Polyimide material makes maintenance-free washers

Edited by the Engineeringtalk editorial team Aug 25, 2008

When Magna Powertrain designed its new integrated transfer cases it followed the advice of Austrian distributor Biesterfeld Interowa, and tested prototype parts made of Vespel.

Thrust washers made from DuPont Vespel SP-21 polyimide are durable, maintenance-free components of the new integrated transfer cases developed by Magna Powertrain of Lannach, Austria.

Used in premium SUVs from Land Rover and Daimler, these ITCs are located upstream of the differential to ensure optimum torque transfer between the front and rear axles.

In addition they provide the choice between two gear ratios for on-road and off-road driving, whereby selection can be made while driving.

Two designs of Vespel thrust washers are used in the planetary gear set for the off-road drive position.

In the first design, six small thrust washers, with outside diameters of 25mm, absorb axial forces, generated by the helical gearing, from the three planet wheels.

In the second, two much larger thrust washers, with outside diameters of 160mm, support the ring gear on both sides against the grey cast iron planet carrier.

Without the use of the Vespel thrust washers, the hardened steel planet wheels and ring gear would rotate directly against the comparably soft grey cast iron housing, causing the housing to wear very quickly.

Potential alternatives to Vespel thrust washers, such as axial needle bearings, would incur relatively high additional costs.

Moreover, such bearings require a larger installation space and are significantly heavier.

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.

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".

The complex interaction of mechanical, chemical and thermal stresses in this application makes very high demands on the material.

The low-friction Vespel components can withstand these stresses long term.

Riegler continues: "We design the Vespel thrust washers used in our ITCs to last a lifetime operating with PV values of over 7MPa.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".

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-optimised 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 1.42g/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 up to 300C.

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 combines excellent mechanical and insulation properties at high temperatures; Vespel SP-21 is enhanced with graphite for very high abrasion resistance; Vespel SP-211 is enhanced with DuPont Teflon PTFE for a very low co-efficient of friction; Vespel SP-22 combines low friction and high abrasion resistance with excellent dimensional stability.