bridge caps and bearing

[Michael Wathen 556-9565]

In Pianotech Digest 1835 Phil Sloffer wrote:


>>>>>>>>>>>>Why should bridges have caps anyway?

>Well bridges don't necessarily need caps.

>It depends how bearing is set.

>In both Steinway and Baldwin the plate is set to a "correct"
>height so that action regulation will be consistent.  The bearing
>is set later.

>Baldwin sets bearing by using accu-just hitch pins where the wire
>is moved up or down the hitch pin to get the correct bearing.

>However with Steinway the bearing is set by planing the bridge
>cap down to the proper height.  If you were to use a bridge
>without a cap this might be a very difficult job especially if
>the bridge had laminations with the grain going in both
>directions.

>Now as far as I know it makes no difference if the bridge in fact
>has a cap or not.

Then having a cap has to do more with the logistical approach to
achieving an end.  A constant grain direction that is found on a
solid cap is in one manufacturer's domain a much easier medium to
layout, to set bearing, and to notch than a bridge that is
multi-laminate and has no cap. On the other hand, the other
manufacturer has no difficulty with the no cap alternative due to
a different type of tooling for notching and a entirely unique
approach to setting downbearing.  In fact the cap is an added
frivolous step and a place where failure could lurk.

My own current belief is that one of the most vulnerable aspects
of the grand piano are all the joined and glued surfaces between
the string and the soundboard.  Most of these surfaces consists
of mated unlike types of wood with differing expansion
coefficients.  Not having a cap as one of these surfaces, in my
opinion, is the one thing that would keep an instrument longer at
its maximum sound production state.

I left a post on Pianotech some time ago entitled "Some Pianos
Just Die".  And what I have said above is what I was hinting at
in that post.  You consider a fine concert instrument where
everything is operating at its potential.  Parts wear; you can
replace and voice hammers and strings..etc....  But you can't
keep that instrument as wonderful as it once was.  You might even
replace the soundboard, dress the capo bar, replace agraffes.
The real failure is hidden just below the surface of the cap.
Every change of season tears at the glue bonds between the bridge
proper and the cap gradually changing it from a cohesive system
to one with loss of energy and loss of motion.

Picture this... suppose you had need of a funny kind of train,
say, consisting of twenty some cars coupled together.  At one end
you have the engine and at the other end you wish to receive some
kind of back and forth movement of the caboose.  This requires
the engine to move back and forth and transmit its movement
across the twenty cars to reach the caboose.  What type of
coupling would transmit the work done by the engine to the
caboose most efficiently?  A loose coupling or a tight coupling?

With a cap things might look okay and you can expect the
downbearing force to hold cap in place.  But you can never be
sure what the real situation is over time is unless you destring
and remove the bridge pins to find out how well it held up.  Next
time you have a chance, check it out.

Next,  with a laminated capless bridge the individual laminates
can have glue failure between them but in the vertical plane no
failure has occurred between the string and the bottom of the
bridge.  A delaminating bridge has it's consequences but it must
be quite severe in comparison to equal that of the cap failure.
The odd fact is that nearly every manufacturer makes a laminated
bridge.  So why cap??


> What I think is a more important question has
>to do with bearing.  If you replace a sound board then this
>assumes that bearing is improved so my question is really about
>those situations where the old sound board is used.

>I know that some technicians will lower plates to get better
>bearing even if the sound board has little or no crown.  I think
>this is wrong.  I say leave the plate right where it is.  Then
>the action will regulate OK.  Any improvement in sound gained
>from lowering the plate will not last if the sound board is bad
>anyway.  Put another way, if the old board sounds bad and it is
>decided to leave it in place then don't make other things worse
>by lowering the plate and making it impossible to regulate the
>action.

>The bearing is only right twice a year.  Once in the fall when
>the board is coming down and once in the spring when the board
>is going up.  The rest of the time it is only close.

What are they trying to achieve?  It is as if everyone has agreed
you must have good downbearing for the instrument to sound good.
But no one has a clue as to how this could be true.


Michael Wathen