Center of Mass… Why?  How?

Since a lot of what we do is pick up things, COM is quite important.  I have a macro that was written by Jim Sculley and critiqued by a group that will put a 3D sketch point at the COM and then update that sketch point on every rebuild.  Quite handy for watching that point as the project progresses.

  WT

Here is a recent example of me having to use COM:

We have a vehicle roof that is extremely heavy. It needs to be moved around the shop floor and placed on skids, tables, carts ect..

It is not a symetric part, therefore it has a lop sided COM. I used the COM to calculate the proper lengths for 4 straps hooked to a single crane hook to lift the part so that when the part was lifted the center of mass would be directly under the crane hook at full suspension.

One drawback, correct me if im wrong...the only way to show the COM as a 'point' is to use a macro.

There should be an option within SWx to create that point and turn it on and off rather than just giving the cooridinates only.

Regards,

NG

Hello Kumar,

I do not think this is a silly question at all. I have over thirty years in aerospace and CG and mass are very important components in the design process. As the design processes so does the mass. In Aircraft the CG and mass are ultimately what everything is based on. You have to know this for all of the components and the various systems.

Most companies have entire groups that are supporting the project with stress, materials, weights, and overall design. Without this info you are doomed to failure.

As far as the silly question is concerned all questions need answers silly and otherwise.

thanks,

Jerry

It concerns me a little that a solidworks employee has to ask this question in the forum.  Does solidworks not have any engineers or even designers on staff that could explain this internally?

Basically COM is critical.  Wayne, thank you for passing on that Macro.

Product Definition Team needs engineers.  Has it occured to Dassault that it might be beneficial if engineers had a hand in developing engineering tools.  There is far more to design than shuffling sketches and features around.

I used to design spring-assist mechanisms for automotive interiors.  CG is essential for determining spring forces needed to lift a console lid.

Currently designing hospital beds.  CG is vital to determine amount of motor power to lift at a given speed.  Also important for determining tipping point.

The stability of anything used in the water depends on the relationship between center of buoyancy and center of mass.

Please excuse my digression, but this has been on my mind since your question first posted...

What is the makeup of the Product Definition Team?  It does not bode well fr the future of SolidWorks that there was no one on your team who could see the importance of center-of-mass in mechanical design.  Does the team not have a mechanical engineer?

Good example; thank you for posting.  But seriously, Boss Ass?  I have seen some funny engineering terms that crack me up when not put in context, and this one will have to be added to that list!

We need both: a visual indicator AND something to mate/dim to/etc.  Thanks in advance...

Oh - and we need to be able to suppress/hide/etc it too.

You can add my name to the list. We need both as kieth parker suggests. Look forward to seeing it in future releases.

Regards

John Matthews

Hi Utpal,

As has been said do not remove the Mass Property dialog box function.

Rather you want to add a function similar to how the CoG macro (which lots of us use) works.

That is a CoG can (on a click of an icon) be placed on either a part or an assembly.

This CoG will update with any changes to part or assembly.

The CoG will also be available (if desired) in a Part / Assembly drawing, dimensions can also be taken to this CoG while in a drawing.

I think if you do a search for CoG you will find a lot of posts on this topic.

Also look to see how Autodesk Inventor does this function.

Thanks

Mark

Uptal,

From the responses to this issue COM has been a huge oversight by SolidWorks.  Perhaps SW can include this functionality in the next SP???

Hi Utpal,

The Center of Gravity functionality DESPERATELY NEEDS to be enhanced.  We need to be able to display the CG on the model at any time (Just like displaying an axis).  We need to be able reference it for:  Mates, Dimensions, and Sketch Relations.  This is probably one of my biggest issues in SolidWorks right now.  Anyone who works in the Aerospace industry can feel my pain on this one.  (As well as folks from many other industries).  The many macros are nice, but this should be standard functionality within SolidWorks.

Best Regards,

Ricky Jordan

Lead Mechanical Engineer

Space Vehicles Department

Dynetics, Inc.

CSWP

http://www.rickyjordan.com

Ronan - I agree with your Inertia-triad concept, having also battled with Mass, Inertia, etc and their axes in the past.

I've had a look at the macro and it seems very promising, however, since I don’t know how to address the first action “1) Reference to Microsoft Excel (version) Object Library”, I get the following error:

"Run-Time error '91'

Object variable or With block variable not set"

I honestly don't know whether this is because I'm running a more recent version of solidworks (2012) or because I have no idea how to set up the macro...

Also it appears this marco only references the CG of the entire assembly rather than each individual part relative to a common reference point (e.g. the assebly origin). Is there any way to expand this to the latter and get a exel file listing each individual parts mass and x,y,z distance relative to the origin?

I'd really appriciate your input as this would literally save me hours of work...

Thanks in advance!

Mark.

The files that people supplied up above are macros.  You run the macro and it will put a sketch point or sometimes a origin to show the center of mass of the part you have on the screen. Macro's are under the tools, macro, run.

I typically use configurations for that.  I suppress or delete bodies selectively to give the mass result I want.  Sometimes I save bodies or groups of bodies out for the same purpose.  Usually because someone wants to know how heavy a specific plate or beam in a weldment is.

Can you set different densities for different bodies now?  I can't in SW2009 but it would be helpful for some parts which feature multiple materials or just aren't worth modelling as an assembly.  This would be necessary for your bottle example.

Hari - I think that if you have a multi-body part, a seperate COM for each body AND a combined COM for the whole collection of bodies is the answer.  With the ability to assign individual materials to the separate bodies, it would be daft not to have individual body COMs available.  I have no immediate example to quote - I'm talking about the general case.

Also, I echo David Anderson's point above about the position and orientation of the COM.