Pocket guide to tightening technique
This booklet provides an introduction to the technique of using threaded fasteners for assembling components, the application of power tools for the assembly and the influence of tool selec- tion on the quality of the joint.
Why threaded fasteners?
There are several ways of securing parts and components to each other, e.g. gluing, riveting, welding and soldering. However, by far the most common method of joining com-ponents is to use a screw to clamp the joint members with a nut or directly to a threaded hole in one of the components. The advantages of this method are the simplicity of design and assembly, easy disassembly, productivity and in the end – cost.
The screw joint
A screw is exposed to tensile load, to torsion and sometimes also to a shear load.
The stress in the screw when the screw has been tightened to the design extent is known as the pre-stress.
The tensile load corresponds to the force that clamps the joint members together. External loads which are less than the clamping force will not change the tensile load in the screw. On the other hand, if the joint is exposed to higher external loads than the pre-stress in the bolt the joint will come apart and the tensile load in the screw will naturally increase until the screw breaks.
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