Load Strength Calculations for threads. 1

[savalakh]

Calculating how much laod a 2"-12 UN thread Nut 2 inch thickness can take. Material is Mild Steel.
Machinary's Handbook give the tensile stress area per sq. inch. But where does the thickness of the bolt plays part.

 

[desertfox]

Hi savalakh

Page 1490 of the machinery's handbook has formula for "stress area's and lengths of engagement of screw threads"


regards desertfox

 

[savalakh]

Hi desertfox,
Thanks for the reply. Also to confirm that for tensile stress only one thread area is to be used for load calculations.

 

[desertfox]

Hi savalakh

All my text books show that the shearing of the threads is
based on each engaged thread taking an equal share of the load. However they all go on to say that in practice that it
is only the last couple of threads that take all the load.
Further for a bolt in tension it should equal the strength of the threads in shear:- so for a ductile material:-

(shear stress = 0.5 tensile stress generated in the bolt)

for a bolt in tension W=(3.142/4)*Dr^2*St

for the threads in shear W=(3.142*Dr*Ss*h)

therefore (3.142/4)*Dr^2*St=(3.142*Dr*Ss*h)

h= Dr/2

where W= load on bolt

Dr= root dia of bolt

h= height of nut required for the strength of the bolt
to equal strength of threads in shear.

St= stress in bolt due to tension

Ss= shear stress which equal St/2

Please note this is an approximation as stresses in threads is a very complex subject, however if you use this method it
will give the approx required height of the nut and used with a reasonble safety margin you should have no problems.

regards desertfox

 

[savalakh]

Hi desertfox,
thanks again for the information. I have another question, what about the outer dia. of the nut. e.g. for 2'-12 UN Nut, what should be outer dia 3" , 4" or more. (actually it is the flat size for spanner)
how can we calculate this based on our load. I tried in the Machinary's handbook but could not find any thing related to that.

Thanks savalakh

 

[desertfox]

Hi savalakh

Have a look at page 1499 in the machinery handbook its got
American National standard and Unified Standard Hex Nuts.


regards desertfox

 

[savalakh]

Hi desertfox,

Thanks. Yes you are right, there are detials at page 1499 on Unified standard hex nuts.

regards
savalakh