Help With Bending Stress Calculation 5

[MarkJ_]

Hello,
I was hoping to get some advice on how to calculate the strength of a new lifting interface. I have 2 existing tools that need to be joined together rigidly for lifting offshore. There are hydraulic lines between the 2 assemblies and access is also needed at the lifting interface for making these up. For this reason and because of space constraints I am looking to use 4 x 16mm OD threaded rods with a support plate in between.

The top assembly with a single lifting point on its end face is rigid and weighs 150Kg. The bottom assembly is also rigid and weighs 250Kg. I have attached a sketch which shows the arrangement. The support plate and lower support rods (359mm long) are already existing which is why the support plate isn’t in the middle.

So the complete assembly will be lifted from horizontal to vertical using the single lifting point on the top assembly.

I was hoping someone could offer some advice on how best to calculate the bending stresses in the rods?

I tried doing this as a simply supported beam assuming the supports are where the rods attach to the top and bottom assemblies. I took second moment of area as being the formula for solid bar and calculated the bending moment halfway between the support rods and support plate. The stresses are very high and I’m pretty sure my calculation is incorrect.

Calculations are not my strong point. I would appreciate any advice?
Thanks
Mark

 

[NRP99]

I guess we all are shooting in the dark. Unless we know the "Tool" really, its difficult to "predict" the behavior of lifting. And besides exact assembly information, there is DAF for offshore lift which will make the lift more risky. The stress we see in those pictures may get magnified based on what the DAF is.

Don't know the about OP's lift, but certainly my brain was lifted with so much insights from members. Thank you - HTURKAK, rb1957, desertfox and MarkJWilson.

 

[desertfox]

Hi NRP99

It certainly is a good thread and certainly got my brain thinking, I hope the OP comes back to tell us about how it is assembled.
I would like to thank all for making it a very interesting thread.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[MarkJ_]

Desertfox/NRP99/rb1957/HTURKAK thank you so much for your help with this.

And thanks for your patience with me plus really helping develop my knowledge on this topic.

It has been very much appreciated.
Mark

 

[desertfox]

Hi Mark

Can you tell us about how you assemble those rods?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[MarkJ_]

Absolutely Desertfox.

Top assembly (150KG) 4 x 16mm rods are screwed into M12 tapped holes on 166mm PCD.

Other end of these rods sit in 17mm diameter counterbored holes that are 10mm deep in the top face of the support plate. This end of the rods is tapped M12 and are fixed in place through the bottom side of the plate with 4 x M12 fasteners.

I maybe didn’t make this clear but the 4 x rods between the support plate and bottom assembly are separate from the ones between the top assembly and support plate. These 4 rods are on the same 166mm PCD equally spaced in between the 4 rods that attach to the top assembly. They are assembled the same way but the 17mm counterbore is on the underside of the plate and they are fastened through the top again with 4 x M12 fasteners.

These 4 x 16mm rods are again screwed into M12 tapped holes on the flange plate on the bottom assembly (250KG).

It needed to be done this way because there isn’t much room and I’m avoiding many existing features on both the top and bottom assemblies. There are hydraulic lines, air lines as well as control data cables that run through the centre of the tool.

The other thing to mention is that the lower rods (that fit into the 250KG bottom assembly) and support plate were designed much earlier and are already existing. The requirement to fit the top assembly came along much later and it needs to be a set distance from the bottom assembly. This is why the support plate isn’t bang in the centre between the 2 assemblies and the rods aren’t of equal lengths.

Thanks again
Mark

 

[desertfox]

Hi Mark

Thanks for updating us on how the rods are assembled, I couldn’t see how it was possible from the original tool sketch but it’s clear now

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[rb1957]

the important detail there is the "M12 tapped hole" ... so the 16mm fasteners are effectively 12mm ,,,

another day in paradise, or is paradise one day closer ?

 

[desertfox]

Very true rb1957 that’s what I was saying about how the detail of fastening the rods was achieved I thought the screwed rod was all M16 however it depends how much bending the screw or bolts actually carry if any but it is significant.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein