Hammer Staples

[Delwin D Fandrich]

----- Original Message -----
From: "Richard Brekne" <rbrekne@broadpark.no>
To: <pianotech@ptg.org>
Sent: May 24, 2001 7:49 AM
Subject: Re: Hammer Staples


> Hi Brian... I suspect you are probably closer to being right then the
> suspicion that the staples help much.. Certainly they are of help for the
> time it takes for the glue to set 110 percent up... but after that.... I
> wonder . . . . . .

> I am faced with this prospect now...and am just wondering if anyone knows
> anything for sure.

------------------------------------------------------------

Richard,

I rather doubt that any studies have been done on the subject. Mostly just
empirical observation.

So, while I've also not done any specific studies on the value -- or lack
thereof -- of hammer staples, I do have a few opinions based on my own
empirical observations while working on the problem of pressing better
sounding hammers.

Staples are an insurance policy used by hammer makers to cover for the
possibility -- probability? -- of poor adhesive bonding of the felt to the
wood hammer molding.

This poor adhesive bonding can be the result of many different factors
inherent in the hammermaking process:
    --  It is very difficult to bond hammer felt to anything using rigid
adhesives. Adhesives that exhibit a lot of cold-flow ('white' glue, pvc-e,
melamine, etc.) cannot be used for obvious reasons and adhesives that don't
exhibit a lot of cold-flow don't work all that well with felt.
    --  The right amount of adhesive must be used at the right viscosity. It
must be allowed to soak into the wool just long enough, but not too long.
    --  If too little adhesive is applied too much moisture will be drawn
into the felt leaving the adhesive too dry to bond to the wood properly.
This was also a problem with the early animal hide glues used many years
ago, hence the various 'reinforcing' solutions use on the shoulders. This
stuff was not applied to reinforce anything except the glue joint.
    --  If the glue is too thick there won't be enough moisture to form the
essential interpenetrating layer necessary for a good adhesive bond. If it
is too thin, too much of the solvent will be drawn into the felt and the
chemical cure of the adhesive will be impaired.
    --  The bond must then be made at the right temperature and pressure. If
the side cauls are too hot the felt is damaged (some will say -- myself
included -- that any high temperatures are damaging to the felt as it is
used in piano hammers). If the side cauls are not hot enough the glue will
not cure properly (assuming a thermal-setting adhesive is used). It will
dry, but not cure and the bond strength will considerably lower than
required.
    --  Side caul pressures are difficult to adjust on most hammer presses.
The amount of pressure they exert against the hammer shoulders -- the area
that is adhesively bonded to the hammer molding -- is dependent on the
thickness of the hammer molding and on the trimming of the hammer felt
wedge. Both are variable. If the side caul pressure (as measured against the
specific piece of hammer felt being pressed along its entire length) is on
the low side the adhesive joint will be thicker than optimum and, since most
rigid adhesives don't gap fill at all well, the bond will be weak and will
get weaker with age. If the pressure is higher than optimum there will be
too much glue squeeze-out and the adhesive bond will be 'starved' and weak.

There's more, but this should give you the idea.

Now, consider the hammer itself. Hot-pressed hammers don't have a lot of
resilience. The felt is pretty much set by the pressing and heating process.
So, under 'normal' conditions there isn't a lot of internal stress trying to
pull the modern hammer apart. The stress on the adhesive bond holding the
felt in place is not all that great in most cases. Put a set of hammers
having a marginal adhesive bond between the hammer felt and the wood molding
into a humid climate, however, and the wool will absorb a considerable
amount of moisture (unless, of course, it has been heavily chemically
hardened in which case it is no longer a piano hammer anyway), the felt will
swell and there will be an increasing amount of stress on the adhesive bond
between the wool and the wood molding. If anything was not optimum during
the hammermaking process it will come apart unless the physical clamping
action of the staple is present.

Obviously, this danger is greatest through the bass and tenor regions where
the felt is the thickest. My own practice is to remove any staples through
the upper third of the set, leaving them in where the felt is thick enough
to create a stress problem resulting from high humidity climates. And we buy
mostly cold-pressed hammers.

Regards,

Del