Bolt Torque Measurement Solution Based on CMCU-09A Wireless Strain Acquisition

1. Solution Overview
This solution employs the RunesKee wireless strain acquisition transmitter CMCU-09A in conjunction with 350–2HA half-bridge torque strain gauges to conduct torsional testing on M10 partially threaded bolts. The strain gauges collect micro-strain signals from the bolts under torsional loads in real-time, transmitting the data wirelessly to an upper computer for analysis and processing, ultimately achieving precise measurement of internal bolt torque.

The core value of this solution lies in directly measuring the internal torque actually borne by the bolt shank, as opposed to traditional torque wrench readings (which include frictional losses at the contact surfaces). By comparing the total torque applied by the torque wrench with the internal torque measured by the strain gauges, quantitative evaluation of frictional losses in bolted connections can be achieved, providing data support for tightening process optimization.

2. Measurement System Components
2.1 Measurement Equipment and Materials

2.2 Acquisition Device: RunesKee Wireless Strain Acquisition Transmitter CMCU-09A

The CMCU-09A is an industrial-grade device dedicated to strain signal acquisition and wireless transmission. Its key features include:
• Wireless Transmission: Comprising a wireless acquisition board (transmitter) and a wireless passthrough board (receiver), it supports automatic pairing upon power-up and enables one-to-one or one-to-many networking.
• Supports three strain gauge connection methods: 1/4 bridge, half-bridge, and full-bridge
• Communication Interface: Uses RS485 protocol for direct data reading via computer connection
• Application Scenarios: Suitable for sensor manufacturing, automotive industry, aerospace, rail transportation, biomedical engineering, civil engineering, and bridge construction, among others

2.3 Strain Gauge: 350–2HA Half-Bridge Torque Gauge

The 350–2HA is a half-bridge strain gauge specifically designed for torque measurement, with the following technical characteristics:
• Nominal Resistance: 350Ω
• Configuration: Half-bridge setup with two integrated sensing grids arranged at a 90° angle
• Mounting Orientation: Must be affixed at ±45° relative to the bolt axis
• Measurement Principle: Under pure torsion, the principal stress and strain on the bolt surface at 45° satisfy material mechanics relationships, and the strain gauge directly measures linear strain in this direction

2.4 Test Object: M10 Partially Threaded Bolt

The nominal diameter of the M10 bolt is 10mm. The shank portion (non-threaded area) of the partially threaded bolt is an ideal location for strain gauge mounting, as the stress distribution here is uniform, yielding more representative measurement results.

3. Strain Gauge Selection Guidelines
3.1 Choosing Between Half-Bridge and Full-Bridge Gauges
In torque measurement, both 2HA half-bridge gauges and full-bridge torque gauges can be used. While their measurement principles and conversion formulas are identical, their applicable scenarios differ:

Notes on measurement results: Under ideal conditions (constant temperature, proper bonding technique, and correct strain gauge setup), the measurement results of half-bridge and full-bridge strain gauges are theoretically consistent. Both reflect the linear strain in the 45° direction, and the conversion formulas are identical.

However, the following differences should be noted:

1. Temperature effect: Half-bridge gauges require additional temperature compensation measures; otherwise, ambient temperature changes may cause zero drift. Full-bridge gauges, with their four symmetrically arranged sensing grids, have self-compensating capability.
2. Lead wire resistance effect: Full-bridge gauges are less sensitive to changes in lead wire resistance, making them suitable for long-distance measurements. Selection recommendations: • For this short-term, destructive test, the 2HA half-bridge gauge fully meets the requirements. • If long-term monitoring of bolt torque is needed while equipment is in operation, it is recommended to use full-bridge gauges.

3.2 Selection of number of gauges to be applied
Either 1 or 2 2HA strain gauges can be used:
• 1-gauge solution: Apply one 2HA gauge on the smooth surface of the bolt shank to measure the shear strain at a single location, which is sufficient to obtain the torque value.
• 2-gauge solution: Apply two 2HA gauges at symmetrical positions (mirrored 180°) on the bolt, and take the average to cancel the effect of bending stress, resulting in more reliable data.

4. Experimental Study
4.1 Pre-experiment Preparation

1. Strain gauge application: Clean and mark the smooth surface of the M10 bolt shank, and apply 350–2HA strain gauges at ±45° directions;
2. Lead wire connection: Connect the strain gauge lead wires to the sensor terminals of the CMCU-09A acquisition board;
3. Connect the wireless transmission receiver board to the computer via USB;
4. Equipment debugging: Power on and pair the devices, confirm normal wireless communication. 4.2 Experimental Procedure

Step 1: No-load condition
• Fix the bolt on the screw torque-to-fracture testing machine, ensuring the screw is not subjected to any load; zero calibrate and record a stable baseline as the reference value for subsequent strain calculations.

Step 2: Torsional loading (tightening torque)
• Steps: Use a torque wrench to apply pure tightening torque to the screw (avoid generating lateral forces), and record the micro-strain data;
• Data analysis: Calculate the internal torque of the bolt at each time point using the conversion formula.

