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ASME B5.26-1950 — Involute Serrations
⚙️ ASME B5.26-1950 is the early American National Standard for 45-degree pressure angle involute serrations. It grew out of a 1945 review of the SAE Serrated Shaft Standard, which disclosed inconsistencies — most notably that many serrations, especially those generated by hobbing, had an involute form even though they were treated as straight-sided. This forced the decision to develop a dedicated 45-degree involute serration standard to replace the old SAE standard. The work was done by Sectional Committee B5, Technical Committee No. 13 (Standardization of Splines and Splined Shafts); the proposed standard was approved on August 29, 1949, and designated an American Standard on March 22, 1950.
💡 The essential difference between serrations and ordinary splines: a steeper pressure angle (45°), finer and denser teeth, and shallow tooth depth — ideal for light-duty torque transmission, precise angular indexing, and adjustment, e.g., steering columns, instrument mechanisms, and a wide range of shaft-to-hub connections.
1. Key Features of the Standard 📌
Compared with the old SAE standard, B5.26 introduced three major improvements:
- Even diametral pitches: a series of even pitch pairs — 32/64, 40/80, 48/96, 64/128, 80/160 — slightly coarser than the old SAE series overall, but covering a far wider range of shaft diameters, offering many more size selections from small to large.
- Allowable errors and effective fits: borrowed from the involute spline standard, the concepts of allowable errors (accumulated pitch error, profile error, lead error, out-of-roundness) and effective fit were incorporated, so unavoidable manufacturing errors are accounted for in fit calculation and true interchangeable assembly is ensured.
- Complete measurement system: basic measurements over pins and between pins are tabulated; pin measurements are given as maximum for the internal member and minimum for the external; new tables for maximum tooth space and minimum tooth thickness let every fit be calculated.
⚠️ Note: in the twin-number pitch notation (e.g., 40/80), the first number is the nominal diametral pitch used for size designation, and the second is the actual diametral pitch used for tooth-form and depth calculations. Always follow the full tabulated parameters of the standard — never infer dimensions from the designation number alone.
2. Diametral Pitches and Size Series 🔧
The standard is built around the even diametral pitch series. The table below summarizes the pitch pairs and their typical application ranges:
| Pitch Pair (Nominal/Actual) | Tooth Density | Typical Shaft Diameter | Typical Application |
|---|---|---|---|
| 32/64 | Coarser, deeper teeth | Larger shafts | Medium shaft-to-hub torque joints |
| 40/80 | Medium | Medium shafts | General adjustment and drive joints |
| 48/96 | Finer | Small-medium shafts | Instruments, precise angular indexing |
| 64/128 | Fine, shallow teeth | Small shafts | Compact mechanisms, micro-adjustment |
| 80/160 | Finest | Very small shafts | Miniature instrument connections |
At the request of the National Aircraft Standards Committee (NASC), the size ranges of 40/80, 48/96, 64/128, and 80/160 were extended, and the 32/64 range was moved downward to agree with the coarser pitches. The series forms a continuous, gap-free coverage of shaft diameters, so designers can interpolate smoothly from small to large sizes.
✅ Selection workflow: set the shaft diameter and (light) torque → look up the pitch pair and tooth count in the standard’s tables → choose tooth space/thickness per the fit type → verify manufacture and acceptance with pin measurements. Complete adherence guarantees interchangeability.
3. Fit Types and Measurement Control 📐
Consistent with the contemporaneous involute spline standard (ASA B5.15-1950), serration fits are classed by centering method — major diameter fit, side (tooth flank) fit, and minor diameter fit — each with several fit classes. The controlling inputs are:
- Basic tooth space / basic tooth thickness: the basic geometry defined by pitch and tooth count.
- Minimum effective space / maximum effective thickness: effective values after adding the error allowance — the true criterion for interchangeable assembly.
- Pin measurement: external serrations are checked with an over-pin measurement (pins in opposite tooth spaces); internal serrations with a between-pin measurement. The standard gives maximum for the internal member and minimum for the external as the acceptance direction.
🚨 In production control, the external member’s tooth thickness may only be reduced from the maximum direction and the internal member’s tooth space only enlarged from the minimum direction; once out of range, the designed fit can no longer be achieved. Select high-precision cylindrical pins of the standard diameter and seat them firmly in the tooth spaces when measuring.
4. Engineering Practice Essentials 🛠️
- 🔍 Manufacturing methods: external serrations are typically hobbed or form-milled; internal serrations are broached or shaped. Hobbed forms are naturally involute and fully agree with the standard’s geometry.
- 🔧 Fit design: prefer side (flank) centering for light-load and adjustment duties; for higher coaxiality, consider major/minor diameter centering with appropriately tighter tolerances.
- 📊 Inspection: use over-pin/between-pin micrometers for size; for indexing uniformity use a pitch checker or a composite gage (see B5.31, Involute Spline and Serration Gages and Gaging).
- 📋 Heat-treatment distortion: carburizing and hardening distort serrations; finish-machine after heat treatment or leave grinding allowance, and make final measurements on the finished part.
5. Frequently Asked Questions ❓
❓ Q: Why do serrations use a 45-degree pressure angle?
A: The 45° pressure angle balances radial force against tangential drive and self-centering, and with shallow teeth it yields a fine, dense tooth form — a compact joint with fine angular adjustment for light loads, the classic choice for instruments and adjustment mechanisms.
A: The 45° pressure angle balances radial force against tangential drive and self-centering, and with shallow teeth it yields a fine, dense tooth form — a compact joint with fine angular adjustment for light loads, the classic choice for instruments and adjustment mechanisms.
❓ Q: What does a notation like “40/80” mean?
A: The first number (40) is the nominal diametral pitch used for size designation; the second (80) is the actual diametral pitch used for tooth-form and depth calculations. The same rule applies throughout the even pitch series.
A: The first number (40) is the nominal diametral pitch used for size designation; the second (80) is the actual diametral pitch used for tooth-form and depth calculations. The same rule applies throughout the even pitch series.
❓ Q: Can serrations and splines be interchanged?
A: No. Serrations (45°, fine and shallow) and splines (30° or 37.5°, coarser teeth) differ completely in geometry and must be designed and made to their own standards; choose by transmitted torque, loading nature, and available space.
A: No. Serrations (45°, fine and shallow) and splines (30° or 37.5°, coarser teeth) differ completely in geometry and must be designed and made to their own standards; choose by transmitted torque, loading nature, and available space.
🔍 Q: How is the serration fit controlled?
A: Establish basis tooth space/thickness, add the error allowance to obtain effective values, then control the actual size direction with over/between-pin measurements (external only decreases, internal only increases) so the assembly achieves the designed effective fit.
A: Establish basis tooth space/thickness, add the error allowance to obtain effective values, then control the actual size direction with over/between-pin measurements (external only decreases, internal only increases) so the assembly achieves the designed effective fit.
