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Bearings for demanding transport applications

Bearings for demanding transport applications As operating conditions in many industries such as aerospace, automotive and rail become increasingly harsh, designers are reviewing components used throughout many applications. Bearings are just one component that need to be able to withstand these hostile environments. Matt Aldridge of Igus reviews the latest polymer bearings, comparing them to metal alternatives, and discusses how they deliver exceptional, reliable performance with a long service life.

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 in bearings 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. 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 exceptionally 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.

There is increased demand to replace metal bearings with plastic bearings in transportation, especially aerospace applications, for their weight advantage. They are significantly lighter, and therefore represent significant cost savings. They are resistant to dirt, dust and chemicals, are self-lubricating and can endure high temperatures, heavy loads and high speeds. All of this means they are reliable and maintenance-free, making them a 'fit and forget' solution.

Plastic bearings have been around for some time now and have become the norm in many applications. But in sectors such as transport, 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.

Polymer-bearing specialist Igus has been conducting its own materials research for more than 25 years. 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 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'. 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.

Temperature resistance
Metal, ceramic and some existing plastic bearings have the capability of withstanding high temperatures of over 315ºC, and have been used successfully in these applications, however, to date these have tended to 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.

Now Igus has developed a new standard triboplastic material with enhanced flame retardance and meets the UL94 V0 flame test requirements. The new Iglidur G V0 material represents a significant breakthrough in both flammability performance and affordability. Iglidur G V0 has been developed with exceptional physical and thermal properties. It has an upper long-term application temperature of 130ºC and an upper short-term application temperature of 210ºC. It has a density of 1.53g/cm3 and is lightweight, with high abrasion resistance to provide a reliable, long service life.

And that raises a key point. It's 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. Iglidur high-performance plastics are tested continuously by Igus - with over 8,000 tests per year for durability, friction and wear. Igus provides an Expert System - a complementary 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 great variety of Iglidur materials has developed over the years as a result of more and more far-reaching customer requirements. Customers have driven Igus' materials development, because, based on their good experience with Iglidur bearings, they want to see new bearing solutions that are increasingly challenging. Now a reliable and affordable solution for high temperature resistant bearings in transport applications is a reality.
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