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What is the Force of the Threaded Connection During the Tightening Process?

Sep. 22, 2020

As a Rail Rubber Part Manufacturer, share with you.

1. Force in threaded connection

The force of the threaded connection during the tightening process:

When we tighten the bolt, the two parts are clamped, and the clamping force is equal to the tension. In fact, the clamping force is what we hope to obtain. This is the pre-tightening force of the threaded connection. Although this clamping force F is difficult to measure, we can control the pre-tightening force by measuring the tightening torque T.

Rail Rubber Pads

Rail Rubber Pads

2. Transformation of force

In the assembly process, the actual applied force is mainly used to overcome friction, but only a little over 10% is actually converted into clamping force, and about 90% of the energy is converted into friction. This is exactly why the bolt will not fall off by itself after the tightening is completed, and the transformation of the force is.

When the bolt is tightened, the bolt rotates until the bottom surface of the bolt head fits the surface of the connected part, and then the bolt starts to be tightened.

For hard connections, when the required torque is reached, that is, when the required clamping force is reached, the assembly process can be completed by only turning a small angle from the assembly surface to the tightening. The moment change rate is relatively large.

For the soft connection, even with the same specifications and dimensions, after the assembly surface starts to fit, it still needs to rotate many turns to tighten, indicating that the torque change rate of the soft connection is small.

The applied tightening torque mainly overcomes the frictional force, one is the friction force in the thread pair (approximately 40%), and the other is the friction force of the fitting end surface (approximately 50%). The tightening force is only about 10%, so the friction force has a great influence on the tightening torque. Therefore, whether the bolts are lubricated during assembly has a very obvious influence on the tightening force, and the bolts without lubrication will produce greater friction when tightening. In other words, if a certain clamping force is required, a larger torque is required when tightening bolts that are not lubricated. If the bolt is lubricated, the friction is reduced. If you continue to tighten the bolt with the same torque as the unlubricated bolt, the bolt may be broken.

In addition, the different surface treatments of fasteners and bolts also have a significant impact on friction. Compared with the surface blackening treatment, the friction coefficient of the former is obviously smaller, and the friction that needs to be overcome during the tightening process is also small. Therefore, when tightening the galvanized bolts of the same specification and grade at the same place, it should be blacker than the surface. It is correct to apply a smaller torque to the bolt.

Whether the supporting surface of the connected part contacting the nut and bolt head is processed smoothly has a great influence on the friction force. Therefore, the processing of the contact surface must be flat to ensure uniform contact. At the same time, the bolt hole axis should be perpendicular to the connection support surface during assembly and should not be inclined. Otherwise, the control of the tightening torque of the threaded connection will be very difficult, which will seriously affect the assembly quality.

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