SW Motion Analysis and Centrifugal Forces

[TheBigMike]

Hello 3DCAD Forums,

New member, first post!

First some background:
I've searched around on Google and can't find help on this. I also registered on a CAD-specific forum and it's been half a week and no one has replied lol. I have a bachelors degree in Mechanical Engineering and I've been to this site many times over the years and finally decided to register.

I am not sure if this is the place to ask about Solidworks questions, but I know this forum is very active and many here use SW, so here goes. (I am using SW2010)


My question:
Do I need to do anything special to my assembly in order for SW to consider centrifugal forces in Motion Analysis?

For example, I have a rotating disk with two face-mounted pegs near its edge spaced 180-degrees apart. I then have two beams with one hole on the end of each beam that fits over each peg. I've added a rotary motor to the disk and set some arbitrary RPM, say 500. I give a material to everything, say 1020 Steel.

Using Motion Analysis, what happens is that when the motion begins, the arms immediately move inwards towards the center and then rotate back out -- and back around again, indefinitely. I could be wrong but I'm pretty sure they would move outward by centrifugal force and would eventually remain straight out forever (at constant speed), not continue to rotate around and around and around literally forever.

I've done this with the pegs on the leading side of the arms (where the pegs are pulling the arms) and with them at the trailed side (where the pegs are pushing the arms), I've done really fast RPMs, I've added gravity, all no change.

I have added friction to the concentric mates, which does allow the arms to remain extended as I'd expect, but with a more complex model that I am working on, friction is just a band-aid and not a centrifugal force enabler in SW.

Am I going crazy or is there some setting I'm supposed to enable before SW will consider centrifugal forces?

Here is a short video of what SW is doing:

http://s704.photobucket.com/albums/ww46/rakudasama/?action=view&current=TestAssembly.mp4

I really don't think this should happen. It continues on forever. I realize there is no air resistance or friction in this video, but what happened to centrifugal forces, how to I enable them?

Thank you very, very much in advance!
BigMike

 

[desertfox]

Hi BigMike

I can't see the video unless I download flash software and when I try its says no software available for my comp.
Any chance you can upload some still pictures of your problem?

desertfox

 

[TheBigMike]

No problem desertfox, thanks for the help!

Here is the video converted to GIF format (I should have just done this in the first place). See attached file.

So it just loops like this forever and ever...

This is with a constant velocity motor applied to the disk. I've also added an acceleration component which confirmed by the equation for centrifugal force, made no difference.

As described above, I've done this with the arms both in front of and behind the pegs, so where the pegs are both pushing and pulling the arms and it makes no difference.

I know there is a way to add a Centrifugal load in a Simulation study, but I need to be able to demonstrate the effects of the centrifugal force in a video, and I will use the analysis feature of the Motion Analysis to check the force values against my hand calculations.

Thanks for the help!
BigMike

 

[desertfox]

Sorry BigMike can't open the GIF file either

 

[TheTick]

SolidWorks Motion can not measure centrifugal force because there is no such thing.

http://phors.locost7.info/phors04.htm

The solver at the root of SW Motion does not like perfectly elastic systems. It anticipates that there will be error correction due to dampening, and has trouble when it can not correct.

You can add dampers as needed to your model. You can also modify individual mates with properties like friction and dampening.

Also try starting your motor from zero and accelerating up to a given speed instead of instantaneously having a speed. Otherwise, you have a singularity in your system due to the motor being at speed but your parts needing to accelerate from zero.

 

[GregLocock]

Well... I'm sure the pins in that simulation will be perfectly happy to demonstrate a centrifugal force, whatever those in a fixed frame of reference might think.

An alternative is to set the initial conditions up for the sim such that it is in equilibrium initially. That means you'll have to give each body appropriate velocities.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[IDS]

SolidWorks Motion can not measure centrifugal force because there is no such thing.

http://phors.locost7.info/phors04.htm

http://xkcd.com/123/

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[zekeman]

"I have added friction to the concentric mates, which does allow the arms to remain extended as I'd expect, but with a more complex model that I am working on, friction is just a band-aid and not a centrifugal force enabler in SW."

Without friction there is no way you can get your ideal centrifugal result.

If you did this analytically, you would get an inhomogeneous nonlinear second order differential equation with the motion depending on initial conditions.

So I don't think the SW solution is necessarily wrong