Under pressure
March 1st 2005

The UK pump industry is under pressure to reduce prices, but increase performance. For many years the standard plain bearing (sometimes known as a bush) used in pumps has been a graphite or carbon sleeve. igus has been developing polymer bearing materials for this specific application. Here, Matthew Aldridge, bearings product director, igus UK discusses the technical results obtained.

In general, 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 0.5 m/s, which, for a shaft of 8mm diameter, equates to a rotational speed of approximately 1200 RPM. In a pump, sliding speeds tend to range from 1 to 1.5 m/s, which, for the same 8mm diameter shaft, equates to rotational speeds of 2400 – 3600 RPM. Secondly, in traditional iglidur applications, the bearing is dry running and requires no lubrication: a major benefit of iglidur polymers. In pump applications, there is a liquid media present, which means the bearing will be under hydrodynamic lubrication. The benefit of this hydrodynamic lubrication is that iglidur polymer bearings can be used at higher sliding speeds than when running dry.

However, for igus to base its knowledge on dry running applications would be misleading, as it does not always follow that the best dry running bearing is the best hydrodynamic bearing.

For many years, igus has been supplying predominantly two materials for use in underwater rotational applications.

These are known as iglidur ‘X’ and iglidur ‘H370’. A test program took these two materials as a bench mark, alongside a conventional carbon bearing commonly used in pumps. The first development was a material now known as iglidur ‘UW’ (UW for underwater). This was released onto the market in 2003. Applications were limited, however, because the temperature resistance of iglidur UW did not allow prolonged use above 90°C. The major breakthrough was the discovery of a compound now known as iglidur ‘UW500’, that can run all day at temperatures of 250°C and work for short periods at a temperature of 315°C.

An example of the test results can be seen in figure 1. This compares the wear rate (measured in microns per km) of iglidur UW, iglidur UW500, iglidur X, iglidur H370 and a traditional carbon bearing when rotating at 1.0 m/s, under a pressure of 0.25 MPa, at a temperature of 25°C, on a hard stainless steel shaft. Tests with different media were also completed, and figure 2 shows the results of the same application criteria with different mediums. Different shaft materials have also been trialled. Figure 3 shows an example of the very slight difference in wear rate when using either soft or hard stainless steel shafts.

A recent customer completed life trials using iglidur UW500 in a pump application.

At a rotational speed of 2900 RPM (over 1.5 m/s), the bearing was used running hydrodynamicaly in potable water. Two loading conditions were tested. The first, at 0.5 MPa, gave a wear result of 0.3 microns per million km. The second, at 0.9 MPa, gave a wear result of 0.8 microns per 1000 km. As with all projects, igus can undertake specific tests according to individual customer applications.

Both iglidur UW and iglidur UW500 are designed for mass production as an injection moulded plain bearing. This also means that customised parts to an individual design are possible, as well providing an economic method of manufacture.

The iglidur UW500 range is designed for use in pump applications. Practical laboratory results provide confidence to recommend this product to the pump industry sector.

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