What Is the Correct Bolt Diameter for a Rigid Flange Coupling?

[ilahac]

A rigid flange coupling, secured by 3 bolts on a PCD of 200mm, is subjected to a torque of 10kN/m. Determine the bolt diameter (to the nearest mm)

Bolt material shear stress = 225N/mm^2
Factor of safety = 2

Hi, I'm chris, I'm an aeronautical engineering student and I'm a little puzzled by a step in the above problem. I don't understand why I have to figure out the bolt area then reverse the step in the following calculation. I'm really just looking for some clarity, y'know, someone to say "yeah, that's correct". Any help appreciated, my work is below.

Working stress = 225N/mm^2 ÷ 2 = 112.5N/mm^2

Torque = Force*Radius => Force = Torque÷Radius
= 10*10^3÷0.1
= 10*10^4 kN

Stress=Force÷Area => Area = Force÷Stress
= 10*10^4÷112.5
= 888.8mm^2

Bolt area = 888.8÷3
= 296.2mm^2

Diameter = √(3*296.2 ÷ ᴨ)
= 16.81mm
= 17mm

 

[mgb_phys]

Looks correct except for the '3' in the bolt diamter - you have already takne into account that there are 3bolts, by dividing 888.8mm^2 by 3.
So I get a bolt radius of 9.7, so a diameter of 20mm