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Steel Designers' Manual - 6th Edition (2003)
852 Bearings and joints
of the pin but not transversely. They have the benefit that they can be designed
to resist uplift. It should be noted, however, that they are unlikely to be anything
other than produced to order and that there may be other means of controlling
comparatively small uplifts (e.g.‘pot’ type bearing with separate vertical
restraints). Leaf bearings have been used in suspension bridges to form the
swing link bearings which are necessary to cater for large movements and uplifts
(Fig. 28.5(e)).
(5) Spherical (PTFE, circular) (Fig. 28.5(f)). These comprise a spherical lower
surface which is lined with PTFE and a matched upper spherical surface of
aluminium or stainless steel. This arrangement allows considerable rotation
capacity in all directions. Horizontal translation is frequently achieved using
another (flat) sliding surface above the upper part of the bearing.
An important consideration with spherical bearings is that in order to
withstand any horizontal loads it is necessary to have a minimum co-existent
vertical load to prevent instability.
Spherical bearings are capable of carrying high vertical loads and also permit
higher rotations than many other types.
(6) Cylindrical (PTFE) ‘anticlastic’ bearings (Fig. 28.5(g)). These are similar in
concept to rocker bearings but instead of using (for example) steel on steel
bearing surfaces they have enlarged bearing areas which are coated with PTFE
on one surface and stainless steel or aluminium on the other. This produces
a bearing with high rotation capabilities about an axis as well as high loadcarrying
capacity. One unit can be combined with another similar arrangement
to provide rotation about an axis at right angles to the first and also with a
sliding plate arrangement to provide translational capability.
(7) Disc or ‘pot’ (Fig 28.5(h)). These are often of similar proportions to spherical
hearings but instead of a sliding spherical surface being used to provide rotation
capability, a disc of elastomeric material is used, confined in a cylindrical
pot. Loading is applied to the surface of the disc via a closely fitting steel piston.
Under these conditions, the confined elastomer is in a near fluid state, and
permits rotation in all directions without significant resistance. Sliding is
achieved by means of a PTFE/sliding surface above the piston, in a similar
manner to spherical bearings. Disc bearings are popular for many applications,
as they tend to be cheaper than spherical bearings but can carry higher
loadings than elastomeric-laminated bearings of comparable plan area. They
have rotation capabilities intermediate between spherical and laminated
bearings.
(8) Fabricated. Fabricated bearings have become less popular largely through the
availability of a wide range of proprietary units. They are used for footbridges
and temporary works applications. There is, however, no reason why properly
designed fabricated bearings should not be used to support a structure, particularly,
say, for a fixed bearing where there is no requirement for sliding surfaces.
Guidance on design of bearings is given in References 2–9.
(9) Special. Special bearings will always be required for particular locations.
Perhaps the most common demands are for: