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ASME B107.28-2005 — Electronic Torque Instruments
📟 ASME B107.28-2005 is the American National Standard dedicated to electronic torque instruments, replacing ASME B107.28M-1997. These instruments electronically indicate the applied torque through a transducer and digital display, and are widely used in aerospace, automotive, and precision assembly applications with strict fastening torque requirements. The Standard was developed under the B107 committee’s Subcommittee 3 — Torque Instruments, with major contributions from the Hand Tools Institute Torque Instruments Standards Committee. The principal change in this edition is the addition of type designators for different accuracy ratings; it was approved as an American National Standard on March 14, 2005.
💡 The key difference between an electronic torque instrument and a mechanical torque wrench is that it converts torque into an electrical signal via a strain-gauge or piezoelectric transducer and processes it digitally, offering peak-hold, tracking display, accuracy classes, and data readout — the core tool for digital torque quality control.
1. Scope and Accuracy Classification 📌
This Standard provides performance and safety requirements for manually operated electronic torque instruments with integral or interchangeable heads, including endurance, torque value ranges, and accuracy. It is not intended to describe products infrequently utilized or those designed for special purposes. Instruments are classified by accuracy:
- Type I: Electronic torque instruments, 1% accuracy.
- Type II: Electronic torque instruments, 4% accuracy.
“Accuracy” is defined as the allowable inaccuracy — also known as permissible deviation, tolerance, or error band; a higher value means a lower certainty of correctness. “Rated capacity” is the maximum intended torque the user shall measure or apply.
⚠️ The difference between Type I (1%) and Type II (4%) matters greatly in critical fastening. Match the accuracy class to the fastener torque tolerance: precision assembly (e.g., critical engine bolts) calls for Type I; general maintenance may accept Type II. Never trade away accuracy for lower cost.
2. Design and Display Requirements 🔩
Section 5 imposes specific design requirements on electronic torque instruments:
- Display: the instrument shall be capable of electronically indicating the applied torque through a transducer and digital display; the readout shall be visible at a viewing angle up to 45 deg from normal.
- Conversion time: when tested per para. 6.2, the maximum allowable digital conversion time shall not exceed 100 msec (minimum 10 readings per second).
- Power supply: it shall have an integral power supply meeting the requirements of para. 5.11 (built-in battery).
- Finish and coatings: exterior surfaces smoothly finished; sharp edges and corners capable of causing injury removed; corrosion-prone parts protected. Coatings shall be tightly adherent and uniform, and shall not peel, crack, or blister in normal use and storage. Cadmium plating shall not be used.
3. Drive Ends and Dimensional Requirements 🔧
Drives shall conform to the requirements of ASME B107.4M, Driving and Spindle Ends for Portable Hand, Impact, Air, and Electric Tools:
| Drive Size | Square (in/mm) | Maximum Rated Torque | Maximum Operating Load |
|---|---|---|---|
| 1/4 in square | 6.3 mm | 250 lbf·in / 30 N·m | 50 lbf / 225 N |
| 3/8 in square | 10.0 mm | 1200 lbf·in / 135 N·m | 125 lbf / 560 N |
| 1/2 in square | 12.5 mm | 3000 lbf·in / 340 N·m | 150 lbf / 670 N |
| 3/4 in square | 20.0 mm | 700 lbf·ft / 1000 N·m | 200 lbf / 900 N |
The square drive shall be oriented so that any two opposite flats are parallel within 5 deg of the longitudinal axis, with the drive ball located as shown in Fig. 1. Where a ratchet drive is incorporated, its operational characteristics shall conform to ASME B107.10M.
4. Operating Range, Resolution, and Accuracy 📊
The rated capacity shall not exceed the limit in Table 1, and the lowest indicated value shall be no greater than 20% of rated capacity. The range between the lowest and highest values is divided into equal increments; the size of each increment is the resolution and must keep readings legible. Accuracy requirements (para. 5.2) apply over the usable range of the transducer, and the instrument shall meet the error band of its Type across that range as verified by the Section 6 test procedures.
💡 Aim to measure within 20% to 100% of rated capacity. Below 20%, the transducer signal is small, signal-to-noise ratio drops, and relative error grows — a general rule for instrumentation, so avoid buying an instrument far larger than the working torque.
5. Engineering Practice Essentials 🛠️
- 🔍 Selection: determine the working torque range and tolerance; choose an instrument whose rated capacity covers the maximum torque and whose accuracy class (Type I/II) satisfies the requirement; match the drive size to your sockets and bits.
- 🔧 Use: calibrate and zero per the manual; keep the reading angle within 45 deg of normal; never use the instrument as a breaker bar or load it beyond rated capacity.
- 📋 Maintenance: charge or replace batteries and schedule recalibration; protect the transducer from drops, vibration, and moisture; power off when idle and remove batteries for long storage.
- 📊 Verification: periodically check accuracy with an electronic tester conforming to B107.29, and keep calibration records for traceability.
6. Frequently Asked Questions ❓
❓ Q: How do I choose between Type I (1%) and Type II (4%)?
A: Check the fastening torque tolerance band. For a narrow band (e.g., ±2% to ±3%), choose Type I so measurement uncertainty does not eat the tolerance; for a wide band (e.g., ±10%), Type II meets the need at lower cost.
A: Check the fastening torque tolerance band. For a narrow band (e.g., ±2% to ±3%), choose Type I so measurement uncertainty does not eat the tolerance; for a wide band (e.g., ±10%), Type II meets the need at lower cost.
❓ Q: Why does the Standard prohibit cadmium plating?
A: Cadmium is a toxic metal subject to environmental regulation; wear and processing of the coating pose health and environmental risks. The Standard mandates alternative corrosion-protection coatings instead.
A: Cadmium is a toxic metal subject to environmental regulation; wear and processing of the coating pose health and environmental risks. The Standard mandates alternative corrosion-protection coatings instead.
🔍 Q: Do electronic torque instruments need periodic calibration?
A: Yes. Transducers drift with use, impact, and aging. Establish a calibration interval (e.g., every 6–12 months or per usage) and verify accuracy with an electronic tester conforming to ASME B107.29, keeping records.
A: Yes. Transducers drift with use, impact, and aging. Establish a calibration interval (e.g., every 6–12 months or per usage) and verify accuracy with an electronic tester conforming to ASME B107.29, keeping records.
❓ Q: Why must the lowest indicated value not exceed 20% of rated capacity?
A: Below that range the transducer output is small and the signal-to-noise ratio poor, so relative error grows and the stated accuracy class cannot be guaranteed. For smaller torques, choose an instrument of lower rated capacity.
A: Below that range the transducer output is small and the signal-to-noise ratio poor, so relative error grows and the stated accuracy class cannot be guaranteed. For smaller torques, choose an instrument of lower rated capacity.
