How much torque is required for proper tensioning of a bolt in a given situation? How can a given amount of work in the form of torque application be translated into the desired tension? A simplifiedscrew jack formula by Farr shows that necessary torque is a function of the tension required, certain dimensions of the nuts and bolts, and the coefficient of friction existing on he bearing surfacesof the threads and nut faces.

T=

Fa  P R f  + t + Rs f   12  2π Cosθ 

Where T = Torque in foot - pounds Fa = Force in pounds of tension required in bolt P = Lead of threads, inches Rt= Average mean radius from axis of bolt out to point where load is applied to thread surface Rs = Mean radius of nut face, or shoulder Cos θ = Cos of one half the included thread angle measured on aplane through axis, degrees θ usually 30°, therefore, Cos θ - 0.86603 F = Coefficient of friction (See figure 1) All terms of this equation are fixed for any given nut and bolt size, once the tension,Fa is established, with the single exception of F, the coefficient of friction. This parameter can vary with the result that the required torque will also vary widely. The efficiency of the situation- the percentage of the effort applied to the nut that is translated into bolt tension-varies widely when the effective coefficient of friction varies.

Fig 1 - A bolt (or stud) is called andexternally threaded fastener and a nut is called an internally threaded fastener. The thread is and example of the inclined plane, one of the basic mechanical principals. All terms of this equation arefixed for any given nut and bolt size, once the tension, Fa is established, with the single exception of F, the coefficient of friction. This parameter can vary with the result that the required torquewill also vary widely. The efficiency of the situation - the percentage of the effort applied to the nut that is translated into bolt tension-varies widely when the effective coefficient of friction... [continua]

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