repetition/backcheck problem

Newton Hunt nhunt@jagat.com
Fri Aug 28 08:01 MDT 1998


OK, you have check all those things so pick the worst offender and let's
do some evaluation.

First, check the pinning of the hammer shank.  Eight swings is the
outside limit on this pin.  If you get ten or more swings the spring
will need to be weaker to prevent rebound upon release. Spring tension
is a major source of repetition failure.  Six to eight swings is the
envelope.

The wippen post pinning is also critical.  Without the spring it should
not move when swung but should move with a light finger touch.  There
should be perceptible resistance by finger pressure, otherwise, the
spring tension will be too weak for good repetition.

Check the touch weight.  Measure the up weight and the down weight.  The
average of the two must not be less than 34 grams and should be 38 to 40
grams.  Much high then 40 gets into the too heavy range for any other
than a professional.  42 to 48 can cause tendentious in anyone.

Determining if the capstan is in the proper location is more complex.
You must have the up weight, down weight, hammer weight, wippen weight,
key weight and exact key ratio.  Subjected to Stanwood's formula will
give you a strike ratio which determines if the capstan is properly
located.

In the Hamburg factory the action is assembled completely before being
fitted into the piano.  In New York they fit the keys to the case, set
the stack to best tone at top and a mark at the bottom and then
determine capstan location.  This procedure does not allow for the
capstan location to be consistent from piano to piano, unfortunately.
BIG difference.

We are past the time when friction is considered to be an enemy and now
realize that it is critical to proper action functioning and needs to be
controlled more precisely than previously thought.

To go any further we need numbers.

Have a great weekend.

        Newton





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