Page 242 - Tribology in Machine Design

Sliding-element bearings 227

categories of self-lubricating bearings can be distinguished and within each
category further subcategories can easily be identified.

A. Homogeneous metal composites
These are porous, sintered-metal bearings. The majority of them are
iron-bronzes, but occasionally lead-bronze, lead-irons or aluminiums are
used. The softer alloys resist galling longest if the lubricant film breaks
down, while the aluminiums can carry nearly twice the compressive load.
A-l. Liquid-impregnated bearings are soaked in oil (often ordinary
machine oil), which may make up 30 per cent of the finished bearing's
volume. In operation, frictional heat and capillary action draw the oil from
the pores to lubricate the shaft or thrust collar. High temperatures and high
speeds can deplete the oil quickly. They are not recommended for operation
in a dusty environment because the oil film can trap grit. Surface finish of
the mating surfaces should be 0.2/^m or better.
A-2. Solid-impregnated bearings are typically sintered from a blend of
powdered graphite and iron-bronze. They are particularly useful with shaft
finishes of 0.4 to 0.8 /im.
B. Metal-backed laminates

Good lubricating materials usually lack structural strength. These lami-
nates consist of a steel backbone on top of which a thin layer of lubricating
material is deposited and hence they can withstand greatly increased
permissible contact stresses. The lubricant layers may consist of:
B-l. Porous, sintered metal impregnated with solid lubricants. Solid
lubricants like PTFE are notoriously difficult to bond to the substrate. One
solution is to avoid glue completely and create a mechanical link, that is, to
hot-press the lubricant into a shallow surface layer of porous bronze. This is
shown schematically in Fig. 5.40. In service, the plastic's relatively high rate
of thermal expansion pushes the lubricant out of the pores to coat the shaft.
The sintered bronze layer also helps conduct heat away from the bearing
surface. Acetal bearings may also be made in this way. Though these
bearings require an initial pre-lubrication with grease, they are often listed
as dry-running bearings.
B-2. Bonded fabric. A warp of lubricant fibres (usually PTFE) is laid
across a woof of bondable fibres or wire, so that the lubricant threads
predominate on one side and the bondable threads dominate on the other.

Figure 5.40