Bass bridge bearing

[Delwin D Fandrich]

----- Original Message -----
From: "Roger Jolly" <baldyam@sk.sympatico.ca>
To: <pianotech@ptg.org>
Sent: November 25, 2000 10:18 PM
Subject: Re:Bass bridge bearing


>
> >On another subject, I've read or heard that the total downward pressure
> >on the soundboard is about 1500 lbs., and that the approximate downward
> >pressure of one string on the bridge is about 7 lbs.    1500/240 strings
> >= 6.25 lbs. per string and  7 lbs. x 240 strings = 1680 lbs. total. , so
> >these two figures are in the same ballpark.   That means a 42-string
> >bass bridge would have about 300 lbs. pressing down on it.  Can that be
> >right?  Many bass bridges are cantilevered out  --  no matter how good
> >the glue joint, and even with screws into the apron from the back of the
> >soundboard, it seems that two 150-lb. people standing on the
> >cantilevered bass bridge would break it right off!
>
> Hi Dave,
>                In many pianos the bass string tension is almost double the
> plain wire tension, with a corresponding increase in down bearing
pressure.
>  However, the strings are wider spaced, compared to say the treble, also 1
> and 2 string unisons,this helps distribute the load over a wider area.
> Greater stiffness and mass per unit length of the taller bass bridge,
helps
> with supporting the load, as does the positioning and height of the ribs .
> An over simplification of a complex subject, that will lead into the old
> Impedance, Reactance, debate.
> I'm sure Ron and Del will contribute.
> Just some thing to muse over.
> Regards Roger
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That's the problem with averages -- on average, they don't tell us much.

As may be...on average, bass bridges are not loaded as heavily as are tenor
bridges.  At least they shouldn't be.  And, yes, there are special cautions
that must be observed when using cantilevered bass bridges.  Enough so that
it probably would be a good idea for the designer to consider ways of not
using them at all (there are several).

I went into the stresses on cantilevered bridges in a Journal article some
time back and have repeated the concepts in several convention and
conference classes.  Not much has changed.  Cantilevered bridges are still a
'feature' to be avoided.

At low frequencies, the stiffness of the bridge/soundboard/rib system has a
greater effect on mechanical impedance than does mass.  This means that,
within reason, a bass bridge can be fairly heavy without the excessive mass
having much effect on the fundamental, or low frequency, sound produced by
the system.  Soundboard stiffness, on the other hand, will have a much
greater effect at these same frequencies.

Excessive string bearing against the bass bridge -- especially with the
typically short backscales found in pianos short enough to sport
cantilevered bass bridges -- will restrict the motion of the bass bridge
quite a lot at low frequencies.  Very simplistically, string downforce works
against the crown of the soundboard to stiffen the complete system.  This
tends to raise the mechanical impedance of the soundboard system and reduce
the transfer of energy from the string to the soundboard at low frequencies.
The problem is made worse by the filtering effect of the bridge cantilever.
The combination reduces the low frequency energy in the sound produced by
these pianos, often to the point that, through the first octave or two, the
string's fundamental frequency is not present in the acoustic wave envelope
at all.

Acoustics aside, most cantilevered bass bridges probably could support a
string downforce of 300 lbs (approx. 136 kgf) evenly distributed along its
length.  Temporarily.  It's the long-term structural viability we have to
worry about.  What will happen to the bridge and soundboard in ten years?
Fifty years?  On average, cantilevered bass bridges do not have a good
long-term track record when they are overloaded.

Regards,

Del