Fastener knowledge: methods to control bolt pre-tightening force

Bolts are one of the commonly used fasteners in machinery, and the pre-tightening force of the bolt is closely related to the performance. The pre-tightening force of the bolt is related to the tightness and reliability of the connected parts. Tightness will affect the quality of the connection. If the bolt pre-tightening force is too large, over-tightening will occur; if the bolt pre-tightening force is too small, the connection strength and quality cannot be guaranteed.

Let's look at the following methods for controlling screw preload:

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

The tightening force has a linear relationship with the bolt pre-tightening force. After controlling the tightening torque, the pre-tightening force value can be obtained through experiments or theoretical calculations. However, in practice, due to the influence of friction coefficient and geometric parameter deviation, under a certain tightening torque, the pre-tightening force changes relatively large, so the accuracy of controlling the bolt pre-tightening force through the tightening torque is not high, and the error is about ± 25%, up to ±40%. Generally speaking, the tools with higher tightening torque accuracy in the control area are torque wrenches and torque-limiting wrenches.

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

Calculate the corner of the nut according to the required pre-tightening force and measure the corner of the nut 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 rotations of the lu mother. The measurement accuracy of the nut rotation angle can be controlled within 10°-15°. (Guide: Knowledge of Fasteners: Methods of Controlling Bolt Pre-tightening)

Method 3. Control the pre-tightening force by 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 connected parts can be excluded. Therefore, high accuracy can be obtained by controlling the pre-tightening force by the bolt elongation. This method is widely used in the pre-tightening force control of the bolt connection in important occasions.

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

Use a hydraulic tensioner to apply tension to the bolts, extend the bolts, and then tighten the nuts. After the load is removed, the shrinkage of the bolts can produce a pre-tightening force equal to the tension in the connection. This method can improve the control accuracy of the pre-tightening force. There is no friction when the hydraulic tensioner applies pre-tightening force to the bolts, so this method is suitable for bolts of any size, and it can apply pre-tightening force to a group of bolts at the same time, and evenly compress the nuts and washers without tilting. Precise control of preload.

Method 5, use the angle of rotation to control the preload

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

The torque angle method to control the pre-tightening force is: first use the tightening torque to control the tightening process, until the tightening torque value is enough to ensure that the nut, bolt and the connected part are really tight, then can start measuring the nut rotation angle, and then Use the nut angle and tightening torque to control the tightening process at the same time. This method uses the information given by the tightening torque and the nut rotation angle, which can accurately control the pre-tightening force of the bolt, and can find the under-tightening or over-tightening phenomenon that may occur during the installation process.

More related stamping machinery processing industry news: