hammer staples--a long answer

Delwin D Fandrich pianobuilders@olynet.com
Sun, 22 Sep 2002 17:41:51 -0700


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  ----- Original Message -----=20
  From: Tony Caught=20
  To: Pianotech=20
  Sent: September 21, 2002 7:43 PM
  Subject: Re: hammer felt (Renner Blues)


  Dave,

  You wrote

  The importance of the staple is somewhat controversial.  There are =
those who argue that it adds tension to the lower part of the hammer and =
those who say it doesn't make any difference.  The Renner blue is, of =
course, without a staple.  =20

  Not having a staple would mean that you cannot use this hammer in high =
humidity conditions. However,
  As the hammer absorbs moisture from the air, the tension created =
varies greatly as does the tone. Possibly being not stapled would allow =
this increase of tension to spread more evenly over the hammer and not =
be restricted by the staple.

  Tony Caught
  caute@optusnet.com.au


Tony, et al.,

In general the staple is an insurance policy against a variety of gluing =
problems. In the early days of piano building, of course, animal hide =
glue was the only adhesive available to bond hammer felt to the wood =
molding. Whatever its other merits, animal hide glue was (is) =
particularly unsuited to this task--it goes on hot and, as the felt is =
pressed against the wood molding, much of the water solvent migrates =
into the wool felt. This leaves the glue too dry to form a reliable =
bond.

Something needed to be done to retard this water migration, hence the =
various shoulder treatments that ultimately came to be called hammer =
reinforcing or shoulder hardening solutions came into use. Despite their =
names, none of these solutions did much to reinforce anything and they =
were not intended to harden the working end of the hammer at all. They =
were designed to retard the rate at which the water solvent component of =
hide glue migrated into the felt and, hence, help prevent starving the =
glue joint. A starved glue joint between the felt and the wood molding =
results in a fairly weak bond and is susceptible to early =
failure--especially under varying stress loads.=20

Like wood, wool felt expands and contracts with variations in moisture =
content creating physical changes in the felt that do continuously vary =
the stress loads on the felt-to-wood glue joint and that lead to its =
rapid breakdown and failure. Under these conditions the staple does help =
to stabilize things--enough, at least, to prevent most felt-to-wood =
bonding failure.=20

Most of the adhesives used to bond hammer felt to the wood molding today =
do not have this problem, or the problem can be controlled in other =
ways.=20

One way is to raise the moisture content of the felt to the point at =
which the water solvent in the glue is not so readily drawn into the =
felt. An unfortunate side effect of this process is that when the hammer =
felt is pressed into a heated caul--as is typical of nearly all hammer =
makers today except for Isaacs and Ronsen (are there others?)--the felt =
takes on a rather permanent set much like your wrinkled wool slacks do =
when they are steam pressed at the cleaners. This pressing process =
results in the much more dense and less resilient hammer that we all =
know and love to voice.=20

Even today not all of the adhesives used to bond hammer felt to wood =
moldings are completely waterproof after they are cured. For a variety =
of reasons some hammermakers use one of several thermoplastic adhesives =
similar to commercial PVAs (white glue) and/or aliphatic resins (yellow =
glue). These adhesives are generally moisture resistant but can fail if =
exposed to very high humidity levels. With these adhesives the reasons =
for failure are different than those encountered with animal hide glues, =
but the point of failure and the type of failure is the same. In these =
hammers the staple is probably an essential structural component. If the =
adhesive is rated as 100% water/moisture resistant--and if it is =
properly handled and applied--the staple is not necessary for structural =
purposes.=20

It is debatable as to whether the staple is desirable--or at all =
effective--at promoting, enhancing or stabilizing tone. Generally the =
staples are inserted below the parting line of the hammer caul and well =
below the point at which the trimming knife has cut deeply into the =
continuity of the outer layers of felt. Personally, I don't see how a =
staple of any variety can do much to support the functioning portion of =
the hammer from down there, though I'm willing to be convinced. (I will =
say, though, that I've taken out enough hammer staples and have =
tried--unsuccessfully--to hear any resulting tonal differences to be =
some skeptical.)

Nor do I see how staples of different types are going to function =
differently, assuming each is installed correctly. Ron has described the =
different types in one of his posts and I have found problems with each. =
The two types of through staples are very often loose (especially the =
T-types), obviously supporting nothing but themselves and hardly that. =
The U-shaped staples are driven in with an air hammer and come in a =
variety of lengths. These lengths must be carefully selected to extend =
through the shoulder felt and into the wood molding. Many of these =
staples are coated with a thermo-activated adhesive--the adhesive is =
heated by friction as the staple is driven home--and once it enters the =
wood molding some reasonable distance it is structurally and permanently =
secure. It is desirable that the staple not extend all the way through =
the molding. Now, look at the side of a hammer and consider how the =
staple is inserted and you'll quickly see how tricky it can be to get =
this just right. Being just a bit heavy or light with the trimming knife =
or slightly high or low on the shoulder from your desired insertion =
point can make quite a difference in how far the staple penetrates the =
molding. As well, the staples only come in a limited variety of lengths. =
Three or four lengths will be used on one set of hammers.

Given some time I could probably think of a few more things to say about =
hammer staples. But I suspect I've all ready tossed out enough stuff to =
generally confuse things.

Regards,

Del



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