Application Description
High reliability, light weight and low assembly costs come with sealing rings made of DuPont™ Vespel® SP polyimide in a continuously variable transmission (CVT).
The stepless automatic transmission for the MINI uses two Vespel® 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. 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 rings are a split design patented by DuPont. 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 residualleakage, to maintain a permanent lubricating film between the ring and the mating surfaces, or to allow lubrication to reach a bearing behind the ring.
Benefits Gained
Assembly cost savings. The rings’ patented split design allows them to slide easily over the shaft and into grooves that hold them. At the groove,
each 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.
Lighter weight. The Vespel® rings weigh about 1/5 as much as cast-iron parts would.
Maintenance-free operation. The exceptional resistance of the Vespel® parts to abrasion, wear and hot transmission oil result in high reliability.
Materials Selected and Why
Vespel® SP-21 meets this application’s needs for low wear and friction at high PV conditions, hot oil resistance, thermal stability and mechanical strength.
Return to Case Studies Index Page
