seeking wood screws

[Dean May]

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A stress concentrator is as its name says, anything that will concentrate a
stress load. It is what enables us to cut glass, for example. The scratch in
the glass acts as a stress concentrator. When a bending load is placed on
the glass it is transfered very uniformly accross the entire surface of the
glass because it is so smooth, unless there is some kind of flaw, like a
scratch. There the outer stress lines will "flow" under the scratch,
combining with the inner stress lines, which produces a higher stress load.
And that is where it cracks.

A keyway in a rotating shaft is another stress concentrator. It will nearly
always be the point of failure if such a shaft breaks.

Think of a shaft projecting from a steel plate. As a load is applied to the
end of the shaft the stress must be transfered to the plate. The stress
lines will concentrate right at the joint of the two members as they bend
around the corner. If you were to make a larger weld area, or even add a
circular boss around the shaft to distribute the load, the shaft would carry
a much higher load before breaking.

Anytime you make a mechanical connection there is a stress concentrator at
the outer perimeter of that connection. Making that contact area larger with
more perimeter is a way of reducing that. So the washer gives both a larger
surface contact area for distributing the load plus a bigger perimeter for
reducing the concentration of stress.

Since cast iron has relatively low tensile/shear strength it is particularly
susceptible to stress concentrators. That is why everywhere you look on the
piano frame you will see no sharp corners. They use plenty of radius. Also
look at the struts; they do not abruptly end, they taper.

None of this is to say that your plates will crack if you don't use a
washer. But in designing heavy equipment we would never allow a bolt head or
nut to rest directly on cast iron. We would use a hardened flat washer. It's
just what I am used to doing. Of course what I designed was subjected to
very high stress loads typically.

Hope this helps.

Dean


  -----Original Message-----
  From: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]On
Behalf Of Terry
  Sent: Saturday, January 08, 2005 2:40 PM
  To: Pianotech
  Subject: Re: seeking wood screws


  Hey, ya learn sumptin' new every day. Please tell me more. How does the
bare nut act as a stress concentrator (and what exactly is a stress
concentrator). How does a flat washer help distribute the load (if the nut
was only touching the plate in one spot, it seems that even with a washer in
there it would still do the same)?

  I know sometimes I will pose a question facetiously, but as I do not have
any background in mechanical engineering, I am genuinely curious to hear
more on this. Thanks.

  Terry Farrell
    ----- Original Message -----
    From: Dean May
    To: Pianotech
    Sent: Saturday, January 08, 2005 12:58 PM
    Subject: RE: seeking wood screws


    It's a throwback to my days as a design engineer. It is bad practice to
place any kind of stress concentrator against cast iron. The washer is a
little insurance to distribute the load, though ideally it should be
hardened to maximize its effectiveness.

    Thanks for the tip on the hole.

    Dean
      -----Original Message-----
      From: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]On
Behalf Of Terry
      Sent: Saturday, January 08, 2005 11:34 AM
      To: Pianotech
      Subject: Re: seeking wood screws


      Why do you put a washer on top of the up-side-down lock nut that goes
on the backside of the plate - assuming of course that the nut has enough
horizontal surface area to mate with the plate (and not want to fall through
the hole)? I would just be afraid of a potential rattle/buzz if one of the
assemblies got a little loose (I realize that none should be). I just
haven't found that to be needed. I do put a washer on the top though - but
mostly to protect the plate finish while tightening the acorn nut on top.

      5/16-inch hole gives me a nice very snug fit.

      Terry Farrell

        All this talk of screw sources has had me browsing some of these
sites. Very cool. I've always tried to maintain an organized supply of
various screw types and sizes which has been a huge asset. But it is not
always easy to get big boxes of the sizes you want any more. Most places
only sell the little 15 piece plastic packages (very expensive way to buy
screws).

        Anyway, I have been wanting to do the plate suspension trick for
adjusting downbearing. Del spoke of it at the recent Indy conference. This
involves threading socket cap screws (headless bolts with recessed allen
socket) into the inner rim, running down a lock nut upside down with a
washer on top (a sufficient clearance well counterbored into the
soundboard), the plate lowered onto the washer, a chrome washer and acorn
nut on top. The height of the plate can then be easily raised or lowered by
removing the acorn nut and turning the stud with and allen wrench.

        Attached is what I ordered. I got the hanger screws just for fun. If
the machine thread doesn't work well, which Del said it would thread right
into the wood, I can always fall back on the hanger screws with their lag
thread.

        Does anyone have a recommended hole size to drill the rim for the
3/8" machine thread to screw into?

        Blessings,

        Dean

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