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Principles of fastener testing methods

October 9, 2023

There are two types of fastener torques: static torque and dynamic torque. Static torque: the instantaneous torque required to continue rotating a fastener in the tightening direction after it is fixed. Measure the torque after loading. Dynamic torque: the maximum peak value measured during the process of securing a fastener. Both torque wrenches and power tools can apply dynamic torque. Dynamic torque cannot be measured after the fastener has been tightened. The torque value obtained through online measurement during dynamic torque loading.


The detection torque for these two types of torques is the same. There are three main methods for testing the torque of fasteners: tightening method (increasing tightening method), marking method (marking method), and loosening method.


1. Tightening method (increasing tightening method)


The principle of tightening method (increasing tightening method): the torque wrench of tightening method (increasing tightening method) steadily applies force and gradually increases the torque (avoid impact). When the nut or bolt first produces slight rotation, its instantaneous torque value is the maximum (due to overcoming static friction). If it continues to rotate, the torque value will fall back to a brief stable state, and the torque value at this time is the torque obtained from the inspection.


Advantages of tightening method (increasing tightening method): simple operation, but must be proficient and experienced.


Disadvantages of tightening method (increasing tightening method):


a. there is a possibility of secondary tightening.


b. it is difficult to determine when the bolt will start, due to subjective factors.


c. when the bolt is started, the actual torque value should be approximately the torque value during installation, so the test value is generally too high.


Static torque: this method is used for testing residual torque and is suitable for detecting tightening quality on assembly sites.


2. Marking method (line marking method)


The principle of marking method (marking method): before inspection, draw a line between the head of the inspected bolt or nut and the connected body to confirm their original positions. Then loosen the bolts or nuts slightly, and tighten them to the original position using a torque wrench (align the lines at the scribed line). The maximum torque value at this time is multiplied by 0.9~1 The value obtained from 1 is the torque obtained from the inspection.


The advantages of marking method (marking method) are low technical level, cumbersome operation, and not suitable for fasteners with anti loosening function.


Disadvantages of tightening method (incremental tightening method): compared to tightening method (incremental tightening method), it is more accurate.


Dynamic torque: the torque given by the designer from a design perspective is called dynamic torque, so it is recommended to use this method when disassembling the vehicle.


3. Loosening method


The principle of the loosening method: use a torque wrench to slowly apply torque to the tested bolt or nut to loosen it, read the instantaneous torque value at the beginning of rotation, and multiply it by 1.1 to 1.2 based on experiments and experience to obtain the inspection torque value.


The advantages of loosening method: simple operation, but must be proficient and experienced.


Disadvantage of loosening method: low torque.