Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME B27.8M-1978 (1999) — General Purpose Metric Tapered and Reduced Cross Section Retaining Rings
⚙️ ASME B27.8M-1978 (1999) is the ASME B27 series standard covering general purpose metric tapered and reduced cross section retaining rings. Approved as American National Standard ANSI B27.8M on November 28, 1978, and reaffirmed in 1999, the Standard uses millimetre dimensions throughout. Compared with B27.7, its three series target heavier loads and more demanding installation conditions: heavy duty external rings 3DM1, reinforced E-rings 3EM1, and C-type external rings 3FM1. These rings are widely used in heavy machinery, power transmission, hydraulic components, construction equipment, and high-speed electric motors where parts must be located axially under high load or high speed.
💡 The three series of B27.8M build up in capability: 3DM1 resists shearing and groove pull-out better than the 3AM1 rings of B27.7; 3EM1 is stiffer and more resistant to centrifugal lift-out than the 3CM1 E-rings of B27.7; and 3FM1 offers a narrow, uniform abutment shoulder for space-limited shaft ends.
1. Scope and Ring Types 📌
The Standard covers three series of general purpose metric tapered and reduced cross section retaining rings for use on nominal-size shafts and in the recommended groove dimensions:
| Type | Series | Construction Feature | Dimension Table | Typical Applications |
|---|---|---|---|---|
| 3DM1 | Heavy Duty External | Greater thickness and section height; shear- and lift-out-resistant | Table 4 | Axial retention of heavy bearings, gears |
| 3EM1 | Reinforced “E” Ring | Three prongs joined by a reduced tapered bridge; stiffer | Table 5 | End retention on high-speed, vibrating shafts |
| 3FM1 | “C” Type External | Taper on the inner surface facing the groove; narrow uniform shoulder | Table 6 | Space-limited shaft ends needing even abutment |
Para. 1.2.4 recommends using the preferred type designations 3DM1, 3EM1, and 3FM1 on drawings, parts lists, and other engineering documentation. As in the other B27 ring standards, listing dimensional data does not imply that every size is a stock production size — consult manufacturers for stock lists (para. 1.1). Para. 1.3 repeats the general applicability rule: the rings are intended primarily for the recommended shaft and groove sizes; altered diameters must not cause permanent set that allows a loose fit.
⚠️ This Standard covers external (shaft) rings only — no internal housing-ring series is included. For internal retention in bores, refer to the 3BM1 basic internal rings of B27.7 or to the appropriate manufacturer’s range; never mix the series.
2. Material and Mechanical Property Requirements 🔧
The standard manufacturing materials are carbon spring steel, corrosion resistant steel, and beryllium copper. Carbon spring steel shall conform to the chemical composition of SAE 1060 to 1090 or AISI 1060 to 1090 (UNS G10600 to G10900) or equivalent. Table 1 gives the hardness ranges for tapered and reduced cross section retaining rings; hardness is tested per the procedure of Section 4. Section 5 sets the permanent set limits, Section 6 the irregularity limitations, and Tables 2 and 3 give the dish and pitch limitations.
| Material | Composition / Grade | Hardness | Typical Applications |
|---|---|---|---|
| Carbon spring steel | SAE/AISI 1060–1090 (UNS G10600–G10900) | Per Table 1 | General heavy duty; cost-effective |
| Corrosion resistant steel | Corrosion resistant steel series | Per Table 1 | Corrosive, marine, and food environments |
| Beryllium copper | Beryllium copper alloy | Per Table 1 | Explosion-proof, non-magnetic, instrument and electrical duty |
🚨 The heavy duty 3DM1 rings are hard and strong — never force them into place with hammer blows or brutal methods, which damage both ring and shaft. And remember that beryllium copper machining dust is hazardous: work only in a well-ventilated area with proper protection.
3. Construction Features and Operating Principle 📏
- 3DM1 heavy duty external rings are spread over the shaft by pliers or a special tool and relax into the circumferential groove, forming an external protruding shoulder. Thanks to greater thickness and section height, they resist shearing and lift-out from the groove markedly better than the 3AM1 rings of B27.7 — the choice for heavy and shock-loaded duty.
