geometry/friction

[Jim_Harvey@yca.ccmail.compuserve.com]

     This is a reply to several threads. I'll try to make a painless
     transition. I'd like some help in resolving these problems in my
     own mind.

     [Dennis Johnson]
     >If this bit of geometric detail regarding the knuckle/jack
     contact point is so brilliant, than why are we compelled to be so
     liberal with the dry teflon powder?

     >I suggest it is because the heavier parts on modern pianos have
     created a problem where previously none existed.
     [end quote]

     While I don't rule it out, I don't necessarily agree with DJ that
     this is due to heavier action parts. I would agree that heavier
     action parts (hammers and shanks specifically) could cause faster
     knuckle core compression, and, without *any* lubrication,
     accelerated leather wear.

     Group, we pride ourselves on (at least discussing) such minutia
     as the proper orientation of the nap on knuckles. We then discuss
     adding lubricant(s) to the knuckle, which, in an effort to
     eliminate squeaks, also serve to eliminate friction. This, to
     some degree, negates the intent of knuckle orientation. This
     seems a contradiction to me.

     Point 1: For knuckle squeaks alone (not friction). Recall that a
     squeak is a result of lack of lubrication between contacting
     parts. In a normal (read historical) situation, the jack and
     repetition lever are the areas that are lubricated.

     Point 2: I've found instances of squeaking where simply
     dry-cleaning the knuckle eliminated the problem.

     My rationale: I feel that knuckle squeaks are a problem of
     knuckle manufacture. I don't know whether it happens in the
     tanning process or in knuckle manufacture, but somehow oil in the
     knuckle leather is involved. Therefore, I feel that resulting
     squeaks may be due to conflicts between dissimilar lubricants.
     Such conflicts are a recognized phenomenon in other areas, why
     not knuckle to jack/rep lever?

     Point 3: I've seen instances where no amount of knuckle
     lubrication helped. In these cases, the squeak sounded exactly
     like it originated from the knuckle; instead, it was coming from
     the contact between the repetition lever leather and the drop
     screw. Since letoff and drop come into play at the same moment in
     time, it is easy to overlook this possibility while diagnosing.

     My rationale: Never mind a brass or plated drop screw contacting
     leather -- with or without oil in the leather. This, IMO, should
     not happen. I've observed that, in properly designed actions, the
     repetition lever leather only contacts the drop screw in a
     straight line, not in a "wiping" action across the drop screw.

     I suggest that we try these things in reverse order the next time
     we have a case involving squeaking knuckles: first, take the
     drop screw and leather out of the circuit, by simply wiping your
     finger across the drop leather. If this is the problem area, the
     squeak should disappear -- for about three key cycles -- then
     return. Once this has been eliminated as a possibility, try
     cleaning a few knuckles, and instead of adding an elixir to them,
     try *further* burnishing the jack and repetition lever tops with
     a hammer shank/other hard surface.

     [Stanwood]
     2. Rotational hammer flange friction set precisely 4.0 grams by
     the guage.
     [end quote]

     I'd like recommendations about the devices/instruments used for
     this and other static tests. I remember your mentioning a
     resettable (to zero) caliper. I must assume in this instance,
     it's the Correx (or equivalent) 2-15 gram tension gauge.

     Thanks for any challenges or discussion.

     Jim Harvey/RPT