New Twists on Torlon

Torlon polyamide-imide (PAI) from Solvay Advanced Polymers already has a good reputation for strength, toughness, and resistance to wear, heat, and chemicals. Those properties, however, don't come cheap. Per pound prices for Torlon can reach $30. Yet in the right bearing and seal applications, Torlon can still manage to overcome its high price by lowering total costs.

Two emerging applications illustrate ways to balance the material's desirable properties against its cost. Solvay, on behalf of one of its automotive customers, has designed a thinner style of Torlon bearing that saves material costs while preserving performance. And Quadrant Engineering Plastics, a Reading, PA-based supplier of engineering plastic stock shapes, has helped its customers apply Torlon to large seals formerly made from aluminum. Here's a closer look at both developments:

Svelte bearings

Solvay has long supplied Torlon as a sleeve bearing material. The company even has a new grade, Torlon 4435, that targets applications whose demanding PV conditions require PAI's wear and thermal resistance. But price has made it difficult for Torlon to compete against ordinary bronze bearings in otherwise suitable applications-such as shaft support in the end caps of small electric motors. "At $25 to $30 per pound, it's been too expensive, relative to bronze bearings that are currently used," says Greg Warkoski, technical service engineer in Solvay's automotive group. But by designing bearings with thinner walls, some of that cost disappears.

In an end cap for a motor used in anti-lock braking systems, to take an automotive example under development at Solvay, the wall thickness of the 0.5-inch-long bearings could drop from as much as 0.15 inches to as little 0.015 inches, explains Warkoski. Solvay engineers have also worked out a way to gain a production advantage by continuously extruding bearings that would normally be injection molded. Together, these design and manufacturing approaches bring the cost down from "something like 7 cents for the bronze bearing down to 4 cents for Torlon," Warkoski estimates.

A straightforward realization is behind this shift to thinner walls: The extra wall stock doesn't do much good in the first place. Surface characteristics determine bearing performance, and even a little wear of the bearing surface can trigger failure long before the bulk of the wall shows signs of erosion, according to Jill Sanders, Solvay's global marketing manager. "The thin wall designs take advantage of the surface properties of Torlon," she explains.

The extra wall stock doesn't even matter much from a stiffness standpoint. Warkoski points out that the thin-wall bearings under development can be insert molded into an end cap made from Amodel polyphthalamide (PPA) end cap. "The bearing is backed up by a material with a modulus around two-million PSI," he says. What's more, the two engineering plastics form a strong bond during the insert molding process thanks to their compatible chemistries-amide groups in both polymers. This bond ties the bearing and end cap together without the need for adhesives or anti-rotation features.

Super-size seals

Quadrant has found a good fit for Torlon on the other end of the part-size spectrum. Quadrant Advanced Engineering Plastics has started making very large stock shapes for the labyrinth seals used to seal shafts in large turbo compressors-the kind used in chemical plants. According to Fred Sanford, Quadrant's business manager, these immense seals are based on 48-inch Torlon rings with 3-inch thick walls. "That's about 50% larger than what we previously made," he says.

These immense seals certainly don't skimp on the Torlon. But the cost play here comes from extending service life and increasing uptime in chemical plants. While labyrinth seals are non-contact during normal operating pressures, there can be some off-axis movement of the shafts during start-up and shutdown. Over time, this play in the shaft can cause metal seals to gall and ultimately fail, Sanford says. One design solution would be to open up the gap between the seal rings and the shaft. "But this approach reduces sealing efficiency," he adds. Torlon, by contrast, temporarily deforms when it briefly comes in contact with shaft. "It's more forgiving," Sanford says.

To produce these big rings, Quadrant had to bump up its manufacturing capacity with larger compression molding equipment. The company now turns out the rings in a single shot, though they are later cut in half for finish machining and installation into seal assembly. In the past, by contrast, any seal rings larger than 32 inches would have to be made from multiple segments, driving up manufacturing costs. "Torlon really wasn't an option for the largest seals," Sanford says.