Retaining Rings: Simple, but Highly Engineered for Multiple Applications

Retaining rings are some of the simplest, most cost-effective and easiest to use retention technologies, especially for controlling axial movement. Every mechanical engineer has seen them, and most have used them, but their simplicity hides a great deal of surprisingly high technology. Specifying retention rings today is more involved than a simple drawing callout for a standard snap ring. Where do you begin?

Ring load capacity is key

Controlling axial motion of rotating assemblies on shafts is the most common application of retaining rings. Advances in low cost gear-making technology, for example, have allowed helical gears to replace spur types in many applications permitting smoother and quieter power flow. They also introduce thrust loads, sometimes significant, which are usually addressed by a lubricated bushing or antifriction thrust bearing. It is common to think of the thrust bearing as “taking the load”, but the bearing is really a device to decouple rotational motion from axial thrust loads. Another device, commonly a ring located in a machined groove retaining the bearing on the shaft, is where the thrust load acts.

Best design practices call for the smallest, lightest, cheapest retention device possible. Similar pressures limit shaft selection to low or medium strength materials like mild steel increasing the likelihood of groove deformation as a possible failure mode. In this case, the ring twists as the groove deforms and, as the twist angle increases, the ring expands. At failure, the ring “dishes” and extrudes or rolls out of the groove. When designing to reduce the risk, Smalley Steel Ring Company recommends a safety factor of two using this equation:

Where:

P_g = Allowable thrust load based on groove deformation (lb)
D = Shaft or housing diameter (in)
d = Groove depth (in)
S_y = Yield strength of groove material (psi)
K = Safety factor (2 recommended)

It is important to know the thrust load on the ring, but what about the ring groove? Since groove deformation precedes failure, the yield point of the shaft material is equally important. Some typical groove material field strengths are:

Note that once the yield point is exceeded by the thrust load, deformation of the ring groove occurs prior to failure… Failure can occur catastrophically by exceeding the yield point by a large amount or more slowly by subjecting the ring groove to progressive plastic deformation until it can no longer retain the ring.

Ring Shear Design Recommendations

If groove deformation rears its head, an obvious no-redesign strategy is to specify hardened or higher strength shaft material. It is a common solution, but at higher loadings, ring shear becomes a possible failure mode. The thrust load formula is very similar to the groove deformation case:

Where:

P_r= allowable thrust load on ring (lbs)

D=shaft or housing diameter (in)

T=ring thickness (in)

S_s=Shear strength of ring material (psi)

                                                                           K=safety factor (3 recommended)

Smalley recommends a safety factor of three; once the maximum thrust is calculated, the ring thrust load capacity is checked from product specification tables.

Considering the groove form

From a retention standpoint, the groove form is also very important. The groove should have square corners, and the retained part should be square to the ring groove to distribute the load concentrically around the entire ring surface. The maximum allowable groove radius is .010 for larger applications and .005 for applications with diameters 1 inch and smaller.

Just as the shape of the ring groove is crucial to retention performance, the retained part must be carefully analyzed. Ideally, the part should have little or no radius or chamfer, spreading thrust loads over as much ring surface area as possible. The maximum allowable chamfer and radius are shown here:

Where:

b = Radial wall (in)
d = Groove depth (in)

Maximum Chamfer = .375 (b-d)
Maximum Radius = .5 (b-d)

Good design practice uses the minimum possible shaft or housing length consistent with good retention, but how close to the end can the groove be? This edge margin, if inadequate, allows failure modes by either shear or bending, so both must be considered for optimal design. Smalley recommends these two equations to calculate minimum edge margin in both shear and bending modes:

Shear case:

Bending case:

Where:

Z=edge margin (in)

P=load (lb)

D_g=groove diameter (in)

S_y=yield strength of material (psi)

D=groove depth (in)

K=safety factor (3 recommended)

How many RPM?

While retaining rings are frequently specified by the bending and shear failure modes, it is possible to spin a shaft fast enough to expand a ring out of the groove by centrifugal forces. According to Smalley, this formula can be used to determine the maximum allowable RPM for a given ring:

Nomenclature:

N= Max RPM

E= Modulus of elasticity (psi)

I= Moment of inertia (in4)

G= acceleration due to gravity (386.4 in/s2)

V=Cling/2 [(DG-DI)/2] (in)

D_g= Groove diameter (in)

D_i= Free inside diameter (in)

Y= Multiple turn factor (right)

n= number of turns

Y= material density (.283 lb/in3)

A= cross sectional area (t b)-(.12) t2 (in2)

T= material thickness (in)

B= radial wall (in)

R_m= Mean free radius [(D_I+b)/2] (in)

Maximum RPM ratings for Smalley standard steel rings can be found here.

Designers looking for a good rule-of-thumb for edge margin should specify a margin of three times the groove depth. Ring shear, groove deformation, and excessive RPM are just three failure modes that can be taken into account during the design process. Performance is affected by the type of ring chosen and the stresses imposed on the ring during installation. Dynamic and asymmetric loadings also add stresses to a retaining ring. There are multiple types, materials and custom designs possible for special applications. Visit www.smalley.com for more details.