Help with constraining rollers on cylindrical surface

[smenzel-2]

Hi Guys,

I’m having huge problems with constraining this assembly…The force is applied to the end of the crane via a long shank eye bolt, and the bottom of the crane is fixed in position.

The first picture is a close up of the roller assembly, I basically need the two rollers to be attached to the tube upright for a static load test.

When I run it through simulate, you can see the deflection has forced the bottom rollers to pass through the main CHS upright…I know CREO exaggerates the deflection but I still don’t think it should pass through like this, considering I quickly made a similar shape with square sections and it did not pass through the main column, the highest stress was in the top beam where the truss support connects (which is what I would expect here). I am also getting next to no stress through the bottom roller plates and the ‘connection point’ between the two rollers and CHS upright, so something isn’t right.

Have you ever come across this at all? Is there any way to pretty much connect the rollers to the upright so it will not pass through if that makes sense. Or somehow constrain a surface area where the rollers touch to be fixed? I'm not sure what to do.

When assembling the parts I have tried almost every combination, tangents ect…it comes up as fully constrained every time.

Any help would be appreciated if you can, cheers

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[GregBrown]

Are you using a contact interface between the rollers and the upright cylinder? Where is the area of interst in terms of looking at stresses? Do you need to use contact on teh rollers or would some other assumption be sufficient?

[smenzel-2]

Hi Greg,

Thanks for your reply. I am relatively new to CREO so I had to quickly search this up but when you say ‘contact interface’ do you mean just defining the constraints of the parts prior to assembly? Would doing this yield a different result from adding the part into the whole assembly and constraining from there? (Which is what I have done)

In terms of stresses, I just need this crane to deform as it would in real life. I need to know the max stress anywhere in the assembly is less then 350MPa. At the moment, because the rollers are passing though the big upright, it is giving a much bigger deflection and resulting in ridiculous stresses at the top hub, (as if it was just the boom arm without any rollers or support arms).

So to answer your question, another assumption could be sufficient, as long as it would give me something close to the real results. I have tried ‘fixing’ the rollers in space where they are, this drastically cuts back the stresses and deflection however as the upright bends it still comes through the rollers, not as much though, but in doing so still gives a larger deflection then it should. If I could somehow ‘fix’ that surface region of the upright, that could work however I have not had any luck with creating a surface region on a cylinder…I have only been successful on flat objects.

Thanks again

[JonathanHodgson · Accepted solution]

Hi Sam,

I think you may need to review all your 'interfaces' between the parts, and possibly simplify your model for analysis.

AFAIK, assembly constraints have no effect in Simulate - all they do is define the (initial) positions of the components.

If you look at Model Setup within Simulate, I think you'll find that the default interface is 'bonded'.  This means that any two surfaces which are coincident are considered to be effectively welded together, as continuous solid material.  I suspect that all your bolts and pins are functioning like this, so various connections that you think are pivots may actually be rigid.  For the rollers, I suspect that they're not quite touching the column and so Simulate has not 'bonded' the two solids.  If you want it to detect when two solids come into contact, you need to create a 'contact' interface.

I haven't tried analysing a structure / mechanism like this in Simulate, so others may have better thoughts on how best to do it; but I would consider suppressing all the bolts and pins and replacing them with combinations of rigid links and spring elements (to transmit shear loads but permit rotation), as well as setting the default interface to 'free' and creating specific interfaces (from the Refine tab, IIRC) wherever they're really needed.

HTH!

[JonathanHodgson]

There are also explicit 'fastener' elements in Simulate, but so far I've found them tricky to use...

[smenzel-2]

Thanks Jonathan, for the time being i'm OK with considering all the bolts and pins as 'bonded' I was just trying to get my head around \why the rollers were passing through the column before I tackled those other elements.

So you were correct, there was the smallest gap between the rollers and the column, after some research I found you can detect whether they are bonded or not under the AUTOGEM tab. I also think, like you said, the correct way to go about this is create a 'contact' interface, however this can only be run through a 'non linear static' analysis, which I don't seem to have access to with the Academic version. So I ended up just extruding the rollers off a datum plane as part of the column and assembled the pieces around them.

I am now getting the result I was looking for

Thanks for your help!!!