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ASME B29.2M-2007 (2017) — Inverted Tooth (Silent) Chains and Sprockets
🧩 ASME B29.2M-2007 (2017) is the American National Standard for inverted tooth (silent) chains and their sprockets. It establishes chain link data, chain widths and sprocket face dimensions, sprocket tooth form, sprocket diameters, over-pin diameter tolerances, and maximum hub diameters for the 3/8 in. pitch and larger series and the 3/16 in. miniature series, with both U.S. customary and metric (mm) tables. The Standard’s roots lie in standardization work of the late 1930s and early 1940s by the Silent Chain Division of the Association of Roller and Silent Chain Manufacturers (ARSCM, now the American Chain Association). Because individual manufacturers used various chain joint designs, the standard was limited to ensuring interchangeability of chains on sprockets of any number of teeth. At a September 1944 meeting of all major American silent chain manufacturers under ARSCM auspices, “Engineering Standards for Industrial Silent Chain and Sprockets” were agreed upon; sprocket design formulas were expanded into tables approved in April 1945, and supplementary cutter information was added to form the comprehensive report Industrial Standards for Industrial Silent Chains, Sprockets, and Cutters. After refinements in 1949 and 1950, the Sectional Committee referred the report to ASME and SAE, and it was accepted by the American Standards Association on November 9, 1950, as American Standard B29.2. A 1957 revision served for many years and was reaffirmed in 1971. Meanwhile, a recommended standard for “small pitch” silent chain (pitch less than 3/8 in.) became American Standard B29.9 in 1958; its only developed size, 3/16 in., was incorporated into the basic standard in the 1982 revision. In 2006 the B29 Committee reviewed B29.2M, deleted the 1-1/4 in. pitch and some uncommon chain sizes, revised the side-guide and center-guide illustrations and the service-factor and horsepower-rating tables, and corrected the formula for G (max.) from the previous errata sheet.
💡 The “silence” comes from the engagement principle: the tooth faces of the link plates slide against the sides of the sprocket teeth instead of rolling impact, giving low noise, smooth running, many load-carrying plates per chain, and high power capacity — the classic choice for high-speed, high-precision power transmission.
1. Chain Construction and Standard Framework 📌
An inverted tooth chain is made of multiple pinned link plates whose outside faces carry teeth that mesh with the sides of sprocket teeth (hence “inverted tooth”). Chains are classified by guiding form:
- Center-guide chain: a central plate with a guide tooth engages a center groove in the sprocket; best centering, preferred at high speed.
- Side-guide chain: the outer link plates guide; simpler construction, common for wide chains and multi-strand arrangements.
The Standard has two series: 3/8 in. pitch and larger (the power-transmission main series) and 3/16 in. pitch (the miniature series originating from B29.9). Each series provides link data (in./mm), chain widths with sprocket face dimensions, tooth-form figures, diameter tables, and over-pin diameter tolerance tables.
⚠️ The primary goal is interchangeability on sprockets, not a full prescription of chain details. Manufacturers may keep different link plate profiles and joint designs as long as the chain engages the standard sprocket tooth form correctly. For replacement, match pitch, chain width, and guide type.
2. Chain Link Data and Width Series 📊
Tables 1 and 2 (in. and mm) give chain link data; Tables 3 through 6 give sprocket face dimensions for each pitch/width. Chain width is graded in inches (e.g., 1/4, 3/8, 1/2, 3/4, 1 in., etc.), and capacity grows with width:
| Item | 3/8 in. Pitch and Larger | 3/16 in. Pitch |
|---|---|---|
| Chain width series | Multiple widths (Tables 3/4) | Per Tables 5/6 (in./mm) |
| Sprocket tooth form | Fig. 6 tooth form | Fig. 7 miniature form |
| Diameter tables | Fig. 8 (e.g., Table 12) | Fig. 9 (Tables 13/14) |
| Over-pin diameter tolerance | Tables 7/8 (in./mm) | Tables 9/10 (in./mm) |
Over-pin diameter is an indirect check of sprocket tooth thickness: two standard pins are placed symmetrically in opposite tooth spaces and the distance over them is measured, verifying cumulative pitch error. A maximum hub diameter table for sprockets of unity pitch supports shaft design.
