Bearing also seals under pressure

Akron, OH--A new thrust bearing and seal for shafts that penetrate a pressure vessel literally got off to a flying start. First conceived of as a way to support helicopter blades, these bearings from Randolph Research Co. have now emerged as a solution for more earthbound sealing problems.

Called Lamiflex, the bearing-seal consists of alternating layers of high-tensile-strength brass and natural rubber joined in a lamination coating process. The brass and rubber layers, which are typically 0.002 inch thick, form a "washer" between a flange on the shaft and a flat-bottomed seat built into the vessel wall. According to Randolph's President Bill Hinks, who designed Lamiflex, the oscillating shaft subjects the rubber layers to shear stresses, causing some degree of torsional strain in the rubber because the twisting shaft shears the rubber rather than slipping within the bearing. "Lamiflex exhibits only a modest spring torque and very little friction," Hinks explains, contrasting the design to conventional sliding contact seals that must overcome high-friction torque in order to operate.

The success of Hinks' design hinges on a careful selection of the width-to-thickness ratio of the 160 or more layers that make up a typical Lamiflex. Keeping that ratio under 50:1 enables the rubber layers to resist bulging under compressive forces to 20,000 psi. "There's barely any rubber extrusion at all, just a reduction in volume dictated by the material's bulk modulus," Hinks says. Thicker layers do make large bearings easier to manufacture but need to have a higher stiffness to resist extrusion under pressure.

While Hinks is seeking real-world validation of Lamiflex's sealing abilities, he has already subjected Lamiflex to extensive testing on a shop-built testing fixture. His results show that under 10,000 psi of external pressure, a Lamiflex with a 2.062-inch outside diameter stands up to more than a million cycles of a shaft oscillating at plus or minus 15 degrees. "Many more cycles are available with a smaller angle," he notes. Lamiflex has also withstood external pressures of 16,000 psi, though with fewer cycles.

Lamiflex does have some temperature restrictions that may limit its application. Hinks notes that the natural rubber will start to overcure, become stiff, and fail at extended exposure to temperatures higher than 160F.

Hinks borrowed the idea for Lamiflex from a helicopter component he invented back in 1956--a similar laminated construction that helps connect the blades to the rotor shaft. Though he licensed the original bearing concept to a well-known ball-bearing manufacturer, he always believed that the bearing could do double duty as a seal in applications where a limited motion shaft must protrude through the walls of a vessel separating a pressure differential. These include not just liquid processing vessels but, potentially, undersea and space craft. "This idea has been knocking about in my head for a very long time," Hinks says.

Additional details...Contact Bill Hinks, Randolph Research, 2449 Kensington Rd., Akron, OH; Tel: (330) 666-1667; E-mail: [email protected].