Bolt Pretension

As function of what ?

What u mean by function?my doubt is when a bolt has to be used how to calculate the max torque that can be given.

Mr are you talking about Manual/ Hydraulic torque wrenchs or Hydraulic bolt tensioning.

All flanges and bolts shall be prepared and pre-assembled according to specified tolerances

1 All bolts shall be torqued to 1/3rd the final torque valueand according with the sequence.

2.- All bolts shall be torqued to 2/3rd the final torque value according with sequence.

3.- Final sequential torque shall be 100% of the final torque value . .

4.- A final 100% torque value verification shall be run in a REVERSE

sequence .

It must be greater than the applied load, with a bit of margin. E.g. if the applied load is 1 kN, pretension the bolts (with no applied load) to 1.2 kN. Then the force between the bolted parts is 1.2 kN. When the 1 kN load is applied, the force between the bolted parts falls to 0.2 kN but the bolt tension remains at 1.2 kN. The bolt therefore will not fail due to fatigue from cyclic stress.

When you know the total pretension you can use bolt data to decide grade, no. and size of bolts, along with the torque needed to give the required pretension.

Cheers...........Codey

Go through this link

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

This will give the pre-load and the approximate torque required to achieve the pre-load.

Please note that here the pre-load is the important factor (ie getting an initial preload approximately but not greater than 80% of the yield strength of the material).

The torque required for this is highly dependent on the collar and the thread frictions and there are numerous literature available that show that these two factors vary widely even for same lot of fasteners assembled on the same component. This is why the normal practice for the critical application is to go for bolt-stretch or use of tension indicating devices (as tension indicating washers).

Certain papers show the reduction in scatter by usage of good thread lubricants.

This is as far as the bolt-loading is concerned.

If you google, you will find the recommended torque values for specific sizes (and pitches), though they, for the reason mentioned above will be approximate to meet the final requirement (bolt stretch). This method exist however due to its inherent simpicity and the designer take care by suitably over stiffening the joint.

On the second aspect, if you are asking for the basics (as mentioned in the previous posts) - you must first calculate the joint loading and the required joint stiffness (so that the joints stay together ubnder worst loading conditions with a suitable factior of safety to address to creep and fatigue).

Then from the stiffness value, the joint pressure requirement is calculated. Then it is partly aret and partly financial.

To get the joint pressure, different bolt sizes, grades and numbers are chosen based on standardisation, availability, cost and space available.

Once the number of fasteners are selected then you get the pre-load o each and calculate the stretch for each or the torue required for each (aftar inflating the requirement as mentioned above to address to the variability in friction etc)