Retaining Rings: Selection, Grooves and Installation: Metric E-Type External Retaining Rings

Metric E-Type External Retaining Rings — Type 3CM1 (Reduced Cross-Section)

E-rings are the go-to for small shafts where a full tapered ring would be impractical. They're designated by series symbol and shaft diameter — 3CM1-10 fits a 10 mm shaft. Size −1 is available in beryllium copper only.

Dimensional Data — 3CM1 E-Ring Series

CM1 Tolerance Summary

Ring Free Diameter Tolerances:

• Sizes −1 through −7: +0.03, −0.08 mm
• Sizes −8 through −13: +0.05, −0.10 mm
• Sizes −15 through −25: +0.10, −0.15 mm

Groove Diameter Tolerances:

• Sizes −1 and −2: −0.05 mm
• Sizes −3 through −6: −0.08 mm
• Sizes −7 through −11: −0.10 mm
• Sizes −12 through −18: −0.15 mm
• Sizes −20 through −25: −0.20 mm

Groove Width Tolerances:

• Sizes −1 and −2: +0.05 mm
• Size −3: +0.10 mm
• Sizes −4 through −25: +0.15 mm

Groove Maximum Bottom Radii:

• Sizes −1 and −2: 0.05 mm
• Sizes −3 through −7: 0.15 mm
• Sizes −8 through −13: 0.25 mm
• Sizes −15 through −25: 0.4 mm

CM1 Performance Data — Allowable Thrust Loads

Pr values apply to SAE 1060–1090 steels and PH 15-7 Mo stainless steel on shafts hardened to Rc 50 minimum (size −1 is beryllium copper only). Safety factor of 4 for Pr, safety factor of 2 for Pg.

Maximum Allowable Assembly Loads (with R max or Ch max):

Heavy Duty External Spiral Retaining Rings — MIL-R-27426

When the practitioner's investigation led her to spiral retaining rings, she found what she called "the quiet professionals of the fastener world." These rings provide a continuous, gapless shoulder — no opening for a retained part to snag or work through — and they're available in both medium and heavy duty configurations.

The heavy duty external spiral rings per MIL-R-27426 cover shaft diameters from 0.469 inches all the way up to 15.000 inches.

Selected Heavy Duty External Spiral Ring Data

Ring Thickness in the supplied reference (Heavy Duty External Spiral)

Heavy Duty External Spiral Tolerance Summary

Ring Free Diameter Tolerances:

• Sizes 0.469 – 1.500: +0.000, −0.013 in.
• Sizes 1.562 – 2.000: +0.000, −0.020 in.
• Sizes 2.062 – 2.687: +0.000, −0.025 in.
• Sizes 2.750 – 3.437: +0.000, −0.030 in.
• Sizes 3.500 – 5.000: +0.000, −0.035 in.
• Sizes 5.250 – 6.000: +0.000, −0.050 in.
• Sizes 6.250 – 7.000: +0.000, −0.060 in.
• Sizes 7.250 – 10.000: +0.000, −0.070 in.
• Sizes 10.250 – 12.500: +0.000, −0.090 in.
• Sizes 12.750 – 15.000: +0.000, −0.110 in.

Ring Thickness Tolerances:

• Sizes 0.469 – 1.500: ±0.002 in.
• Sizes 1.562 – 5.000: ±0.003 in.
• Sizes 5.250 – 6.000: ±0.004 in.
• Sizes 6.250 – 15.000: ±0.005 in.

Groove Diameter Tolerances:

• Sizes 0.469 – 0.562: ±0.002 in.
• Sizes 0.594 – 1.023: ±0.003 in.
• Sizes 1.062 – 1.500: ±0.004 in.
• Sizes 1.562 – 2.000: ±0.005 in.
• Sizes 2.062 – 5.000: ±0.006 in.
• Sizes 5.250 – 6.000: ±0.007 in.
• Sizes 6.250 – 10.000: ±0.008 in.
• Sizes 10.250 – 12.500: ±0.010 in.
• Sizes 12.750 – 15.000: ±0.012 in.

