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Plain bearings are used across a wide range of aerospace applications to help achieve better fuel efficiency, extend maintenance intervals, and lower carbon emissions. These applications include installation in aircraft wing systems (flaps, spoilers, and slats), flight controls, cockpit controls, auxiliary power units, landing gear, door systems, and aircraft interiors (seats, bins, latches, and hinge points). “Our plain bearings even have a footprint on Mars,” said Brett Ricci, GGB Aerospace Strategic Account Manager for North America. “Operating in temperatures between -200°C to +280°C, our plain bearings have served as the primary suspension components in the robotic drilling arm of NASA’s Curiosity Mars Rover since 2012.”

Just as impressively, all of these applications are served by just two types of plain bearings: metal-polymer and fiber reinforced composite (FRC).

Metal-Polymer Plain Bearings

GGB’s HPMB® fiber reinforced composite bearing

Metal-polymer bearings consist of an outer metal backing with a porous bronze inner structure that is coated with a polymer-resin lining. Each part of this structure contributes to the overall characteristics of these bearings: the polymer liner provides lubricating properties with low friction and wear; the bronze inner structure provides the mechanism to contain the polymer liner while also transferring load and heat; and the metal backing provides mechanical strength. Metal-polymer bearings are made in two varieties: self-lubricating and pre-lubricated.

Self-lubricating bearings have a smooth, PTFE-based liner that is transferred to the mating surface during operation, forming a lubricant film. This results in very good wear and low friction performance over a wide range of loads, speeds, and temperatures in dry running conditions. Pre-lubricated bearings utilize different materials for this liner and include circular indents that are filled with grease before operation.

“Self-lubricating metal polymer bearings are GGB’s most popular products with the aerospace industry—particularly the DU-B,” says Kim Evans, one of GGBs aerospace application engineers. “I’d say it’s the industry standard for aircraft landing gear struts.”

Fiber-Reinforced Composite (FRC) Bearings

Fiber reinforced composite bearing materials structure

FRC bearings consist of a self-lubricating liner backed by continuously wound high-strength fiberglass. To ensure they fit a variety of applications, FRC bearings use two different forms of liners: fiber and tape. A fiber liner offers high abrasion resistance and improved ability to handle shocks and misalignment. Due to a greater PTFE content, a tape liner additionally offers higher speed capability and improved machinability. Regardless of the liner, all FRC bearings are self-lubricating through the use of dry lubricants. This method of lubrication results in a low coefficient of friction, low wear rates, and extended maintenance intervals, as re-lubrication is unnecessary. In addition, FRC bearings are able to operate in a wide range of temperatures and are also resistant to acids, bases, salt solutions, oils, fuels, alcohols, solvents, and gases.

FRC bearings can have the self-lubricating liner on the inner or outer diameter and can contain flanges or grooves with or without a liner as well. FRC washers, plates, and other custom forms are also available to serve different applications. “The versatility and specifications of these FRC products make them an excellent choice for most heavy load, low speed, oscillating applications,” says Yuri Klepach, GGB’s FRC Product Manager.

Plain Bearing Manufacturing

Metal-polymer bearing material structure

Metal-polymer bearings are produced using a series of technologies that combine the steel or bronze metal backing, bronze powder, and polymer liner. To start, a coil of backing metal is fed through a machine that applies bronze powder to one side through the use of heat—a process known as “sintering.” The sintered strip is then cooled and ready for impregnation. Impregnation is the application of a polymer liner onto the sintered strip and can be done with the polymer in either mush or tape form. According to Evans, “For GGB’s self-lubricated bearings, a mechanical arm drops the polymer onto the sintered strip, which is then rolled out through downstream machines to create a smooth self-lubricating liner. GGB pre-lubricated bearing liners are made with a polymer tape that is applied directly onto the sintered strip.”

The impregnation process for both forms of polymer includes a series of mill-rolling, heating, and cooling operations in order to create a smooth-surfaced metal-polymer strip. After impregnation, the strip is finished and coiled for later shaping into bearing products. As Evans explains, “This shaping process utilizes roll-forming or pressing, depending on the bearing size, to create a smooth, cylindrical-shaped product.”

GGB’s DU-B bearing

FRC bearings are manufactured through a winding process that utilizes automated winding machines. For bearings with the liner on the inside diameter, the liner material must first be applied to the length of a mandrel. For wound liner products, this is done by a winding machine that is continuously fed strings of high-strength fibers encapsulated in an internally lubricated epoxy resin. For tape liner products, a PTFE tape is applied on a mandrel. After this base liner is applied, the fiberglass backing is wound around the mandrel through the use of automated winding machines. Once this backing reaches the required thickness, the mandrel is removed and later cured in an oven. This process hardens the winding around the mandrel into a solid tube. At this point, the inside diameter is smooth and finished while the outside diameter is rough and oversized. In order to finish the outside diameter, it is ground down to its desired final size. The tube is then cut to produce multiple finished bearings whose edges are deburred as needed. “The length and diameter of the mandrels used in this process can vary,” Klepach says, “to achieve different dimensions based on customer need.”

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