BEARINGS FEATURE FEEL THE HEAT!
With operating conditions in industries ranging from aerospace to automotive and rail becoming harsher, designers are reviewing the components they use. Rob Dumayne, director at igus, examines the latest polymer bearings, comparing them to metal alternatives, and discusses how they can deliver reliable performance with a long service life
I
n the underground, trains, aerospace, automotive and other forms of
transport, safety is paramount. The bearings selected for use here need to be able to operate safely at high temperatures. They therefore must have exceptional thermal properties and a resistance to high temperatures for the prevention of fire, especially where passengers and human life are at risk. Here, linear bearings are used
extensively, for applications including on the seating systems in aircraft, or in the underground on escalators and moving advertising boards. Metal, ceramic and some existing
plastic bearings are able to withstand temperatures of over 315˚C. These, however, are high cost. To withstand fire, plastic bearings should be classified to UL94 – V0. UL 94 is the recognised standard for safety of flammability of plastic materials for parts in devices and appliances testing. One solution that meets these
requirements is the iglidur G V0, a new triboplastic material with enhanced flame retardance that represents a significant breakthrough in both flammability performance and cost effectiveness. The base of each iglidur polymer
bearing is, depending on the specific requirements, a carefully selected thermoplastic matrix material with exceptional properties. These usually feature reinforcing fibres which increase the compressive strength, and solid lubricants that optimise
resistance to wear and friction. iglidur G V0 has a maximum long-
term application temperature of 130˚C and a maximum short-term application temperature of 210˚C. It has a density of 1.53 g/cm3
and is lightweight, with
high abrasion resistance to provide a reliable, long service life.
PLASTIC BEARING BENEFITS Within transportation, especially aerospace applications, there is increased demand to replace metal bearings with plastic bearings 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; and ceramics have also been popular for their ability to withstand exceptionally rapid accelerations and high speeds. However, periodic maintenance to prevent premature failure is essential for many bearings. This is especially true for those used in high cycle operations, where 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 are resistant to dirt, dust and chemicals, are self-lubricating and can endure high temperatures, heavy
/ DESIGNSOLUTIONS
To withstand fire, plastic bearings should be classified to UL94 – V0. UL 94 is the recognised standard for safety of flammability of plastic materials
loads and high speeds. These are also reliable and maintenance free. Engineers, however, hesitate to use plastic bearings in their designs because of the knowledge and experience they have of the more conventional materials. iglidur plain bearings contain solid
lubricants which are very important for the dry-operating performance. They are integrated homogenously 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’. With an injection-moulded iglidur plain bearing, however, this does not occur, and the friction and wear are almost constant over the entire wall thickness.
BEARING LIFE It is, however, 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. The iglidur high-performance plastics are tested continuously – with over 8,000 tests per year for durability, friction and wear. In addition, 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. The variety of iglidur materials has
developed over the years as a result of more and more far-reaching customer requirements. Customers have driven materials development and want to see new bearing solutions. Now, a reliable and cost effective solution for high temperature resistant bearings in transport applications is a reality.
igus UK T: 01604 677240
www.igus.co.uk Enter 227
DESIGN SOLUTIONS | MAY 2015 29
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