[Public-List] Testing knee/bulkhead compression

public-list at lists.alberg30.org public-list at lists.alberg30.org
Wed Mar 25 11:10:34 PDT 2009

I'm basing this entirely on what I know of physics and mathematics, and
have never explicitly seen or heard anything about this bolt test before
(but the physics is easy, so I'm pretty sure I've got it right).

You make the conversion with the thread pitch and a factor of 2pi.

If you were to rotate the nut/bolt a full turn, you'd have taken it
through an angle of 2 pi radians, so the energy you've put into the
system (assuming the torque remains constant) is 2*pi*T (where T is

Now the distance the nut has actually moved is the thread pitch
(remember with imperial threads that the pitch is the inverse of the
number given, so 24tpi is a 1/24" thread). Noting that energy = work =
force * distance, we have our thread pitch for d, and get

2*pi*T = F*d

or F = (2*pi*T)/d

where F is your tensile strength.

Now this being an average over a full rotation, we can multiply the
whole thing by the fraction of the rotation that we actually pass
through, take the limit as that fraction goes to zero, factor the "zero"
out of both sides and have the exact same formula for a measurement
taken at a single point. (You can trust my calculus here, I've taught
the subject in university so many times that...)

Build in your fudge factor so you aren't actually tightening your bolt
to the limit of its tensile strength (the size of this fudge factor is
something that I'd have to defer to an actual engineer on) and there you
have it.

Candy Cane #583

On Wed, Mar 25, 2009 at 12:42:30PM -0500, Roger L. Kingsland wrote:
> Don,
> I certainly agree with you and guess you missed the last paragraph of my
> 3/23-17.32 post; 
> "Another issue is the compressive strength of the bulkhead (marine plywood)
> and knees (hardwood).  This could be field tested by torquing(sp?) down a
> bolt (does anyone know how to convert tensile strength to bolt torque) with
> a couple of layers of wide washers on each side and observe if the wood
> crushes.  To me, this is perhaps the most important test since several
> owners have mentioned water leaks and water damaged bulkheads/knees would be
> the weak link in any connection detail."
> I figured I would use a torque wrench for "the test" but don't know how to
> convert nut torque to bolt tension.  Do you, or any other Albergers, happen
> to know?
> All the best,
> Roger 148

Kris Coward					http://unripe.melon.org/
GPG Fingerprint: 2BF3 957D 310A FEEC 4733  830E 21A4 05C7 1FEB 12B3


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