CHRISTOPHER M. PIEKNIK
Engineering/Quality Assurance Manager
Spirolox Div.
Kaydon Corp.
St. Louis, Mo.
Spiral-wound retaining rings are used in a host of applications from toys and sports equipment to automobiles and aircraft. That’s because they offer a range of features attractive to design engineers. They are strong, light, and in many cases offer the ability to simplify product design and reduce manufacturing costs.
While primarily used to hold components on a shaft or in a housing, spiral-wound retaining rings are also used as vibration dampers, back-up rings, grease shields, and spacers or shims. The rings are fabricated from flat wire that has been coiled on edge. This provides more design flexibility compared with other types of retaining rings. For instance:
• Ring thickness can be varied by changing the number of turns or wire thickness.
• Changing wire size varies the height of the retaining shoulder.
• Using thin-section material, manufacturers can form tabs, prongs, or other devices for special assembly or disassembly requirements.
• Flat-wire, multiturn construction creates rings that are more flexible and therefore easier to assemble.
Spiral-wound rings offer several additional advantages over other types of retaining rings. Multiturn spiral-wound rings have a full 360° retaining surface and, unlike stamped retaining rings, do not require expensive tooling to manufacture. They also have a uniform profile that minimizes clearance required for installation, and permits automated assembly for high-volume applications.
Different end configurations provide a means of removing the rings from grooves. Internal and external notches, and slots are standard end configurations, while beveled and tabbed ends are examples of specials. Beveled ends reduce the possibility of the ring unwinding out of the groove when rotating action opposes the direction of winding. Tabbed ends lock into a notch in the groove and prevent the ring from spinning during high acceleration and deceleration.
Spiral-wound retaining rings are manufactured with either left or right-hand windings. When mating parts rotate, external rings should be wound in the direction of rotation. Internal rings, in contrast, should be wound against the direction of rotation to prevent the ring from unwinding out of the groove.
Carbon-spring steel and 302 stainless are the standard materials for spiral-wound rings. However, they are also made from nonstandard materials such as 316 stainless, A-286 superalloy, and beryllium copper, as well as with special surface finishes, for example phosphate or black-oxide coating, and cadmium plating. Nonstandard materials and finishes provide greater corrosion resistance and strength at elevated temperature.
Rings are also available in a number of special design configurations to meet specific needs. These include:
Self-locking rings incorporate a precision tab and slot at each end of the ring. Tabs bottoming at the ends of the slot limit ring expansion and contraction. Self-locking rings are a necessity for applications where rings are subject to high centrifugal forces, vibration, impact, or oscillating loads. Otherwise, the ring can lose contact with the groove bottom, which reduces thrust capacity.
Self-locking spiral-wound rings are especially well-suited for high-speed external applications and cyclic loading. For example, they can handle speeds five times greater than that for standard spiral-wound retaining rings using a safety factor of 2.
They also generally perform better when exposed to dust and dirt. That’s because a retaining ring flexes under axial load. This flexing lets dirt and other foreign matter work its way into the bottom of the groove and can eventually force the ring out of the groove. Self-locking rings limit the amount of dirt that accumulates in the groove and help prevent premature failure.
Dished rings differ from standard spiral-wound rings in that they have a conical shape, making them extremely useful in applications where end play cannot be tolerated. End play is usually caused by a tolerance stack-up of ring thickness, groove width, groove location, or retained part thickness. Dished rings can also provide a controlled force on the retained part, if necessary.
Balanced rings are suited for high-rpm applications where centrifugal forces are a concern. The rings are statically balanced to a fraction of an oz-in. by a series of slots located opposite the ring gap. The slots counteract the slight material variance of the gap. The rings may also be self-locking for shafting applications.
While functional requirements such as operating environment and manufacturability play a role in selecting the best retaining ring, among the most important design factors are static and dynamic loads and the thrust capacity of the ring and groove.
Thrust capacity is determined by the shear strength of the ring or yield strength of the groove material. The lower of these two values is the limiting strength. Load limits for ring shear normally incorporate a safety factor of 3, and load limits for groove yield normally use a safety factor of 2. Equations for calculating load limits are found in retaining-ring manufacturers’ design catalogs. In general, Ps, the allowable thrust based on shear strength of the ring material is found from
Ps= | AFSsπ |
K |
In most cases, the groove material yields before the ring shears. Commonly, the ring experiences a twisting moment when the compressive yield strength of the groove material is exceeded and the ring deflects axially.
This uniform twisting moment puts a tensile stress on the ring ID. If stressed beyond its yield point, the ring will tend to grow in diameter and become dish shaped. This, in turn, deforms the groove. To determine thrust-load capacity based on groove deformation, first calculate the allowable angle of ring deflection u. For internal rings,
In this equation, C1 = the percent change in ring diameter from the free to installed state, found from
C1 = (G – C)/G. In addition, Dn = G – E and Rg = C/2.
For external rings
where C1 = (C – G)/G, Dn = G + E, and Rg = C/2.
The thrust load, based on groove yield, can then be found using
Calculated thrust capacities are based on three important considerations: square corners at both the groove and retained part; minimal radial clearance between the retained part and the bore or shaft; and static loading conditions. Excessive clearance between the retained part and shaft or bore, and large chamfers on the groove and part can affect load capacity. In addition, rings subject to eccentric or dynamic loading must be tested to ensure safe performance.