PISTON RINGS

P.T.F.E. compounds have long and ever-lengthening list of uses. Those discussed briefly below are representative of the main areas of application, but are by no means exhaustive.

COMPRESSOR PISTON RINGS

Filled P.T.F.E. is often used for piston rings in compressors. The main benefits are:

• Ability to operate without lubrication or in marginally lubricated conditions; no catastrophic failure if lubricant starvation occurs.


• Reduced wear rates, both of the piston ring and of the mating surface.


• Resistance to chemical attack by corrosive gases.


• Compatibility with unround surfaces.


• No need for running-in procedures.


• Ease of installation; non-brittle resilience allows one-piece construction.

Reciprocating compressors having piston rings made of filled P.T.F.E. have operated successfully in dry, unlubricated service up to 350 bar and in cylinders having diameters as large as 1 metre. With minimum lubrication, pressures of 400 bar and piston velocities of 300 m/min. can be achieved. In oil-free compressors P.T.F.E. is the most widely used material for piston rings. It has replaced straight carbon, which has the disadvantage of being extremely brittle and failing often during installation of the part.

Rod Packings

Much of the above also applies to compressor rods packings. Unlike piston rings, rod packing rings are usually made in sections, held by a spring, and either radially or tangentially cut. (fig 2) Because of very high pressures at this end of the compressor, P.T.F.E. compounds may be supported by a non-sealing metal ring that also dissipates heat.

Rider Rings

Rider rings on a piston keep the piston in position. They prevent metal-to-metal contact between the cylinder and the piston, and enable the sealing rings to function properly. They frequently have the same composition as the sealing rings, but are made wider to increase the bearing area. The load on such a ring is much less than on the sealing piston ring; hence a material with a higher wear rate but lower abrasion may be used.

Piston Rings in Hydraulic Systems

In hydraulic systems, P.T.F.E. compounds are frequently used for piston rings, and rider rings. Carbon/graphite, graphite, glass and bronze have all been successfully used. Rings made with graphite compounds are generally used in automotive shock absorbers, while bronze compounds are favoured for piston rings in large hydraulic systems. Unlike compressor piston rings, hydraulic seal rings are usually continuous (nonsplit) and need to be installed over a tapered mandrel. Some designers, however, prefer compounds containing various combinations of carbon, graphite, molybdenum disulphide, glass and ceramics. In vacuum applications, polymer-filled types are sometimes used; under these conditions, graphite-containing compounds are unsatisfactory. Fig 1 Design for Piston Rings Step Cut Straight Cut Angle Cut Fig 2 Design for Rod Packing Rings Because of very high pressures at this end of the compressor, P.T.F.E. compounds may be supported by a non-sealing metal ring that also dissipates heat.

Shaft Bearings

Shaft bearings of P.T.F.E. compounds have several advantages over the more traditional roller bearings or bronze bearings: low friction, hence low starting torque; absence of stick-slip motion and the fact that they can be made either electrically insulating or conductive, as required. But by far the most important advantage is that they can operate without lubrication.

Bearings of P.T.F.E. compounds :

• Reduce maintenance.


• Can be used over a temperature range between -270 ° C and +260 ° C.


• Can be used in places where lubricants could cause unacceptable contamination, e.g. in the food, textile or pharmaceutical industry.


• Can be exposed to or immersed in corrosive media.


• Can operate in full vacuum.


• Can be installed in inaccessible places where maintenance and lubrication are difficult or impossible.


• Will not fail catastrophically if lubrication is accidentally interrupted.