General: The importance of torque in many applications can not be overemphasized. Critical applications such as automotive engines,
brakes, aircraft, and structural installations are torque design - manufacturing sensitive. Under-torque can result in unnecessary wear of nuts and fasteners as well
as the parts they are securing together. When insufficient pressures are applied, uneven loads will be transmitted throughout the assembly which may result in
excessive wear or premature failure due to fatigue. Over-torque can be equally damaging because failure of a nut or bolt from overstressing the fastener and secured
areas. Torque requirements for particular applications should be determined and the proper torque applied.
Design Considerations
The first requirement in determining the amount of torque to apply is a knowledge of the desired bolt stress. This
stress based on the yield strength of the bolt material. It is recommended that the induced stress not be allowed to exceed 80% of the yield strength. In the design
of a fastener application which will be subject to external loading, whether static or dynamic, it will be necessary to establish bolt size and allowable stress in
accordance with current engineering practice.
The mathematical relationship between torque applied and the resulting tension force in the bolt has been determined to be
as follows:
T = Torque required (inch pounds)
F = Bolt tension desired (Axial Load) (pounds).
D = Nominal bolt diameter. (major dia.)
EQUATION: T = .2 D F
This relationship is based on the assumption that regular series nuts and bolts with rolled threads are used, acting
on surfaces without lubrication.
