DuPont material offers abrasion resistance in bus gearboxes

  • 31-Mar-2008 06:14 EDT
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Low-abrasion thrust washers made of DuPont Vespel SP21 polyimide prolong maintenance intervals of the new Voith Turbo DIWA.5 automatic gearboxes, installed in buses worldwide.

The planet wheels in Voith Turbo’s new DIWA.5 automatic gearboxes rotate against 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 provide a low-abrasion function while working in an environment with high relative rotating speeds of more than 20,000 rpm between the components. Their performance results in longer maintenance intervals for the gearboxes.

“In developing our DIWA automatic gearboxes we focused on maximum economy. We aimed not only at optimized operation of the buses in which they are installed, but also at extending the intervals between their maintenance visits to the workshops,” Markus Zwickel, Project Leader at Voith Turbo, said in a statement. “The Vespel thrust washers help us to attain this objective. They are so abrasion-resistant that no further replacement is needed as a rule.”

Another benefit of the DuPont polyimide is it does not interact harmfully with the automatic transmission fluid, which was a possibility with the bronze disks Voith Turbo used previously, said Zwickel. “As a result, we can reach up to 180,000 km between oil changes.”

Voith Turbo offers the new DIWA.5 gearboxes in two versions, depending on the torque to be transmitted. Among the differences are the size of the planet wheels and the diameter of the thrust washers. For both versions DuPont recommended using Vespel SP21, which is a polyimide filled with 15% graphite. According to DuPont, the material does not melt and retains its high abrasion resistance even when the gearbox fluid exceeds 120°C (248°F).

Voith Turbo optimized the shape of the thrust washers in close cooperation with DuPont. “DuPont produces these thrust washers with a special direct-forming technique similar to powder metallurgy,” Zwickel explained. “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 supplied Voith Turbo with machined prototypes for testing, enabling the transmission supplier to optimize the number, dimensions, depth, and position of the grooves. After determining the best possible geometry, the direct-forming production method was adopted, said Zwickel. “The direct-formed thrust washers meet our requirements for close tolerances without any post-finishing.”

This is not the first such application for Vespel SP21 polyimide, according to Harald Class, Marketing, DuPont Vespel Parts & Shapes; thrust washers for planetary gears are already commercial in Vespel.

“The washer [has been] commercial since a few years as spare parts at Voith,” said Class. “Because of the great success and the durability, they use it now also for the serial production.” The switch to serial production began in January 2007.

Class would not reveal how DuPont thrust washers compare to the bronze disks they replace in terms of weight and cost. “Vespel was chosen because of [its] long-term performance,” he noted.

The gearboxes are being installed by most major bus makers, including Neoplan, Solaris, Mercedes-Benz, Volvo, MAN, and Van Hool.

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