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Maintenance-free for life

By Jürgen Krey
DuPont Engineering Polymers, Germany

Thrust washers made of DuPont™ Vespel® polyimide provide axial support to metal transmission components running at different speeds and help compensate for their relative rotary movements. If lubricated properly, they show almost no abrasion – a benefit that led Magna Powertrain to use them in the production of maintenance-free transfer cases for 4-wheel drive, premium SUVs from Landrover and Daimler.

SUVs have become increasingly popular in Europe over the last 10 years. The majority of these vehicles offer very high performance. So the greatest challenge for designers is the optimum distribution of power from the engine to the four wheels in all conditions, whether on-road or off. It is solved by the use of differentials on each axle, which equalize power between the left and right wheels, and an upstream transfer case, used to balance torque between the front and rear axles.

For on-road and off

The new Integrated Transfer Cases from Magna Powertrain are a prime example of such technology. They are equipped with an electronically-controlled, steplessly variable differential lock and provide the choice of two gear ratios for on-road or off-road driving. The simple selection between ratios (using an electromagnetic actuator) can also be carried out when driving. The new ITCs, designed for an input torque of up to 2,700 newton meters, are installed in premium SUVs such as the Mercedes ML and GL models, the Landrover Discovery as well as numerous Range Rover models, where they ensure optimum distribution of torque between the front and rear axles in all driving conditions.

The complete gear set of the planetary gear train, responsible for power equalization, rotates against thrust washers made of Vespel® SP-21 from DuPont, which separate the wheels from each other and from the planet wheel carrier. Despite the high relative rotating speeds and forces, the thrust washers fulfil their function virtually without abrasion.

Cost-efficient wear prevention

Generally speaking, when metal rubs against metal, the contacting surfaces wear, with the softer friction partner abrading considerably faster. The wear-determining load parameter in this process is the pv value, which is the product of the contact pressure p and relative speed v. The higher this value is, the more the surfaces of the friction partners will be tribologically stressed (tribology is the science and technology of interacting surfaces in relative motion, taking into account lubrication, friction and wear).

If no other measures were taken, the steel planet wheels and ring gear would rotate against the gray cast iron housing, causing the housing to wear very quickly because of the very high pv values created. To prevent this, additional components are required. Possible solutions include separate housing covers made of steel with a nitrided thrust surface or axial needle bearings positioned between the gearwheel and housing. Both these options incur relatively high additional costs. Such bearings also require a larger installation space and are significantly heavier than polymer solutions.

In the quest to find a lightweight, space-saving alternative that was also cost-efficient, the manufacturer followed the advice of Austrian distributor Biesterfeld Interowa and tested prototype parts made of Vespel® SP-21 from DuPont. The results were impressive, as Ernst Riegler, department manager at Magna Powertrain, confirms: “We know of no other material that combines similarly outstanding tribological properties over a wide temperature range with such low space requirement and light weight.”

Lightweight, thin and extremely wear-resistant

Magna Powertrain uses two designs of wear-resistant Vespel® thrust washer in the planetary gear set for the off-road drive position (see picture above). In the first design, six small thrust washers with an outside diameter of 25 mm (approximately 1 inch) absorb the axial forces, generated by the helical gearing, from the three hardened steel planet wheels. In the second, two much larger thrust washers (with an outside diameter of 160 mm or 6 inches) support the hardended steel ring gear on both sides against the gray cast iron planet carrier.

DuPont produces the thrust washers in-house from polyimide using a direct-forming method similar to metal powder sintering. Directly integrated oil grooves, with geometry flow-optimized by Magna, ensure a permanent thin film of lubricating oil on the thrust washers. The sintered parts require no post-finishing and are ready for immediate installation. At the same time, they meet the very close tolerances specified. The application advantages of Vespel® SP-21 are not confined simply to low wear. The polymer has a specific gravity of only 1.42 g/cm³ for light weight parts, excellent chemical resistance for stability in a broad range of automotive lubricants and extremely high heat resistance, permitting continuous service temperatures of up to 300°C (572°F).

The complex interaction of mechanical, chemical and thermal stresses in this application makes very high demands on the material. The Vespel® components can withstand these stresses long-term. As Riegler continues: “We design the Vespel® thrust washers used in our ITCs to last a lifetime operating with pv values of over 7 MPa·m/s. In other words: the reduction in thickness that the thrust washers sustain due to wear is so low that they never need to be changed throughout the entire vehicle service life. The axial positioning of the gear wheels remains within close tolerances and the formation of an optimum lubricating film is guaranteed.”

• Vespel® SP-221: Created for highly specialized applications, for example against soft metals in “non-lube” conditions.
• Vespel® SP-224: combines the benefits of SP-22 with additional load-carrying capability
• Vespel® SP-3: low out-gassing for use in vacuums and dry environments, e.g. aerospace

Further information on these grades and other members of the Vespel® S family, as well as many other Vespel® families, can be found at vespel.dupont.com

The Vespel® S Family

Vespel® S is a family of highly-durable polyimide parts and shapes that deliver exceptional wear resistance, insulation, and a low coefficient of friction.

• Vespel® SP-1: excellent mechanical and insulation properties at high temperatures, e.g. for the semiconductor industry
• Vespel® SP-21: very high abrasion resistance due to graphite enhancement, e.g. for the automotive industry
• Vespel® SP-211: low co-efficient of friction due to DuPont™ Teflon® PTFE additive, slightly reduced thermal and abrasion resistance, also for non-lubricated applications
• Vespel® SP-22: low friction and high abrasion resistance combined with excellent dimensional stability