duplex bearing angles

[Dennis Johnson]

At  7:06 PM 3/27/95 -0700, David Porritt wrote:
>What tool do you use to measure the angle?
>


___________

       Guess I should have given more detail. This is not a straightforward
measurement, but I have a technique I've used for some time and can't even
remember where it came from. I am not compulsive about measuring the duplex
angle however, and anyway the issue is more about solving problems when
they occur. If you do find a problem in this area though, it certainly is
easiest to deal with it before stringing.
        The technique for measuring is simple. Get a short piece of stiff
wire, maybe 6"-7" long, and about the same diameter of wire in that section
(or slightly heavier if necessary) then carefully bend it with your bending
tool and set the outside surface angle of the wire to be your guide. Set
this gauge at a good ideal angle to start (19o- 20o), then carefully fit it
between unisons (it's easier with the action out) and hold it in place
while you compare it to what is in the piano. You may then compare more or
less deviation from this standard gauge, say 19o, and rebend the wire by
trial and error until you match exactly the angle that the wire follows in
the piano. In this way we are measuring the angle taken by the underside of
the wire. Obviously, it is important to be careful while bending the gauge
wire, the bends must be only in one spot. I keep a nice wire gauge in my
desk which is used only for this purpose.
        Measuring this while the piano is unstrung is similar. In this
case, just tie up heavy gauge string line as tight as you can around the
bar and over the bridge, to simulate the string line.
        I am not the only one who has tried to increase this angle, and I
believe that some (Ed McMorrow at least) strongly discourages it. However,
on one S&S D, which happens to have been the preferred performance piano
here for 5 years now, I did this with fine results. I have forgotten the
original angle now on that piano, but I *carefully* carved out a round
groove in the duplex string rests by hand with my dremel grinding tool, and
fit in brass rod of the proper diameter to get a 20o angle duplex bearing.
The reason I say that 22o is a good limit is because I took that
opportunity to experiment with larger diameter brass rods. I found that
whatever inherent sympathetic resonant problems exist in the design of the
duplex system they are not improved by increasing the duplex bearing beyond
about 22o, and at 25o tuning becomes more difficult.
        In case you wondering, even though I have measured many pianos, I
had some fun with this one in particular because after rebuilding and
stringing the pins came out a little tight anyway, so I was not concerned
about loosening one duplex section at a time and popping in a different
size brass rod in the groove which had already been neatly carved to
receive it.
        I only did this on that one piano however, and I don't plan to make
a habit of it, but I believe the duplex on that piano was improved. I have
also heard Willis Snyder say that they routinely route out a groove in the
cappo bar itself and install a hard brass rod.


Dennis Johnson
St. Olaf College
johnsond@stolaf.edu