[Public-List] Testing knee/bulkhead compression

Roger L. Kingsland r.kingsland at ksba.com
Thu Mar 26 02:52:46 PDT 2009

Thanks Kris, I'll get my calculator out and get to work.

Roger 148
PERFECT intentions 

Roger Kingsland, Managing Partner
Kingsland Scott Bauer Associates



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r.kingsland at ksba.com

-----Original Message-----
From: public-list-bounces at lists.alberg30.org
[mailto:public-list-bounces at lists.alberg30.org] 
Sent: Wednesday, March 25, 2009 1:11 PM
To: Alberg 30 Public List -- open to all
Subject: Re: [Public-List] Testing knee/bulkhead compression

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 torque).

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
> 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/
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