Measuring center pin resistance

[Stéphane Collin]

Hi Phil.

I liked this post.
Precision of measurements is a great concern.  Even better would be defining 
clearly (and in my opinion this only can be done by empirical tries) how 
precise you need to be to be pertinent in what you are doing.  Every 
measurement you do in a piano should be accompanied by a relative error of 
measurement discussion (which should be easy) and an evaluation of which 
precision of measurement is still pertinent in relation to the goal you 
reach (which would need a lifetime careful observations).

Best regards
(and admiration for your scientific abilities)

Stéphane Collin.

----- Original Message ----- 
From: "Phillip Ford" <fordpiano@earthlink.net>
To: <pianotech@ptg.org>
Sent: Sunday, December 19, 2004 4:53 PM
Subject: Measuring center pin resistance


>I wanted to make some additional comments about center pin friction. Rather 
>than try to respond to various posts I thought I would start a new thread. 
>Comments below:
>
>     With regard to measuring with a gage vs. measuring with your finger 
> (or whatever part of your anatomy you choose to measure with) - the end 
> result is to get an accurate measurement within an appropriate tolerance 
> that is consistent and repeatable.  If you can do that with your finger 
> more power to you.  I've been doing this a fairly long time and feel that 
> my fingers are fairly sensitive.  But I've repeatedly proven to myself 
> that my fingers are nowhere near as accurate as a Correx gage.
>
>     Speaking of gages - I'm not talking about one of these $5 leaf spring 
> gages sold by the supply houses (I assume they still sell those things). 
> I'm talking about an accurate gram gage of the appropriate range, such as 
> a Correx gage.  Such a gage will set you back $100 (US) or more.
>
>     I can see a difference between checking to see if a jack is 
> functioning on a console in the field and setting up a performance piano 
> action on the bench.  In the first case I probably wouldn't 'whip out a 
> gage', but would check with my finger.  But I certainly don't see anything 
> wrong with using a gage.  In the second case I would certainly use a gage.
>
>     I think some of the confusion apparent in the recent exchange is 
> evidence that we haven't developed or put in place the proper words and/or 
> numbers for unambiguously communicating information about center pin 
> resistance.  To begin with, I don't believe there is consensus about what 
> word is used to describe what we're trying to measure.  Friction, 
> resistance, stiffness, other?  Any votes on which seems most appropriate?
>
>     The use of a gram number to describe center pin resistance is 
> incomplete at best, and misleading or simply wrong at worst.  This harks 
> back to discussions of measuring or describing downbearing.  To say that a 
> center has 5 grams of resistance is like saying that a piano has 5 
> thousands of downbearing.  What does it mean?  Nothing without further 
> information.  In both cases you have to know the measuring point for the 
> number to have any meaning.  Describing downbearing with an angle is 
> unambiguous because it removes the measuring point from the description. 
> Describing center resistance as a torque is unambiguous because it removes 
> the measuring point from the description.  Perhaps it's time we started 
> using torque to specify center resistance.  Rather than specify 4 grams 25 
> mm from the center pin, why not specify that the resistance should be 100 
> gram-mm?  This seems more accurate and less subject to confusion between 
> sender and receiver of the information.
>
>     With regard to the traditional tests that have been mentioned - the 
> swing test and the 'Steinway test' (I assume that what is meant by the 
> Steinway test is to tap the shank and see if the flange will rise) - I did 
> a few measurements on a Steinway flange to give some perspective to this. 
> I repinned the flange several times to make it match the extreme ends of 
> these two traditional tests.  All numbers below were taken on a old 
> Steinway shank and flange in good condition (I didn't try this on a new 
> shank and flange, so your mileage may vary).  All gram measurements were 
> taken with a 2-15 gram Correx gage at a measuring point on the flange to 
> the side of the screw hole, 23 mm from the center pin.
>
> SWING TEST - depending on your source I've seen anything from 3 to 7 
> swings specified:
>
> For 3 swings:  7 grams at the measuring point or 161 gram-mm torque.
> For 7 swings:  3 grams at the measuring point or  69 gram-mm torque.
>
> STEINWAY TEST - tap hammer shank and see it flange will rise:
>
>    Loosest pinning at which I could get flange to rise:  below the lowest 
> indicated mark on the gage (somewhat less than 2 grams - for the sake of 
> argument let's call it 2 grams) at the measuring point or 46 gram-mm 
> torque.
>
>    Tightest pinning at which I could get flange to rise:  8 grams at the 
> measuring point or
> 184 gram-mm torque.
>
> RESULT:  The range in torques between minimum that meets one of these 
> tests and max that meets one of these tests is 46 gram-mm torque and 184 
> gram-mm torque.  Or in other words, if you had two adjacent flanges that 
> met these traditional tests you could have a potential difference of 184 - 
> 46 = 138 gram-mm torque.
>
> To put that number in perspective - for those who do hammer weight or 
> strike weight smoothing per Stanwood - the tolerance that I work to when 
> matching strike weights to a curve is 0.1 gram (I don't allow any 
> individual SW to be more than 0.1 gram different than the number specified 
> by the curve).  So, max difference in SW between two adjacent hammers is 
> 0.2 grams.  Typical distance from hammer center line to hammer center pin 
> is 130 mm.  Variation in torque between two adjacent flanges caused by 
> variation in SW = 0.2 x 130 = 26 gram-mm.
>
> As you can see, the possible difference in torque between two adjacent 
> flanges from using traditional tests is more than 5 times the difference 
> caused by the variation in SW.  In my opinion, if we're going to go to all 
> the trouble to smooth SWs, and to do the sorts of things that Jon Page 
> describes, such as sorting shanks by weight, and knuckles by size, in 
> order to get as smooth a progression as possible of touch from one end of 
> the keyboard to the other, then saying - don't bother with a gage, just 
> use the traditional tests - doesn't make sense.
>
> Phil Ford
> San Francisco, CA
>
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