Snap rings locate and retain components in a wide range of applications, from rotating equipment to valve bodies. There are two basic types of rings, internal and external. An internal ring, for example, keeps a spool valve in a bore, while an external ring may locate a splined gear on a mating shaft. Rings typically incorporate a pair of "ears" to which an appropriate pair of snap-ring pliers engages. The pliers expand or contract the ring to unseat it from its locating groove.
An alternative, spiral-wound design from Smalley Steel Ring Co., eliminates the ears and the special pliers, both advantages in certain applications. Such rings can wind into a groove by hand or with an automated process. Inserting a regular screwdriver in a notch removes them. Spiralwoundretaining rings use multiple-turns of material to maintain position in a ring groove and resist thrust loads. Smalley suggests using the following equations to calculate loads and stresses on spiral retaining rings. First, consider shear stress resulting from thrust loads:
where K = 3 for carbon-steel rings
Ring shear may limit thrust loads when softer carbon-steel rings install in hardened-steel grooves. Likewise, the groove itself can deform under excessive thrust loads, which is a more common design limitation. Grooves that begin to permanently deform twist the ring. An increasing angle of twist enlarges the ring diameter. At even-higher twist angles, the ring "dishes" and rolls out of the groove. The load at which initial groove deformation takes place is found by:
where K = 2 (recommended).
For maximum load capacity, corners in the ring-groove bottom must be square, or nearly so. Corner radii should not exceed 0.005 in. for shaft and housing diameters 1 in. and under, and 0.010 in. for those over 1 in. Keep retained components square to the ring groove to maintain an even, concentric load against them. The corner of the retained part touching the ring should as well be square. Maximum chamfer and radius dimensions on retained parts are figured by:
Maximum chamfer = 0.375(b d)
Maximum radius = 0.5(b d).
Leave enough material or edge margin forward of a ring groove when it locates near the end or edge of a shaft or housing. Check both shear and bending loads and select the larger of the two values as the edge margin, Z:
where K = 3 (recommended).
Centrifugal forces on external retaining rings mounted on rotating shafts limit rpm. The rpm at which the force holding the ring on the groove goes to zero, N, is found by:
The above equations compute recommended rotational speed limits for standard retaining rings. Rings with a self-locking feature work beyond these speed limits. Self-locking rings incorporate a small tab on the inside turn that locks into a slot on an outside turn. The arrangement better withstands vibration, high acceleration rates, and impact loads as well.
Obviously, balance can be an important design consideration in rotating machinery. Statically balanced retaining rings contain a series of slots opposite the end gap. The slots account for the missing material in the gap and lower eccentric loading.
The forming process by which spiral-wound retaining rings are made places residual tensile stresses in the radial wall and compressive stresses in the outer edge. Installation also stresses the rings. To avoid permanent deformation, installation stresses should stay below 80% of the material's tensile strength for external rings fitted on shafts. For internal rings in housings, that number is 100% of tensile strength. Special designs that push installation forces beyond elastic limits need rings that yield a predetermined amount. Once installed, the rings exert the proper amount of force on the groove.
In general, stress analysis probably isn't necessary when standard rings are manually installed in properly sized grooves. However, stresses associated with special rings and installation methods should be checked:
|YIELD STRENGTHS OF TYPICAL GROOVE MATERIALS|
Yield strength (psi)
|Hardened 8620 steel|
|Cold-drawn 1018 steel|
|Hot-rolled 1018 steel|
10,000 to 40,000
Allied Systems Co., www.alliedsystems.com
Smalley Steel Ring Co., www.smalley.com