Hartman Downbearing Gage vs. Lowell Gauge

[Phillip Ford]

>David,
>
>Didn't the photos show it all? It is a straight stick of wood with a notch 
>cut in it. You hold it flat on the string plane and measure the gap 
>between the end and the string rest. I showed the stacking gauges I use. 
>This tells me what the bearing condition is. I am only interested in the 
>total bearing not whether the bridge is tilted or what. In the type of 
>pianos I work on I have never sean a situation where a slight tilt to the 
>bridge was found to be the source of a tonal problem. It may be a symptom 
>but not the cause. I guess somewhere there are pianos with serious bridge 
>role, but expect that by the time this develops many other things have 
>gone wrong as well.
>
>David Skolnik wrote:
>>John Hartman - John Hartman
>>Can you elaborate as to how you use the method and tools to which you 
>>refer below in measuring downbearing in a strung piano?
>
>John Hartman RPT

      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