Plastic Bearings

Nov. 15, 2002
Although many different types of plastics have properties which make them suitable for bearing applications, the most commonly used are phenolics, acetals, UHMWPE, and nylon.

Although many different types of plastics have properties which make them suitable for bearing applications, the most commonly used are phenolics, acetals, UHMWPE, and nylon. The major limitations involved in the use of plastics have to do with high temperatures and possible cold flow under heavy loads.

Phenolics: Composite materials that consist of cotton fabric, or other fillers bonded with phenolic resin. The compatibility of the phenolics makes them easily lubricated by various fluids.

They have replaced wood bearings and metals in such applications as propeller and rubber-shaft bearings in ships and electrical switch-gear, rolling-mill, and water-turbine bearings. In small instruments and clock motors, laminated phenolics serve as structural members as well as a bearing material. They have excellent strength and shock resistance coupled with resistance to water, acid, and alkali solutions.

Some precautions must be observed with phenolic bearings. Thermal conductivity is low, so heat generated by bearing friction cannot be readily transmitted through the bearing liner. Consequently, larger, heavily loaded bearings must have a generous feed of water or lubricating oil to carry away heat. Some swelling and warping of these bearings occurs in the larger sizes, so larger-than-normal shaft clearances are required.

Nylon: Although the phenolics have predominated in heavy-duty applications, they are frequently replaced by nylon, which has the widest use in bearings. Nylon bushings exhibit low friction and require no lubrication. Nylon is quiet in operation, resists abrasion, wears at a low rate, and is easily molded, cast, or machined to close tolerances. Possible problems with cold flow at high loads can be minimized by using a thin liner of the material in a well-supported metal sleeve.

Improvement in mechanical properties, rigidity, and wear resistance is obtained by adding fillers such as graphite and molybdenum disulfide to nylon. While the maximum recommended continuous service temperature for ordinary nylon is 170°F, and 250°F for heat-stabilized compositions, filled-nylon parts resist distortion at temperatures up to 300°F.

PTFE: Has an exceptionally low coefficient of friction and high self-lubricating characteristics, immunity to almost all types of chemical attack, and ability to operate over extremely wide temperature range (-330 to 360°F continuous, to 550°F short-term).

New reinforcing constructions, particularly tin bronze and stainless-steel interlayer reinforcement (metal fabric), have raised PV ratings, minimized cold flow, and extended wear life substantially. These have outperformed properly applied metal sleeve bearings at PVs of 30,000 or more. Other reinforcing constructions include fabric, powder metal filler, and steel backing in various combinations.

Cost of PTFE bearing material is high relative to plain metal or other resins. But, applications for PTFE center in low rpm, oscillatory or intermittent service or where reliable service without lubrication is vital. Load capacity depends on construction and reinforcing material. Many applications include exposure to weather, chemicals, or vapors which attack metals, lubricants, and some plastics. Others, such as sluice gates, involve the need to operate smoothly, reliably, and without sticking after prolonged idle periods.

Successful applications have included pedal mechanisms, linear actuators, process valve stems, trunnions, pivots, reciprocating mechanisms, automotive knuckle and ball joints, office equipment, health-care equipment, production equipment, farm machinery, textile machinery, and aircraft accessories. An important application (for unfilled types) is in food and packaging machinery where FDA regulations prohibit most lubricants for sanitary reasons.

Since lubricant clearances are not needed, metal fabric reinforced PTFE bearings are used increasingly to improve stiffness and reduce bearing play in a mechanism. Interference fits are even possible; the metal fabric keeps the PTFE in place. Generally, higher loads are possible at lower speeds.

PTFE reinforced with tin bronze fabric exhibits particularly long wear life a high load because interlayer reinforcing fabric is itself a bearing material. Applications and tests have involved static loads to 50,000 psi and dynamic loads of 35,000 psi at moderately low rpms. The bearing still functions and friction coefficient remains low even when worn to the point where bronze fabric is exposed. The shaft rides mainly on PTFE lodged in pockets of the fabric and contact of the shaft with bronze in the fabric is acceptable; an equilibrium of sorts results. The reinforcing metal fabric also helps dissipate heat. In extremely corrosive applications, stainless steel fabric adds protection against unwanted chemical reactions if wear exposes the metal.

In metal fabric-type reinforcements, mechanical interlock between PTFE and fabric has the advantage of holding the PTFE in place and reducing cold flow, resulting in prolonged life. Metal and glass fabrics create a more economical, more supple bearing material. Metal fabrics add substantial strength, dimensional stability, wear life, and heat dissipation. Fabric-reinforced PTFE is readily handled and applied, and is adequate for many moderate-PV applications such as automotive thrust washers, ball and socket joints, aircraft controls and accessories, bridge bearings, and electrical switch gear. To provide a strong bond to steel or other rigid backing material, a secondary fiber such as polyester, cotton, or glass is commonly interwoven with the PTFE and then bonded to the backing.

Improved versions have woven or braided "socks" of PTFE and bondable material. The bearing sleeve is then filament wound with a fiberglass epoxy shell. These bearings may carry static loads up to 50,000 psi. Another bearing material combines low friction and good wear resistance of filled PTFE with strength and thermal conductivity of bronze and steel-supporting structure. A plated-steel backing is covered with a thin layer of sintered spherical bronze particles. The porous bronze is impregnated with a mixture of PTFE and lead to provide a thin surface layer. Service temperatures of -330 to 536°F are possible. For higher loads and PVs, stainless-steel fabric reinforced PTFE bonded to a steel backing provides durability and low friction.

A self-lubricating material employs a proprietary bonding process that allows a filament-wound PTFE liner to be fabricated without a secondary bondable fiber. This places more PTFE at the wear surface. In addition, the material includes a support backing of filament-wound fiberglass which is epoxy bonded. This filament-wound material can carry static loads to 25,000 psi.

Design approach is similar to plain metal bearings except for lubrication provisions. As a class, PTFE bearing materials are a little more forgiving of rough mating surfaces, tolerances, and misalignments than plain metal bearings. For dynamic applications, smoothness of the mating surface should be 32RMS or better. Out of roundness for shaft or hole should not exceed half the diameter tolerance. Although not essential, a one-time initial lubrication application, where permitted, will prolong life and enhance performance.

Acetal: Has been used for inexpensive bearings in a wide variety of automotive, appliance, and industrial applications. It is particularly useful in wet environments because of its stability and resistance to wet abrasion.

Polyimide, polysulfone, polyphenylene sulfide: High-temperature materials with excellent resistance to both chemical attack and burning. With suitable fillers, these moldable plastics are useful for PV factors to 20,000 to 30,000. Polyimide molding compounds employing graphite as a self-lubricating filler show promise in bearing, seal, and piston ring applications at temperatures to 500°F. Polyphenylene sulfide can be applied as a coating through use of a slurry spray, dry powder, or fluidized bed. These coating techniques require a final bake at about 700°F.

Ultrahigh-molecular-weight polyethylene: Resists abrasion and has a smooth, low-friction surface. Often an ideal material for parts commonly made from acetal, nylon, or PTFE materials.

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