Plain Bearings For Aerospace Applications
- May 02, 2018 -
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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
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
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 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.”
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.”
The Trend Toward Self-Lubricating Bearings
As mentioned previously, many plain bearing models are used in aerospace applications—most of them self-lubricating. Self-lubricating bearings utilize a pre-applied dry lubricant, usually PTFE, in place of traditional liquid lubricants. Dry lubricants do not require reapplication and thus entail less maintenance than traditional bearings. This makes them extremely effective in applications where re-lubrication maintenance would prove difficult. Dry lubricants are also able to operate in conditions where fluid lubricants are ineffective, such as environments susceptible to corrosive gases, dirt, and dust; high temperatures; cryogenic temperatures; radiation; extreme pressures; or vacuums—all of which are hazards found in the aerospace industry. Due to these benefits, it is no surprise self-lubricating bearings are being used over traditional metal bearings both here on Earth and beyond in the following applications:
In aircraft landing gear struts, or shock absorbers, where they eliminate ladder cracking and heat damage on the strut rod surface. GGB’s DU-B bronze-backed metal-polymer bearings were chosen by one of the world’s leading commercial aircraft manufacturers to be used in all current production of their landing struts due to their high load capabilities, resistance to corrosion, and increased component life.
In aircraft ground support, which requires reliable equipment to ensure flights leave safely and on time. According to Klepach, “GGB’s HSG [High-Strength GAR-MAX®] FRC bearings are found in scissor-lift-type applications, which handle significant loads during intermittent operations, often while being exposed to harsh environmental conditions.” This type of bearing offers ultimate compressive strength up to 620 MPa (90,000 PSI) and more consistent friction than greased bronze bearings—with the added benefit of being both abrasion and corrosion resistant. This helps extend maintenance intervals and improve the efficiency of aircraft ground support services.
In NASA’s Curiosity Rover, the largest and most successful Mars Rover to date. As Ricci says, “Curiosity’s arm-drill required bearings that could withstand the harsh Martian temperatures—ranging between -153°C and 20°C—and atmosphere. DU®bearings were chosen due to their high wear resistance, ability to operate comfortably in the temperatures of Mars, and resilience towards dust and debris.” The operation of this arm-drill was critical to the discovery that Mars once had conditions suitable for microbial life.
GGB has also worked with Airbus, Airbus Helicopters, Boeing, Lockheed Martin, private spaceflight companies, the military, and other private aircraft manufacturers to create custom solutions for their plain bearing needs.
Plain bearing solutions provide the aerospace industry with weight and space reduction, enhanced energy efficiency, improved strength and safety, and increased operating temperatures for its ground, air, and outer-space applications.