Polymers enhance race-car performance

Brent Gilliatt
Chris Walker
Greene, Tweed & Co.
Kulpsville, Pa.

Edited by: Kenneth J. Korane

In racing, a split second can make the difference between winning and losing, or between a serious accident and a trouble-free ride. That's why race teams in CART, Formula 1, Indy Car, and NASCAR are always on the lookout for parts that perform better, are more reliable, and weigh less.

Much of this burden falls to the materials suppliers, who apply years of research and experience gained in demanding aerospace, semiconductor, automotive, and oil-exploration applications to tackle the unique challenges of performance racing.

High-performance polymers
Advanced polymers and elastomers are gaining wide acceptance in racing circles because they boost performance and cut weight. Components made from these materials can be manufactured in complex geometries and offer high-temperature resistance and exceptional durability. It is for that reason they're replacing metals in many traditional sealing, cylinder, and housing applications.

An added benefit is that polymers also offer the possibility to consolidate parts, simplify assemblies, and add functionality versus more traditional metal constructions.

Seals for suspension and hydraulic systems provide a case in point. Components constructed of advanced polymers reduce galling and abrasion caused by metal-to-metal contact.

Housings and surrounding components made with engineered polymers and composites provide a more-unified assembly that improves performance and reliability. Advanced polymer compounds resist the high temperatures and aggressive chemicals common in racing vehicles.

Innovative applications
In critical systems such as suspensions, transmissions, drivelines, brakes, clutches, and hydraulics, components made of polymers and elastomers offer many advantages. Here's a look at some of the latest innovations and sealing advances.

Separator piston. A conventional racing gas damper uses a damping chamber and separate aluminum canister containing a free-floating aluminum piston, which acts as a barrier between the damping fluid and gas energizer. Typically, the aluminum piston has a filled polytetrafluoroethylene (PTFE) bearing strip to prevent metal-to-metal contact, and a conventional O-ring seal on the pressure side. A separate gland houses each component.

This design has several drawbacks. The highly squeezed O-ring seal generates significant friction within the canister. Avoiding metal-to-metal contact requires large clearances between the aluminum canister and piston. And the PTFE bearing adds to breakout friction of the assembly.

A new design that combines a tough, high-temperature thermoplastic Arlon polyetheretherketone (PEEK) and a patented advanced concept T-ring (ACT) can replace the standard separator piston assembly.

This offers several advantages over traditional aluminum pistons: It's half the weight and the lubricated material does not require a separate bearing. Eliminating the bearing permits tighter tolerances between the piston and bore — as much as 50% less clearance. This results in less rocking of the piston in the bore and less squeeze on the seal, which reduces friction and improves sealing. The ACT seal uses thermoplastic backup rings to prevent elastomer extrusion. It cannot roll like an O-ring and forms a more stable, lower friction seal. The ring also offers a higher temperature capability and as much as a 20% lower coefficient of friction than conventional nitrile elastomers.

Compared with the conventional solution, the combination of lighter weight, improved sealing efficiency, and reduced friction improves response time by as much as 50%.

Damping piston. In a conventional gas damper, an aluminum piston ring operates in an aluminum cylinder. Typically, a graphite-filled PTFE bearing ring with an O-ring energizer runs in a wide, stepped groove machined in the piston OD. The O-ring acts as a spring — forcing the bearing against the cylinder ID and letting it function as a seal. This ensures that fluid flows through ports in the piston, rather than around the outside of the piston, to maintain proper damping qualities.

Replacing the conventional piston assembly with a thermoplastic piston and a self-actuating, pressure-activated Ener-Cap seal enhances sealing while reducing bearing surface area, which cuts friction and overall weight.

This advanced seal consists of a solid thermoplastic cap ring energized by a custom-designed elastomer, operating in a simple groove. The cap material has a low coefficient of friction (comparable to that of graphite-filled PTFE), wide temperature range, and good deformation resistance.

It is much narrower than the conventional seal yet offers higher sealing efficiency than a traditional O-ring cap seal. That's because the energized design distributes force over a wider area than an O-ring and better contains the cap. The cap is solid, rather than split, which further improves sealing efficiency without compromising friction.

In addition, tight piston clearance reduces squeeze on the energized ring during side-load conditions. This has far more impact on overall friction than the coefficient-of-friction of the bearing material.

Piston rod seals. The piston-rod system performs two sealing functions: Containing fluid in the damper body and preventing dirt and other contaminants from entering. It also provides a bearing surface should the rod experience side loads. The conventional solution is often an aluminum bearing and seal housing containing a metal-backed PTFE bearing and a filled PTFE O-ring cap seal.

A more-efficient solution replaces the O-ring cap seal with a metal spring energized seal (MSE), a low-friction design for unidirectional seal applications. This design offers several enhancements that combine to significantly reduce friction. First, the MSE seal uses a stainless-steel spring energizing a thermoplastic jacket, which offers a narrow contact surface. Second, replacing the standard metal-backed PTFE bearing with a thermoplastic bearing also improves frictional and wear characteristics. Third, the design locates the bearing above the seal so that it runs in the damper fluid, which lubricates surfaces and reduces running friction.

This solution and the alternative piston assembly reduce overall friction of the damper unit by as much as 50%. To further decrease weight and increase strength, the damper could be constructed from a filament wound, thermoplastic reinforced carbon-fiber composite, such as Greene, Tweed's WR525 material. These are just a few examples of innovative sealing solutions. Other components such as high-temperature seals, low-friction seals, wear bands, strut wipers, brake seals, transmission seals and rings, structural components, housings, and electrical-pin and fiber-optic connectors can provide similar benefits in this arena. Using advanced polymers and elastomers in critical parts improves performance, reliability, and lowers component weight.

Arlon, ACT, Ener-Cap, MSE, and WR525 are materials and products developed by Greene, Tweed & Co. For more information on the company's sealing solutions, visit www.gtweed.com