Reagrding bolt Torue Calculation

[Arbu]

Dear All,

I am having a doubt about bolt torque.
I am designing a steel structure in that I have M-24 (24 mm diameter) bolt 8.8 grade with maximum tension of 125 kN. Is it mandatory to calculate torque for this case or simply I can follow the Standard torque chart available? Is torque value is based on tension in bolt ?? or it depends on friction coefficient and material of bolt??

Please suggest its urgent.

 

[Ron]

The relationship between torque and tension in bolts is variable. It depends on the type of bolt, its strength, if there is rust on the threads, if there is oil on the threads, if the bolts are coated....with hot dip galvanizing (zinc) or with some other corrosion inhibitor. The point of tightening bolts is to induce tension in the bolts that creates a clamping force to the members it passes through. Empirical testing has shown that developing about 70 percent of the tensile strength of the bolt for clamping force provides long term tensioning capability without excessive creep or loss of tension.

There are several ways to determine the tension developed in the bolt. Calculating the tension using torque-to-tension theoretical relationships is not considered extremely accurate but it gives on a place to start when specifying a tightening torque. A calibrated wrench method then can be used to determine the field torque achieved. Another method is the Turn of Nut method. This process involves the establishment of a nebulous "snug tight" condition where the faying surfaces of the clamped member are brought into full contact, and then the nut is turned an additional 1/2, 2/3 or 3/4 of a turn to achieve additional tensile clamping force. None of this method actually quantifies the tension achieved in the bolt.


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[drawoh]

Arbu,

Do you understand what assumptions went into your torque chart? What percent of proof or yield stress was used? Is the joint assumed to be lubricated?

Your chart analyzes the bolt. There could be issues with the material being clamped. If nothing else, I would do some of the chart's calculations by hand, and make sure I understand their assumptions.

I am doing a quick calculation here. If I assume my bolt tension is 75% of yield, I am getting 160kN as my maximum tension. The chart on Engineer's Edge agrees with me.

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JHG