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Critical bending moment
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Posted by: IMoo

06/09/2005, 09:26:31

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Anyone any idea of the formula to calculate the critical bending moment of a hollow cylinder (eg. a plastic tube) or could point me to text book where I could find this info. Cheers.






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Re: Critical bending moment
Re: Critical bending moment -- IMoo Post Reply Top of thread Forum
Posted by: zekeman

06/10/2005, 09:46:21

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You probably mean the yield stress, so you use the formula
s=Mc/I where
I=moment of inertia, in^4
M= bending moment, lb-in
c= outer radius of cylinder
s= yield stress in lb/in^2
For the cylinder,
I=pi/64(Do^4-Di^2)
C=Do/2
If you find the yield stress s, then
M=sI/c
If this is a design problem you have to put in factor of safety, at least 3, so that your design stress is 1/3 of the yield stress or less and therefore the allowed M is 1/3 of the value calculated above.






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Re: Critical bending moment
Re: Re: Critical bending moment -- zekeman Post Reply Top of thread Forum
Posted by: ex03134

11/26/2005, 09:12:18

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hi,im doin something similar, basically im investigating a type of can crusher. i need to know how to calculate a force acting down on a can that is positioned on its side..it looks like a simply supported beam however because its cylindrical in shape i dont know the formula to calculate the force for this...i was wondering if u can help me..thank you






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Re: Critical bending moment
Re: Re: Critical bending moment -- ex03134 Post Reply Top of thread Forum
Posted by: ex03134

11/26/2005, 09:16:30

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sorry again..there was a formula you gave to Imoo.Since i'm from england im unsure of what your symbols mean..
I=pi/64(Do^4-Di^2)
C=Do/2
what is Do, Di and C?






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Re: Critical bending moment
Re: Re: Critical bending moment -- ex03134 Post Reply Top of thread Forum
Posted by: devitg

11/27/2005, 17:59:37

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about formulae

It shall be as follow

S=M * C/I where
I=moment of inertia, in^4
M= bending moment, lb-in
C= outer radius of cylinder
S= yield stress in lb/in^2
For the cylinder,
I=pi/64(Do^4-Di^4), Di is rised to the 4 not to the 2
I= moment of inertia of a hollow cylinder
as the substraction of the hollow or empty from the solid
where Do =outside diemeter
and Di = inside diameter
C=Do/2
C= of course is the outside radious

If you find the yield stress S then
M =S * I/C
The therm I/C it is call resistent module or W






Modified by devitg at Sun, Nov 27, 2005, 18:00:18

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Re: Critical bending moment
Re: Re: Critical bending moment -- zekeman Post Reply Top of thread Forum
Posted by: IMoo

06/15/2005, 12:44:36

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Thanks for the reply zekeman. What I am doing is kink testing plastic tubing. The tubing is bent into a semicircle and placed between 2 parallel plates. The top plate moves downwards until the tube collapses /kinks. If I know the force when the tube collapses, the distance between the plates and the cross sectional area of the tube what would be the correct formula to calculate critical bending moment?






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Re: Critical bending moment
Re: Re: Critical bending moment -- IMoo Post Reply Top of thread Forum
Posted by: devitg

11/27/2005, 18:05:51

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Your aplication is far from a bending, despite you BEND it .
It is near a BUCKLING , and maybe you are asking for the force to be excerted by the plates.
I sugest to do a try and error procedure or fit the plates on a hydarulic press , read the pressure need to bend it , and get the force by the area of the press cylinder.








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