- 3EM1 reinforced E-rings have three prongs joined by a reduced tapered-section bridge; installed radially, usually with an applicator, they provide a high abutment shoulder. Because they are stiffer, they resist centrifugal lift-out substantially better than the 3CM1 E-rings of B27.7, making them the right choice for high-speed rotation.
- 3FM1 C-type rings have a tapered section on the inner surface facing the groove; installed radially, they provide a narrow, uniform shoulder for the retained part — suited to shaft ends with limited axial space where even load distribution is needed.
Allowable loads, maximum radii and chamfers, clearance dimensions, gaging diameters, and rpm limits for the three series are provided in Appendixes A, B, and C for design verification.
4. Engineering Practice Essentials 🛠️
- 🔍 Load check: heavy duty does not mean “the bigger the better” — verify axial thrust and groove-wall bearing stress against the allowable load tables of Appendix A to avoid crushing the groove edges.
- 🔧 High-speed check: above the allowable rpm, centrifugal force opens the ring and lifts it out of the groove, letting the retained part shift axially. Check the rpm limits step by step for high-speed motors and drive shafts.
- 📊 Installation tools: 3EM1 and 3FM1 are installed radially with a dedicated applicator; 3DM1 takes expanding pliers. Never hammer rings into place — it deforms the ring and scars the shaft.
- 📐 Procurement identification: drawings and purchase orders must cite the full designation ASME B27.8M-1978 (1999), the type code (3DM1/3EM1/3FM1), material, and finish, and confirm stock availability with the manufacturer.
💡 Selection rule of thumb: heavy/shock load → 3DM1; high speed → 3EM1; limited shaft-end space → 3FM1. All three are metric sizes and, together with the 3AM1/3BM1/3CM1 series of B27.7, cover most shaft-ring applications.
5. Frequently Asked Questions ❓
❓ Q: B27.8M and B27.7 are both metric tapered-ring standards — are they interchangeable?
A: No. B27.7 covers basic external 3AM1, basic internal 3BM1, and E-rings 3CM1; B27.8M covers heavy duty external 3DM1, reinforced E-rings 3EM1, and C-rings 3FM1. Groove dimensions and ring sections differ at the same nominal size, so select the series by the actual load.
A: No. B27.7 covers basic external 3AM1, basic internal 3BM1, and E-rings 3CM1; B27.8M covers heavy duty external 3DM1, reinforced E-rings 3EM1, and C-rings 3FM1. Groove dimensions and ring sections differ at the same nominal size, so select the series by the actual load.
❓ Q: 3EM1 and 3CM1 both look like “E” rings — what is the real difference?
A: 3EM1 is the reinforced version: the bridge between prongs uses a reduced tapered section, giving higher stiffness so the ring resists centrifugal lift-out at speed. 3CM1 is the standard E-ring with larger elastic travel and easier assembly, but lower high-speed capability. For high-speed duty, prefer 3EM1.
A: 3EM1 is the reinforced version: the bridge between prongs uses a reduced tapered section, giving higher stiffness so the ring resists centrifugal lift-out at speed. 3CM1 is the standard E-ring with larger elastic travel and easier assembly, but lower high-speed capability. For high-speed duty, prefer 3EM1.
❓ Q: What does “taper on the surface facing the groove” mean for the C-type 3FM1?
A: The taper guides the C-ring into the groove as it is pushed on radially, while keeping the abutment shoulder narrow and uniform in width, so the retained part receives a stable, evenly loaded bearing face without local stress concentration.
A: The taper guides the C-ring into the groove as it is pushed on radially, while keeping the abutment shoulder narrow and uniform in width, so the retained part receives a stable, evenly loaded bearing face without local stress concentration.
🔍 Q: Why does the Standard stress that “dimensional data do not imply stock production sizes”?
A: Ring sizes are numerous, and manufacturers normally stock only the common ones; special sizes are made to order. Confirming stock and delivery with the manufacturer before design freeze avoids long procurement lead times for non-standard sizes.
A: Ring sizes are numerous, and manufacturers normally stock only the common ones; special sizes are made to order. Confirming stock and delivery with the manufacturer before design freeze avoids long procurement lead times for non-standard sizes.
