Polymer bush offers near zero wear

Dean Palmer takes a look at a new polymer compound for bearings and bushes that promises to reduce costs and improve the performance of industrial pumps

A new polymer bearing material has been developed that looks ideal for industrial pumping applications, replacing traditional graphite or carbon sleeve bearings and bushes. The developer of the new polymer, Igus, claimed in its recent white paper entitled 'iglidur UW500 in pump applications' that using the new polymer bearing material (iglidur UW500) would help pump manufacturers "reduce costs, whilst at the same time improve pump performance". Igus has predominantly supplied two materials for underwater rotational applications, iglidur X and iglidur H370. An extensive test programme resulted in the development of iglidur UW500, which was released onto the market in 2003. However, applications were limited, because the temperature resistance of the new material did not allow prolonged use above 90°C. The major breakthrough came when the iglidur UW500 compound was discovered, which is capable of running all day at temperatures of 250°C and work for short periods at 315°C. In its practical laboratory tests, Igus benchmarked a traditional carbon bearing normally used in pumps, against its iglidur X and iglidur H370 polymers. The first development resulting from this work was iglidur UW but the latest development iglidur UW500 came recently. In tests, the bearing materials were all rotated at 1m/s under a pressure of 0.25MPa, at a temperature of 25°C, on a hard stainless steel shaft. Tests with different media were also completed and with different shaft materials. According to Matthew Aldridge, product manager polymer bearings at Igus UK, a recent customer has also completed life trials using the new polymer in a pump application. At a rotational speed of 2,900rpm, over 1.5m/s, the bearing was used running hydrodynamically in potable water. Two loading were tested. He explained: "The first, at 0.5MPa, gave a wear result of 0.3 microns per million kilometres. The second, at 0.9MPa, gave a wear result of 0.8 microns per 1,000 kilometres." In other words, the wear was almost immeasurable. Both iglidur UW and UW500 are designed for mass production as an injection moulded plain bearing. This also means that customised parts to a unique design are possible, as well as an economic method of manufacture. As Aldridge said: "In general, plain bearings used in pumps have two significant differences from traditional iglidur plain bearing applications. First, 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 of 8mm diameter, equates to a rotational speed of around 1,200rpm. In a pump, sliding speeds tend to range from 1 to 1.5m/s, which for the same size shaft, equates to rotational speeds of 2,400 to 3,600rpm. "Second, in traditional iglidur applications, the bearing is dry running and requires no lubrication, which is a major benefit. In pump applications, there is a liquid media present, which means the bearing will be under hydrodynamic lubrication. The benefit of this is that the bearings can be used at higher sliding speeds than when running dry." But Aldridge did qualify this statement by pointing out that it does not always follow that the 'best' dry running bearing is the best hydrodynamic bearing.