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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.

walterwalter 1st April 2016 10:26 AM

Effective mass mess
 
4 Attachment(s)
Effective tonearm mass ( E.M.) seems to be very uncertain and messy theme. It would be highly appreciated, if some serious professionals share some light here. There are many tonearms with unknown effective mass, as well as recommended cartridge compliance (not mentioned in manuals). Even in manual on classic SME3009 it is not there.
Question is: how to determine said E.M. of tonearm already owned? Or self-made?
I would guess, that E.M is equal to momentum of inertia. If we accept tonearm mass equal to its weight, than E.M should be equal to it's mass applied on stylus point. So, if we take off tonearm rear section with counterweight (pic. #2), and than measure gravity force applied on stylus point, using stylus force gauge (pic.#2), we should get E.M of tonearm (not counting E.M of rear section with counterweight)? Of course, E.M. together with cartridge, shell and mounting screws. Tonearm here is Micro MA101, and it's E.M. is unknown. Results are: arm+shell+ cart= 27,3g, shell+cart=20,9g, so

Am I on a right path?

DF96 1st April 2016 10:48 AM

My understanding is that effective mass is the integral of 'mass x distance from pivot', divided by 'distance from pivot to stylus'. It is mass rescaled according to distance, but unlike calculating downforce all mass counts as positive.

If you could remove everything beyond the pivot (i.e. the counterweight end), and assume that there is nothing above the pivot (i.e. everything either contributes to downforce or upforce) then measuring the downforce at the stylus would give you approximately half the effective mass. This is because the end you have chopped off must contribute roughly the same weight (but in the opposite direction) in order to get balance, but it adds to effective mass while subtracting from downforce.

Hans Polak 1st April 2016 04:40 PM

It is hard to calculate the effective mass from weighting the single elements, but it is more accurate to calculate the effective mass from:

Fres=1/{2*Pi*SQRT[C*(m+M)]}.

Fres is the resonance frequency of your arm including element, C the compliance of the cantilever suspension, m the effective mass of the cartridge and M the effective mass of the arm.
m and M in Kilograms and C in meter/Newton.
As an example: suppose m=10 gram, M=5 gram and C is 15um/mN, then Fres is 10.6 Hz.
The resonance frequency can be measured with a test LP, and should be between 9 and 12 Hz.
11Hz seems to be the optimum according to several publications.

Hans


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