💡 Width sets power: select by reading the “horsepower rating per inch of chain width” at the design speed, then multiply by width for allowable transmitted power. Widening the chain at the same pitch is the most direct way to increase capacity.
3. Sprocket Tooth Form and Diameters ⚙️
The Standard provides tooth-form data on a unity-pitch basis, converted by formula for different tooth counts:
- Tooth elements: pressure angles, tooth space shape, tip and root radii, and profile corrections are set by figures and formulas for smooth sliding engagement of plate faces against tooth sides.
- Working diameters: pitch diameter from pitch and tooth count; tip diameter, root diameter, and pin-measurement diameters are taken from the standard tables.
- Hub limits: a maximum hub diameter is given for unity-pitch sprockets to avoid interference with the link plates; combine with keyway strength checks when sizing the bore.
🚨 Tooth-form precision directly controls noise and life: pitch error and tooth-thickness deviation cause plate impact and climbing. Verify with over-pin diameter tolerances, and control tooth surface finish and heat-treatment hardness, or high-speed noise will deteriorate rapidly.
4. Service Factors and Drive Selection 📈
Nonmandatory Appendix A supplies service factors by service class and horsepower ratings per inch of chain width. The basic selection flow:
- Identify driver and load type and read the service factor;
- Compute design power = nominal power × service factor;
- Read the horsepower-per-inch rating at the design speed and determine the required width;
- Determine chain length and number of pitches from center distance and tooth counts, and check sprocket diameters against space.
💡 For high-speed duty prefer a small pitch with a center-guide chain: small pitch gives high engagement frequency but lower impact energy, and center guiding improves centering. The 2007 edition deleted the 1-1/4 in. pitch, so use the current tables.
5. Engineering Practice Essentials 🛠️
- Installation alignment: keep sprocket face alignment deviation small; silent chains are sensitive to axis parallelism, and deviation worsens side friction on the plates.
- Tension control: maintain a moderate slack-side tension and sag; too tight causes abnormal joint wear, too loose causes skipping; use an idler when needed.
- Lubrication: for high-speed applications use spray or drip lubrication so the pinned joints receive adequate oil film.
- Inspection: check tooth thickness by over-pin measurement and tooth-form templates; inspect plate wear and pin-hole elongation.
6. Frequently Asked Questions ❓
❓ Q: Why is the inverted tooth chain called “silent”?
A: Because the link plate faces slide against the sprocket tooth sides instead of rolling impact, engagement is smooth and noise is significantly lower than roller chain, hence the name “silent chain.”
A: Because the link plate faces slide against the sprocket tooth sides instead of rolling impact, engagement is smooth and noise is significantly lower than roller chain, hence the name “silent chain.”
❓ Q: How do I choose between center-guide and side-guide chains?
A: Choose center-guide for high speed, precise centering, and low noise; choose side-guide for simpler construction, wide chains, or tight retrofit spaces.
A: Choose center-guide for high speed, precise centering, and low noise; choose side-guide for simpler construction, wide chains, or tight retrofit spaces.
❓ Q: Where does the 3/16 in. miniature series come from?
A: It originated as American Standard B29.9 (small-pitch silent chain) in 1958 and was incorporated into B29.2M in the 1982 revision for instrument and light miniature drives.
A: It originated as American Standard B29.9 (small-pitch silent chain) in 1958 and was incorporated into B29.2M in the 1982 revision for instrument and light miniature drives.
❓ Q: What changed in the 2007 edition versus 1982?
A: The 1-1/4 in. pitch and several uncommon widths were deleted, the center/side guide illustrations and service-factor and horsepower tables were revised, and the G (max.) formula errata was corrected.
A: The 1-1/4 in. pitch and several uncommon widths were deleted, the center/side guide illustrations and service-factor and horsepower tables were revised, and the G (max.) formula errata was corrected.
