The stepless automatic drive in the sporty MINI car can operate maintenance-free, thanks in part to the exceptional abrasion resistance and other high-performance capabilities of sealing rings made of DuPont™ Vespel® SP polyimide. The drive, a continuously variable transmission (CVT) type, is made by ZF Getriebe GmbH of Germany.
The Vespel® sealing rings help save costs, space and weight without compromising reliability. The drive uses two such rings of rectangular cross-section to provide a dynamic high-pressure seal at differential pressures up to 800 psi (55 bar) and rotational speed differences up to 12,000 rpm. One ring is mounted on the driving shaft, the other on the driven shaft in the transmission’s variator. This key CVT component uses two opposed pairs of conical discs to vary the speed and torque of the driven shaft.
A locking tab on the inner diameter of each ring prevents relative movements between the CVT’s abrasion-sensitive aluminum housing and the rings. The ring’s circumference is split diametrically opposite to the tab (see photo). During transmission assembly, the split design (covered by a DuPont patent) allows the ring to slide easily over the shaft and into a groove designed to hold it; at the groove, the ring snaps firmly into place, thanks to the high elasticity of Vespel® SP. This elastic feature permits automated assembly of the rings, which would be impossible with cast-iron parts.
In the drive’s warm-up phase, the ring’s split remains slightly open, so that expanding air can escape; after warm-up, the gap closes, producing maximum sealing force. With Vespel® SP, the temperature at which the gap 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 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 and 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 multifunctional 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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Photo caption: Stepless automatic transmission in the Mini relies on DuPont™ Vespel® sealing rings for effective sealing of high-pressure oil.
This release is based on information from:
ZF Getriebe GmbH
Contact: Frank Adriaenssens