Groove Width Tolerances:

• Sizes 0.469 – 1.023: +0.003, −0.000 in.
• Sizes 1.062 – 2.000: +0.004, −0.000 in.
• Sizes 2.062 – 5.000: +0.005, −0.000 in.
• Sizes 5.250 – 6.000: +0.006, −0.000 in.
• Sizes 6.250 – 7.250: +0.008, −0.000 in.
• Sizes 7.500 – 15.000: +0.008, −0.000 in.

Inch Series Stamped Retaining Rings — Complete Dimensional Data

MS16632 — Crescent-Type External Retaining Rings

Source: Industrial Retaining Rings, 2000 Series. Standard material: carbon spring steel SAE 1060–1090. Safety factors: Ring = 4, Groove = 2. Groove wall thrust loads are for grooves machined in cold-rolled steel with a tensile yield strength of 45,000 psi; for other shaft materials, the thrust load varies proportionally with the yield strength.

MS3215 — Open-Type External Tapered Retaining Rings

Source: Industrial Retaining Rings, 1200 Series. Standard material: SAE 1060–1090. Safety factors: Ring = 3, Groove = 2.

Self-Locking External Retaining Rings (7100 Series)

These rings are designed for applications where grooves are not recommended on smaller shaft sizes. Standard material: SAE 1060–1090. Safety factors: Ring = 1, Groove = 2.

Note: For shaft sizes below 0.248 inch, the use of grooves is not recommended. The ring relies entirely on its spring action to grip the shaft.

Internal and External Self-Locking Rings (6000/6100 Series)

Source: Industrial Retaining Rings. All dimensions in inches, thrust loads in pounds. Standard material: SAE 1060–1090.

Selected Internal Ring Data:

Selected External Ring Data:

Thrust Load Capacity — The Most Critical Design Parameter

Here is where the practitioner's story intersects with engineering science. The most important criterion in determining which ring is best suited for a specific application is thrust load capacity. Both the strength of the ring and the strength of the groove must be considered to determine which will fail first.

The Two Failure Modes

The ring and the groove are two separate structures, and either one can fail independently. The published thrust load tables in this guide already include safety factors, but understanding the underlying mechanics is essential for non-standard applications.

Formula 1: Ring Shear Capacity

When a ring is loaded by a retained part and groove both having compressive yield strength greater than 45,000 psi, or when sharp-corner line-to-line contact exists, or when the ring is too thin relative to its diameter, shear failure of the ring becomes the concern.

$$P_s = \frac{\pi \cdot D \cdot t \cdot S_s}{K}$$

Where:

• $P_s$ = Allowable thrust based on ring shear (lbf)
• $D$ = Shaft or housing diameter (inches)
• $t$ = Ring thickness (inches)
• $S_s$ = Shear strength of ring material (psi)
• $K$ = Factor of safety

Formula 2: Groove Deformation Capacity

The most common groove failure is yielding of the groove material when the thrust load exceeds the compressive yield strength at the groove corner. This causes the ring to tilt, dish, and eventually escape.

For spiral-wound rings, the thrust load that initiates groove deformation:

$$P_G = \frac{\pi \cdot D \cdot d \cdot S_y}{K}$$

Where:

• $P_G$ = Thrust load at onset of groove deformation (lbf)
• $D$ = Shaft or housing diameter (inches)
• $d$ = Groove depth (inches)
• $S_y$ = Yield strength of groove material (psi)
• $K$ = Factor of safety

For stamped rings, estimate $P_G$ by multiplying the equation above by the fraction of the groove circumference that contacts the ring.

The Golden Rule: Always use the lower of the values from the ring shear equation and the groove deformation equation as your allowable thrust load. Increasing groove material strength may shift the failure mode from groove deformation to ring shear.

Formula 3: Minimum Groove Distance from End

The groove must be located far enough from the end of the shaft or bore to prevent shear-out:

$$Y = \frac{K \cdot P_t}{\pi \cdot D \cdot S_c}$$

Where:

• $Y$ = Minimum safe distance from end of shaft/bore (inches)
• $K$ = Factor of safety
• $P_t$ = Thrust load on groove (lbf)
• $S_c$ = Shear strength of groove material (psi)
• $D$ = Shaft or housing diameter (inches)

General rule of thumb: Locate the groove at least two to three times the groove depth from the end of the shaft or bore.

