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IZHAKKATZ 5th August 2007 02:25 PM

effective mass
 
2 Attachment(s)
Good people,

Attached is a file for getting an arm eff. mass I downloaded from the net.
however counter weight doesn't seem to effect the effective mass.
Doesn't make sence does it?

Thanks
IK

SY 5th August 2007 02:43 PM

It really doesn't have a huge effect- think of the lever arm which minimizes the effect of that mass on the moment of inertia. The counterweight is close to the pivot, maybe a cm or two. The stylus is something like 25cm away. That's a serious lever.

AndrewT 5th August 2007 03:15 PM

Hi,
cell E22 references a blank cell.
This may be an error.

The counterweight does have a significant effect.
That is why a few manufacturers chose tungsten for this component, simply to allow a more compact weight that could be brought closer to the pivot and thus lower the overall effective mass, even though the very high density resulted in a heavier counterweight.
Has anyone seen advertising for an osmium counterweight?

I cannot see why the horizontal moving mass is so much higher than the vertical effective mass.
What am I missing?

maxro 6th August 2007 05:40 AM

Quote:

Originally posted by AndrewT
I cannot see why the horizontal moving mass is so much higher than the vertical effective mass.
What am I missing?

The spreadsheet appears to be for a linear tracking arm. The horizontal moving mass is simply the total mass of the arm.


edit: typo correction

IZHAKKATZ 6th August 2007 07:37 AM

thank you for the info...

does anyone have a spreadsheet for a pivot arm?
can anyone give the formula for calculating the effective mass?

maxro 6th August 2007 08:04 AM

I would think this one ought to work just fine for a pivoting arm using only the vertical results.

AndrewT 6th August 2007 09:48 AM

Quote:

The counterweight is close to the pivot, maybe a cm or two
Hi Sy,
I don't think so.
The gap between the counterweight and the bearing housing could be less than 10mm. But then one must add half the housing diameter and the distance to the counterweight CoG from that near face. This COG of counterweight to arm bearing distance could well be 30mm to 60mm.


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