Since the first use of P.T.F.E. structural bearing pads in the early ‘sixties, slide bearings incorporating P.T.F.E. have been specified extensively to accomodate movement caused by expansion and contraction, vibration, ground settlement, and changes in degree of loading in major stuctures such as bridges, chimneys, buildings, tunnels, pipelines and heavy industrial plant. Slide bearings can be considered as falling into two categories, simple and complex. Examples of simple bearings are those used for bridges where movement in one plane only is required, as temporary bearings during construction and as pipeline supports. Bearings of this type may or may not be sealed.
Complex bearings are capable of movement in more than one direction and can also accommodate rocking and rotating deflections. Bearings of this type are produced as standard equipment in a range of sizes by specialist manufacturers using P.T.F.E. in their own designs and this note does not attempt to set out detailed design data for these bearings. It deals with the principles involved in the design of P.T.F.E.-based slide bearings and indicates construction methods. The case histories serve to illustrate these principles and methods.

GENERAL PRINCIPLES

In its simplest form a slide bearing consists of a load-carrying pad in sliding contact with a fixed pad. The load-carrying pad is usually attached to the upper part of the structure, and the pad to the lower or foundation member. To maintain constant pressure over the whole area of the fixed pad, the sliding pad is made large enough to cover the fixed pad for the full extent of its travel. This overhanging construction also helps to prevent ingress of dirt between the bearing surfaces, but, to eliminate all risk of dirt penetration, a seal should be incorporated. Side stops are fitted to control the movement of the sliding element where such limitation is a design requirement.

CHOICE OF MATERIALS

Established materials for slide bearings are P.T.F.E., metals, carbon, and interlocking structures of porous metal and filled P. T. F.E. Although rubber does not operate as a sliding component it is also used as a structural bearing because of the rotational and shear deflections obtainable. Each material has advantages and disadvantages which make it more suitable in some applications than others, but P.T.F.E. has certain outstanding features which make it particularly suitable in a wide range of applications. Listed below are the advantages offered by slide bearing surfaces based on P.T.F.E.:

Low Friction

Low coefficients of friction - lower than for lubricated metal or any other self-lubricating solids - maintained over long periods of time.

Chemical Inertness

Resistance to corrosion by virtually all chemicals, solvents and fuels.

Weather Resistance

Complete immunity to outdoor exposure, a non-wetting surface, and no water absorption.

Thermal Stability

A continuous operating temperature range of -250°C to +250°C (-450°F to 450°F). Slide bearing surfaces incorporating P. T. F.E. will never cold-weld to themselves or to other mating materials.

Low and Even Wear

The wear rate of P.T.F.E.-based slide bearings is extremely low and not normally a design limitation.

Electrical and Thermal Insulation

Electrical and thermal insulation automatically provided; this can be of value in preventing galvanic corrosion and heat loss.

Abrasion Resistance

P. T. F.E. allows small foreign particles to become embedded without long-term detriment to frictional properties and bearing life.

Resistance to Fatigue

No fatigue failure with P.T.F.E.-based slide bearings.

Maintenance - Free Life

Requires no lubrication or maintenance for the life of the bearing.

Simplicity and Flexibility of Design

Design capability for rotating and rocking movements as well as translation. Bearing pads incorporating P.T.F.E. can be produced to almost any shape and size and can be combined with elastometric components to allow for mis-alignment and to absorb shock and vibration.

DESIGN PRINCIPLES FOR P.T.F.E. SLIDE BEARINGS

The basic requirement is for a bearing which will remain maintenance-free for a very long time and show constant lowfrictional behaviour with negligible deformation and wear. This is achieved in P.T.F.E.-based slide bearings by exploiting the low-friction, non-weathering properties of the material and designing at stress levels which reduce creep to negligible amounts. Both unfilled and filled P.T.F.E. can be used in slide bearings, and the latter is chosen when minimum deformation is required. The friction coefficient of P.T.F.E. is increased when fillers are incorporated. Slide bearings can be designed to move in translation and in rotation about any axis.