Automobile manufacturers are feeling the pressures of obtaining improved fuel economy and lowering maintenance costs for owners. One solution that automotive engineers developed is to use smaller, high-output engines to reduce weight. The materials specified for these smaller engines must be able to maintain properties at high temperatures especially with manufacturers offering extended vehicle warranties.
When Melling, a manufacturer of fluid pumps for the worldwide transportation industry, was developing its innovative electric water pump for the automotive industry, it needed a small sliding bushing that would support the pump's impeller in the highly stressful environment of today's engine cooling systems. Melling chose an innovative wear grade, Victrex WG polymer, for the bushing because of its ability to withstand the mechanical stresses of a high-heat environment where metals had previously failed. Victrex WG polymer also reduces frictional losses, leading to a reduction in the power consumption of the pump. The reduced stress on the component should increase the lifetime of the bushing, resulting in lower vehicle emissions from the energy efficiency improvements.
"The electric water pump is a vital component of cooling systems in automobiles and motorcycles," explained Gilberto Fanaro, director of sales and marketing at Melling. "Inside the pump's housing is the impeller which pumps coolant continuously through the engine. A failure of the system can lead to overheating and the breakdown of internal components resulting in costly maintenance. Mechanical integrity and dimensional stability are, therefore, essential material requirements due to continual immersion in water containing chemical additives."
According to David Cislaghi, injection molded parts development manager at Ensinger, the molder of the sliding bushings, "Traditionally, metal sleeve bearings, and in some cases, ball bearings, are used in these systems. However, when these components are immersed in a 50/50 mix of water and ethylene glycol at 150 °C (302 °F), corrosion can occur which can lead to failure of the cooling system. Using the PEEK-based Victrex WG polymer for the sliding bushing has eliminated the possibility of that corrosion."
Another major benefit of using Victrex WG polymer is that the impeller and its integrated sliding bushing can then be overmolded. "This eliminates the traditional assembly operation when using ball bearing or metal sleeve bearing supports, resulting in faster production cycles," said Fanaro, "The ability to injection mold Victrex WG polymer has made it possible to create a single bearing system leading to the miniaturization of the cooling pump components."
The Victrex WG polymer used to injection mold the sliding bushing is specially formulated for higher speed/higher load applications. According to Cislaghi, "It is easily processed and affords excellent part-to-part repeatability."
In addition to good hydrolysis and chemical resistance, Victrex WG polymer also satisfies a number of other requirements including: up to 70 percent in weight reductions compared to metals leading to better fuel economy, good dimensional stability at 150 °C (302 °F), a coefficient of thermal expansion similar to steel, high abrasion resistance for loads at 4,500 rpm, low coefficient of friction, and the possibility of re-designs to achieve part miniaturization that reach tight tolerances during the injection molding process.