Fastener Screw Torque Clamp Force Design Calculator - Engineers Edge

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Fastener and Screw Torque Design and Equation and Calculator

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:

Equation T = c D F

Torque Axial Force Calculator - for steel and zinc plated threads.

T = Torque required (inch pounds)
F = Bolt tension desired (Axial Load) (pounds).
D = Nominal bolt diameter. (major dia.)

c = Coefficient of friction constant

Steel and/or zinc plateds threads = .2
Cadmium threads = .16
Lubricated threads = .16 to .17

This relationship is based on the assumption that regular series nuts and bolts with rolled threads are used, acting on surfaces with industry standard thread pitch and flank angle.


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  • Alexander Blake, What Every Engineer Should Know About Threaded Fasteners Materials and Design. Marcel Dekker, Inc., New York (1986).
  • Industrial Fasteners Institute, Fastener Standards, fifth edition. Industrial Fasteners Institute, Cleveland, Ohio (1970).
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  • Joseph Edward Shigley, Mechanical Engineering Design. McGraw-Hill Book Company, New York (1977)