Plastic plain bearing specialist, igus has undertaken a lot of research work in the pump sector, where its iglidur range of plain bearing materials has been used to advantage, improving performance and reducing pump costs. Matthew Aldridge provides a technical summary of this workThe standard plain bearing used in a pump - sometimes called a 'bush' - has traditionally been either a carbon or graphite sleeve. For a number of years, igus has been developing materials for pumping applications based on its highly successful plain bearing polymer, iglidur. But the plain bearings used in pumps have two significant differences from traditional iglidur plain bearing applications.Firstly, the sliding speeds tend to be higher. In a standard dry-running iglidur plain bearing application, the maximum sliding speed would be around 0.5m/s, which, for a shaft diameter of 8mm, equates to a rotational speed of approximately 1,200rpm. In a pump, sliding speeds tend to range from 1 to 1.5m/s - equivalent to a rotational speed of 2,400 to 3,600rpm for the same 8mm shaft.Secondly, in traditional iglidur applications, the bearing is dry running and requires no lubrication - one of the material's major benefits. In pump applications, liquid is present, which means that the bearing is under hydrodynamic lubrication. This has the benefit of allowing iglidur polymer bearings to run at higher sliding speeds than when running dry. However, it does not always follow that the best dry-running bearing is the best hydrodynamic bearing.A test programme was initiated, with iglidur X and iglidur H370 (materials supplied by igus for underwater rotational applications) as a benchmark, alongside a conventional carbon bearing. The first development was a material subsequently branded iglidur UW (underwater) and released to market in 2003. Applications were limited, however, because UW could not be used for long periods above 90oC. The breakthrough came with the discovery of an iglidur compound subsequently branded UW500, which will run all day at temperatures of 250oC and for short periods at 315oC.One of igus' recent customers completed life trials using iglidur UW500 in a pump application. At a rotational speed of 2,900rpm (corresponding to a sliding speed in excess of 1.5m/s), the bearing was used running hydrodynamically in potable water. Two loading conditions were tested. The first, at 0.5MPa gave a wear rate of 0.3 micron per million kilometres. The second, at 0.9MPa, gave a wear rate of 0.8 micron per thousand kilometres. This small amount of wear is almost immeasurable. Both iglidur UW and UW500 are designed for mass production as an injection moulded plain bearing. This is an economic method of manufacture that also allows the production of customised parts.Matthew Aldridge is bearings product manager at igus UK