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ASME B29.23M-1985 (2004) — Flexible Chain Couplings
🔗 ASME B29.23M-1985 (2004) is the American National Standard dedicated to flexible chain couplings — couplings that use a length of roller chain or silent chain as the coupling medium between two sprocket hubs. It controls the overall coupling dimensions, misalignment capabilities, cover requirements, and lubrication practices, and provides metric equivalents for all dimensions and capacities. Developed by the ASME Standards Committee B29 (Chains, Attachments, and Sprockets for Power Transmission and Conveying), the Standard draws on information supplied by member companies of the American Chain Association as well as AGMA Standards 510.02, 511.02, 512.03, 513.01, and 515.01 covering flexible couplings. Approved by the American National Standards Institute on April 10, 1985, reaffirmed in 1995 and again in 2004, it remains the basic reference for selecting and manufacturing chain-type shaft couplings.
💡 A chain coupling combines the compliance of chain — it absorbs moderate misalignment — with the positive, slip-free, constant-ratio character of chain drive. It is a proven, economical choice for connecting prime movers to driven machinery.
1. Scope and Terminology 🔍
This Standard covers the chain-type flexible coupling: one member of a broad family of mechanical shaft connectors that transmits torque without slip while accommodating moderate misalignment between axially oriented driving and driven machine members. Two hubs having sprocket teeth are engaged by a length of roller chain or silent chain.
Key terms defined by the Standard include:
- Sprocket hub: the part of the coupling machined for mounting on a machine shaft; sprocket teeth are cut on the periphery of the hub flange to positively engage the chain.
- Chain: either (a) an ANSI B29.1 type double-strand chain, (b) a special single-strand roller chain designed for coupling applications only, or (c) a silent chain. Only double-strand ANSI B29.1 type chains are interchangeable; single-strand roller chains and silent chains are generally not.
- Pitch: the average distance between the joint centers of an assembled chain, normally used to state chain size.
- Cover: the part that encloses the chain to retain lubricant, protect the working parts (chain and sprocket teeth) from the environment, and protect workers from rotating chains.
⚠️ Interchangeability has limits: only double-strand ANSI B29.1 chains are interchangeable in service. Single-strand coupling chains and silent chains differ in link and pin design and should not be mixed. Always verify the chain number and type before replacing parts.
2. Overall Dimensions and Hub Bores 📏
The Standard uses a uniform set of dimension symbols to define the coupling envelope (see Fig. 2, Overall Coupling Dimensions):
| Symbol | Meaning | Notes |
|---|---|---|
| A | Maximum length over both sprocket hubs | Including clearance between sprocket faces |
| B | Maximum diameter of chain coupling cover | Cover outside-diameter limit |
| C | Nominal diameter of clearance path around chain | Clearance space for chain articulation |
| D | Maximum diameter of sprocket hubs | Hub outside-diameter limit |
| E | Maximum sprocket hub depth | Applies where hub depths differ |
For bores and keyways, the Standard requires that finished bores, taper bore hubs, and keyways have dimensions and tolerances in accordance with the latest issue of the AGMA Bore and Keyway Standard for Flexible Couplings. The overall length of the cover shall not exceed dimension A.
💡 By referencing the AGMA bore and keyway standard rather than writing its own, B29.23M keeps chain couplings consistent with gear and steel couplings in the same family — simplifying purchasing and parts inventory.
3. Misalignment Capabilities and Installation 📐
The Standard stresses that couplings should be aligned as perfectly as possible, because any amount of misalignment will decrease coupling life. The values below are for guidance only:
- Parallel offset: when two shafts are not coaxial but their axes are parallel, the offset Y shall not exceed 2% of the chain pitch.
- Angular misalignment: when the axes are not coaxial and not parallel, angular misalignment should not exceed 1/2 deg for maximum life.
- Combined offset-angular misalignment: when the axes are neither parallel nor intersecting, the combined misalignment must be substantially less than the values above for maximum life.
| Misalignment Type | Limit | Notes |
|---|---|---|
| Parallel offset Y | ≤ 2% of chain pitch | Parallel axes, offset shafts |
| Angular | ≤ 1/2 deg (max life) | Non-parallel axes |
| Combined | Substantially less than above | Offset plus angular together |
🚨 Misalignment is a major cause of premature coupling failure. Although chain couplings tolerate more deviation than rigid couplings, do not treat them as an excuse to skip alignment: measure with a dial indicator or laser alignment tool at installation and re-check after the machine reaches operating temperature.
4. Covers 🛡️
A cover is optional, provided the operating speed does not exceed the maximum limit recommended by the manufacturer and safety codes are met. Covers may be made of any material with sufficient strength to withstand centrifugal force — typically plastic, aluminum, or sheet metal — and may be split either axially or radially. The cover shall have a feature to retain lubricant (for example, seals or an oil reservoir).
⚠️ Although the cover is described as optional, it is strongly recommended for occupational safety: a high-speed chain that breaks and flies out can cause serious injury. Even below the manufacturer’s speed limit, perform a risk assessment before omitting guarding.
5. Lubrication and Maintenance Practice 🛠️
- Lubrication method: chain couplings rely on oil or grease retained inside the cover to keep the chain joints wet; the cover must therefore retain lubricant.
- Service intervals: follow the manufacturer’s schedule to check lubricant level and replenish with clean lubricant free of corrosive additives.
- Inspection points: check for chain elongation, sprocket tooth wear, and cover seal condition; stop and inspect immediately on abnormal noise or temperature rise.
- Re-alignment: re-check parallel offset and angular misalignment after major maintenance, and confirm values are within the limits given in Section 3.
6. Frequently Asked Questions ❓
❓ Q: Which types of chain coupling does B29.23M cover?
A: Three types: double-strand roller chain, single-strand roller chain (special coupling chain), and silent chain. Only double-strand ANSI B29.1 chains are interchangeable.
A: Three types: double-strand roller chain, single-strand roller chain (special coupling chain), and silent chain. Only double-strand ANSI B29.1 chains are interchangeable.
❓ Q: Why is the parallel offset limit expressed as a percentage of chain pitch?
A: Because the misalignment capability of a chain coupling derives from the clearance in the chain joints, and pitch is the fundamental size parameter of the chain — expressing the limit as a pitch percentage lets it apply to all coupling sizes.
A: Because the misalignment capability of a chain coupling derives from the clearance in the chain joints, and pitch is the fundamental size parameter of the chain — expressing the limit as a pitch percentage lets it apply to all coupling sizes.
❓ Q: What materials are covers made of, and are they mandatory?
A: Covers are typically plastic, aluminum, or sheet metal and may be split axially or radially. They are optional when speed is within the manufacturer’s limit and safety codes are satisfied, but they are strongly recommended.
A: Covers are typically plastic, aluminum, or sheet metal and may be split axially or radially. They are optional when speed is within the manufacturer’s limit and safety codes are satisfied, but they are strongly recommended.
❓ Q: What governs the bore and keyway dimensions?
A: The latest AGMA Bore and Keyway Standard for Flexible Couplings governs standard finished bores, taper bore hubs, and keyways.
A: The latest AGMA Bore and Keyway Standard for Flexible Couplings governs standard finished bores, taper bore hubs, and keyways.
