Bearings are just one component that needs to be able to withstand the hostile environments faced by industries such as aerospace, automotive and rail. Matthew Aldridge, director at igus UK, reviews the latest polymer bearings and discusses how they deliver reliable performance with a long service life in such conditions.
In transport such as the underground, trains, aerospace and automotive, safety is paramount, and being able to operate safely at high temperatures is a key quality for bearings. Exceptional thermal properties and a resistance to high temperatures are critical for the prevention of fire, especially where passengers and human life are at risk.
Linear bearings are used extensively throughout transportation – for example, on the seating systems in aircraft, or in applications in the underground, from escalators to moving advertising boards. While metal, ceramic and some existing plastic bearings have the capability of withstanding high temperatures of over 315°C, and have been used successfully in these applications, they can be high cost.
To withstand fire, plastic bearings should be classified to UL94 V0, meaning that they are fire retardant. UL 94 is the recognised standard for safety of flammability of plastic materials for parts in devices and appliances testing.
A new triboplastic material with enhanced flame retardance that meets the UL94 V0 flame test requirements, has been developed by igus: iglidur G V0.
The base of each iglidur polymer bearing is, depending on the specific requirements, a carefully selected thermoplastic matrix material with exceptional properties. There are usually reinforcing fibres embedded in it to increase the compressive strength, and solid lubricants that optimise resistance to wear and friction.
Iglidur G V0 has an upper long-term application temperature of 130°C and an upper short-term application temperature of 210°C. It has a density of 1.53g/cm3 and is lightweight, with high abrasion resistance to provide a reliable, long service life.
Be confident in plastic
There is increased demand to replace metal bearings with plastic versions in transportation, especially aerospace applications, as they are significantly lighter and therefore represent cost savings.
Designers have trusted the reliable and robust qualities of metals, such as stainless steel and bronze, to meet their bearing needs for many years. Ceramics have also been popular for their ability to withstand rapid accelerations and high speeds. However, periodic maintenance to prevent premature failure is essential for many bearings, and this is especially true for bearings used in high cycle operations where periodic lubrication and cleaning, as well as the occasional adjustment, are needed to minimise the effects of wear.
Time and time again, the main reasons given as the causes of bearing failure are lubrication issues. Bearing failure can be catastrophic, leading to hot spots and potential sources of fire.
Plastic bearings have been around for some time. They are resistant to dirt, dust and chemicals, are self-lubricating and can endure high temperatures, heavy loads and high speeds. The only problem is that some engineers hesitate to use plastic bearings in their designs because of the extensive know-how and experience they have already with the more conventional materials.
Plastic bearings are reliable and maintenance free, making them a ‘fit and forget’ solution. iglidur plain bearings contain solid lubricants which are very important for the dry-operating performance. They are integrated into the plastic matrix and so always perform at the contact surface between shaft and bearing. With microscopic particles embedded in millions of tiny chambers in the matrix material, the plain bearings release tiny quantities of solid lubricants, which are sufficient to lubricate the immediate area.
The great advantage of this type of design is its homogeneity and lack of layers. Layering means that during the transition into the next layer – for example, by putting wear on the actual gliding layer – a more or less distinct change in properties occurs. With traditional composite bearings, this usually means the end of the bearing, since the rates of friction and wear lead to a dramatic increase in ‘seizing up’. Withan injection-moulded iglidur plain bearing, however, this does not occur, and the friction and wear are almost constant over the entire wall thickness.
It is important not to confuse high-performance plastic bearings with plastic bearings from a local injection moulder. Designers can accurately calculate the life of an iglidur plastic plain bearing according to wear rates, actual testing results and specific application parameters. igus’ iglidur high-performance plastics are tested continuously – with over 8,000 tests per year for durability, friction and wear. Igus provides an Expert System – a complimentary online tool where users enter the maximum loads, speeds, temperatures, and shaft and housing materials. The software then calculates the appropriate plastic bearing and its expected lifetime based on real-world testing.
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