According to statistics, in the final assembly tightening process of a certain brand of automobiles, more than 60% of the bolts are tightened by the torque method. In other non-precision manufacturing industries such as construction machinery, the torque method is more widely used. When the torque method is used for tightening, the bolts generally do not exceed the yield point.

As shown in the figure, the tightening area of the bolt is shown as the shaded area in figure, that is, below the yield point.

So, in the area of the tightening curve, what is the most suitable torque for tightening?

TorcStark will give a detailed introduction

Phase Analysis of Tightening Curves

The stages of the complete tightening curve are mainly divided into 5 stages: find the cap, screw in to fit, fit to yield, yield to the highest torque, and the highest torque to break. If you are not clear about the tightening stage, there is a detailed analysis in the previous column article.

The process point corresponding to the torque method is generally more suitable between the fitting and the yield stage. It will also be explained later: Why is the torque method not suitable for exceeding the yield point?

The figure above shows the difference in clamping force when tightening to 120Nm for different friction coefficient states. The difference in clamping force between fully lubricated (clamping force about 60KN) and original state (clamping force about 30KN) is up to 50%.

This is also the disadvantage of the torque method: the dispersion of the clamping force is greatly affected by the friction coefficient. What needs to be reminded here is that if the tightening position requires the clamping force accuracy, be sure to control the friction coefficient.

Moreover, when the friction coefficient is the upper-line value, it is necessary to consider whether the clamping force can meet the requirements of the minimum clamping force by tightening according to the set process torque.

The figure above shows the complete curve of the bolt screwed to fracture failure. Point A is the yield point of the bolt. If the torque method is used to tighten over the yield point, assuming that the torque is 180Nm, for this connection point, the maximum torque from the yield point to the maximum torque is only about 10Nm.

Because the clamping force is greatly affected by the friction coefficient, when the friction coefficient of the bolt is low, the tightening curve yields at 150Nm, and the maximum torque is 160Nm. Then, until the bolt breaks, the process torque of the tightening point cannot be reached.

Therefore, the torque method cannot achieve over-yielding tightening, and there is a large rotation angle from yielding to breaking, so the angle method is more suitable.

Appropriate torque value for torque tightening

How to evaluate whether the tightening torque value is appropriate?

The above picture shows a relatively complete tightening curve. Select the linear segment of the tightening curve, the lowest point of the linear segment is the fitting torque MJ, and the highest point of the linear segment is the linear maximum torque Mlin. When the tightening point does not exceed Mlin, the bolt is still in the elastic stage, that is, no irreversible deformation occurs.

For the torque method, it is also generally required that the tightened bolts can be reused. Therefore, the tightening point is recommended to be below Mlin. Taking into account the deviation of the clamping force accuracy of the torque method, so multiply it by the safety factor, that is 0.9*Mlin.

So what is the appropriate lower limit value? Considering that the bolts must have a certain utilization rate, if the utilization rate is too low, you can consider using bolts with smaller diameters and lower grades.

Comprehensive bolt elastic limit and utilization considerations:

The most suitable range of MT (process torque) is: 0.5*Mlin≤MT≤0.9*Mlin

In order to ensure sufficient clamping force, the fitting torque MJ is not recommended to be too high. The recommended range is: MJ≤0.5*MT

E.g:

The tightening process of a connection design is: 90Nm, according to the formula: 0.5 Mlin≤90Nm≤0.9*Mlin,

then 100Nm≤Mlin≤180Nm.

The linear maximum torque Mlin of the recommended tightening curve is between 100-180Nm, which not only ensures that the bolts can be used effectively, but also has a sufficient safety margin.

Fitting torque MJ: according to MJ≤0.5*90Nm, then MJ≤45Nm. It is recommended that the fitting torque MJ should not be higher than 45Nm to ensure insufficient clamping force.

The tightening process of a bolt tightening design is 8Nm, and the design requirement is the torque method. The above picture shows the curve of tightening to failure. The tightening process of this bolt is 8Nm, which is indicated by the arrow.

The fitting torque MJ is 2.4Nm≤0.5*MT=4Nm. According to 0.5Mlin≤8Nm≤0.9*Mlin, it is obtained that 8.89Nm≤Mlin≤16Nm, the Mlin of this tightening curve is 12.6Nm, and the process torque MT is suitable.

If you want to increase the clamping force, it is also suitable to set the process torque MT to 9-10Nm. You can calculate by yourself.

Подвести итог

The torque method is widely used in automobiles, construction machinery and other fields. The torque method is not suitable for tightening over the yield point. The fitting torque MJ of the tightening curve is recommended not to be higher than 0.5 times the process torque MT, and the process torque MT is recommended to be between 0.5 times the linear maximum torque (0.5Mlin) and 0.9 times the linear maximum torque (0.9Mlin).