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LOW FRICTION PLUS LOW WEAR: VESPEL® SEALING RINGS FOR ZF’S CVT DRIVE
One ring of rectangular cross-section is mounted on the driving shaft and one on the driven shaft in the variator, where they seal off the oil at differential pressures up to 55 bar and rotational speed differences up to 12 000 rpm. They help to save costs, space and weight without compromising reliability. The Mini, in which the VT1F is transversally mounted, affords a current example of its use. A locking tab on the inner diameter of each ring prevents relative movements between the abrasion-sensitive (but light and therefore economical) aluminium housing and the rings. Before a ring is mounted on its shaft, a butt joint is open on the diametrically opposing side to the tab. This allows the ring to be slid easily over the shaft and into the groove designed to hold it; at the groove, the ring snap-fits firmly into place, thanks to the high elasticity of Vespel®. This feature of the DuPont polyimide permits automated assembly of the rings, which would have been impossible with cast-iron ones. In the drive’s warm-up phase, the butt joint remains slightly open, so that expanding air can escape; then the split closes as it warms up, producing maximum sealing force. With Vespel® SP, the temperature at which the split closes can be accurately set, and the ring can also be designed so as to allow some residual leakage, to maintain a permanent lubricating film between the ring and the mating surfaces, or to allow lubrication to reach a bearing behind the ring. DuPont Vespel® SP polyimides have no glass transition temperature and they do not melt. This means that they can be used anywhere from very low to very high temperatures: up to about 290°C in continuous use and about 480°C for short periods. Very good wear resistance, both in lubricated and unlubricated environments, is common to all types. For manufacturers of automatic drives worldwide, graphite-filled Vespel® SP-21 und SP-22 are the preferred types for seal rings and washers. DuPont fabricates these materials into high-precision finished parts with a direct-forming technique. Thermoplastic Vespel® TP types offer even greater freedom to make multi-functional parts of complex shape. Their glass-transition temperatures of about 250 °C are substantially higher than current requirements in CVT engineering. Depending on the application, DuPont offers types with high temperature and abrasion resistance combined with especially high strength or improved chemical resistance.
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