Worked Example: the practitioner's Conveyor

the practitioner's failed application used a 2.000-inch bore with a stamped internal ring in low-carbon steel housing.

Given:

• Housing bore: D = 2.000 in.
• Groove depth: d = 0.068 in.
• Groove material yield strength: $S_y$ = 45,000 psi
• Safety factor: K = 2

Groove thrust capacity:

$$P_G = \frac{\pi \times 2.000 \times 0.068 \times 45{,}000}{2} = \frac{19{,}226}{2} = 9{,}613 \text{ lbf}$$

But the practitioner's groove had an oversized bottom radius. Testing data consistently showed that improper groove geometry — radii exceeding specification, walls not perpendicular to the axis, rounded top corners — significantly reduces thrust capacity below published values. The published value assumed ideal groove geometry. Her groove delivered perhaps half that capacity.

Centrifugal Capacity and High-Speed Applications

A retaining ring only works if it stays seated in the groove. For external rings, this depends on the ring "clinging" to the groove bottom — the ring's inside diameter is slightly smaller than the groove bottom diameter, creating an interference that holds the ring in place.

At high speeds, centrifugal force can overcome this cling and throw the ring out of the groove.

Formula 4: Allowable Steady-State Speed

For external spiral retaining rings:

$$N = \sqrt{\frac{0.466 \cdot C_1 \cdot E^3 \times 10^{12}}{R_n^3 \cdot (1 + C_1) \cdot (R_o^3 - R_i^3)}}$$

Where:

• $N$ = Allowable speed (RPM)
• $C_1$ = Minimum ring cling to groove bottom
• $E$ = Ring radial wall (inches)
• $R_n$ = Free neutral ring radius (inches)
• $R_o$ = Free outside ring radius (inches)
• $R_i$ = Free inside ring radius (inches)

For external spiral rings: $C_1 = \frac{C - G}{G}$, where $C$ is the mean groove diameter and $G$ is the maximum ring free ID, both in inches.

High-Speed Design Strategies

• Use self-locking rings specially designed for high-speed applications
• Increase ring cling by specifying a ring with a smaller interior diameter (provided installation stress remains within limits)
• Use locking rings that engage a hole in the groove bottom, physically preventing rotation and ejection
• For very large rings subject to sudden acceleration/deceleration, specify extra cling to prevent spinning in the groove

Rotation Between Parts — A Hidden Failure Mode

This is the failure mode that catches experienced engineers off guard. Spiral-wound rings can unwind from their grooves if the retained part rotates in the wrong direction.

The Rules for Spiral Rings and Rotation

• External rings: Wind the ring in the same direction as the rotation of the retained part. The rotation tends to tighten the spiral into the groove.
• Internal rings: Wind the ring against the direction of rotation of the rotating part.
• Failure to follow these rules will cause the ring to spiral out of the groove under rotation.
• Spiral rings are available in right-hand (standard) and left-hand (reverse) configurations.

When Stamped Rings Win

Stamped retaining rings do not have this directional limitation. They can be used for applications requiring rotation of the retained part in either direction, or in applications where rotation direction may change. If your application involves bidirectional rotation, stamped rings are the safer choice.

Ring Materials — Matching the Ring to the Environment

Standard Materials

Surface Finishes

Standard rings are supplied uncoated. Special finishes available include:

• Cadmium plating — for carbon steel rings requiring corrosion protection
• Phosphate coating — for carbon steel, provides lubricity and mild corrosion resistance
• Zinc plating — general-purpose corrosion protection
• Black oxide — for carbon steel, mild corrosion resistance and appearance
• Passivation — for stainless steel rings

Plating Note: For plated, phosphate-coated, and stainless steel rings, the maximum ring thickness may be exceeded by 0.002 inch. Always verify that groove width accommodates the plated ring.

Retaining Ring Failure Analysis — A Complete Diagnostic Guide

When a retaining ring application fails, the root cause falls into one of several categories. Here is the practitioner's diagnostic framework, refined over hundreds of inspections.

