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13/01/2011
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Tom Shelley reports on some recent advances in solid state bearings that allow them to replace rolling element metallic types.
 Polymer bearings based on PTFE are now able to replace rolling element types in many bicycle applications, maintaining performance, avoiding corrosion and reducing cost and the need for lubrication maintenance. At the same time, bearings based on a completely different polymer technology can now be used in the food industry, thanks to their being made metal detectable, meaning there is no risk of fragments from broken bearings appearing in the end product.
PTFE-based bearingsmade by Saint Gobain Performance Plastics, already employed in bicycle front suspensions, have been developed to the point where they can be used in headsets – the assemblies that allows front forks to rotate relative to frames, bicycle pedal assemblies, and braking systems. For the headsets, the solution is a PTFE friction surface with a rubber backing. For the bicycle pedal assemblies, they replace axle shaft rolling elements bearings and plastic bushings. For the braking systems, they find application in the brake levers, where they provide low clearance, low friction replacements for conventional lever pivots, and in cantilevers, where they offer a high-performance, corrosion resistant option for brake pivots. A low friction cylinder, flange or washer with an interference fit improves feel. The properties of the bearings can be tailored to application by incorporating metal backing and/or the addition of suitable fillers. Glass fibres increase the load carrying capability and reduce wear and creep, while graphite minimises initial wear.
Carbon can be made to assume a similar function to glass fibres, but is less abrasive on the mating contact surface. A special 'Ekonol' aromatic polyester improves wear resistance and lowers the coefficient of friction for high-speed sliding operations and soft mating surfaces. It is also possible to tailor the electrical conductivity of bearings for electrostatic discharge and to assist in the cathodic dip-coating of assemblies. Electrically non-conductive bearings in the paint shop, on the other hand, reduce paint build-up on bearing surfaces and help improve paint quality. In a similar manner, igus, which uses a totally different proprietary set of polymer technologies, has managed to make its 'Igubal' range of plastic bearing products in metal detectable form for the food and beverage industries.
The range includes lubricant and maintenance-free rod end bearings and clevis joints, flange, pivoting and pillow block bearings. Both the housing and spherical balls are made of detectable plastic. The new plastic variants are easy to install, adjust to all angle deviations and, in many applications, replace traditional metallic components, saving up to 80% weight can be saved. This allows machines and systems to work faster and more efficiently. The igus bearings are suitable for dry use at application temperatures ranging from -30 to +80°C, can run in liquids as well as chemicals, and are corrosion resistant, so they are not adversely effected by regular wash downs.
In comparison to traditional metallic bearings, the range also offers better damping characteristics since the material more effectively absorbs vibrations. Saint Gobain Plastics, incidentally is the same company that supplies 'Norslide' cable liners made by paste extrusion and 'Rencol' tolerance rings, three dimensionally patterned inserts made of spring steel. These have a set of applications that range from accommodating differential thermal expansion and small amounts of movement to noise and vibration reduction and slippage at known loads to provide a simple method of mechanical overload protection.
Design Pointers
• PTFE-based bearings can replace most rolling element bearings on a bicycle, especially in headsets, pedals and brakes, maintaining performance while reducing cost and the need for grease maintenance
• A new grade of plastic bearing material for the food and beverage industry has been developed which is metal detectable
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Author
Tom Shelley
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