Experienced inspection engineers can quickly determine the qualified tightening force of a bolt according to the specification level of the bolt. Below, we take the common tightening force of seat bolts in the automobile industry as an example to see how much tightening force of bolts with common dimensions and specifications should reach.

Of course, the most intuitive way to understand the fastening force is still to do the corresponding test, generally need to prepare 20 groups of corresponding fasteners to make test samples. The dimensions of the sample shall cover all dimensions of the bolt. The test environment should be constantly changing under various torsional limits. And each set of time can only be used for one test and can not be repeated measurement of torque, so that the data can be the most convincing,

In the process of testing the value,

Minimum torque of tightening: Ta* proportional coefficient of fixed point torque value (generally 1.2 for plastic parts, 1.3 for metal parts)

Maximum torque of tightening: slip tooth torque value Tb* proportional coefficient (generally 0.5 for plastic parts and 0.8 for metal parts)

Tightening torque: Calculate the +3sigma value of the screwing point and the -3sigma value of the yield point with statistics to obtain the torque interval, and then select the tightening torque according to the torque tolerance control ability (generally select the middle value of the torque interval).

The tightening torque value of bolts and nuts under the law is calculated by empirical method in the drawing stage. After OTS parts are produced, joint test of threaded fasteners is still needed to verify whether the torque is reasonable.

Similarly, this method is often used to determine the final tightening torque.

According to VDI2230 standard, the calculation formula of tightening torque is as follows: T=FM[0.16*P+0.58*d2*uG+(Dkm*uk/2)]FM: assembly preload, P: pitch, d2: diameter of screw thread, uG: friction coefficient of screw thread, Dkm: effective diameter of frictional torque at screw head, uk: Friction coefficient of screw head support surface

Wherein, the formula for calculating assembly pretightening force is FM=V*Rp02*A0V: utilization factor of yield point stress in tightening process, usually V=0.9Rp02: according to DIN Eniso898-1,0.2% elastic limit stress of screw (yield strength) A0: minimum cross-sectional area of screw

Using calculation method to determine torque involves the determination and calculation of friction coefficient of different materials and friction coefficient of thread and Angle. This process is very complicated, and the selection of many factor coefficients will also affect the final result of torque value.