[pianotech] Re Green piano bridge repair with carpet tacks

[David Love]

Can't argue with comparisons to Big Dig however I will say that I've done
many bridge repairs with epoxy (West System not Marine Tex) with high and
medium density fillers that are similar to this one (maybe not quite so
severe) and after many years (some of them are 15 years old now) there is no
evidence of even the slightest bit of creep, deformation or any change
whatsoever in bridge pin angles.  FWIW

David Love
www.davidlovepianos.com


-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Douglas Gregg
Sent: Friday, November 12, 2010 6:31 AM
To: pianotech at ptg.org
Subject: [pianotech] Re Green piano bridge repair with carpet tacks

I agree that Marine tex is a better choice for a filled epoxy and is
almost identical to JB Weld, if you must use epoxy. Both are filled
epoxies with iron filings used as filler that can withstand
compression better than unfilled epoxy.

That said, epoxies are still a form of plastic that is polymerized
monomer chemically bonded by an  epoxy bond into a polymer ( ie. a
plastic.) and have all the same characteristics of plastic. They
undergo tensile deformation and creep which increases with heat. Most
epoxies are designed for impact resistance and have good elasticity to
rebound. What they are worst at is constant pressure or tension  over
time. With most epoxies, this is the worst in the range of 20-25
degrees C and reach the maximum creep at times in the order of 700 to
1000 hours. These conditions can easily be reached in any average
summer. There are many scholarly articles on this subject and it is a
serious concern for engineers. Just Google epoxy , compression, creep.
It is the reason that the Big Dig tunnel in Boston collapsed and
killed a person. The epoxy embedded bolts suffered creep due to
constant tension and slowly gave way over time. They did not come out
but crept due to the epoxy deforming under constant stress. By using
hard fillers in epoxy, like iron filings and silicates, compression
and creep can be reduced but not eliminated.

This is why I suggested using a mechanical solution like 5/8 inch long
steel wedges made from large carpet tacks to wedge the bridge pin back
into place and not have to worry about the long term effects of
compression, temperature, and creep when using a product not well
designed for withstanding compressive forces. I agree that epoxy can
work in some cases, but it can fail too, as in the Big Dig tunnel in
Boston.

Douglas Gregg PhD
Classic Piano Doc
Southold, NY 11971