Fastener Thread Tightening Principle II

  1. The relationship between tightening torque and tightening axial force

When force is applied to the nut, a moment is generated. Torque: T=F*L;


The tightening torque consists of two parts:

1) Bearing surface friction torque Tw;

2) Thread pair friction torque Ts;

In the elastic zone, the relationship between the tightening torque and the tightening axial force (pre-tightening force) is as follows:

T=Ts+Tw=K Ff d



Among them, K: torque coefficient, which is related to the roughness and lubrication of the bearing surface, the accuracy of the thread pair and the lubrication.


5. Factors affecting preload

When tightening with the same torque:

I. If the friction coefficient increases and the K value becomes larger, the pre-tightening force Ff is insufficient;

II. When the friction coefficient decreases and the K value decreases, the pre-tightening force Ff increases, which can lead to failure of the threaded connection.


6. The essence of tightening fasteners

Fasteners are tightened with threads, and the essence of tightening is to control the tightening axial force (pre-tightening force) of the threads to ensure the clamping force required by the connected parts.

The clamping force needs to be moderate: too small a clamp is not tight, the connected part is easy to loose; too large a clamp, the connected part is easy to damage, and at the same time, it is easy to cause damage to the threaded part.

I. The lower limit of the axial preload: determined by the function of the connecting structure, this value must ensure that the connected parts always fit reliably during the working process;

II. The upper limit of the axial preload: it is determined by the strength of the bolt (nut) and the connected part. This value must ensure that the bolt and the connected part will not be damaged during the pretension and service process.

(E.g. bolts elongated, unscrewed, tripped, compressed/ruptured by the connected parts, etc.)