is it Pure Shear?

[Mece79]

Im hoping someone can help me with a design im currently working on.
I am designing a shaft to take 200 Tons (400Kips) now the dilemna is that do i consider the shaft to be in pure shear or do i treat it as simply supported or maybe fixed-fixed end condition?
my initial calcs considered bending but i do kind of beleive that it might be a pure shear situation... check out the attachment and let me know what you think.

Thx

 

[Ron]

The shaft is in double shear.

 

[JStephen]

I think you'd need to take bending into account as well. The bosses there just act as spacers, and support for the pin is at the end.

 

[Mece79]

There is no doubt in my mind that the shaft sees bending but the question is how much! The reason I beleive that shear would be the design constraint is because in order for the shaft to bend the block, bosses and plates would also need to bend due to the tights tolerances and taking those section modulus' into consideration would make the bending stress negligible. You agree??

Thx for the input guys

 

[Tmoose]

Without knowing the dimensions of the side plates to which the "bosses" are welded, the amount of bending that will occur is unknown.

It is fairly typical of heavy duty tackle to extend the width of the part you call "block" out to the side plates, like this green one -

http://www.anker.in/images/products/Chain-Pulley-Block-Sprinted.jpg

One result is to make the bending loads low

 

[Mece79]

Tmoose thx for the input but i dont need help calculating the bending stress I am debating whether bending exists by modeling the shaft as simply supported. this question should be possible to answer without dimensions. also as for changing the design not practical.. the geometry must look like this because it connects to other equipment.

 

[BobM3]

If you apply enough axial bolt preload then the assembly will act as one solid piece. It doesn't matter if there is radial clearance between the bolt and the end plate, bosses or block. If the bending stiffness is infinite then you have simple supports. In real life you haven't something between simple supports and rigid supports.

 

[Mece79]

Bob i agree with you on the axial load part but i will have to disagree with you on "It doesn't matter if there is radial clearance between the bolt and the end plate, bosses or block" if you draw a deflection curve of the shaft you will see that minimizing deflection will minimize stress basic stress-strain relationship so how can it be that allowing the shaft to deflect inside the bosses and block not have an effect!!

 

[rb1957]

you have load applied to the block (2*200kip). it looks like you're trying to align the load points with the attachments to the rest of the world, so these will be only axially loaded (ie no bending, which is a good thing). the block loads the shaft over a portion of it's length (obviously). the shaft reacts this load (where the block applies it) and carries it to the attachment to the rest of the world ... therefore there is bending in the shaft. it gets complicated when you consider closing of the small gaps (between the block and the bosses).

 

[Wrenchbender]

Only going by the sketch, and assuming all parts have the same modulus, I think the shaft is such as short fat beam with such a well distributed load that any bending can be neglected. Treat it as Ron suggested: double shear. Then apply a safety factor.

Also a couple of casual observations -
The shaft looks a lot heftier than the combined areas of the two pins holding the 200 kip loads. And I’m sure you did, but maybe check the shear areas on the end plates as well.

 

[Mece79]

thanks Bestwrench I agree with your observations everything on the assembly has been checked for shear, bending, tear-out and bearing. this shaft was a little bit of a concern since i am designing to API 8c spec and factor of safety needs to be 2.9 and if i was to consider bending on the shaft(simply supported) the bending stress would be way over based on the shaft size that i am using. thanks again everyone
more opinions are always welcome