Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
Laboratory study shows that, under 3.7 GPa pressure, GKN advanced grease has similar friction behaviour to the commercial grease on smooth contact surfaces, (Fig.11). When changed the surface conditions in to ground surfaces with Ra = 0.6 µm, the advanced grease shows a large reduction in friction from the synergic effect after a running period under 2.7 GPa pressure.
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weight of TPEs decreases. Consequently, a control of molecular weight change from interactions between lubricants and polymer materials plays one of the most important roles for CVJ life.
Figure 11. Friction behaviour under different test conditions. Notes: ribology tests performed at 80°C with 1.5 mm stroke and 40 Hz frequency on SRV test machine.
Challenge from compatibility with sealing materials related to reliability Greases are sealed by boots made from rubbers or thermoplastic elastomers (TPEs) in CVJ systems for life. A failure of boots would result in the end of CVJ life. In one way, in order to improve performance of lubricants, an enhanced additive package is required in formulation. In another way, the additives for enhancement of grease performance could attack the polymer chains to deteriorate the properties of boot materials as a result of reducing the life of boots.
Fig.12 shows a correlation between molecular weights (MW) of TPEs, their property change in laboratory tests and the normalised life of CVJ boots in bench tests. When TPEs contact with lubricants at high temperatures, chemical and physical interactions could take place so that a reduction in molecular weights occurs dependent on different lubricants, temperatures and durations. GKN work indicates a tendency in which a reduction appears in the life of CVJ boots where the molecular
30 LUBE MAGAZINE NO.134 AUGUST 2016
Figure 12. Effect of molecular weight change on boot life.
Figure 13. Additive effect on sealing materials
Fig.13 shows an effect of 4 different additives on changes of molecular weights of a particular TPE material during interactions in laboratory compatibility tests when mixing them in same basic grease respectively. In the compatibility tests,
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