Plain bearings carry load by sliding. They are compact and lightweight and have a high load-carrying capacity. While most plain bearings require lubrication of some kind, polymer plain bearings can be made self-lubricating and dry-running. In this article, Matthew Aldridge describes some new developments in polymer bearing technology, which allow plain bearings to be used at a higher pressure and temperature than before
Until now, polymer bearings have been limited by load capacity and temperature. The traditional pressure limit for high-performance polymer bearings is 150MPa, which is a load equivalent to a Ford Focus hanging on a bearing with a diameter of 10mm and a length of 11mm. New developments in polymer materials double this pressure to 300MPa, as well as retaining a press-fit at temperatures of up to 240oC.
Design engineers are usually trying to do two things when they use plain bearings: make the unit smaller and run the system faster. These two factors increase the pressure on the bearing and increase the running temperature. The maximum pressure figure stated for use in plain bearings refers to the projected area of the bearing. This is the nominal inside diameter multiplied by the length, so a 20x25mm bearing has a projected area of 500mm2. In this case, a pressure of 350MPa would equate to an applied radial load of 175kN or 17.8 tonnes.
The author's company has developed a new material in its 'iglidur' range, called Iglidur Z510, which redefines the term 'high load capacity' in relation to polymer plain bearing performance. It is an homogenous, self-lubricating material that is designed to run dry as well as having the ability to run in dirty, dusty and wet environments. It has almost universal chemical resistance and a moisture absorption (by weight) of 0.3% at 23oC, 50% humidity. Z510 can be used long term at temperatures of up to 260oC, with short term temperatures of up to 315oC possible.
Figure 1 shows the wear of Z510 at different temperatures, compared with two other 'highload/temperature' iglidur bearing materials. It shows how favourably Z510 compares; the values are almost identical to the established iglidur Z material. The test results under high load, slow speed and oscillating motion are also interesting and are presented in Figure 2. Even at a pressure of 350MPa, the Z510 material has a wear rate of around 350 microns/km.
iglidur Z510 allows designers to achieve their aim, and there are some interesting applications that have successfully exploited the material's properties. A leading manufacturer of mounting arms for flat plasma screens, for example, uses Z510 as it allows a very small arm to be used. Manufacturers of conveyor chain often require a bearing that retains press-fit at high temperatures and takes high loads when the chain passes through, for example, an oven during a paint curing process. Applications for high-performance, maintenance-free bearings on truck tail lifts are also now possible for the first time, according to engineers in this sector.
Z510 material is manufactured by injection moulding and is designed for mass production. A standard range is available (diameters from 8 to 30mm), and new tools can be produced to mould as little as a few thousand pieces. The high performance of iglidur Z510 means that it is more expensive to produce than most iglidur bearings, but the high load capacity means that smaller bearings can be designed into the application in the first place, so cost savings are still possible.
Custom parts to a unique design are easily manufactured, while still maintaining an economic method of manufacture
The iglidur Z510 plain polymer bearing material opens up new opportunities for designers by pushing the envelope of material performance to new limits of load and temperature.
Matthew Aldridge is bearings product director at igus UK