Another bearing question - call for expertize

[John Delacour]

At 10:00 pm +0200 10/5/07, StŽphane Collin wrote:

>Maybe you can enlighten poor me on a good procedure to make these 
>important measurements.

I have lengths of square hollow section aluminium tubing about 10 x 
10mm which you can slip between the framing and the bars.  These will 
not sag under their own weight over the distances involved, and 
besides, the job is easiest done with the piano on edge.  Drill and 
tap the aluminium rod in the middle and put a screw there.  Align the 
rod with a rib and gradually turn the screw out until when you tap on 
the rod it makes no sound.  Then remove the rod and measure with the 
depth gauge of a vernier caliper the distance to which the screw 
protrudes.

It's worth noting that the bridge does not, of course, run along the 
crown, but, for most of its length quite a bit downhill from it.

The length of your rod (L) is the chord of a circle and the amount 
the screw protrudes(H) is the perpendicular from the chord to the 
perimeter of that circle, called the 'segment height'.  Suppose your 
rod is 740 mm. long and the screw is turned out 3.7 mm. Circle 
formulas can be found at <http://www.1728.com/circpart.htm> and there 
is also a calculator <http://www.1728.com/circsect.htm> if you don't 
want to use the formula.  Check 'Chord & Segment Height' and type in 
0.740 and 0.0037 and it will calculate the radius to be 18.502 metres 
(60.7 feet).

>>Q2: Among your experiments to improve the sound, did you include any
>>tapping down of the strings at the bridge?
>
>I must say I didn't.  Like you said, intuition plays a large role in 
>diagnose of any problem.  My intuition told me there was no string 
>to bridge contact problem, as there was unusually few false beats in 
>this piano.

Fine, but I think the 5 minutes spent tapping down are worth it in 
order to remove at least one source of possible unpleasantness, which 
may not be only falseness.

>Q3: Have you removed the metal frame yet and if so, did you get any
>unpleasant surprises?
>
>Not yet.  Do you mean the tension you often find in Bechstein frames 
>?  I indeed found out that there was some tension in the frame in 
>many Bechstein pianos I restored.  Why should this be an unpleasant 
>surprize ?

It was for me when a Bechstein B frame rose 2 or 3 inches above the 
seating at the toe when I undid the rim screws.  I could not risk 
replacing it in that state, since cast iron is not meant to be bent 
that much.  As a result, and after checking with an engineer, I made 
adjustments to the seating to bring the frame as close as was safe to 
its proper seating.  All in all the job turned out well enough, but 
it was impossible to get the downbearing right at the bass/tenor 
break and this was all too audible to the trained ear.

>Right you are, of course.  This is also my first thought of it.  But 
>I thought twice and considered that even if the overall downbearing 
>force was less, the individual downbearing of the strings in the 
>middle section was more, as the board went up when unstringing the 
>trebble and the bass.

Yes, but my idea was that excessive downbearing on the bass bridge 
was depriving the middle of pressure.  It's a while since I saw an 8' 
Bechstein, so it would be useful to see a picture of how the bridges 
are arranged.  At any rate, you seem to be satisfied that there is 
plenty of resilience in the board, so one way or another the load can 
be spread to give a good result, it seems to me, so long  as that 
poor old green iron frame allows it.

JD