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Steel Designers' Manual - 6th Edition (2003)
(a) (b)
D
Bearings 847
bearings, a figure of 0.06 is usually assumed for the coefficient of friction, and the
value is checked later when the bearing selection is complete.
(d) Phosphor bronze
For particular applications, such as bearing guides, phosphor bronze may be used,
BS 5400: Section 9.1 suggests a coefficient of friction of 0.35 for phosphor bronze
sliding on steel or cast iron.
(e) Elastomers
An elastomer is either a natural rubber or a man-made material which has rubberlike
characteristics. Elastomers are used frequently in bearings; they either constitute
the bulk of the bearing itself or act as a medium for permitting rotation to take
place (see sections 28.2.2.2 and 28.2.2.3(7)).
Elastomers are principally characterized by their hardness, which is measured in
several ways, the most common of which is the international rubber hardness
(IRHD). This ranges on a scale from very soft at 0 to very hard at 100. Those elastomers
used in bearings which are to comply with BS 5400: Part 9 have hardnesses
in the range 45 IRHD to 75 IRHD.
The tensile capacity of most elastomers is considerable.As an illustration BS 5400:
Part 9 specifies a minimum tensile elongation at failure of between 300% and 450%
depending on IRHD.
When considering the behaviour of a block of elastomer under vertical compression
it is assumed that the material is securely bonded to top and bottom loading
plates. In this case (which is representative of most bearing situations) the vertical
behaviour is related to the material’s ability to bulge on the four non-loaded faces
and is expressed in terms of the shape factor for the block, which is the ratio of the
loaded area to the force free surface area (see Fig. 28.2).
LB
S
tL B
S D
=
2 ( + )
=
4t
for a rectangle
for a circle
Fig. 28.2 Elastomeric bearing dimensions for (a) a rectangular block, (b) a circular block