X-Y-Z: Retaining Rings Make Shaft Assemblies a Snap
Retaining rings are circular fasteners that, when installed in a groove, hold components on a shaft or inside a bore. They hold gears, cams and pulleys on shafts, and they keep bearings, valves, nozzles and locks inside housings. Also known as snap rings, these fasteners are removable and reusable. They often do the job of several parts by replacing cotter pins, nuts and washers.
Retaining rings can be axial, radial or self-locking. Axial retaining rings are slipped, horizontally, over a shaft along its axis. Some axial rings are flat, coiled springs. Others are stamped, with lugs and lug holes to ease installation. Once installed, they make almost complete circular contact with the groove, which enables them to withstand significant thrust loads.
Also known as open rings, radial retaining rings are for low-thrust load applications. They don't have lugs or lug holes, and they do not extend as far around the circumference of the groove as axial rings. Self-locking retaining rings do not require a groove for installation. They are used in small applications with very low thrust loads.
Axial retaining rings can be further classified as external or internal. An external ring goes over a shaft, snaps into a groove, and exerts radial tension toward the ring center. An internal ring goes through a housing or bore, snaps into a groove and exerts radial tension outward from the ring center.
All radial retaining rings are external. They are applied vertically toward the center of the circular shaft along its radius. Although they handle less force than axial rings, radial rings have their own advantages. They can be installed on a stepped shaft in place of an axial ring, which could be overstressed during installation. The same size radial ring can be installed on a larger shaft by deepening the groove. And because they're made of less material, radial rings cost less than axial ones.
Retaining rings are usually made from carbon spring steel with a hardness rating of 42 to 53 on the Rockwell C scale and a tensile strength of 192,000 to 280,000 psi. Rings can also be made from stainless steel, Inconel, Elgiloy, A286, beryllium copper and phosphor bronze. Many finishes are available.
Standard sizes range from 7/16 to 16 inches ID, and specials can be made in sizes ranging from 3/8 to 85 inches ID. The rings can be made in various cross-sectional profiles, depending on their function and the shapes of the mating components. The most popular cross-sectional shapes are rectangular, square and round. However, they can also be made in tongue, beveled, tapered, chamfered and other shapes.
In addition, both internal and external axial retaining rings can be bowed to help assemblers overcome problems with accumulated tolerance variations. If the components on the shaft are slightly less than the specified dimensions, the assembly will be loose if it is fastened with a flat retaining ring. A bowed retaining ring solves the problem. Its curved shape acts as a spring, compensating for the undersized parts and holding the assembly firmly in place.
When ordering retaining rings, engineers should specify the diameters of the housing, shaft and groove; the depth and width of the groove; ring thickness; shaft speed; and operating temperature. They should also specify the shaft material and the thrust load that will be exerted on the ring.
Thrust load is the maximum force that can be applied to the ring without shearing it. To maximize the amount of thrust load the ring can take, the groove must be sharp, with minimal clearance between the retained components. In general, as the shaft radius and groove width increase, the maximum thrust capacity of the ring decreases. The load should be applied uniformly to the bearing surface of the ring. In most cases, the load-carrying capacity of the groove will be less than the ring, due to the lower tensile strength of the groove material.
Retaining rings can be installed with hand tools or pneumatic tools, or with fully or semiautomatic equipment.