Measure the center (axis) of my mass to the center (axis) of my cylinder

[Ralph2]

I have modeled a cylinder with a spiral pattern of teeth. I would like to measure, for the purpose of dynamically balancing, the distance between the axis of the mass and the axis of the cylinder. Is this possible.. and how would one do this?

Mass properties seems to deal only with the absolute center. I want the distance between those two axis measured at the ends of the tube.

Thank you for your time
Ralph

 

[ctopher]

I have done this before by faking it (sort of).
Add a 3dsketch point at the coordinates of each, then measure between the two.

Chris
SolidWorks 11
ctopher's home
SolidWorks Legion

 

[Jboggs]

I have done the same as ctopher in the past myself. My purpose was to locate a connection point for lifting a platform of an unusual shape. All you do is read the X,Y, and Z offsets in the Mass Properties dialog box and create a sketch (2d or 3D) that places a sketch point on or near those offsets. That way that sketch point will be very close to the center of mass. Of course it will not update if you change geometries.

I have read that SW 2013 will allow you to treat centers of mass like a sketch point. VERY HANDY INDEED!

 

[Ralph2]

Thanks Chris
I did not know that was possible.. will try it. As I understand it though (without trying yet)that will only give me the distance at the center of gravity which will be close to the mid point of my rotor. I would like to determine the distance at the ends of the rotor.

I don't know how practical this info will be. This rotor is what is known as a brush mulcher, attaches to the front of an all terrain vehicle (caterpillar.. BobCat) and spins at somewhere in the 1800 to 2700 RPM range. It then drives through the brush.. and anything less than ~4" diameter gets pulverized into chips. Used to clear land or seismic trails. Anyway.. the tooth pattern varies but in general it is a two start spiral. As the teeth and holders each weigh ~18 odd pounds any unsymmetrical placement creates a balance problem. As I am the guy who has to balance these I have often wondered if one could design one that is balanced by design.

Thanks again for your time

 

[Jboggs]

The Center of Mass is expressed in three dimensions: X,Y, & Z. Whichever one of those axes is parallel to the axis of your shaft is the one that will apply at the bearings at the ends.

 

[Ralph2]

Thanks Jboggs.
Unfortunately the axis of the mass will only be parallel to the axis of the rotation when one has a true static unbalance. If it is not one has a dynamic unbalance and can only be corrected by utilizing two or more planes. The distance these two lines are apart when measured at the bearing planes represents the amount of unbalance in what ever units one is using (typically in the US / Canada gram.inches). This distance is very small and for this rotor, 1300 pounds at 2200 RPM and balanced to ISO 1940 G2.5 works out to 125.6 gram.inches per plane (@ each bearing), at tolerance the two axis are <0.000213" apart.