Bolt strength

[Delwin D Fandrich]

----- Original Message -----
From: <BobDavis88@AOL.COM>
To: <pianotech@ptg.org>
Sent: September 20, 2001 10:57 AM
Subject: Bolt strength


> Subject: Everett studio, pinblock separating from back
>
> I need to bolt this sucker back together. I have done this repair twice
> before with half-inch bolts (w/long-term success), and I know others have,
> but I would like to take a fresh look at this from the perspective of the
> engineers out there.
> 1) I chose bolts empirically, and they worked. However, is there anyone
out
> there who knows the tensile strength of various diameters of bolts? Of
> hardened bolts?

Ungraded cap screws/bolts can be made of anything, but usually they are made
of some kind of metal. Often something resembling steel. They are supposed
to be either Grade 1 or Grade 2 and be made of a low or medium carbon steel.
But those from you local hardware store might have come from anywhere and
rarely conform to any standard. Grade 5 cap screws (3 ridges on the head)
are made from medium carbon steel (tensile strength in the 100,000 psi
range) and are quenched and tempered. Grade 8 cap screws (6 ridges on the
head) are made of medium carbon alloy steel ( in the 150,000 psi range) and
are quenched and tempered. If we knew they met any standard Grade 2 would be
adequate for this application. Since we don't we try to use Grade 5. If your
local hardware store doesn't have a reliable selection of fasterns, check
your yellow pages for an industrial fastener store or check one of the
national outlets like McMaster-Carr.


>
> 2) How much strength do I actually need to resist breaking or bending of
the
> bolts?

Not all that much.


>
> 3) Is there an advantage or disadvantage to hardened bolts in this
> application?

No.


>
> 4) Is the separating force the only one I need to worry about, or should I
be
> worrying about the shearing force tending to collapse the plate?

Separating only.


>
> 5) In re: above, I have always assumed the bolts did all the work, and
that
> the glue line was contaminated/dirty enough it wouldn't cinch up
completely.
> Is there a glue better than Titebond for resisting creep in a glue line
which
> is both dirty and not as thin as it should be? Weldwood? Polyurethane
> (Gorilla) glue? Are both the adhesive and cohesive strengths of epoxy
> sufficient?

It's a combination of both. The bolts are going to bring things back
together, probably with the string tension somewhat relaxed. The glue and
bolts together are going to hold it there. My adhesive of choice for this
application is epoxy. You're going to end up with gaps unless you
disassemble the entire piano and refit the pinblock and neither Weldwood
(I'm assuming you're talking about the powdered variety) nor Gorilla have
much gap-filling strength. Epoxy will penetrate the little cracks well and
will bond to most anything and is quite good at gap-filling. Use a very
slow-setting hardener to give it plenty of time to creep into as many of the
little cracks as possible. Keep filling the void until you're convinced it's
gotten everywhere then start tightening things up. Make sure the bolts get
into the backposts, not just the spacer blocks.

Del