Hammers

[Delwin D Fandrich]

----- Original Message -----
From: "Tony Caught" <caute@optusnet.com.au>
To: <pianotech@ptg.org>
Sent: December 29, 2000 9:19 PM
Subject: Re: Hammers


> I don't know what type of glue they use but in this day of cost cutting I
> would not be surprised if this glue also failed. Be it water penetration
> into the glue or the extra tension put on by swollen felts or a
combination
> of both.
>
-----------------------------------------------------

Tony, et al,

There are many excellent adhesives available at relatively nominal cost and
there is very little difference in the cost of a given adhesive type from
one vendor to the next. There are so many much more creative and interesting
ways to have problems gluing hammer felt to wood moldings--I doubt it's
necessary for the company to try saving money by using an inferior product.

To start with, these are all two-part adhesives--one part can be water--and
there can be problems if this mixing in not done properly. They are also age
and temperature sensitive. As process-conscious as Yamaha is, though, I
would quite surprised if Yamaha was having problems with either mixing these
adhesives or with using out of date materials.

Moving on, moisture content (MC) of both the wood and the felt--especially
the felt--is critically important during the actual hammer press process.
Especially that of the felt. The felt strip must have some minimum level of
moisture before it is pressed. If there is not enough, and the felt is some
on the dense side--as is most felt used in 'modern' hammer construction--it
will break and tear as it is pressed into the caul. If there is too much
moisture in the felt, the excess moisture will tend to block the glue liquor
and not enough adhesive will penetrate the felt surface to form an adequate
bond.

A much more common problem is having the felt MC excessively low--i.e., just
barely enough to keep the felt from breaking. In this case excessive
solvent--most commonly water--will be drawn out of the adhesive mixture into
the felt and there will not be enough solvent left in the felt
inter-penetrating layer (the transition layer of felt and adhesive which
should be thoroughly penetrated by glue liquor of the proper chemical mix)
to insure proper chemical cure of the adhesive mix. The adhesive will still
dry--it will be quite hard and brittle--but it will not have reached full
adhesive bonding strength due to improper chemical cure.

This same problem is evident even if the felt started out at the right MC
but the temperatures of the heated side cauls is excessively high. The felt
layer between the side caul and the wood molding is relatively
thin--especially toward the treble end of the hammer set--and the heated
cauls force moisture out of the felt very rapidly. If the heat is
excessively high too much moisture is forced out too rapidly. It will be
replaced by moisture drawn rapidly from the adhesive with the end result
being a 'starved' glue joint.

Most of the heat-cured adhesives of the type used in modern piano hammer
production cure to a relatively rigid glue line. This is always a problem
when bonding substrates that are not moisture-stable. Obviously, neither
wood nor felt are moisture stable. As they both expand ad contract during
the various seasonal cycles quite a strain is put on the adhesive joint. If
there is any weakness in any part of this bond, the joint will fail.

This problem, incidentally, was also common with so-called 'cold-pressed'
hammers made using animal hide glue. The adhesive worked best when it was
fairly hot and quite thin. When too much water leached into the felt too
quickly the resulting adhesive bond was often not strong enough to hold the
felt to the wood molding over a long period of time. Especially when
confronted with more extreme climate swings of non-air conditioned homes.
Various techniques were developed to solve the problem, the most effective
being the addition of various pre-hardening solutions being added to that
portion of the felt that was actually being glued to the wood molding--the
shoulder of the hammer. This so-called 'reinforcing' had nothing to do with
the voice of the hammer and was only an aid to gluing the felt to the hammer
molding.

Insurance was then provided by inserting a staple into, or through, the
shoulder felt and molding. This staple also had/has nothing to do with the
'voice' of the hammer. It is there only to hold the shoulder felt to the
molding and help prevent glue failure.

Del