Polymer bearings take the load
Tom Shelley assesses recent developments and some of the limitations in polymer and composite polymer bearings
Polymer-based bearings are the solution of choice for low cost and many heavy-duty applications
New materials and combinations of materials are constantly being developed, raising maximum working temperatures as high as 300 deg C, and maximum working pressures to 600MPa, or 39 tonnes per square inch.
For many years, the best material for plain bearings was considered to be phosphor bronze, but even 20 years ago, asbestos-reinforced phenolic was able to withstand similar load pressures and last many times longer with or without lubrication.
Asbestos is now looked on with extreme disfavour, and is about to be almost totally banned throughout Europe. ACM Bearings of Rotherham says that its materials now have up to 35 per cent more compressive strength than asbestos phenolic and can be impregnated with solid lubricants such as graphite, molybdenum disulphide or PTFE, which remains for the life of the bearing.
The majority of its bearings are made of polyester resin reinforced with polyester fibre, though some use aramid fibres for additional strength, or Peek (polyetheretherketone) for high temperature performance. The companys latest products are bridge bearings able to carry loads of up to 355 MPa or 23 tonnes per square inch.
Other applications include: Boss lift trucks; automotive coating lines; aircraft simulators; Helvic and Mechtech International rock breakers; and water/oil emulsifiers.
In the Boss lift trucks, ACM bearings are used for mast pivot bushes, side thrust rollers, hydraulic wear rings, steer axle pivots and wear pads for side lift frames and top lifts. Two developments coming through are the replacement of rolling element bearings and solid steel chain rollers.
Polymer composites from DSM are the key to 5 to 60 tonne Steerman Load Moving Systems machinery moving skates. The skates have a steerable front assembly with adjustable rear units which allows substantial loads to be easily moved across industrial floors.
Repositioning plant and equipment in most industrial environments has traditionally meant employing specialist contractors or the use of metal skids liable to damage floors. The new skates, on the other hand, use load wheels and turntables made of polymer composite. The wheels are machined from solid blocks. Because there are a number of these, they ensure weight is evenly distributed and less likely to cause damage. The turntables on the steerable front assemblies are also made from the same composite and are said to give greatly improved impact resistance over any steel equivalent.
The company says that many makers of capital equipment supply the systems as part of their overall packages, allowing end users to easily alter the layout of their premises as production needs change.
Bob Clare is managing director of Yale Industrial Products, which designed and made the Steerman. He says: "The performance of these innovative wheels and turntable is fundamental to the world-wide success of the Steerman."
Another company with considerable expertise in polymer and composite polymer-based bearings is Railko, which particularly designs and makes products for the railway industries. Specialist components include: anti-roll bar bushes, secondary suspension air spring pads, gangway facings, treadplate liners, brake gear bushes, door guide strips and bushes, centre pivot liners and side bearer liners. Customers range from light rail systems through goods wagons to the TGV and ICE high speed passenger trains.
Iglidur, headquartered in Cologne, Germany, has introduced polymer-encased polymer plain bearings. Because they have inner and outer sleeves, the shaft material and surface finish have no effect on performance. Sleeve bearings are available in sizes to suit shaft diameters from 5mm to 20mm.
Iglidur Z is a concoction with high wear resistance at radial loads of 50 to 100MPa and can operate at up to 250 deg C. Other materials are available for less arduous conditions at lower cost. The company also now makes split bearings and spherical thrust bearings. The latter are made from combinations of different bearing materials in a range of sizes from 6mm to 20mm internal diameter. There is more information on the internet ( www.igus.de ).
A word of warning about plastic bearings. Many low cost polymer bearings are made of nylon. Like most thermoplastics, they exist as a randomly arranged, tangled mass of molecules at room temperature and are in effect supercooled liquids. As the temperature rises, the molecules gain thermal energy, allowing the chains to move freely enough for the material to behave as a viscous fluid. The temperature at which the material changes from being hard and glass like to soft and rubber like is known as the glass transition temperature.
A particular problem with nylon is that it absorbs significant amounts of moisture, which causes swelling. Moisture also acts as a plasticiser, lowering the glass transition temperature. In the case of fully saturated nylon 6, this can be as low as room temperature.
Nitto Kohki makes a large number of reciprocating shuttle pumps with PTFE seals cum bearings. The pistons are driven by electromagnet attraction at mains frequency in one direction, with a spring to return them. They are mostly low cost, and many run 24 hours a day for up to 12 years without repair. Applications range from artificial heart assistance to vending machines, bubble baths, sewage treatment plants and fish farms.
The majority of Sarnatech BNL bearing assemblies are made from injection moulded POM (acetal) with steel rolling elements.
Martensitic stainless steel balls are the most common, but carbon steel, glass and POM are also available. Race materials for chemical immersion can be PVDF (polyvinylidene fluoride), PP (polypropylene), UHMWPE (ultra high molecular weight polyethylene), or PK (polyketone). PBT (polyester) is a flame retardant and radiation resistant option as is PA6/12 (nylon). Nylon also resists chlorides. Peek (polyetheretherketone) is, as always, the material of choice for high temperatures in applications which can justify its cost.
Speeds are limited by heat dissipation. In the case of acetal, actual working limits are from -50 deg C to +100 deg C. For practical applications, the range should be restricted to -20 deg C to +70 deg C. For radial bearings, acceptable speeds are: up to 3,000 rpm with intermittent running; or up to 2,000 rpm continuous if lightly loaded; and up to 100 rpm for full complement bearings with no cage. Depending on application needs, bearing life may be designed for between 1 million and 500 million cycles. Outer race size limits range from a minimum practical size of 11mm to a maximum practical size of 150mm. Manufacturing technology s now being tested to reduce the minimum size to 8mm and below. Bearing track sizes have to be controlled to better than 0.1mm depending on bearing size.
Typical applications include: CCTV security cameras, digital printers, copiers, gaming machines and spa baths.
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