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Bolt is one of the most commonly used standard parts in mechanical equipment. Bolt failure is also common in equipment. There are many reasons for bolt failure. One of the main reasons for unqualified bolt installation is over tightening or insufficient pre tightening force of bolts, which is caused by improper installation. Today, let's talk about how to tighten the bolts and the tightening methods.

In fact, the bolt tightening method mentioned today is not completely correct. The purpose of bolt tightening is to obtain an initial pretightening force. In some cases, the pretightening force of bolts is not obtained by tightening, but by direct drawing. Therefore, bolt tightening is more rigorous. What are the methods of bolt tightening?

1. tightening with torque control method

The purpose of bolt tightening is to obtain a certain pre tightening force before the bolt bears external load. The reason why the bolt should obtain a certain pre tightening force will be explained later. What is the principle of tightening the bolt with torque control method? The thread on the bolt is a helix. When the nut is tightened clockwise, the nut moves downward along the helix on the bolt, the distance between the nut and the hexagon head of the bolt becomes shorter, and the nut and the hexagon head bolt tighten the connected parts. When the nut is tightened continuously, the bolt is lengthened, resulting in bolt pre tightening force.

When the torque method is used to control the bolt tightening force, the friction coefficient of each contact surface of the coupling pair has a great impact on the bolt tightening force. For more important occasions, the torque control method also needs to monitor the rotation angle during tightening. If the rotation angle exceeds the reasonable range, it is necessary to evaluate whether the coupling pair has the possibility of failure in other aspects.

Generally, the surface roughness and lubrication of bolts tightened by torque control method are strictly controlled. The torque coefficient and the standard deviation of torque coefficient are specified in the project, that is, the torque coefficient must be within a certain range. If it is too large, it cannot be accepted.

According to jgj82-2011 technical specification for high strength bolt connection of steel structures and gb50205-2017 code for acceptance of construction quality of steel structures, the torque coefficient of high strength bolts shall be controlled within the range of 0.11 to 0.15, and the standard deviation of torque coefficient shall be controlled within 0.01. This can ensure that the pre tightening force of the bolt is within the design range when a certain tightening torque is applied to the bolt.

1. angle control method

The angle control method controls the pre tightening force of the bolt by controlling the tightening angle of the nut. We know that when the nut rotates on the bolt for one circle, the distance between the nut and the bolt head is close to or away from a thread pitch. When tightening, the thickness of the connected part changes little, and the bolt is stretched. According to the elastic theory, the product of the bolt elongation and the stiffness coefficient is the pre tightening force of the bolt.

When using the angle control method to tighten the bolt, the proportional relationship between the bolt elongation and the bolt preload is established only when the connected piece is fully fitted and the connected piece and the bolt do not have plastic deformation. Therefore, before controlling the angle, it is necessary to ensure that the connecting surface is tight. Therefore, an initial torque is required. The contact surface of the connecting piece is tightly fitted. After the initial torque, the angle is controlled, At the same time, in order to judge the quality stability in the tightening process, the torque is usually monitored, and the general torque is also within a certain range.

Angle control method and torque control method are commonly used in practical applications. The operation is relatively simple, and the tools are also commonly used.

Compared with the torque control method, the angle control method can control the bolt pretightening force in a relatively narrow range, and the pretightening force control is more accurate. Compared with the torque control method, the angle control method is more troublesome to operate. When each bolt is tightened, the rotation angle must be measured, and the efficiency is relatively low. The torque method only needs to set the torque value of the tightening tool.

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