Hartman Downbearing Gage vs. Lowell Gauge

[John Hartman]

Phillip,

Was this post addressed to me? If so thanks for the trigonometry lesson. 
Believe it or not I was aware of these issues. If I want to find the 
deflection angle I can simply look it up on my chart. I could even find 
the bearing force if I wanted to on another chart. BUT I generally don't 
bother with this since the bearing tools I use measures the bearing in 
just the same way I set it. When setting bearing on a new board I don't 
try to find the angles or force I simply use the stacking gauges. The 
same stacking gauges are used to measure the bearing after the strings 
go on to monitor the bearing over time. If I used the bubble gauge I 
would have to figure out how it relates to the stacking gauges to get 
the information I need.

Phillip Ford wrote:

>       The static downbearing load from a given string is dependent on the 
> string tension and the angles the front and back segments of the string 
> form with the bridge, and is independent of the lengths of the front and 
> back string segments.  From what I can see, what your tool is measuring is 
> the vertical distance from the back end of the tool to the aliquot.  This 
> means that for different backscale lengths the same reading would result 
> from different string angles (and thus different downbearing loads).  I see 
> that your gage blocks are marked 1, 1.5, etc.  I'm assuming that those are 
> millimeter thickness.
>       By way of illustration let's say that the front segment of the string 
> has a zero angle (any angle will do - the result will be the same).  Let's 
> say string one has a backscale length of 100 mm.  Let's say your tool and 
> gage show 1 mm of 'bearing'.  The angle of the back segment of the string 
> is 0.573 degrees.  The downbearing load is sin (0.573 degrees) x string 
> tension.  Let's say string tension is 150 lbs.  The downbearing load is 1.5 
> lbs.
>       Now let's say string two has a backscale length of 150 mm.  Let's say 
> your tool and gage once again show 1 mm of 'bearing'.  The angle of the 
> back segment of this string is 0.382 degrees.  The downbearing load is 
> sin(0.382 degrees) x string tension.  For string tension of 150 lbs the 
> downbearing load is 1.0 lbs.  The downbearing load for string two is 2/3 of 
> that for string one, yet your tool says that the 'bearing' for both is 
> the same.
>       The Lowell gage, on the other hand, because it is measuring string 
> angles, would indicate that there is a difference in downbearing between 
> these two strings.  I don't see that there is anything difficult or time 
> consuming about using the Lowell gage.  You place it either on the front or 
> back segment of the string.  Zero it.  Measure the angle of the other 
> segment.  That angle gives you the bearing.  About the only way it could be 
> simpler was if the bubble gage was calibrated in more sensible increments 
> of angle than 1/6 degree and if the degree units were actually marked on 
> the gage (and if he hadn't included instructions with the gage which make 
> something simple seem complicated). 
> 
> Phil Ford
> 
> 
> 
> Phillip Ford
> Piano Service and Restoration
> San Francisco, CA
> _______________________________________________
> pianotech list info: https://www.moypiano.com/resources/#archives
> 
> 



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John Hartman RPT

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