Page 243 - Tribology in Machine Design

228 Tribology in machine design

This fabric may then be bonded to a steel backing. Such bearings are limited
by their adhesives. Though they are relatively insensitive to high pressures,
glues may give way to environmental chemicals or high temperatures -such
as those generated by high sliding speeds.
B-3. Bonded plastic-based layer. Thermoplastic tapes, thermosetting
phenolic or polyamides filled with PTFE are bonded to a steel strip. The
plastic layer may be moulded around wires or bondable cloth to facilitate
welding or glueing to the backing.
B-4. Unbonded liners. Cylinders of moulded nylon (filled or unfilled),
acetal or reinforced PTFE are easily installed and replaced in metal sleeves.
They generally cannot take as much load or speed as bonded liners, though
setting a reinforcing fabric into the polymer helps to improve the situation.

C. Homogeneous non-metallic composites
C-l. Unfilled base resins are usually nylons, acetals, polyethylene (espe-
cially high density), polyamides and PTFE. Each of these has its own
special advantages, though they can only support relatively low loads.
C-2. Single lubricant fillers. These are made of nylons, acetals, poly-
ethylenes, polyimides, PTFE, phenolics and polyphenylene sulfides with
lubricant fillers - MoS 2, PTFE or graphite. Additions of silicone or oil are
not very popular.
C-3. Single reinforcing fillers, typically fibreglass, in proportion from 10
to 30 per cent, can increase compressive strength, cohesion and temperature
resistance.
C-4. Multiple fillers. Various combinations of the materials already
mentioned are used, plus bronze powder, metal oxides and sometimes
carbon fibres.
C-5. Fabric-and-filler composites are usually compression moulded from
phenolic resins filled with PTFE or MoS 2 onto an open-weave reinforcing
fabric.

D. Filament wound
Manufacturers can make sleeves of glass or other fibre, using techniques
developed for the fabrication of pressure vessels. The sleeves are then lined
in the same way as the metal sleeves.
D-l. Fibre-lined. A strand of bondable material is twisted together with a
strand of lubricant polymer. The resulting thread is wound on a mandrel
and encapsulated in epoxy.
D-2. Bonded fabric is of the same construction as that described in B-2.
D-3. Bonded tape is described in B-3.

5.11.2. Design considerations
There are a number of situations where self-lubricating and pre-lubricated
bearings of some kind should be considered by a designer. The need to
reduce maintenance or to increase reliability is frequently encountered in