Bolt strength

[Ron Nossaman]

>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?

Del quoted some figures, but generally, too much is better than too little.


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

Not as much as you might assume. I've always used regular old vanilla 3/8"
carriage bolts with a big enough washer under the head that the squared
shank fits into the hole. It gives you a big "footprint" at the head, so
you don't suck the head into the back of the piano when you crank the nuts
down. I've seen it done with 5/16" bolts, but I like the overkill of 3/8".
Bending the bolts isn't a problem, since I've often pre-bent the bolts
before driving them in so the heads will sit square to the back. I've also
waled away at the heads from the back side with a large hammer as I
tightened the nut to produce the same effect. It depends on where you are
and who's watching. 


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

Yes, the nominally grade 2 bolts are a lot easier to saw to finished
length, and a lot easier to bend as per the above paragraph.


>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?

Just the separating force. The plate is keeping itself from collapsing
without the bolts' help.


>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? 
>Thanks, engineers,
>Bob Davis

Epoxy's fine, with it's teriffic gap filling qualities. I prefer Titebond
because it's adequate for the job, and a lot easier to clean up after. The
way I slop glue around, that's a weighty consideration for me. I've seen
backs that were bolted back together lo these many moons ago without a lick
of glue of any sort that were doing fine 3mm of dust accumulation later, so
the glue choice doesn't seem to be all that critical. The bolts are doing
the work. And yes, as Del said, get them through the posts.


Ron N