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Does the Surface Treatment Affect the Bolt Tightening Characteristics?

Sep. 07, 2020

In order to reduce the dispersion error of the bolt torque coefficient and improve the corrosion resistance, the fasteners are usually surface treated. However, different surface treatments have a great impact on the friction coefficient of threaded fasteners, and ultimately affect the specific tightening of the bolts. The following Tram Fastening Manufacturer leads everyone to analyze the effect of surface treatment on bolt tightening characteristics.

Hex Bolt

Hex Bolt

Theoretical analysis of the influence of friction coefficient on bolt torque coefficient:

1. Tightening torque of bolt connection

1.1 Friction in the triangular bolt spiral pair

The friction of the moving pair can be divided into plane friction, inclined surface friction and groove surface friction according to the shape of the contact surface. In order to simplify the calculation of the friction force of the moving pair, regardless of the geometric shape of the two moving pair elements of the moving pair, the contact of the two members with different geometric shapes can be regarded as the moving pair contacting along a single plane, and the friction force is calculated The formula is expressed as the unified calculation formula shown in the formula:

Groove surface friction: The movement of the nut on the screw can be approximately regarded as the movement of the wedge-shaped slider on the inclined groove surface, that is, the combined form of groove surface friction and inclined surface friction. At this time, the angle of the groove surface is equal to 90° -β.

1.2 Tightening torque of bolts

The total torque required during the bolt tightening process consists of two parts: the tightening torque to overcome the friction of the thread pair and the friction torque between the bolt head or nut and the supporting surface.

2. Torque coefficient of bolt connection

The total torque of bolts is divided into three parts, namely, friction consumption of bolt support surface, thread friction consumption and pre-tightening force consumption.

It can be seen from Table 1 that during the tightening process, the energy consumed by the friction of the bolt support surface is about 50%, the thread friction is about 40%, and the pre-tightening force is about 10%. Under the same tightening torque, when the friction coefficient changes by 0.05, the pre-tightening force changes as high as 43.1%. That is to say, if the surface treatment of the bolt is slightly different, assuming that the friction coefficient is increased by 0.05, the axial pre-tightening The force is only 57% of the original force, which will bring major safety hazards to the reliability of the bolt connection. Therefore, we must pay full attention to the study of the friction coefficient of the thread pair.

Analysis of the influence of surface treatment on torque coefficient

Through the multifunctional bolt tightening analysis system, the clamping force, total torque and torque on the thread pair can be measured during the bolt tightening process, and the relationship between the clamping force and the torque can be accurately reflected in real time, and the bolt thread can be measured at the same time Coefficient of friction with the bearing surface of the bolt head. After data analysis, it can be concluded that the degree of galvanization has little effect on the friction coefficient of the bolt head, but it has a greater impact on the thread friction coefficient, and ultimately has a greater impact on the torque coefficient.

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