Autodesk Inventor

[#1]

I would really like to know why when one uses flexibility in an assembly why the constraints fail? This is really annoying because one has to open the assebly or assemblies, suppress all the constraints then unsupress them to fix it. Why does this happen

[#2]

Are you saying that the assy you place in the top level assy has constraints in it. and then when you are placing into the top level, making it flexible, and trying to constrain it in the top level, it gives you an error? So then you have to go into the subassy and suppress the constraints in it?

Just curious, you at the Westinghouse here in Round Rock/Georgetown or whatever(kind of in the middle) Edited by: jlackey4740 on May 11, 2009 9:27 PM

[#3]

let's say you have a pneumatic or hrydaulic cylinder. a simple model of a cylinder includes two parts the main body and the rod. when contrained the rod should be able to move in and out of the cylinder body. now you take that assembly and drop it into another assembly and make the cylinder assembly flexible. it might be fine today or tomorrow but sooner or later that single contraint in the cylider assembly fails because the cylinder is flexible. to fix it you have to open the cylinder assembly supress that single constraint then unsupress it in order to fix it

[#4]

That's pretty odd, we use a similar set up with suspa gas springs, and constrain them the same way, but I've yet to see the constraints malfunction over period of time. The only time I see them act up is when the surface that they are mated to in the top level chnage and that constraint reference is lost, however, I've never had to fix the constraint in the spring assy istself. Are there any aspects of the two assys that are changing over time, size if the rod, cylinder body, the parts that they are being mated to or anything like that?

[#5]

other that the position of the cylinder rod nothing else changes... I just posted something here because a much more complex flexible assembly that I am working with is having problems and I needed to vent. I guess I need to get used to the fact that Inventor is a low quality piece (cheap) of software and that I just need to learn to deal with that fact.

[#6]

Ah, yes, many frustrations. Of course, I've only worked with Inventor, so I have no frame of reference as far as other programs are concerned, but I have my days as well. Well, hope you can figure it out, there are a few threads by Shastu that go into some detail with problems concerning flexible assy, although the problem he describes are different than what you have here, there may be some good advice from some of the other posters there that may help you find a solution.

Oh yeah. The only reason I had asked about the location of the Westinghouse you work at is because I work at a place about 5 minutes from there, so I was just curious. Hope if you get a chance to look at the other threads I told you about you find something that helps. Take it easy.

[#7]

My experience with failures of this type almost always
indicates poor constraining technique. Too bad you feel this way about
Inventor.

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[#8]

Which is why I have almost entirely given up on mating via part or assembly features, rather I will ensure that each part has work features that can be constrained to. This way, an entire part's features can be blown away (accidentally or on purpose!) and your assemly won't fall to bits.

[#9]

Thats sweet as but what do you tie the work features down to? I make sure that the first thing in do in any new part is create a work plane for my first sketch, that way i can pick the part up (usign the workplane) and move it whereever i like in 3D space without the sketch failing. This means i have to have the horizontal&vertical contraints turned off, otherwise things tend to turn themselves inside out. A bad habit of inventors i might add.

Maybe thats what is happening with the flexible assy problem???

[#10]

If you are using just axis/axis constraint then there is probably to much freedom in the flex assy.
The rod will occasionally flip over.
A more robust way, as mentioned in another post, is plane constraints. If the rod and cylinder are drawn centred at the origin then two constraints on two of the origin planes will allow sliding without flipping.

It's fun when your ram goes inside out!!