Joining bolts and nuts

Mechanical joining with bolts and nuts

Bolts and nuts are commonly used for mechanical joints. Bolts have external threads – these are grooves on the outside of the shaft (or shank), running down around it. They allow a nut, which has an internal thread, to be screwed onto the bolt. For this turning motion you use a wrench (or spanner).

In order to tighten a bolt, you need to apply turning force, or torque. By increasing the tension (stretching force) in the shaft, you make the bolt grip tightly, and the components being bolted together are pressed towards one another. This pressing force is called preload. The right amount of preload will prevent the components from sliding apart in different directions. Too much of this sliding may cause the bolt to break as a result from shear force (scissoring force) which is exerted on the sides of the bolt. You can use a torque wrench to indicate how much torque you are applying when tightening or loosening a bolt. Thus, you make sure that the amount of torque applied is neither too high nor too low and the bolt is tightened enough, yet not over-tightened. With a torque wrench you can also check that bolted joints do not work loose over time.