What is the Correct Bolt Torque to Use in a Given Situation?
What is the correct bolt torque to use? Great question – remember – every bolted joint must be tested.
The basic formula T = K x D x P provides you with a starting point for establishing an initial target tightening torque. The variables represented in the above formula are:
• T = Target tighten torque (the result of this formula is in inch pounds, dividing by 12 yields foot pounds
• K = Coefficient of friction (nut factor), always an estimation in this formula
• D = Bolts nominal diameter in inches
• P = Bolt’s desired tensile load in pounds (generally 75% of yield strength)
The reason all applications should be evaluated to determine the optimum tightening torque is that the K factor in this formula is always an estimate. The most commonly used bolting K factors arc 0.20 for plain finished bolts, 0.22 for zinc plated bolts, and 0.10 for waxed or highly lubricated bolts.
The only way to know the correct bolt torque is to simulate the application with a test. Test the bolt with a tension device. The bolt is tightened until the desired P (load) is indicated by the tension device. The tightening torque required to achieve the desired tension is the actual tightening torque that should be used for that given application. It is extremely important to realize that this tightening value is valid only so long as all of the aspects of the application remain constant.
The size, threads, metal, and whether it is dry or lubricated all have a bearing on the bolt torque limits.
