Retaining Ring Standards — Complete Reference
Military Standards
| Standard | Designation | Description |
|---|---|---|
| MIL-R-21248B | MS-16633 | Open-type external uniform cross-section |
| MIL-R-21248B | MS-16634 | Open-type external uniform cross-section, cylindrically bowed |
| MIL-R-21248B | MS-3215 | Open-type external tapered cross-section |
| MIL-R-21248B | MS-16632 | Crescent-type external |
| MIL-R-21248B | MS-16625 | Internal |
| MIL-R-21248B | MS-16629 | Internal cylindrically bowed |
| MIL-R-21248B | MS-16624 | Closed-type external tapered cross-section |
| MIL-R-21248B | MS-16628 | Closed-type external tapered, cylindrically bowed |
| MIL-R-21248B | MS-16627 | Internal inverted |
| MIL-R-21248B | MS-16626 | Closed-type external tapered cross-section |
| MIL-R-21248B | MS-90707 | Self-locking external tapered cross-section |
| MIL-R-21248B | MS-3217 | External heavy-duty tapered cross-section |
| MIL-R-27426 | Type 1-External, Type 2-Internal | Uniform cross-section spiral retaining rings |
Aerospace Standards
| Standard | Description |
|---|---|
| AS 3215 | Ring, Retaining — Spiral, Internal, Heavy Duty, Stainless Steel |
| AS 3216 | Ring, Retaining — Spiral, External, Heavy Duty, Stainless Steel |
| AS 3217 | Ring, Retaining — Spiral, Internal, Light Duty, Stainless Steel |
| AS 3218 | Ring, Retaining — Spiral, External, Light Duty, Stainless Steel |
| AS 3219 | Ring, Wound — Dimensional and Acceptance Standard for Spiral Wound Retaining Rings |
ANSI Standards
| Standard | Description |
|---|---|
| B27.6-1972, R1983 | General Purpose Uniform Cross-Section Spiral Retaining Rings |
| B27.7M-1977, R1983 | General Purpose Tapered and Reduced Cross-Section Retaining Rings (Metric) |
| B27.2M-1977, R1983 | General Purpose Metric Tapered and Reduced Cross-Section Retaining Rings — Types 3DM1 (Heavy Duty External), 3EM1 (Reinforced E-Rings), 3FM1 (C-Type Rings) |
ANSI/SAE Standards
| Standard | Description |
|---|---|
| MA4016 | External Spiral Wound, Heavy and Medium Duty, Crescent, Metric |
| MA4017 | External Spiral Wound, Heavy and Medium Duty, Crescent, Metric |
| MA4020 | External Tapered, Type 1, Class 2, AMS 5520, Metric |
| MA4021 | Internal Tapered, Type 1, Class 1, AMS 5520, Metric |
| MA4029 | Internal, Beveled, Tapered, Type 2, Class 1, AMS 5520, Metric |
| MA4030 | External, Reinforced E-Ring, Type 1, Class 3, AMS 5520, Metric |
| MA4035 | Spiral Wound, Uniform Section, Corrosion Resistant, Metric |
| MA4036 | Tapered Width, Uniform Thickness, Corrosion Resistant, Metric |
DIN Standards
| Standard | Description |
|---|---|
| DIN 471, 472, 6799, 984, 5417, 7993 | Normal and heavy type, internal and external retaining rings and retaining washers |
| LN 471, 472, 6799 | Aerospace standards for internal and external retaining rings |
The Decision Matrix: Choosing the Right Retaining Ring
When you're standing at the design desk or the maintenance workbench, use this matrix to narrow your selection:
| Application Requirement | Best Ring Type | Why |
|---|---|---|
| General shaft retention, metric | 3AM1 Tapered External | Broadest size range, well-documented loads |
| General bore retention, metric | 3BM1 Tapered Internal | Covers 8–250 mm bores |
| Very small shafts (1–25 mm) | 3CM1 E-Ring | Reduced cross-section, minimal footprint |
| Gapless shoulder required | Spiral-wound (MIL-R-27426) | No gap for parts to snag through |
| High thrust loads, inch | Heavy Duty Spiral (Class 2) | Highest capacity, shaft to 15 in. |
| Bidirectional rotation | Stamped (any type) | No unwinding concern |
| High-speed applications | Self-locking rings | Designed to resist centrifugal ejection |
| No groove allowed | Self-locking (7100 Series) | Grips shaft by spring action alone |
| Corrosive environment | Type 302 or 316 SS ring | Resists rusting and chemical attack |
| High temperature (to 900°F) | A286 superalloy | Retains spring properties at temperature |
| Extreme temperature (to 1,200°F) | Inconel X-750 | Maximum temperature capability |
| Food industry | Type 316 SS | FDA-compatible corrosion resistance |
| Aerospace / military | Per applicable MS or AS standard | Qualified to military/aerospace specs |
Quick-Reference Formula Card
Cut this out and pin it to your wall. Every formula you need for retaining ring design:
| Formula | Equation | Use For |
|---|---|---|
| Ring Shear Thrust | Maximum load before ring shears | |
| Groove Deformation Thrust | Maximum load before groove yields | |
| Minimum Groove Distance | How far from shaft/bore end to cut groove | |
| Max Total Radius | Whether full thrust loads apply | |
| Max Total Chamfer | Whether full thrust loads apply | |
| Allowable Speed | Maximum RPM for external spiral ring | |
| Ring Cling (External Spiral) | Input for speed calculation | |
| Design Thrust Load | Always use the lower value |
the practitioner's Final Audit Checklist
After rebuilding the conveyor and auditing every retaining ring application in the plant, the practitioner created this checklist. She laminated copies and posted them at every maintenance workstation. It has prevented every repeat failure since.
Before Specifying a Retaining Ring
Before Machining the Groove
Before Installing the Ring
After Installation
The Lesson That Cost Six Figures
the practitioner's conveyor failure was caused by ten thousandths of an inch in the wrong direction on a groove bottom radius. The ring was correctly specified. The material was correct. The load was within published limits. But the groove — that single machined feature that the ring depends on entirely — was out of specification by an amount invisible to the naked eye.
That is the fundamental truth of retaining rings: They are only as reliable as the grooves they sit in, the loads they're subjected to, and the attention to detail of the people who specify, machine, and install them.
The next time you reach for a snap ring or specify a retaining ring in a design, remember the practitioner's rule: "The ring is cheap. The groove is critical. The failure is expensive."
Your Next Step
Print the formula card. Laminate the practitioner's checklist. The next retaining ring you specify or install, run the numbers — ring shear, groove deformation, minimum groove distance, maximum total radius. Compare your actual loads to both the ring and groove capacities and use the lower value.
And if you have a colleague who still thinks retaining rings are "just snap rings that don't matter," share this guide. Because that tiny, invisible shoulder is the only thing standing between a running machine and a catastrophic, costly, and completely preventable failure.
What's the most common retaining ring failure mode you've encountered in your facility? Is it groove deformation, ring shear, or something the handbooks don't cover?
This guide is based on data from ANSI B27.7M-1977 (R1983), MIL-R-21248B, MIL-R-27426, and standard industrial retaining ring engineering references. All thrust load values include the safety factors noted in each table. For critical applications, always verify with current editions of applicable standards and consult the ring manufacturer's engineering data.