Machine Design

Wire-formed Rings

Split rings formed and cut from spring wire of uniform cross-sectional size and shape are known as wire-formed rings.

Rings are available in many cross-sectional shapes, but the most common have rectangular or round cross sections. Rectangular-section rings are made for internal or external applications. Round-section rings are for external use on shafts only and are used where the expected load is light.

Ring assembly and disassembly tools range from a screwdriver to automatic tools, depending on the application.

Carbon spring steel (SAE-1060) is the standard material used in retaining rings. In addition, a wide range of carbon steels (SAE-1010 to SAE-1095) are used, including pre-oil tempered and alloy steels. Hardnesses range from 43 to 53 Rockwell C, with tensile strengths from 200,000 to 280,000 psi. Other materials used are stainless steel, yellow brass, silicon or phosphor bronze, Inconel X, beryllium copper, aluminum, and K-Monel.

Design Considerations

Wire-formed rings are available for shafts or housings in diameters from 1/8 to 30 in. The controlling factor in designing a wire-formed ring (other than size) is the amount of thrust the ring is to absorb. In most cases, retaining rings withstand greater shear loads than the groove material because of the strength of the hardened spring-steel ring section.

The controlling factor in size selection is thrust capacity of the ring. Wire-formed retaining rings, because of their configuration, usually do not fail in shear. Under an increasing load, the ring deflects and finally jumps out of the groove.

This type of failure can be reduced by increasing ring width (section height measured in plane or diameter). Wider rings have more resistance to expansion forces that would work the ring out of the groove.

Although rings can be made with almost any combination of section width and thickness, rectangular-section thicknesses are usually 2 to 5% of the shaft or bore diameter. Width of these sections varies from two to three times the thickness.

Ideally, the shaft and the retained part should have strength equal to or better than the ring. The groove should be square, concentric, and held to such dimensions that a close slip fit is provided with the sides of the ring. The bearing surface of the retained part should be perpendicular to the shaft axis with no corner radius.

If these conditions cannot be met in practice, the wire-section corner radius, mating-part radius, and tolerances should provide an adequate flat surface for the ring to bear against.

Shear strength of the groove material can be increased by increasing the distance from the edge of the groove to the end of the shaft or by increasing the strength of the groove material.

Sufficient design clearance should be allowed to assemble the rings with a minimum of effort. Radially assembled, or clip, retaining rings require accessibility to the side of the shaft. Rings used internally in bores require adequate space for tool insertion.

External closed rings are assembled axially over the end of the shaft. A radius or 30° chamfer on the end of the shaft greatly assists the assembly operation. A small edge margin also makes assembly easier.

Tolerances of ring thickness, groove location, and retained components may add up to cause objectionable end play. Tolerances can be taken up, to some extent, by using tapered-keystone or beveled sections. The narrow portion of the section thickness is in the groove, and the wide portion bears against the part.

External clip rings are easier to install than axially assembled rings, and require fewer tools. These rings are designed to be driven on gap ends first, over the bottom of the groove.

Axially assembled rings usually have better external appearance and higher rotational speed limits than the external clip rings.

Grooves for retaining rings vary according to the type of ring used and the type of service. Radius (rounded) grooves are used for round-section retaining rings. For external rectangular-section clip and closed rings, groove width should be 1.15 times the ring-section thickness. This width is the absolute minimum that will accept standard rings. It may be desirable to specify larger minimums and tolerances to be consistent with design requirements.

TAGS: Fasteners
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