High-strength bolts are connected, and equipment accidents often occur due to the impact of the quality and installation of mechanical parts or equipment bolts. Among them, the insufficient pre-tightening force of the bolts causes the bolts to loosen, the vibration of the machine parts or equipment to be excessive, and the bolts to shear fracture under long-term service, and eventually cause a large proportion of major accidents. The reason is that firstly, the quality of the bolt itself is not enough, and problems occur in the design and manufacturing process; secondly, the pretension force of the bolt is insufficient during installation and maintenance, and caused by vibration during operation.
In the case of bolt fracture failure, by examining the bolt's chemical composition, mechanical properties, metallographic structure, etc., it was found that various indicators were basically qualified, which eliminated the quality problem of the bolt. Excluding the special working conditions at the installation site, the focus of bolt failure is on installation, construction, commissioning and maintenance.
During the installation process, if the bolt preload exceeds the upper limit of the range, the bolt will plastically deform or even be broken; if the bolt preload is less than the lower limit of the range, a gap will be created between the two connected parts. During operation, especially for severe Under normal conditions, the stress of the machine is increased by about 95% than normal, causing brittle fracture of the bolt.
The tightness of the bolt is directly related to the service life of the parts or equipment. In view of the current situation of bolts, it is necessary to test the axial force of the installed bolts; and the large labor intensity and errors in the measurement of the pre-tightening force cause accidents of the bolts.
Several methods of axial force detection
Now, we are trying to apply some new methods to test the axial force of high-strength bolts to ensure the quality of the installation. The three new detection methods are briefly described below.
1. Calibration method
Nowadays, the so-called tension calibrator is the axiometer. In order to ensure the installation quality of high-strength bolted joints, a mobile axial force gauge for detecting axial force at the installation site is composed of a spoke pressure sensor and a digital display instrument.
The mobile axial force meter is mainly applicable to the installation site, and the pre-tightening force after screwing is compared with the torque applying tool of the field construction in order to timely correct the field torque. The calibration of the on-site screwing tools by the mobile axonometer provides a certain quality guarantee for the quality of the on-site construction.
Shandong Hanpu Machinery Industry Co., Ltd. is a professional power tool manufacturer. The produced axial force gauge is usually used to detect the axial force of torsional shear bolts. Tooling.
Axial force meter features:
Equipped with a digital display, you can change three units of force value.
Battery-powered, the power is 8 AA batteries, which can stand for 6 months continuously.
The integrated design effectively saves volume and weight, and can be carried to the scene.
The side is designed with a fixing device, which is convenient for fixing on the steel structure and can be detected at any time.
Attach the strain gauge to the measured object, and make it expand and contract with the strain of the measured object, so that the metal foil inside will expand or contract with the strain. Many metals change resistance when they are mechanically elongated or shortened. Strain gauges use this principle to measure strain by measuring changes in resistance.
The strain method is to use a strain gauge to form the working principle of a Wheatstone bridge, and then package it to form a bolt axial force sensor device. Then install the sensor between the bolt and the connected part, and place a gasket between the bolt and the sensor. The sensor is evenly stressed. Because the sensor is pressed by the bolt pre-tensioning force, the strain gauge inside the sensor changes, and displacement strain occurs. Strain is measured by the above principle method, and then the pretension force is calculated from the relationship between strain and stress.
3, ultrasonic measurement method
People have theoretically researched and explored the relationship between acoustic waves and stress for many years. In the 21st century, there are already bolt stress ultrasonic measuring instruments on the market. The ultrasonic stress measurement method can be directly measured, has high accuracy, and is simple to measure. Especially, the bolts in service can be tracked and inspected regularly, so it has developed rapidly.
The calculation principle of the ultrasonic stress detection method. First, measure the sound time required for ultrasonic conduction to go back and forth under stress and no stress; then use a torque machine and a tensile machine to compare with the ultrasonic stress meter. The results obtained by the measuring instrument are good. , The error is very small.
The ultrasonic stress measuring instrument can detect the bolt installation process and the current service bolt axial force detection, bolt in-service detection, and can monitor the bolt pre-tightening force of the machine at any time, which plays an important role in protecting the equipment or the safe operation of the vehicle .
In short, the calibration method is only applicable during the installation process; the ultrasonic measurement method is suitable for detecting the axial force of the bolt in service; the strain method is suitable for presetting during the installation of the bolt, and can be monitored online or stopped at any time during the operation. Each of the three methods has its application, which requires overall consideration by engineering and technical personnel, and a reliable method can only be selected for design.