplace, widely used, most simple and most
inexpensive mode of lubrication for rolling-element bearings is grease
lubrication. Lubricating greases consist of a fluid phase of either a
petroleum oil or a synthetic oil and a thickener. Additives similar to those in
oils are used, but generally in larger quantities.
The lubricating process of a grease in a rolling-element bearing is such
that the thickener phase acts essentially as a sponge or reservoir to hold the
lubricating fluid. In an operating bearing, the grease generally channels or is
moved out of the path of the rolling balls or rollers, and a portion of the fluid
phase bleeds into the raceways and provides the lubricating function.
However, it was found that the fluid in the contact areas of the balls or
rollers and the raceways, appears to be grease in which the thickener has
broken down in structure, due to its being severely worked. This fluid does
not resemble the lubricating fluid described above. Also, when using grease,
the elastohydrodynamic film thickness does not react to change with speed,
as would be expected from the lubricating fluid alone, which indicates a
more complicated lubrication mechanism. Grease lubrication is generally
used in the more moderate rolling-element bearing applications, although
some of the more recent grease compositions are finding a use in severe
aerospace environments such as high temperature and vacuum conditions.
The major advantages of a grease lubricated rolling-element bearing are
simplicity of design, ease of maintenance, and minimal weight and space
requirements.
Greases are retained within the bearing, thus they do not remove wear
debris and degradation products from the bearing. The grease is retained
either by shields or seals depending on the design of the housing. Positive
contact seals can add to the heat generated in the bearing. Greases do not
remove heat from a bearing as a circulating liquid lubrication system does.
The speed limitations of grease lubricated bearings are due mainly to a
limited capacity to dissipate heat, but are also affected by bearing type and
cage type. Standard quality ball and cylindrical roller-bearings with
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stamped steel cages are generally limited to 0.2 to 0.3 x 10 DN, where DN is
a speed parameter which is the bore in millimetres multiplied by the speed
in r.p.m. Precision bearings with machined metallic or phenolic cages may
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be operated at speeds as high as 0.4 to 0.6 x 10 DN. Grease lubricated