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Is the First Loose Bolt in the Bolt Connection Not Easy to Break?

Oct. 09, 2020

The bolt that is loosened first in the bolt connection is more difficult to break. Do you know why? The following Railway Fastening System Exporter will share why the first loose bolt in the bolt connection is not easy to break.

First of all, we must figure out the degree of looseness and what kind of load the bolt group bears in order to judge whether the first loose bolt will break first. Discuss in three situations.

Bridge Bolt

Bridge Bolt

1. If it only bears the static load, the bolt that is loosened first will not break first. If the bolt is loose, it means that the pre-tightening force has become smaller or the pre-tightening force is zero. The smaller the pre-tightening force, the external load Under the action of the bolt, the smaller the comprehensive working stress, the bolt with the smallest working stress will certainly not break first under static load conditions.

Under normal circumstances, the bolt is applied with the same pre-tightening force. Assuming that the pre-tightening force of the bolt is 98KN, if one bolt is loose, it means that the pre-tightening force is reduced or there is no pre-tightening force. The pre-tightening force is set to 30KN, which is 68KN less than the pre-tightening force of other bolts. When the flange bears a 600KN tension, see the difference in the bolt force;

2. If the bolt group bears a fatigue load, the bolt is completely loose (that is, there is no pre-tightening force), even under the action of an external load, the bolt will not be stressed, and the bolt next to it bears the force it should bear, so it is loosened first The bolt will not break first, and the bolts adjacent to it may break first.

Taking the flange connection as an example, one bolt is completely loosened, that is, the pre-tightening force is zero, and the pre-tightening force of the other bolts is still 98KN.

It can be seen from the above results that the completely loose bolts do not bear the tension when the flange connection is subjected to external load, that is, the tension is zero.

When the flange is subjected to an alternating load of 0 to 600KN, the range of the tensile force of each bolt is:

It can be seen from the fatigue characteristics of bolts that the greater the range of bolt stress changes, the earlier the fatigue fracture will occur. And the shorter the fatigue life. Therefore, the two bolts next to the two sides of the completely loose bolt will fatigue fracture first, and the completely loose bolt will not break.

3. If the bolt only reduces the pre-tightening force, then under the action of fatigue load, its comprehensive stress variation range will become larger, the fatigue life will be lower than the design life, and fatigue fracture may occur first.

From the above three points of analysis, we can know why in the bolt connection, the bolt that is loose first is not easy to break. Because the bearing force of the loose bolt will be greatly reduced, the force of the next bolt will increase, so that other bolts will fatigue and fracture due to the large force, but the first loose bolt will break later.

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