Fastener Thread Tightening Principle I

The tightening of threaded fasteners is not what we imagined. In fact, there are many ways to fasten threaded fasteners, but the simplest method is to use a hand wrench tool to tighten. Although this tightening method is easy to operate, it is not suitable for high-light or important connection fasteners. Some principles of fastener screw tightening are listed below.

1. The principle of tightening fastener threads

1. Thread principle

Look at the picture below, when the line with an inclination angle β moves around a cylinder or cone, a spiral line is formed on the cylinder or cone. The grooves and protrusions produced along the spiral are the threads. The outer thread of the cylinder is called the outer thread. The inner thread of the cylinder is called the internal thread. The thread is used simultaneously in the inner and outer groups.


The main parameters of the thread:

1) Pitch P: The distance between the two thread centers.

2) Lead L: The distance that a point on the thread advances along the axis direction when the thread rotates once.

Note: The thread produced by a helix is called a single-head thread, and the pitch is equal to the lead. The thread produced by two or more equally spaced spirals is called a multi-start thread.



2. Tighten the thread

Tightening is to connect the parts with bolts and nuts, which cannot be loosened. In order to prevent it from loosening, an appropriate pulling force (tightening force) called "axial force" must be applied to the bolt.

In actual operation, because it is difficult to directly measure the axial force when tightening bolts and nuts, use "torque" instead.


3. Force in threaded fasteners

When the threaded fastener is tightened, its force has three aspects:

1) Axial force F: The axial tension acting on the external threaded part (the clamping force of the connected part);

2) Bearing surface friction Fw: the friction between the nut or bolt and the surface of the connected part;

3) Thread pair friction Fs: the friction between the side of the external thread and the side of the internal thread.


Under normal circumstances, the three force ratios are: