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<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2>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. </FONT></SPAN></DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2>A
keyway in a rotating shaft is another stress concentrator. It will =
nearly always
be the point of failure if such a shaft breaks. </FONT></SPAN></DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2>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. </FONT></SPAN></DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2>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. =
</FONT></SPAN></DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2>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. </FONT></SPAN></DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2>Hope
this helps.</FONT></SPAN></DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2>Dean</FONT></SPAN></DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=781361220-08012005><FONT face=Arial color=#0000ff =
size=2></FONT></SPAN> </DIV>
<BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px">
<DIV class=OutlookMessageHeader dir=ltr align=left><FONT =
face=Tahoma
size=2>-----Original Message-----<BR><B>From:</B> =
pianotech-bounces@ptg.org
[mailto:pianotech-bounces@ptg.org]<B>On Behalf Of =
</B>Terry<BR><B>Sent:</B>
Saturday, January 08, 2005 2:40 PM<BR><B>To:</B> =
Pianotech<BR><B>Subject:</B>
Re: seeking wood screws<BR><BR></FONT></DIV>
<DIV><FONT face=Arial size=2>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)?</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>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.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Terry Farrell</FONT></DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>
<A title=deanmay@pianorebuilders.com
href="mailto:deanmay@pianorebuilders.com">Dean May</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A =
title=pianotech@ptg.org
href="mailto:pianotech@ptg.org">Pianotech</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Saturday, January 08, =
2005 12:58
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> RE: seeking wood =
screws</DIV>
<DIV><BR></DIV>
<DIV><SPAN class=671354917-08012005><FONT face=Arial =
color=#0000ff
size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=671354917-08012005><FONT face=Arial =
color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=671354917-08012005><FONT face=Arial =
color=#0000ff
size=2>Thanks for the tip on the hole.</FONT></SPAN></DIV>
<DIV><SPAN class=671354917-08012005><FONT face=Arial =
color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=671354917-08012005><FONT face=Arial =
color=#0000ff
size=2>Dean</FONT></SPAN></DIV>
<BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px">
<DIV class=OutlookMessageHeader dir=ltr align=left><FONT =
face=Tahoma
size=2>-----Original Message-----<BR><B>From:</B> <A
=
href="mailto:pianotech-bounces@ptg.org">pianotech-bounces@ptg.org</A>
[mailto:pianotech-bounces@ptg.org]<B>On Behalf Of
</B>Terry<BR><B>Sent:</B> Saturday, January 08, 2005 11:34
AM<BR><B>To:</B> Pianotech<BR><B>Subject:</B> Re: seeking wood
screws<BR><BR></FONT></DIV>
<DIV><FONT face=Arial>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.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>5/16-inch hole gives me a nice very snug
fit.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Terry Farrell</FONT></DIV>
<BLOCKQUOTE
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial"> </DIV>
<DIV><SPAN class=906561315-08012005><FONT face=Arial =
color=#0000ff
size=2>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).</FONT></SPAN></DIV>
<DIV><SPAN class=906561315-08012005><FONT face=Arial =
color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=906561315-08012005><FONT face=Arial =
color=#0000ff
size=2>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. </FONT></SPAN></DIV>
<DIV><SPAN class=906561315-08012005><FONT face=Arial =
color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV class=OutlookMessageHeader align=left><FONT =
face=Tahoma><FONT
size=2><SPAN class=906561315-08012005>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.
</SPAN></FONT></FONT></DIV>
<DIV class=OutlookMessageHeader align=left><FONT =
face=Tahoma><FONT
size=2><SPAN =
class=906561315-08012005></SPAN></FONT></FONT> </DIV>
<DIV class=OutlookMessageHeader align=left><FONT =
face=Tahoma><FONT
size=2><SPAN class=906561315-08012005>Does anyone have a =
recommended
hole size to drill the rim for the 3/8" machine thread to screw
into?</SPAN></FONT></FONT></DIV>
<DIV class=OutlookMessageHeader align=left><FONT =
face=Tahoma><FONT
size=2><SPAN =
class=906561315-08012005></SPAN></FONT></FONT> </DIV>
<DIV class=OutlookMessageHeader align=left><FONT =
face=Tahoma><FONT
size=2><SPAN
class=906561315-08012005>Blessings,</SPAN></FONT></FONT></DIV>
<DIV class=OutlookMessageHeader align=left><FONT =
face=Tahoma><FONT
size=2><SPAN =
class=906561315-08012005></SPAN></FONT></FONT> </DIV>
<DIV class=OutlookMessageHeader align=left><FONT =
face=Tahoma><FONT
size=2><SPAN =
class=906561315-08012005>Dean</SPAN></FONT></FONT></DIV>
<BLOCKQUOTE><FONT face=Arial
=
size=2></FONT> </BLOCKQUOTE></BLOCKQUOTE></BLOCKQUOTE></BLOCKQUOTE=
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