Analysis of bolt preload

There are various reasons for the damage of the bolt. In summary, the damage of the bolt is generally caused by stress factor, fatigue, corrosion and hydrogen embrittlement. The super stress is produced by any one or a combination of shearing, stretching, bending, and compression. The first consideration for most designers is the combination of tensile load, preload, and additional utility load.

The preload is essentially internal and static, which stresses the joint assembly. The utility load is external and is typically a reciprocating force applied to the fastener. The tensile load attempts to pull the joint assembly apart. When these loads exceed the yield limit of the bolt, the bolt changes from elastic deformation to plastic zone, causing the bolt to be permanently deformed, so that the original state cannot be restored when the external load is removed.

For similar reasons, if the external load on the bolt exceeds its ultimate tensile strength, the bolt will break. The bolt tightening is obtained by twisting the preload.

Excessive torque results in excessive torque during installation, and also causes the fastener to be overstressed and reduces the axial tensile strength of the fastener, ie, the bolts that are continuously twisted and the same bolts that are directly tensioned. Ratio, the yield value is lower. In this way, it is possible for the bolt to yield without a minimum tensile strength of the corresponding standard.

The large torsional moment can increase the bolt pre-tightening force and reduce the joint slack. In order to increase the locking force, the pre-tightening force generally takes an upper limit. Thus, unless the difference between the yield strength and the ultimate tensile strength is small, the bolt generally does not yield due to twisting.

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