Several methods for measuring the axial force of high-strength bolts

Several methods for measuring the axial force of high-strength bolts

In the case of bolt fracture failure, by checking the chemical composition, mechanical properties and metallographic structure of the bolt, it is found that the indicators are basically qualified, which excludes the quality problem of the bolt. Excluding the special working conditions of the installation site, the focus of bolt failure falls on the installation construction and commissioning maintenance.

During the installation process, if the bolt preload exceeds the upper limit of the range, the bolt will undergo plastic deformation or even be pulled off. If the bolt preload is less than the lower limit of the range, a gap will occur between the two connecting parts. During operation, especially under severe working conditions, the stress of the machine parts will increase by about 95% compared with normal conditions, resulting in brittle fracture of the bolt.

High strength bolt is a very important connecting part in equipment and engineering parts. The degree of fastening of bolts is directly related to the service life of the equipment. In view of the present situation of bolts, it is necessary to test the axial force of the installed bolts, and the large labor intensity and the error of the pre-tightening force measurement cause the bolt accident. Now began to try to apply some new methods to the detection of bolt shaft force, in order to ensure the quality of the installation.

1. Calibration method

Now commonly referred to as the tensile force calibration device is the axis force meter. In order to ensure the installation quality of the high-strength bolt connection pair, the mobile axial force meter used to detect the axial force at the installation site is composed of a spoke pressure sensor and a digital display instrument matched with it.

The mobile axial force meter is mainly applicable to the installation site, and the pre-tightening force after tightening is compared with the torque applying tool for on-site construction, so as to correct the on-site tightening torque in time. Through the calibration and verification of the on-site screwing tool by the mobile axial force meter, it provides a certain quality guarantee for the on-site construction quality.

2. Strain method

Stick the strain gauge on the measured object so that it stretches and contracts with the strain of the measured object, so that the metal foil inside stretches or shortens with the strain. Many metals change their resistance when they are mechanically elongated or shortened. Strain gauges apply this principle to measure the strain by measuring the change in resistance.

The strain method is to use strain gauges to form the working principle of a Wheatstone bridge, and then package it to form a bolt axial force sensor device, and then install the sensor in the middle of the bolt and the connected piece, and place a gasket between the bolt and the sensor to make The sensor is evenly stressed. Because the sensor is squeezed by the bolt pre-tightening force, the strain gauge inside the sensor changes, resulting in displacement strain. The strain is measured by the above principle method, and the preload is calculated by the relationship between strain and stress.

3. Ultrasonic measurement method

In theory, the relationship between sound waves and stress has been studied and explored for many years. In the 21st century, the ultrasonic measuring instrument for bolt stress has been on the market. Because the ultrasonic stress measurement method can be measured directly, and the accuracy is high, the measurement is simple, especially the bolts in the service state can be regularly tracked and tested, so the development is fast.
The calculation principle of ultrasonic stress testing method, first measure the sound required for ultrasonic conduction back and forth under stress state and without stress state, and then compare the torque machine and tensile machine with the ultrasonic stress meter respectively, the result of the measuring instrument is very good, the error is very small.

The ultrasonic stress measuring instrument can detect the bolt installation process, the current service bolt axial force and the bolt in service. It can monitor the bolt preload of the machine at any time and play an important role in protecting the safe operation of equipment or vehicles.

In short, the scope of application of the calibration method is only in the installation process; the ultrasonic measurement method is applicable to the detection of the axial force of the bolts in service; the strain method is applicable to the preset of the bolts during installation, and can be monitored online or stopped at any time during operation. Each of the three methods has its own applicability, which requires engineering technicians to consider the overall design and choose a reliable method.

Source: Golden Spider Fastener Web