Bolts are designed to securely fasten objects together. Each bolt has specific tolerance levels before failure. This tolerance level is determined by the material make-up, width, depth, and size of a bolt. Too much stress on a bolt will cause it to break. The pressure that is placed on a bolt is called torque. Each bolt has a specific torque capacity that defines how much pressure it can handle.
The breaking pressure of a bolt is represented in a formula known as pounds per square inch (PSI). Each bolt is designed to support a specific torque tolerance in PSI before it will fail. This is identified as the pressure of one-pound of force placed on one-inch of area of a bolt.
There are publicly available specifications charts that document the PSI capacity of bolts. These specifications are based on bolt material, number of threads, and overall size. The charts should be used as a guide for an engineer to ensure over-tightening does not occur.
A torque wrench is a special tool designed to measure the PSI placed on screws or bolts. This wrench has special springs that do not permit over-tightening of bolts. The operator of the torque wrench sets the specification of the pressure prior to tightening a bolt. When the torque capacity is reached, the wrench prevents the operator from adding pressure to the bolt.
A car motor is a precision piece of mechanical engineering. There are hundreds of bolts in the motor of an automobile. Many of these bolts have capacity requirements that vary depending on their location in the motor. Precision tightening is typically required in the areas of heads, crank, intake, and drive train. These parts receive the most stress during normal operations. A mechanic will use a torque wrench to tighten the critical areas of a motor.
Head bolts are the bolts that hold the heads on a motor. These bolts ensure the heads stay secure when the piston explodes against the wheel cylinders. Each automobile has a specific torque capacity for the head bolts of a motor. This specification is based on the size of the motor and bolts used for tightening.
Lug bolts are used as a device to attach wheels to an automobile. These lugs are designed to ensure the wheels stay securely fastened to the automobile. The lug bolts have a specific torque tolerance depending on the wheel size and number of lugs. When wheels are replaced it is important to verify the torque specification has been set. This reduces the long term stress of lug bolts. Most automobiles require a minimum of 50 foot pounds of PSI.