Hello
I'm getting a very strange behaviour with a Basic Motion study. I have a study with 2 parts with no mates, no motor, no gravity, and set solid contacts. As soon as the study begins, the parts fly off the screen. How come the parts move even when there's no external force exerted on them?
Here's a video: Parts flying off - YouTube
Thanks
John
John was going to take a look at this but you havent attached your part files along with the assembly
Hi Martin,
Sorry about that. I've added the parts now.
Thanks
I think the problem is with the solid body contact. If I am reading this right the bodies don't actually touch each other? Removing the contact removes the strange motion
Edit: I believe (sorry if I am wrong) that by defining contacts you are telling SW that these parts contact at some point and this might be causing your issues.
I've added a tangent mate between the parts, so they are now meant to touch. But they still fly off.
It has to be to do with the solid body contact. I believe that applying a solid body contact (from SW help "Include contact in a motion study to apply a force that prevents components from passing through each other during the motion calculation:") that it creates a force to stop bodies going through each other. If the bodies are unconstrained i.e no mates to hold them in place then they might have unexpected movement.
Hi Martin,
If SW somehow applies a force to prevent pass-through, the study could become quite wrong since there would be latent forces all over the bodies.
Isn't it fairly often that the bodies are unconstrained? For instance, in my second motion study, I'd expect the connector to stay inside the ring. That'd be the only intuitive result - right? The two bodies are touching though.
The forces would be small and if the bodies are unconstrained completely (obviously some level of non constarint is necessary for movement) including no gravity then if any force acts upon them then movement of those bodies is inevitable?
It seems it is the contacts. Just had a check if you go to:
And set the contact resolution to max
The parts no longer fly off
Oh! That's interesting. I looked up resolution in help, but it's not entirely clear what differences low and high are meant to produce. It seems to suggest that a high resolution would give a smoother motion. Does that mean it's equivalent to lower friction on surface? Thus, resolution is essentially friction?
For solid bodies, shouldnt a high resolution be always used then? Are there downsides to using a high resolution?
Thanks
As with most Simulation the higher the resolution the longer it will take to solve. With this motion it's obviously not too intensive but if there are lots of contacts and motion it might be a trade of between resolution and time to solve.
Actually, it seems like setting Geometry Accuracy to High already keeps the bodies still:
I wonder why the geometry of the bodies affect the exerted forces.
It seems it is the contacts. Just had a check if you go to:
And set the contact resolution to max
The parts no longer fly off