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ASME B18.27-1998 (2017) — Tapered and Reduced Cross Section Retaining Rings (Inch Series)
⭕ ASME B18.27-1998 is the consolidated inch-series retaining ring (snap ring) standard of the ASME B18 fastener family. It integrates the former B18.27.1 through B18.27.5 sectional standards into a single document providing complete general and dimensional data for general-purpose tapered and reduced cross section retaining rings. A retaining ring is a split elastic ring that seats in an annular groove in a shaft or bore to position and retain components axially — one of the most widely used axial-fastening elements in machinery. The Standard was issued July 30, 1999 with automatic addenda B18.27a-1999 and B18.27b-2000, and was reaffirmed in 2017.
💡 The core value of a retaining ring is removable axial retention: compared with shoulders, sleeves, or set screws, rings take little space, assemble and disassemble easily, and are inexpensive — ideal for shaft-hub joints that need regular maintenance. Matching the correct groove width and depth is the precondition for reliable ring performance.
1. Scope and General Framework 📌
This Standard covers three series of general-purpose tapered and reduced cross section retaining rings for use with the nominal shaft and housing sizes listed, seated in grooves of the recommended dimensions:
- External rings: seated in shaft grooves; Table 6 gives basic external ring and groove dimensions for various shaft sizes; Nonmandatory Appendix A provides equivalent metric data.
- Internal rings: seated in bore grooves; Table 7 gives basic internal ring and groove dimensions for various housing sizes; Nonmandatory Appendix C provides equivalent metric data.
- External E-rings: Table 8 gives E-type external ring and groove dimensions; Nonmandatory Appendix E provides equivalent metric data.
The Standard also includes the formulas and tolerances on which the dimensional data are based, plus three appendices covering assembly guidance and recommended standard drawing formats. Note that inclusion of dimensional data does not imply all sizes are stock production sizes — confirm availability with suppliers.
⚠️ B18.27 rings are designed to work in the recommended groove dimensions. Widening or deepening a groove at will lowers axial capacity and invites fatigue fracture; an oversized groove-bottom radius also prevents full seating. Design the groove strictly per the standard tables.
2. Type Family Overview 🔧
B18.27 consolidates all type styles of the five sectional standards, numbered in the NA series:
| Sectional Std. | Types (Series No.) | Design Feature | Typical Use |
|---|---|---|---|
| B18.27.1 | External NA1, Internal NA2, E-ring NA3 | Basic tapered-section rings | General shaft/bore retention — most common |
| B18.27.2 | Heavy-duty external NA4, reinforced E-ring NA5, C-ring NA6 | Thicker sections, higher capacity | Heavy-duty axial retention |
| B18.27.3 | Bowed external NA7, bowed internal NA8, bowed E-ring NA9 | Bowed elastic preload | Eliminating axial play, noise control |
| B18.27.4 | Inverted external NA10, inverted internal NA11, beveled external NA12, beveled internal NA13 | Inverted/beveled construction | Special assembly and loading direction |
| B18.27.5 | Self-locking external NA14, interlocking external NA15, bowed locking prongs NA16 | Self-locking/interlocking prongs | High-vibration duty, pop-out prevention |
The series spans small to large shaft and bore diameters in inch units; exact size boundaries are per the standard tables.
3. Dimensions, Tolerances, and Materials 📏
The Standard gives ring free diameter, section thickness, groove width, groove depth, groove-bottom radius, and more, with tolerances derived from a formula system (including simplified models for material modulus and axial load capacity). Common materials and finishes:
- Carbon spring steel: the most common base material, often phosphate-treated for corrosion control.
- Corrosion-resistant steel: for moist and corrosive environments.
- Beryllium copper: for non-sparking, non-magnetic, corrosion-resistant, and high-elasticity requirements.
- Surface treatments: phosphate, zinc, cadmium, and others per order agreement.
💡 Material selection: carbon steel balances cost and performance but has poor low-temperature toughness; beryllium copper gives excellent elasticity and is non-magnetic but costs more. For corrosive environments prefer corrosion-resistant steel, or well-coated carbon steel, and watch hydrogen embrittlement (de-embrittle high-strength plated steel).
4. Designation Method 📋
Rings are designated by: ring series type number; size; material; protective finish, if required — or optionally by the ASME B18.24.3 PIN Code. Examples:
- NA1-37, carbon spring steel, phosphate
- NA2-75, corrosion resistant steel
- NA3-S118, beryllium copper
With the PIN system, codes in the R271NAA0037NN056NNAA style uniquely express size, material, and finish for procurement and traceability.
5. Assembly and Use Essentials 🛠️
- 🔍 Groove design: set groove width, depth, and bottom radius per the standard tables; deburr shaft/bore groove mouths to avoid scraping the ring during installation.
- 🔧 Tools: expand external rings with dedicated ring pliers and compress internal rings with pliers; never pry with a screwdriver — that causes plastic deformation and loss of spring.
- 📊 Load check: axial capacity depends on section thickness, groove depth, and material strength; verify per the appendices and formulas, and consult manufacturers when in doubt.
- 📋 Inspection and storage: sample-check dimensions and hardness per the B18.18 series; store rings protected from rust and mechanical damage.
🚨 A retaining ring is a single-use elastic element: once noticeable plastic expansion/compression occurs during installation, its spring action is permanently lost. Do not reuse removed rings, especially in high-vibration or heavy-duty service.
6. Frequently Asked Questions ❓
❓ Q: How do I tell external from internal rings?
A: External rings seat in a groove on the shaft and retain parts mounted on the shaft (preventing them from sliding off); internal rings seat in a groove in the bore (housing) and retain parts inside the bore.
A: External rings seat in a groove on the shaft and retain parts mounted on the shaft (preventing them from sliding off); internal rings seat in a groove in the bore (housing) and retain parts inside the bore.
❓ Q: What is special about E-rings?
A: E-rings have a small section and tiny space requirement, installed in shallow shaft grooves for light-duty axial retention; their capacity is lower than basic external rings — use NA1 or heavy-duty NA4 for heavy loads.
A: E-rings have a small section and tiny space requirement, installed in shallow shaft grooves for light-duty axial retention; their capacity is lower than basic external rings — use NA1 or heavy-duty NA4 for heavy loads.
❓ Q: What are bowed rings used for?
A: Bowed rings are arched in the free state and apply elastic preload to the retained part after installation, eliminating axial play, rattle, and noise — ideal for “zero-play” positioning.
A: Bowed rings are arched in the free state and apply elastic preload to the retained part after installation, eliminating axial play, rattle, and noise — ideal for “zero-play” positioning.
🔍 Q: How does B18.27 map to metric retaining ring standards?
A: B18.27 is the inch series (metric equivalent appendices are only reference data); metric rings follow DIN 471/472 or ISO-related standards. Groove sizes and sections are not interchangeable — never mix systems.
A: B18.27 is the inch series (metric equivalent appendices are only reference data); metric rings follow DIN 471/472 or ISO-related standards. Groove sizes and sections are not interchangeable — never mix systems.
