Bearing gifts
March 1st 2006 New developments in polymer bearing technology allow plain bearings to be used at a higher pressure and temperature than before. Matthew Aldridge, bearings product manager, igus UK looks at the iglidur Z510 material for high load capacities
Plain bearings are defined as "a bearing which carries load by sliding... [and are] compact, light weight... and have high load-carrying capacity". Most plain bearings require lubrication of some kind, but igus has, for many years, been developing and supplying a range of self lubricating, dry running polymer plain bearings.
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 limit to 300MPa, as well as retaining a press fit at temperatures up to 240°C.
The iglidur Z510 material is a homogenous, self-lubricating material. It is designed to run dry and has 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 23°C, 50% humidity. The material can be used long term at temperatures up to 260°C, with short term temperatures up to 315°C possible.
Figure 1 (below left) shows the wear of iglidur Z510 at different temperatures compared to two other 'high load/temperature' iglidur bearing materials.
It shows how favourably iglidur Z510 compares; the values are almost identical to the well known iglidur Z material.
The test results under high load, slow speed and oscillating motion are very interesting, and are presented in figure 2 (below right). Even at a pressure of 350MPa, the new iglidur Z510 material is showing a wear rate of around 350µm/km.
Pressure calculation
The maximum pressure figures stated for use in a plain bearing refer to the projected area of the bearing. This is the nominal inside diameter multiplied by the length, so a 20 x 25mm 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 metric tonnes.
Potential and existing applications
Design engineers are usually trying to do two things when using plain bearings: make the unit smaller and run the system faster. These two factors increase the pressure on the bearing and increase the application running temperature. The use of iglidur Z510 allows the designer to achieve their aim.
A leading manufacturer of mounting arms (used for plasma flat screens, etc.) uses iglidur Z510 as it allows a very small arm to be used.
Manufacturers of conveyor chain often require a bearing which retains pressfit at high temperatures and takes high loads when the chain passes through an oven during the paint 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.
Production and economics
The iglidur 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. Customised parts to a unique design are easily manufactured, while still maintaining an economic method of manufacture.
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