55 Replies Latest reply on Dec 3, 2012 7:15 AM by Mark Thompson

    Center of Mass… Why?  How?

    Utpal Kumar

      I know that this might sound like a silly question, but I would like you to tell me why you need a Center of Mass / Center of Gravity (COM) feature and how you would use it in your design process. For starters, consider these questions…

      • Why is a COM needed?
      • How would you like to use COM in your design process?
      • Do we need COM in drawings? How? Why?
      • Current workarounds and problems associated with COM?
      • Anything else you like to share / contribute to this topic.
          • Re: Center of Mass… Why?  How?
            Dinesh Gandhi

            Hello,

             

            As far as my knowledge, these are very good features from Solidworks,

            with these center of mass we will know the actual coordinates of the mass center and weight distribuation of the given mass,

             

            consider a unsymmetric rotating part which has to be balanced,( to avoid virations)

            unless and untill we know tha mass center, we may not be able to add or remove the mass from the part,

            when we have balanced part, there will be less vibration in the motion,

            • Re: Center of Mass… Why?  How?
              Dougal Hiscock

              Three common uses for the COM.

               

              Stability, transport and lifting.  There are many more.

              • Re: Center of Mass… Why?  How?
                Roland Schwarz

                I'm having trouble believing an engineer would ask such a question.  There is a multitude of problems that require center of mass.  As previously noted: stability, transport and lifting.  Add buoyancy, mounting, determining motor power required to lift or move, return spring design, anything involving calculations of speed and acceleration.

                • Re: Center of Mass… Why?  How?
                  Utpal Kumar

                  Well! Thanks for your answers explaining why COM is needed and excuse me for asking such a silly question.

                   

                  I am part of SolidWorks Product Definition team and we are trained to ask such silly questions.

                   

                  Now as you guys have shown that COM is an important part of design. Can you guys please share your experiences on how you currently use SolidWorks tools especially to capture design intent which revolves around COM of the component/assembly and its drawbacks (if any?)

                    • Re: Center of Mass… Why?  How?
                      Wayne Tiffany

                      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

                      • Re: Center of Mass… Why?  How?
                        Nick Goodrum

                        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

                        • Re: Center of Mass… Why?  How?
                          Jerry Eden

                          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

                          • Re: Center of Mass… Why?  How?
                            Dougal Hiscock

                            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.

                              • Re: Center of Mass… Why?  How?
                                Mark Greenwell

                                Hi Dougal,

                                 

                                With a bit of luck the question is being asked so the ability to add a CoG will be available in either a New Release or Service pack.

                                This ability has been asked for many times.

                                 

                                He is probably gauging the importance of this function so it can be added.

                                 

                                As has been said CoG is critical.

                                 

                                Thanks

                                 

                                Mark

                              • Re: Center of Mass… Why?  How?
                                Roland Schwarz

                                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.

                                • Re: Center of Mass… Why?  How?
                                  Roland Schwarz

                                  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?

                                    • Re: Center of Mass… Why?  How?
                                      Utpal Kumar

                                      Well... Just for the records every member in the Product Definition Team in SolidWorks holds a Mechanical Engineering Degree with ton of experience in Product Design.

                                       

                                      Now, SolidWorks very well understand the importance of center of Mass in mechanical design that’s why  Center of Mass calculations is there in SolidWorks from DAY 1 Click on Tools ==> Mass Properties to see it.

                                       

                                      Finally, my objective in this post was to gauge how this CoM information should be better presented to designers so that it perfectly gels in their design process and make their life easier.

                                        • Re: Center of Mass… Why?  How?
                                          Roland Schwarz

                                          Apologies.  They way I understood your initial post and also your subsequent comments, it was not clear that you are an engineer or had access to engineers with design experience.  I understand now that the question is geared to the specifics of how the feature is actually employed, not the broader and more basic question of how CG is used in design.

                                    • Re: Center of Mass… Why?  How?
                                      Mark Greenwell

                                      Hi Utpal,

                                       

                                      Attached are a couple of PDF files showing an ongoing contract.

                                       

                                      Boss Ass shows internal stiffeners etc, Assem 1 lid on shows same piece with the soffit plate lid added.

                                       

                                      This piece is about 40Tons and is a small part of a bridge.

                                       

                                      To add lifting lugs to be able to turn / move around the shop, we need to know CoG at certain stages of Fabrication.

                                       

                                      All Major Fabrication structures which are moved now require some sort of lifting Plan, which are based around knowing where the CoG is located.

                                       

                                      Thanks

                                       

                                      Mark

                                      • Re: Center of Mass… Why?  How?
                                        Keith Parker

                                        Just to add my two-pence-worth, I used to do a lot of work on full-motion flight simulators that are almost as mass-sensitive as aircraft.  A COG indicator would have been invaluable during the design process to ensure that the motion-platform loads were kept within limits.  This applies to part, assembly & drawing use.  Workaround is either a macro or information from mass properties.

                                        • Re: Center of Mass… Why?  How?
                                          Utpal Kumar

                                          From the responses it looks like a CoM/CoG visual indicator similar to what we see when we launch mass property dialog (see image below) can suffice the need.

                                          Or

                                          Do we need the CoG/CoM to be more than a visual Indicator, A entity which can be referred to create dimensions, mates, sketch relation etc?

                                           

                                          COM Indicator.jpg

                                          • Re: Center of Mass… Why?  How?
                                            Ronan Carrroll

                                            We design motion control camera movers for the film industry. The rigs weigh anything from 100Kg to 2500Kg

                                            Many have counter balanced arms. I have spent many hours pouring over the Mass and Inertia data that solidworks calculates.

