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S.Harris1-Visitor
1-Visitor
November 7, 2013
Solved

Multiple sub-level mechanisms do not function in top level assembly

  • November 7, 2013
  • 4 replies
  • 22104 views

Platform: Creo 2.0 M030

Problem: Mechanism constraints broken on a simple assembly with two synchronized hydraulic cylinders that extend/retract and pivot around a common axis.

http://content.screencast.com/users/mr.shaun/folders/Jing/media/4cfc4aac-7484-4887-ac17-785ed567b8cc/2013-11-07_1733.png

Additionally, there is a guide-plate that controls the arc of the devices on the ends of the cylinders. That guide plate also has the ability to move back and forth in a slot. The model did at one point perform nearly as intended.

I would appreciate some feedback on how this assembly could be created.

Thanks!


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Best answer by vzak

Hey Shaun,

I just took a smaple look at 5000-400-a17000.asm - and this basic assembly already has connection error. problem seems to be in a bad tangent constraint, between the 2 first components, that also seem to be a wrong one - pin goes through the whoel solid material.

In top assembly you just inherit same error.

I changed tangent to Coincedent (aligns slot side with a hole) and error goes away - also in top assembly. Try putting the fixed assembly into your data and check if top assembly works better.

Regards

- Vlad

4 replies

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TomD.inPDX17-Peridot
17-Peridot
November 8, 2013

I never liked that little "disconnnected" error.

The only thing someone pointed out that fixes this is by starting with a "snapshot" in order to "reset" the assembly.

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TomD.inPDX17-Peridot
17-Peridot
November 8, 2013

And welcome to the forum, Shaun!

    V
    vzak12-Amethyst
    12-Amethyst
    November 8, 2013

    Shaun,

    Here are some general notes on connections and related errors in Mechanism design - see if this may help you. Given you talk about sub-level mechanisms problems in top level assembly this might be related to disruption in bodies scheme / body locks.

    http://communities.ptc.com/message/222656#222656

    Regards

    - Vlad

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    TomD.inPDX17-Peridot
    17-Peridot
    November 8, 2013

    Vladimir; I really appreciate the insight you provided in the linked post. I read it at the time and it makes perfect sense.

    I think here is a different issue. Yes, the model is a mess (sorry Shaun!), and when you are in this state, where do you even begin to clean it up? Shouldn't there be -some- kind of log that tells you where the "disconnected" error emanates from?

    Consider this scenario:

    Someone left an assembly file in a lousy state and they went off on their new job and left me to resolve it during a routine Engineering Change Order operation. Now, rather that a 10 minute drawing update, I have a 2-3 day assembly rebuild to do and probably a fully messed up exploded assembly drawing, assuming I even know -how- it was meant to work in the 1st place.

    This is all too common. When you contract with someone to do simple work, then telling them that their files are a mess is typically cause to get you replaced in a hurry. Clients just want -results-!

    Is there not any kind of diagnostic tool within Creo that can help sort out these types of constraint errors? Model tree columns that will highlight the failure? "FIXIT" button?

    In the case of the file included in this thread, the sub-components do not fail on their own, they fail at the top level. But no indication what-so-ever as to -what- is failing. Someone probably dragged a component and things cannot be put back where they were... and then they saved it! ...and purged the previous versions. Etc.

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    S.Harris1-VisitorAuthor
    1-Visitor
    November 8, 2013

    Antonius Dirriwachter wrote:

    In the case of the file included in this thread, the sub-components do not fail on their own, they fail at the top level. But no indication what-so-ever as to -what- is failing.

    Yea that's what I'm struggling with.

    I can strip everything out of the top assembly one by one until I've isolated the issue, but I still can't get the mechanism to function, and I don't know what happened to cause the failure.

    The model is definetly a mess. It is in-progress created from a lot of client iges parts, so working from the top-down would've been counter productive. I hoped there would be something obvious for mechanism modelling in Creo, like a set of best practices or something.

    Would it make more sense to model the assy's/subs as trsaditional models (ie not mechanisms), and drive them with family tables or something?

    Edit: And thanks for the link, Vladimir! I'll read through that thread carefully.

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    S.Harris1-VisitorAuthor
    1-Visitor
    November 11, 2013

    Vlad has found the error culprit, but I'm convinced this assembly should be modeled not using mechanism constraints (its not necessary for us).

    Antonius recommended using flexible constraints for the cylinder and driving them with family tables - I would really appreciate if someone could get me started; what components are flexible? Where do the flexible dimensions belong? How does the hinge function? I have not modeled a flexible component or sub assy before..

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    TomD.inPDX17-Peridot
    17-Peridot
    November 12, 2013

    I saw that tangent constraint as well but it didn't dawn on me to try to delete it. I still don't know how one can see that this was the offending feature. Glad it was resolved, however.

    Shaun... sorry I left you hanging there. It was time for me to go for my daily bike ride. As for making things flexible at the assembly level, you right click on the component in the assembly tree and select "make flexible". From there you have several option to control a parameter of the cylinder model. For instance, you can have the shaft in the cylinder constrained with a distance. You can then -overwrite- the distance with the value you specify in the flexible model dialog. Once the assembly is regenerated, it will take on the new position. I do something very similar with springs where the part model can be driven with a single height measurement. Making the spring flexible allows me to make a uncompressed spring drawing, but have the spring compressed in the assembly.

    Unfortunately, there is a flexible modeling extension so searching for help is rather difficult.

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    TomD.inPDX17-Peridot
    17-Peridot
    November 12, 2013

    ...here is a quick overview on an compressed/uncompressed o-ring model

    Making o-rings flexible in assemblies

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    staitinger1-Visitor
    1-Visitor
    May 5, 2014

    I have ran into this a number of times. Just to clear up one thing for those frustrated, just because the error message says it is Connection X doesn't mean it has anything to do with that Connection! The only thing it does seem to tell you that is useful is that it does have to do with the components that Connection X references.

    In the case I just figured out Connection 2 was fine but some other component down the structure referenced the part hosting connection 2 as well as another part. EVERY part in a mechanism hopeful assembly needs to have references to only one other part. If you have a part with non-mechanism connections that references two parts it seems to fail one of those parts in mechanism. My guess is the first part in the regen order will have the connection error.

    If I can ever figure out how to get the VBAPI to work with Creo maybe I will write a simple script that checks this and notifies you of the perpetrating part. The logic behind this is extremely simple as far as I understand it.

    -All parts reference only one other part?

    If yes it should work. If not, probably it won't work.

    TIP if you are getting that Connection X will be suppressed because of invalid constraints error, zoom out so you can see most of the assembly the error probably resides in, then try to enter into mechanism mode. It gave me an error saying the highlighted constraints are invalid and will be supressed if you continue. You should be able to see highlighted boxes representing each origin of the offending components. You can't zoom or pan (thanks Creo!) so you might have to hit cancel a few times and adjust the zoom then try to enter back into mechanism to see the highlighted boxes.

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