Bolt Torque

Tightening bolts and nuts to a specified torque value, as measured with a torque wrench, is a way to ensure that they are tight enough without being in danger of going too far and breaking the fastener or stripping the threads.

Vibration of assembled parts causes stress. A fastener which is simply holding things together will be subjected to stress alternating in opposite directions which will eventually cause it to become loose. To counter this loosening, fasteners are tightened more, and actually stretched, in order to pre-stress them. When tightened this way, they are always stressed in one direction, in spite of vibration, and will not work loose.