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THRUST-WORTHY WASHERS.

Take a country road with a 10 to 12 percent grade, add a pickup truck and trailer whose combined weight flirts with allowable maximums, mix in a couple of test drivers who are eager to break things, and what do you get? When the Syracuse, N.Y-based New Process Gear division of New Venture Gear Inc. followed this recipe, it wound up with a perfect field lab for evaluating thrust washer durability under heavy loads.

The New Process Gear division designs and builds transfer cases used in four-wheel-drive pickups. Driven on snow, through mud, or over ice, such trucks rely on transfer cases to shunt engine power toward their front axles. The transfer case effectively doubles the grip of a truck's four tires.

According to Timothy Filkins, program manager at New Venture, prototype thrust washers used on either side of the low-range planetary gear were melting under heavy loads. The washers, manufactured from polyetheretherketone, or PEEK, resins, suffered not only from the effects of high heat and loads but from occasional lubricant starvation as well.

The solution to the problem came by way of E.I. du Pont de Nemours and Co., of Wilmington, Del. Richard Van Ryper, senior development engineer at the company's engineered polymers division, suggested that the transfer case manufacturer try washers made from Vespel, DuPont's polyimide resin. The New Process Gear division had used Vespel washers successfully in other transfer case applications.

The chief advantage of the Vespel washer is that it is formed just as a powdered metal is--by high pressure in a mold. And unlike the injection-molded PEEK, Vespel acts as a thermosetting plastic. So, it tolerates heat better than PEEK.

At first, the engineers tried duplicating the design of the PEEK washers in Vespel, but found that tabs used to prevent the washer from rotating broke off under high thrust loads. Deciding to remove the tabs, the engineers thought it would be better just to let the new washer rotate. Now, under severe loads or low-lubricant conditions, the Vespel washer simply spins relative to one of the two machined faces that sandwich it.

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Author:Sharke, Paul
Publication:Mechanical Engineering-CIME
Geographic Code:1USA
Date:Nov 1, 1999
Words:350
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