(Last Updated On: March 1, 2023)

Bolts are one of the most commonly used fasteners in machinery, and the pre-tightening force of bolts has a close relationship with their performance. The bolt pre-tightening force is related to the tightness and reliability of the connected parts, the too-large or too-small pre-tightening force will affect the connection quality. If the bolt pre-tightening force is too large, overtightening will occur; if the bolt pre-tightening force is too small, the connection strength and quality cannot be guaranteed.

## What is the control bolt pre-tightening force?

The purpose of using bolts to connect parts is to make the two connected parts closely fit, and in order to withstand a certain dynamic load, there is also a need for sufficient compression force between the two connected parts to ensure reliable connection and normal operation of the connected parts. In this way, sufficient axial pre-tightening force(that is, axial tensile stress) is required after tightening.

## The method of controlling the bolt pre-tightening force:

### Method 1. Control the pre-tightening force by tightening the torque

The tightening force has a linear relationship with the bolt pre-tightening force, and the pre-tightening force value can be obtained through experiments or theoretical calculations by controlling the size of the tightening torque. But in practice, due to the influence of friction coefficient and geometric parameter deviation, under a certain tightening torque, the pre-tightening force changes greatly. Therefore, the accuracy of controlling the bolt pre-tightening force by tightening torque is not high, and its error is about ±25%, and the maximum can reach ±40%. Generally speaking, the tools with higher tightening torque accuracy in the control area are torque wrenches and limit wrenches.

### Method 2. Control the pre-tightening force through the nut rotation angle

Calculate the nut rotation angle according to the required pre-tightening force. When tightening, measure the nut rotation angle to achieve the purpose of controlling the pre-tightening force. The easiest way to measure the nut rotation angle is to engrave a zero line and measure the nut angle according to the number of times the nut turns. The measurement accuracy of the nut rotation angle can be controlled within 10°-15°.

### Method 3. Control the pre-tightening force through the bolt elongation

Since the elongation of the bolt is only related to the stress of the bolt, the influence of variable factors such as friction coefficient, contact deformation, and deformation of the connected parts can be excluded. Therefore, high precision can be obtained by controlling the pre-tightening force through bolt elongation, and this method is widely used in the pre-tightening force control of bolted connections on important occasions.

### Method 4. Control the pre-tightening force through the hydraulic tensioner

Use a hydraulic tensioner to apply tension to the bolts to elongate the bolts, and then screw the nuts. After the load is unloaded, a pre-tightening force equal to the tension can be generated in the connection due to the shrinkage of the bolts. This method can improve the control precision of the pre-tightening force. Hydraulic tensioners apply preload to bolts without friction. Therefore, this method is applicable to bolts of any size and can apply pre-tightening force to a group of bolts at the same time, compress nuts and washers evenly, and avoid inclination and affect the precise control of pre-tightening force.

### Method 5. Use the corner to control the pre-tightening force

Using the relationship between tightening torque and rotation angle to control the pre-tightening force is to apply a certain torque to the bolt, then turn the nut through a certain angle, and check whether the final torque and rotation angle meet the proper relationship, so as to avoid insufficient pre tightening or excessive tightening.

The torque angle method for controlling the pre-tightening force is as follows: firstly, the tightening torque is used to control the tightening process until the tightening torque value is sufficient to ensure that the nuts, bolts, and connected parts are really tight. At this time, the nut rotation angle can be measured, and then the nut rotation angle and tightening torque can be used to control the tightening process at the same time. This method uses the information given by the tightening torque and the nut rotation angle to accurately control the pre-tightening force of the bolt, and to find possible under-tightening or over-tightening phenomena during the installation process.

## In the end

There are many ways to control the pre-tightening force of bolts. Among the five methods mentioned above, controlling the pre-tightening force through a hydraulic tensioner is the most accurate method. If your project requires high accuracy of bolt preload, it is recommended that you purchase a hydraulic tensioner. If you are less strict, you can try the other four methods. TorcStark provides a one-stop bolting solution. Whether it is controlling bolt pre-tightening force, bolt tightening, or bolt removal, we have rich project experience and can provide you with professional solutions and product equipment.