How to control the preload of Stainless Steel Hex Cap Bolt

1. Use a torque wrench
Principle and operation: The torque wrench is the most commonly used tool for controlling preload. By setting the appropriate torque value, when tightening the bolt, when the set torque is reached, the wrench will send a signal to prompt the operator to stop. There is a certain relationship between torque and preload. According to the size, material and connection requirements of the bolt, the corresponding torque value can be found in the relevant standards or manuals. For example, for an M10 stainless steel hex cap bolt, a torque of 50-80N・m may need to be applied under normal connection conditions.
Accuracy and precautions: High-quality torque wrenches can provide higher accuracy, but they should be calibrated regularly during use. At the same time, pay attention to the tightening speed. Too fast tightening speed may cause inaccurate torque measurement, which in turn affects the control of preload.
2. Measure the elongation of the bolt
Principle and operation: The preload is controlled by measuring the elongation of the bolt before and after tightening. Special elongation measurement tools such as bolt extensometers can be used. According to the elastic modulus of the material and the size of the bolt, the elongation of the bolt when the required preload is reached is calculated in advance. During the tightening process, the elongation is monitored in real time. When the calculated value is reached, the expected preload is reached.
Application scenarios and limitations: This method is more applicable in some occasions where the preload accuracy is extremely high, such as the steel structure connection of large bridges and the assembly of high-end precision machinery. However, it requires accurate knowledge of the material properties and dimensional parameters of the bolt, and the measuring equipment is relatively complex and the operation requirements are high.
3. Use the angle method
Principle and operation: After the bolt is initially tightened, it is rotated at a certain angle to control the preload. Usually, the bolt is tightened to the fitting surface first to eliminate the gap, and then it is tightened at the specified angle. The size of the angle is determined by factors such as the diameter, length and material of the bolt. For example, for some high-strength stainless steel hexagonal head bolts, it may be necessary to rotate 90° - 120° after initial tightening.
Advantages and applicability: The angle method is more effective for controlling situations where the friction coefficient varies greatly, because it does not rely on the friction coefficient to determine the preload, and to a certain extent reduces the preload error caused by factors such as surface treatment and lubrication. It is often used in the assembly of automobile engines.
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