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Bearings take the trials of transportation

02 August 2012

As the aerospace, automotive and rail sectors become more innovative in their approaches to product design, component choice is, not surprisingly, under constant review. 

Durability, weight, resistance to corrosion, fire - all have to be taken into consideration and bearings are no exception. Matt Aldridge takes a look at one particular type: polymer bearings, comparing them with their metal counterparts and explaining how they are able to deliver reliability and long life even in the most demanding applications.

Safety is possibly the paramount consideration in any transportation design exercise; the ability of bearings to continue operating safely at high temperatures, for example, is critical for the prevention of fire, and the preservation of passenger safety. Take, as an example, the linear bearings that are used extensively throughout transportation - on the seating systems in aircraft, and in underground railway systems, from passenger escalators to moving advertising boards. 

Metal, ceramic and some existing plastic bearings are capable of withstanding high temperatures of over 315ºC, and have been used successfully in these applications, however, they are high cost. To withstand fire, plastic bearings should be classified to UL94 V0, meaning that they are fire retardant.

Plastic bearings specialist, igus has developed a new triboplastic material with enhanced flame retardance that is UL94 V0 flame test compliant. This new material - iglidur G V0 - represents a significant breakthrough in both flammability performance and affordability. iglidur G V0 has an upper long-term application temperature of 130oC and an upper short-term application temperature of 210oC. It has a density of 1.53 g/cm3, is lightweight, and offers high abrasion resistance for long life.

At the heart of each iglidur polymer bearing is a thermoplastic matrix material with exceptional properties, selected according to specific end-user requirements Reinforcing fibres are usually embedded in this matrix material to increase its compressive strength, along with solid lubricants that optimise resistance to wear and friction.

Metal replacement
There is a growing trend to replace metal bearings with plastic alternatives in transportation, especially in aerospace applications where their lighter weight brings many advantages, not least being their contribution to lower fuel consumption.

Designers have trusted the reliable and robust qualities of metals 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 occasional adjustment are needed to minimise the effects of wear.

Time and 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 now and in many applications. 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 their familiarity with the more conventional materials

Self-lubrication
Plastic bearings are not only reliable but also maintenance free. Taking the example of iglidur again, plain bearings made from this material contain solid lubricants which are integrated within 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, these 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 an eventual ‘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.

It is important not to confuse high-performance plastic bearings with those that might be ordered from your friendly local injection moulder. The life of an high-performance plastic plain bearing can be accurately calculated according to wear rates, actual testing results and specific application parameters.

igus' iglidur high-performance plastics, for example, are constantly being monitored, with over 8,000 tests per year for durability, friction and wear. A complementary online tool is provided on the igus website, which allows users to enter the maximum loads, speeds, temperatures, and shaft and housing materials for their applications. The software then calculates the appropriate plastic bearing and its expected lifetime based on this extensive real-world testing programme.

Challenging customer applications have driven development and a great variety of iglidur materials has been formulated to meet these demands. Now a reliable and affordable alternative to metal is available – not just for high temperature resistant bearings in transport but for a fast growing range of applications across many sectors.

Matt Aldridge is a director at igus UK




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