Shear stress: average vs. max stress?

[kingnero]

for instance when calculating bolts for shear stress:

I normally use the average shear stress (tau = F/A) because, in combination with large tensile stresses, the FoS for shear becomes automatically much higher than needed.

When a cylindrical object (bolt) is loaded in pure shear, should I take the max. shear stress into account? (tau = 1.5 F/A) ?
This is for an installation where the FoS needs to be as small as possible because of the extra weight involved.


I need a FoS of (minimal) 3.5 before yielding, and as low a weight as possible.
Application is a bolt in double shear for attachment of a double-working hydraulic cylinder, fatigue not to be considered as it's a first prototype.

 

[prex]

In a circular section tau_max=1.33 F/A (or more rigorously tau_max=1.385 F/A for [ν]=0.3).
However in the case of bolts or pins, normally tau=F/A is taken, as in those cases the sections under shear may be assumed to remain plane, and this condition leads to tau=F/A.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes and launchers for fun rides

http://www.levitans.com

: Air bearing pads

 

[desertfox]

hi kingnero

This site use:-

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

 

[kingnero]

prex : thanks for that.
desertfox : I do know the basics and I know that site, but it doesn't mention anything about the 1.33 or 1.385 factor. It also doesn't say you can ignore the max. stress or you should be using the average stress. It's a very informative site nevertheless.

I've heard of these factors before but I haven't used them before and google didn't came up with much useful stuff (that can be used in reality). So I asked it here for confirmation...

 

[rb1957]

shear stress due to torque on a solid section is covered in every text ... stress = TR/J, J = 2*I