5. Experimental Data Analysis
First set of data:

Third set of data:

5.1 Test Result Data

5.2 Method for Converting Micro-strain to Torque
According to the principles of material mechanics, under pure torsional loading of the bolt, the relationship between the maximum shear strain on the surface and the torque is:

Where:

5.3 Simplified Calculation
Substituting typical parameters of M10 bolts and steel, the simplified formula is obtained:

That is: every 1000με ≈ 27.5 N·m

Example: If the strain gauge reading is 500με, then the internal torque of the bolt ≈ 500 × 0.0275 = 13.75 N·m

What is the actual internal torque corresponding to the "wrench torque" and "microstrain"? We can back-calculate using the measured strain-torque ratio (approximately 0.031 N·m/με).

Taking the maximum load as an example:

• Wrench reading 35 N·m
• Microstrain 1183 με
• According to the simplified formula M internal ​ = 1183 × 0.0275 ≈ 32.5 N ⋅ m

Note: This 32.5 N·m is the internal torque of the bolt calculated from the strain. The wrench reading is 35 N·m, and the two are very close! This indicates that under this experimental condition, the end face friction and thread friction are very small.

Under this experimental condition (pure torsion, no axial force), the wrench torque is almost entirely converted into the internal torque of the bolt, with extremely low friction loss. The data conforms to the theoretical formula.

5.4 Physical Significance of the Two

5.5 Why Are the Two Not Equal?
When tightening a bolt with a torque wrench, the total input torque is consumed in three aspects:
• Thread friction (approximately 40–50%): Converted into torsional deformation of the bolt (the measurement object of this experiment)
• Bearing surface friction (approximately 40–50%): Friction between the bolt head/nut and the connected parts, not transmitted to the bolt shank
• Effective preload (remaining portion): Converted into axial tensile force
Therefore, the torque wrench reading ≠ the internal torque of the bolt.

5.6 Advantages of This Solution
The traditional torque method cannot determine the actual torque borne inside the bolt, whereas this solution directly measures it through strain gauges, offering the following advantages:

1. Precisely obtain internal torque: Can quantitatively calculate the shear stress at the critical section of the bolt
2. Evaluate friction loss: By comparing the wrench reading with the internal torque, the friction coefficient can be calculated
3. Failure warning: Real-time monitoring of strain values, providing early warning when approaching the material's yield limit
4. Process recording: Wireless acquisition can fully record data throughout the entire process from loading to fracture

6. Application Scenarios
This measurement solution has significant application value in the following industrial scenarios:

6.1 Optimization of Tightening Process Parameters
During machine production and processing, this solution can be used to precisely determine the appropriate installation force. By establishing a relationship curve of "applied torque - internal torque - preload force," the optimal tightening parameters can be determined, avoiding loosening due to insufficient torque or bolt fracture due to excessive torque.

6.2 Bolt Material Quality Verification
For high-strength and high-safety scenarios (such as automotive chassis, wind turbine towers, and rail transit), it is essential to ensure that bolt materials meet design requirements. This solution enables direct measurement of the mechanical response of bolts under torsional loads, verifying whether the actual material performance meets standards.

6.3 Finite Element Analysis (FEA) Verification
After establishing a finite element model of the bolted connection, experimental data is required to validate the model's accuracy. The microstrain-internal torque measurement data provided by this solution serves as a benchmark for FEA model calibration and verification.

6.4 Bolt Fracture Failure Analysis
When bolts fracture during use, it is necessary to determine the failure mode (overload fracture, fatigue fracture, hydrogen embrittlement, etc.). This solution helps establish the mapping relationship between torque and stress, providing data support for failure analysis.

6.5 Intelligent Tightening Quality Monitoring
By combining torque-angle curves and strain data, an intelligent recognition model for tightening quality can be established to identify abnormal states such as normal tightening, floating locks, and tilting, enabling real-time warnings during the tightening process.

6.6 Full Lifecycle Torque Management
In applications requiring long-term monitoring, such as wind power and high-speed rail, a "digital twin assembly" system can be established using wireless strain acquisition technology, enabling torque data traceability and full lifecycle management for each bolt.

7. Experimental Precautions

1. Strain gauge bonding process: The surface of the elastic element must be cleaned and polished, and specialized adhesive should be used to ensure firm bonding and accurate alignment.

2. Zero-point calibration: Record initial values under no load to perform zero-point correction.

3. Wireless interference: Avoid use in environments with strong electromagnetic interference and ensure no metal obstructions between the transmitter and receiver.

4. Loading speed: Gradual step-by-step loading is recommended to record a complete torque-strain curve.

5. Safety precautions: For bolt torsion fracture experiments, be aware of potential splintering upon fracture and install protective covers.

6. Summary
   This solution, based on the RunesKee wireless strain acquisition transmitter CMCU-09A and the 350–2HA half-bridge torque strain gauge, achieves precise measurement of internal torque under torsional loads for M10 bolts. By distinguishing torque wrench readings (total torque) from strain gauge measurements (internal torque), it quantitatively evaluates frictional losses in bolted connections, providing a scientific basis for tightening process optimization, quality control, and failure analysis.

The solution is applicable to fields such as automotive manufacturing, rail transit, and wind power equipment, offering advantages like high measurement accuracy, data traceability, and convenient wireless transmission, making it an effective tool for bolt connection quality assurance.

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