                                            Both CoM and Inertia (as I calculate inertia relfection through gear reduction) are of major interest.

                                            It would be very helpful to display the different co-ordinate inertia triads in seperate colours. Have 3 different icons that can

                                            display the triads (with their respective Ixxs and Lxxs) by clicking on them. Every time i have to refigure these out as to which

                                            axis is which. See attached graphic.

                                            A help link that explains the definitions Px,  Ixx, Lxx etc would be good too.

                                            • Re: Center of Mass… Why?  How?
                                              Tobin Sparks

                                              Howdy,

                                              Attached is what we use when we can. It's not really ready to release, so you'll be a tester :-) .

                                              It's limited to SW2010 x32 and Excel is required :-( . There may be other unknown limitations since I'm very limited as to what I can test with.

                                              Read the CofG.txt carefully because setting up all the associated files is critical.

                                              This was put together to automate features we use in order reduce the time it takes to complete some drawings.

                                               

                                              Hope this helps

                                                • Re: Center of Mass… Why?  How?
                                                  Mark Thompson

                                                  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.

                                                • Re: Center of Mass… Why?  How?
                                                  Scott Parker

                                                  I use the center of mass all the time in designing vibratory conveyors.  The center of mass of each component is important for proper running characteristics of the machine, as is the overall assembly center of mass for stability.  The center of mass is currently used in the design process as follows:

                                                   

                                                  • The CG of the conveyor pan is noted and a sketch line is drawn from that point.  Dimensions from the origin to the CG point are placed
                                                  • The center of the drive system is mated to the sketch line, so that it is always directing its energy through the pan CG
                                                  • As the design is iterated, the CG is recalculated and the sketch dimensions are updated manually

                                                   

                                                  What would be useful is to be able to use the CG location in an equation or as a parameter driving other dimensions.

                                                  • Re: Center of Mass… Why?  How?
                                                    Jeff Wierengo

                                                    We recently manufactured a machine that was adjustable for parts that ranged from 6" long to 48" long. The part fixture at one end of the machine was fixed and the other adjustable. Both of the fixtures together biased the center of mass quite a bit to one end of the machine. Due to a leg in the center of the base we were limited in where it was possible to pick the machine up with our fork lift to load onto a truck for shipping. I used the center of mass feature to determine where the adjustable end of the machine should be positioned and where the fork lift should lift, which was off center, to be able to load the machine safely. I agree that it would be helpful to have a permanent feature in the model to indicate center of mass rather than just coordinates.

                                                    • Re: Center of Mass… Why?  How?
                                                      Scott Layman

                                                      Add me to the list as well. I am a complete newb to SW but one of the benefits to me will be the ability to locate CoG on assemblies of the compressed air skids we build. We provide packages to the Navy and nuclear industries so we need to provide data for seismic and shock and vibe analysis.

                                                       

                                                      Thanks

                                                       

                                                      Scott

                                                      • Re: Center of Mass… Why?  How?
                                                        Mike Vis

                                                        Very important to the ergonomics of workers handling parts by hand also.  Not just in lifting large items with cranes or other means  I would like to see that i can turn it on and off both in assemblies, parts and drawings.  Also be able to make mates to it would be wonderful.

                                                        • Re: Center of Mass… Why?  How?
                                                          Tyler Kemp

                                                          im glad this topic came up because I am interested in using it, however have not been able to figure out how. I am the only SW operator in my company and often have to learn things on my own, and i havent had any luck using the help menu to learn. Anyone willing to give me a crash course? Thanks

                                                          • Re: Center of Mass… Why?  How?
                                                            David Anderson

                                                            i lift things up and put them down.

                                                             

                                                            the necessity for knowing COM should answer itself unless you design software

                                                             

                                                            without knowing the COM, i could not have balanced this camshaft prior to build. sure would have been nice to have a feature that could be tracked as the "lightening" holes were tweaked on each individual cam.

                                                            camshaft.JPG

                                                             

                                                            please add the capability to place a coordinate system that is centered on the COG and aligned to the principle axes of inertia.

                                                            • Re: Center of Mass… Why?  How?
                                                              Hari Padmanabhan

                                                              From the thread I see that there would be one COM per document. What about multi-body parts? Would we just internally treat it as an assembly with many parts and just give you one COM for the entire part or would you prefer that we would allow you have many different COM's for one or more bodies? The only use case I came come across for this type of scenario is "Bottles (one COM for the hollow plastic bottle and one COM for fluid volume and one COM for the final volume (solid+fluid))"? Any other used cases for multiple COMs per document?

                                                                • Re: Center of Mass… Why?  How?
                                                                  Dougal Hiscock

                                                                  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.

                                                                    • Re: Center of Mass… Why?  How?
                                                                      Philip Lewis

                                                                      You can do it in weldments for sure. I think you can do it in weldments in 2009. You have to change the material in the cut list ares and not in the features of the feature manager. If you go into the cut list and right click on a body you can apply a material there. What we do here is turn off the automatic cut list and order our pieces more logically (outside beams first then full length beams then support beams) than the random way solidworks does it. We can then pick all the pieces we put together and give them a material at once. This probably isn't as necessary now that you can re-order the cutlist on the drawing but I find it helps me think things through.

                                                                       

                                                                      BTW: If the parts don't seem to be taking the material check what configuration the cut list on the drawing is using. The "as welded" derived configuration may also need to be updated.

                                                                      • Re: Center of Mass… Why?  How?
                                                                        Keith Parker

                                                                        Philip - The ability to assign materials to separate solid bodies was a new feature in SW2010.

                                                                      • Re: Center of Mass… Why?  How?
                                                                        Keith Parker

                                                                        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.