Is The First Loose Bolt Not Easy To Break In Bolted Connection? Why?

There is a saying, “The first bolt that is loosened in a bolt connection is more difficult to break.” Do you know why? As a professional fastener manufacturer, Xinchi Fastener Company will share why the first loose bolt is not easy to break in the bolt connection.

First, it is necessary to find out the degree of loosening and what kind of load the bolt group bears, so as to determine whether the first loose bolt will break first. It can be discussed in three cases.

In case one, if it is only under static load, the bolt that is loosened first will not break first. If the bolt is loose, it means that the preload is smaller or zero. The smaller the preload is, the smaller the comprehensive working stress of the bolt under external load. The bolt with minimum working stress will not break first under static load conditions.

Now take the bolt connection in the flange connection as an example to see what will happen to the stress of the bolt if one bolt is loose. The following flange is connected by 8 M16 10.9 bolts, bolt numbers are shown in the figure below:

Normally, the same preload is applied to the bolt. If one bolt is loose, it means that the preload is reduced or no preload. Here, the preload of bolt 1 is set to 30KN, which is 68KN less than the preload of other bolts. When the flange is under the tension of 600KN, let’s see what the difference is in the force of the bolt.

The stress of bolts is shown in the following table:

According to the analysis results, due to the relaxation of bolt 1, the force distribution of all bolts is changed. The relaxed bolt bears less tension, while the two bolts next to the relaxed bolt (bolt No. 2 and bolt No. 8) bear more tension, which means that the internal stress is relatively large. If the load continues to increase, these two bolts will break first.

In case two, if the bolt group is subjected to a fatigue load and the bolt is completely loose (that is, there is no preload), the bolt will not be stressed even under the external load. The bolt next to it bears the force it should bear. Therefore, the bolt that is loose first will not break first, and the bolt next to it may break first.

Also, take flange connection as an example, one bolt is completely relaxed, that is, the preload is zero, and the other bolt’s preload is 98KN.

It can be seen from the above results that the completely relaxed bolt does not bear 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 tension variation range of each bolt is:

It can be seen from the fatigue characteristics of bolts that the greater the stress variation range of bolts, the earlier fatigue fracture occurs and the shorter fatigue life will be. Therefore, the two bolts immediately adjacent to the completely relaxed bolt will suffer fatigue fracture first, while the completely relaxed bolt will not break.

In case three, if only the preload of the bolt is reduced, then under the action of fatigue load, its comprehensive stress variation range will be larger, and fatigue life will be lower than the design life, so fatigue fracture may occur first.

Assuming a preload of 98KN for bolts, if one bolt is loose, it means that the preload has decreased or no preload. Here, set the preload for bolt 1 to 30KN, which is 68KN less than the preload for the other bolts. When the flange bears the alternating tension of 0 to 600KN, calculate the force and tension range of the bolt, as shown in the table below:

As can be seen from the calculation results in the above table, the force variation range of the incomplete relaxed bolt and its two adjacent bolts is relatively large, so the incomplete relaxed bolt and the two adjacent bolts on both sides will suffer fatigue fracture first.

From the above three points of analysis, we can know why in bolt connection, the first loose bolt is less likely to break. Since the load bearing of the loose bolt will be greatly reduced, the stress of the bolt next to it will be increased, so that the other bolts will be tired and break because of the heavy stress, but the bolt loose first will break later.

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