Lowell Component Downbearing Gauge

[Frank Emerson]

I'm with you, Terry!  I hear all too much talk about measuring down bearing
in linear dimensions, and all too little in angular dimensions.  Even with
Ron's apt explanation of the instructions, I take some exception to the
idea of 0.5 degrees, across the board for down bearing.  (Maybe Ron should
have written the instructions for the gage!)  Don't get me wrong.  I really
like the bubble gage for measuring down bearing.  You can't get more
accurate than this, as a practical method for measuring what needs to be
measured.  My pet peeves on this subject are:  Failure to distinguish
between front and back bearing.  Failure to distinguish between loaded and
unloaded bearing (before and after the full string tension is applied). 
Using linear dimensions to specify down bearing, and totally loosing sight
of the fact that these specifications are only an expedient to achieve what
is really important, the angle of deflection, front and back.

I have seen technical drawings that clearly indicate that it was the
designer's intent that all of the down bearing should be at the front of
the bridge, with zero back bearing.  In a piano built with this type of
bearing design, and if the bridge were supported only directly under the
center line of the bridge, when the string were drawn to tension, the
bridge would immediate roll forward until the front and back bearing were
equal.  Of course, the ribs and supporting structure may not allow this to
fully occur, but the stresses are there attempting to move the bridge in
this direction, stresses that don't need to be there, if the design called
for equal front and back bearing, in the first place.

I have been in production environments where the previous designer had
built into the culture of the company the idea that distinguishing between
front and back bearing is not important, and that the correct overall
bearing is all that matters.  I have seen in this same company, pianos
going out the door with SIX TIMES the front bearing intended in the design,
and negative back bearing FIVE TIME what the positive bearing should have
been.  In round numbers, if the correct overall bearing should be one
degree, the front bearing is a positive six degrees, and the back bearing
is a negative five degree, so the overall bearing is one degree.  It meets
the specification, so it must be correct.  Right? ....WRONG!   The overall
bearing measures correct, so they went out the door, nobody even knowing
that there was a serious problem with the bearing.  All that was ever
checked was the overall bearing.

In yet another company, with more traditional methods, the front bearing
was largely neglected, and the overall bearing was largely determined by
the back bearing.  Their method was to draw a cloth string through the
agraffe, over the bridge and then over the string rest or duplexer.  A saw
kerf is cut in the bridge until the string, drawn through the saw kerf,
just touching the bridge, results in the specified linear dimension above
the string rest.  This works, if the operator is aware of the importance of
the string touching the front of the bridge first, then, lowered slightly
further to touch the back of the bridge, and finally taking the linear
measurement above the string rest.  The problem in this instance was that
the operators, and those who trained them, had totally lost sight of front
and back bearing considerations, resulting in the front bearing being all
over the place, but the back bearing was always correct.  Another problem
that crept into the culture of this company was a disregard for the tail
lengths.  The linear dimension measured at the string rest followed a
smooth curve from note to note, even though, in one case, the tail length
suddenly jumped from about 6" to 14".  While the linear dimension remained
the same, by virtue of the radical difference in the tail length, and the
ultimate loading of the bridge with full string tension, the down bearing
suddenly changes from minimal to negative bearing.

Typically, in the field, judgements are made about down bearing with the
system loaded with full tension on the strings.  On the other hand, in the
manufacturing environment such judgements are made with the system
unloaded, with no string tension.   OK, now I will get specific enough to
stimulate some controversy.  On an unloaded system, I like to see overall
downbearing as little as 0.3 degrees at notes 1 and 88, and as great as 1.2
degree near the center of the treble bridge, depending on the size of the
piano and the proximity to the perimeter of the soundboard.  Once up to
full string tension, I like to see this range reduced to 0.3 - 0.8 degrees.
In every case, with a balance between front and back bearing, being equal,
front and back, regardless of the specific value for overall bearing.  In
no case do I want to see 0.5 degrees for every note.

Of course, we must always take into account that the down bearing will
change over time, with changes in the moisture content of the soundboard.
For this reason, I like to maximize the tail length.  The closed the tail
length is to being equal to the speaking length, the greater the
equilibrium between front and back bearing, as it changes over time.

Frank Emerson