Contact sets keep breaking - how to fix?

[AnthonyPaul2010]

Attached is a screen snipe of design that has a hydraulic cylinder in it. I've created contact sets for the rod and the cylinder however they keep breaking appart especially when I insert them into an assembly and drive to drive the constraints in a positional representation.

Does anyone know what I am doing wrong?

[harco]

You're probably using mate axis constraints between rod and cylinder as logic would suggest but this allows flipping as there is no angle constraint.

You could try an angle constraint to force direction but I have had trouble with that in the past.

If your end pin axes are always parallel (which it looks like they are) then a more robust constraint technique is to mate/flush both origin planes that run the length of you parts, make sure both parts are constructed centred on the origin axis.

Should slide nicely after that.

[SBix26]

I'd avoid contact sets for this application.  Make the hydraulic cylinder a subassembly, and mate the piston to the inside face of the cylinder with limits.  Then place in your assembly and mark as flexible.  Much more reliable than contact sets.

[AnthonyPaul2010]

Hey guys I tried both solutions provided in this thread. Not sure what the problem is but I'm still having problems. I tried both methods and it seemed to work with one cylinder in one sub assembly and not with another cylinder in a different sub assembly, see attached screen snipe. Suffice to say I'm still having problems.

I've noticed that if I click and drag either the rod or the cylinder it will snap back around the other way.

What does this mean? I've tried everything but I can't get rid of the instability, i.e it still keeps flipping apart.

 One thing I also noticed was that it is happening while trying to create a positional representation where the whole machine (model) is slewed around 180 degrees. However when I slew it at angle less than 180 degrees the cylinder and rod don't flip apart.

[SBix26]

Without the files it is really hard to know what's happening.  But it looks from the picture as if the cylinder isn't properly constrained.  Did you follow my recommendation above?

[Ray_Feiler]

Try constraining it like this attached example and then make it flexible in your assembly. Note files attached are IV2012 format.

[AnthonyPaul2010]

Yes I followed your recommendation however it didn't work. I've tried all the solutions proposed in this thread and I am still having the same problem. I've created a positional representation of the crane slewed at 0, 90, 180 and 270. They all work fine until I insert the luff cylinder and activate the positional representation at 180 degrees. The crane doesn't move but the cylinder and rod flips apart at 180 degrees. However the cylinder constrains correctly when I activate the 0, 90 or 270 degree slew position of the crane.  It only seems to have a problem at 180 degree slew position.  I have attached a screen snipe of the positional representation at  0 180 degrees to hopefully illustrate the problem.

I have completely rebuilt the model ( a couple of times now) and tried many different constraint options without any success.  This is driving me mad. Is it a bug in the program or is it something I'm doing wrong. I am using explicit reference vector angle constraints to control the slewing angles of 0, 90, 180 and 270. After I've created the positional representation I am then using the drive constraint to change to the required angle. The luff cylinder is a sub assembly and is flexible. 

Unfortunately I can't upload the model for confidentiality reasons however I'd really appreciate it if there is anything anyone can see I am doing wrong.

[AnthonyPaul2010]

Further to my last post is the attached screen snipe that shows an error message I get.

Adaptivity settings. Where can I find them?

Could this be the problem. (I try to avoid adaptive parts in my approach to modelling but not sure if these are the same things this error message is referring to.)

[feame]

I get all sorts of random problems associated with positional representaions and angles.

Driving constraints can be okay, inventor seems to understand what its doing, where something came from and where its going.

Pre-defined positions however just make a model move any parts it deems necessary to get 'some sort' or result.

I can set up models with 2 pre defined positions, yet simply by changing from one to another and nothing more, make the models go into as many as 6 different variations.

This current release of inventor has had issued for me with rotaional constraints too. For example, clockwise and anti-clockwise rotaion is the same result.

Similarly, with angles, positive and negative angles are the same thing. +30, -30, +330 and -330 can all give the same result. Did somone forget to program inventor with a reference direction for angles the same as they got gravity backwards a few years ago?  

[Ray_Feiler]

Yes, I have see this before with angle constraints. Try 179.9 degrees instead of 180.

Adaptivity settings. Where can I find them?

[AnthonyPaul2010]

Yeah I've tried angles of just less than 180 and it works so I'm almost convinced it has something to do with the z direction of faces used for constraining.

When you place an angle constraint which method do you use of the three choices?

1. Directed
2. Undirected
3. or the Explicit Vector Reference (which is the default selection).

I'm assuming because EVR is the default that this is the one you are supposed to use the most.

I'm thinking that if I learn the proper application of each of these three choices then maybe it might help me solve the problem.

What type of angle constraint do you use the most?

[Ray_Feiler]

When I use angle constraints I always use directed. Maybe someone with more experience can help you with the other choices.

[harco]

Yes use EVR it was introduced to overcome the flipping problem.

It positively tells Inventor which way is up so that positive and negative directions can be specified.

Generally apply the 2 selections of edges or faces for the required angle and then select another edge or face perpendicular to the plane of the first 2 selections.

On a side note I'm not seeing how you are slewing you assembly using the ram that's flipping, it looks like a lift ram to me.

If this is so then surely you are hitting overcentre at 180º and serious loading.

Can you simulate the problem with 4 very simple basic parts, an arm, a turntable, a rod and cylinder.

Sometimes it's a bit trial and error trying to get a set of constraints to perform the function you want, especially if there are multiple movements.

It can be the order in which Inventor solves the constraints too, I have seen things not work and delete an early constraint but put it back in later on and the assembly then works.

Try not to drive the ram itself but drive the pins it is attached to then the ram follows where the pins go.

Hope these suggestions help.

[feame]

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harco wrote:

Yes use EVR it was introduced to overcome the flipping problem.

It positively tells Inventor which way is up so that positive and negative directions can be specified.

Generally apply the 2 selections of edges or faces for the required angle and then select another edge or face perpendicular to the plane of the first 2 selections.

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Still doesn't work. My cylinders and assemblies still flip inside out. I'm sure I'm not the only to find that.

[AnthonyPaul2010]

After spending nearly a week on this problem I am convinced there are other IV users having the same problem.

I am convinced that it is a bug in the software?