Fastener selection (3): Force and strength of bolt parts

    During the use of fasteners, it is necessary to understand the force of each part of the bolt. In order to choose a better quality bolted connection.

(1) Shear bolts

When the shear force is transmitted, the dangerous part is in the polished part; the diameter of this part should ensure the shear strength of the bolt and the crushing strength of the rod and the wall of the connected piece. The threaded portion or diameter of the bolt is slightly smaller than the polished rod portion, or its nominal diameter is the same as the polished rod portion; the length of the thread should be such that the polished rod has as much contact as possible with the wall of the hole (ie, the length into the joined interlayer is as small as possible). The head size of the bolt is generally slightly smaller than the tensile bolt. Due to the fact that it is not subject to tension, the size of the threaded portion and the head are generally determined by the structure and process.

(2) Tensile bolts

The bolt structure is divided into: head, polished rod, thread; the thread is divided into two parts: the rod body and the thread tooth. When the tensile force is transmitted, the dangerous part is in the threaded part (the first turn of the thread from the direction of the force transmission), and the diameter of this part should ensure the tensile strength. The diameter of the polished rod portion is determined by structural and process factors, and the strength is generally higher than the threaded portion. The bolt head is pressed against the connected piece or the gasket, and the outer diameter of the shape contact surface is mainly determined by the crushing strength, and the height thereof should meet the needs of the wrench to be pulled, and the cracking of the joint with the polished rod portion can be prevented from occurring. But bent or cut. For standard threads, the dimensions of these parts are based on the strength of the threaded part, taking into account structural and process factors, and the necessary strength calculations are generally performed only for non-standard bolts or for designing new standard size bolts. For standard bolts, the strength of the thread is ensured by the use of a standard nut with a sufficient screw length for the nut; for non-standard bolts or for short bolt lengths or for designing new standard sizes, strength is required Check. Both the bolt and the nut must be considered during the check, as the thread parameters and dimensions of the bolt may be determined by the need to screw the nut.

(3) Tensile shear bolts

  The force, structure and dimensions of this bolt combine the characteristics of the first two bolts.

(4) The main failure mode of bolted connection

 

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