Failure Mode 1: Ring Shear

Symptoms: Ring found fractured, clean shear surfaces, retained part displaced.

Root Causes:

• Groove and retained part both have compressive yield strength > 45,000 psi, concentrating load on the ring
• Sharp corners on both groove and retained part creating line-to-line contact
• Ring too thin in section relative to its diameter

Diagnosis Formula: Calculate $P_s$ using the ring shear equation and compare to actual load.

Failure Mode 2: Groove Yielding (Most Common)

Symptoms: Ring found intact but dished/cupped in shape. Groove walls show deformation — rolled edges, material displacement. Ring may still be partially seated but tilted.

Root Causes:

• Low compressive yield strength of groove material
• Thrust load exceeded groove capacity
• Groove bottom radius exceeded specification (the practitioner's conveyor)
• Groove walls not perpendicular to shaft/bore axis
• Groove corners not square

What Happens: The thrust load, applied through the ring against the groove corner, exceeds the compressive yield strength. The groove material yields, the ring tilts, and a bending moment develops across the ring cross-section. Tensile stress concentrates at the ring ID. If this stress exceeds the ring material's yield strength, the ring ID grows permanently and the ring becomes permanently dished.

Failure Mode 3: Centrifugal Ejection

Symptoms: External ring missing entirely. No groove damage. Application is high-speed.

Root Causes:

• Speed exceeded allowable steady-state RPM
• Ring cling insufficient for operating speed
• Sudden acceleration/deceleration events

Failure Mode 4: Spiral Unwinding

Symptoms: Spiral ring found partially or fully unwound from groove. Application involves rotating parts.

Root Causes:

• Ring wound direction opposes rotation direction
• Bidirectional rotation used with spiral ring instead of stamped ring

Failure Mode 5: Dynamic/Shock Loading

Symptoms: Intermittent failures under apparently adequate static loads. Ring or groove shows fatigue indicators.

Root Causes:

• Impact loading from mass and velocity of retained part striking ring
• Vibration frequency coinciding with ring resonant frequency
• Eccentric loading from misaligned parts, cocked retained parts, or improperly machined surfaces

Failure Mode 6: Assembly Damage

Symptoms: Ring deformed before failure. Groove shows scoring or tool marks.

Root Causes:

• Excessive assembly force exceeding maximum allowable assembly load
• Improper tooling damaging ring or groove during installation
• Retained part corner radius or chamfer exceeding maximum total radius specification

Groove Design and Machining — Where Most Failures Begin

the practitioner's most important lesson: A properly designed and machined groove is just as important as the ring itself.

Groove Location Rules

• Position the groove at least 2× to 3× the groove depth from the end of the shaft or bore
• If too close to the end, the groove wall may shear or yield under load
• Calculate the minimum safe distance using the groove distance formula ($Y$ equation)

Groove Geometry Requirements

• Walls must be perpendicular to the shaft or bore axis
• Top edges must have square corners — rounded top edges reduce thrust capacity
• Bottom must have specified radii — too large reduces capacity (the practitioner's root cause), too small creates stress concentrations

Maximum Bottom Groove Radii — Spiral Rings

For stamped rings, the maximum bottom groove radius varies with ring size and style — always consult the specific table for your ring series.

Maximum Total Radius and Chamfer

The maximum total radius or chamfer determines whether you can use full published thrust load values or must derate:

$$\text{Max Total Radius} = 0.5 \times (b - d)$$ $$\text{Max Total Chamfer} = 0.375 \times (b - d)$$

Where $b$ is the radial wall thickness and $d$ is the groove depth.

If your application meets these conditions, full published thrust loads apply:

• Minimum clearances exist between retained part and shaft/housing
• Groove and retained part have square corners
• Contact occurs close to the shaft or housing

If conditions are met but side clearances, radii, and chamfers are less than the max total values: Divide the tabulated thrust load by 2.

The Complete Installation Geometry

Maximum Total Radius or Chamfer =
    Max Groove Chamfer or Radius
  + Max Side Clearance
  + Max Retained Part Chamfer or Radius

Every one of these dimensions matters. A designer who specifies the ring correctly but ignores the retained part corner radius is inviting the same kind of failure the practitioner experienced.