Principle And Characteristic Analysis Of Fastener Torque Testing Methods

Fastener torque is divided into two types: static torque and dynamic torque. Static torque: the instantaneous torque required to continue rotating the fastener in the tightening direction after it is fixed, and the torque needs to be measured after loading. Dynamic torque: the maximum peak torque measured during the process of fixing the fastener; both torque wrenches and power tools can apply dynamic torque, and dynamic torque cannot be measured after the fastener is tightened, and must be measured online during the dynamic torque loading process.

The detection standards for the two types of torque are consistent. There are three main commonly used fastener torque testing methods: tightening method (re-tightening method), marking method (line marking method) and loosening method.

1. Tightening Method (Re-tightening Method)

Principle of the tightening method (re-tightening method): Use a torque wrench to apply force stably and gradually increase the torque (impact is strictly prohibited); when the nut or bolt starts to produce slight rotation, its instantaneous torque value is the largest (because it is necessary to overcome static friction); after continuing to rotate, the torque value will drop to a short-term stable state, and the torque value at this time is the torque obtained from the test.

Advantages of the tightening method (re-tightening method): Simple operation, but the operator needs to be skilled and experienced.

Disadvantages of the tightening method (re-tightening method): a. There is a possibility of secondary tightening; b. It is difficult to determine the timing when the bolt starts to rotate, and there is subjective judgment error; c. When the bolt starts, the measured torque value is slightly higher than the actual torque value during installation, so the test result is generally on the high side.

Application scenario: This method is used for testing static torque (residual torque) and is suitable for testing the tightening quality on the assembly site.

2. Marking Method (Line Marking Method)

Principle of the marking method (line marking method): Before inspection, first draw an aligned line on the head of the inspected bolt or nut and the connected body to confirm their original positions; then slightly loosen the bolt or nut, and then use a torque wrench to tighten it to the original position (the marked lines are aligned); multiply the maximum torque value measured at this time by 0.9~1.1, and the result is the test torque value.

Advantages of the marking method (line marking method): Compared with the tightening method (re-tightening method), the test result is more accurate; it has low requirements on the technical level of the operator.

Disadvantages of the marking method (line marking method): The operation is relatively cumbersome, and it is not suitable for fasteners with anti-loosening function.

Application scenario: The torque given by designers from the design perspective is dynamic torque, so this method is recommended for vehicle disassembly inspection.

3. Loosening Method

Principle of the loosening method: Use a torque wrench to slowly apply torque to the inspected bolt or nut to gradually loosen it, and read the instantaneous torque value when the bolt or nut starts to rotate; multiply the instantaneous torque value by a coefficient of 1.1~1.2 according to test data and practical experience, and the result is the test torque value.

Advantages of the loosening method: Simple operation, but the operator needs to be skilled and experienced.

Disadvantages of the loosening method: The measured torque value is generally on the low side.