Low-Abrasion DuPont™ Vespel® Thrust Washers Lengthen Maintenance Intervals in Automatic Gearboxes of Buses
The planet wheels in Voith Turbo’s new DIWA® .5 automatic gearboxes rotate against low-abrasion DuPont™ Vespel® thrust washers separating them from the planet-wheel carrier. Developed for public transit buses by Voith Turbo, of Heidenheim, Germany, the thrust washers made of DuPont™ Vespel® SP21 polyimide fulfil their function virtually without abrasion, in spite of the high relative rotating speed of more than 20 000 rpm between these components. This permits extremely long maintenance intervals for the highly stressed gearboxes, which are being installed today by all major bus makers.
Markus Zwickel, project leader at Voith Turbo, comments: “In developing our DIWA®1 automatic gearboxes we focused on maximum economy. We aimed not only at optimised operation of the buses in which they are installed, but also at extending the intervals between their maintenance visits to the workshops. The Vespel® thrust washers help us to attain this objective. They are so abrasion-resistant that no further replacement is needed as a rule. They also provide a further advantage. With the bronze disks we used previously, there could be a harmful interaction with the automatic transmission fluid. That cannot happen with this high-performance DuPont polyimide. As a result, we can reach up to 180 000 km between oil changes.”
Depending on the torque to be transmitted, Voith Turbo offers the new DIWA®1.5 gearboxes in two versions. These differ, among other things, in the size of the planet wheels and, with them, of the diameter of the thrust washers. For both versions DuPont recommended using Vespel® SP21, which is a polyimide filled with 15 % graphite; it does not melt and retains its high abrasion-resistance even when the gearbox fluid exceeds 120 °C, which it may do in exceptional cases.
Voith Turbo optimised the shape of the thrust washers in close co-operation with DuPont. “DuPont produces these thrust washers with a special direct-forming technique similar to powder metallurgy,” Zwickel explains. “This allows a great deal of design freedom. The new thrust washers had to be a direct replacement for the previously used bronze disks, so their diameter and thickness were fixed from the start. But we could design radial oil grooves into the surfaces without any additional production costs; these grooves ensure that a fine oil film is formed permanently on both sides of the washer. DuPont first supplied us with machined prototypes for testing, so as to allow us to optimise the number, dimensions, depth and position of the grooves. The direct-forming production method was then adopted after our tests determined the best possible geometry. The direct-formed thrust washers meet our requirements for close tolerances without any post-finishing.”
The Voith AG Group has 34 000 employees, sales of over € 3,7 billion and has plants or offices in 250 locations. Within the group, Voith Turbo specialises in hydrodynamic transmission, clutch and braking systems, which are used on road and rail, in industry and marine engineering — mechanical, hydrodynamic, electrical and electronic solutions which drive machines efficiently, move vehicles comfortably, save energy and reduce emissions.
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Image Caption: Extremely low-abrasion thrust washers made of DuPont™ Vespel® SP21 polyimide substantially prolong maintenance intervals of the new Voith Turbo DIWA® .5 automatic gearboxes, installed in buses worldwide.
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