<!doctype html public "-//W3C//DTD W3 HTML//EN"> <html><head><style type="text/css"></style><title>Re: More on soundboard crown</title></head><body> <blockquote type="cite" cite> <blockquote type="cite" cite>At 11:35 PM -0700 15/8/03, Phillip Ford wrote:</blockquote> <blockquote type="cite" cite><br> <blockquote type="cite" cite>. . . Perhaps 10% of the load could be taken by the arch and 90% by bending. ...</blockquote> <blockquote type="cite" cite><br></blockquote> <blockquote type="cite" cite>A proposed experiment:  Take a crowned section of panel with a rib attached.  Glue one end to a rigid support.  Glue the other end to a block which is guided by bushings or rollers...</blockquote> </blockquote> <blockquote type="cite" cite><br></blockquote> <blockquote type="cite" cite>Hi Phil and all,</blockquote> <blockquote type="cite" cite><br></blockquote> <blockquote type="cite" cite align="center">I've been developing a new sound board for our next 225 piano. Firstly, I built an 880 mm long CC rib to use as a control (all the prototype ribs are machine crowned). The first test I did was to investigate the assistance or otherwise of the crown, with regard to its ability to resist down bearing...</blockquote> <blockquote type="cite" cite>In this particular case I measured 5% less deflection 400 mm out from the rib end, when the test rib was clamped to the RHS steel section which supported the rib ends. So your guestimate of 90/10 wasn't too far off the mark Phil.</blockquote> <blockquote type="cite" cite><br></blockquote> <blockquote type="cite" cite>I also proved that sound board deflection under an increasing load is not linear. But the real surprise was that the rate of deflection increased as a greater load was applied. At least it does when dealing with down bearing loads up to 24 Kg on a single rib (the load on our rib no. 11 if the down bearing angle is at 1% when the piano is at pitch).</blockquote> <blockquote type="cite" cite><br></blockquote> <blockquote type="cite" cite>Ron O.</blockquote> </blockquote> <blockquote type="cite" cite><br> <br> </blockquote> <blockquote type="cite" cite>Hi Ron,</blockquote> <blockquote type="cite" cite><br></blockquote> <blockquote type="cite" cite>Thanks for sharing this info.  Very interesting.  An increasing rate of deflection as the load is increased is what I would expect if there was arch behavior going on.</blockquote> <div><br></div> <div>Certainly.</div> <div><br></div> <blockquote type="cite" cite>  As the arch is deflected down, the arch height is decreasing, which causes an increase in the thrust load at the supports.  Are those deflections in mm by the way?</blockquote> <div><br></div> <div>Yes, the first line of data is with the rib unclamped, while the second line of data is with both rib ends clamped.</div> <div><br></div> <blockquote type="cite" cite>  Care to try my experiment where you put a load on the guided edge to see how much force is required to deflect the 400 mm point up by 0.13 mm (the difference in deflection between the guided and fixed case)?  This would give an idea of the compressive stresses caused by this effect and whether they would cause compression set over time.</blockquote> <div><br></div> <div>There's a lot of force going outwards when the two ends are clamped. I know this because I was holding the rib ends with a couple of F clamps. I had to make them really tight or the dial gauge wouldn't return to zero after removing a test load. This indicated that the rib end was creeping along its support block when the load was applied, so that when the load was removed the test panel remained under some tension which was pulling it down (often around 0.05 to 0.075 mm - 2 to 3 thou')</div> <div><br></div> <div>I'm am sure that as Del and Ron N. have mentioned the arch effect will be reduced to near zero in a very short time, after installing a board, due to compression set. Initial reduced board response, due to initial arch assistance, could go some way towards explaining why instruments never sound at their best when new. It seems to take about a year for them to really bloom. Remember that the rib I tested was built one day and tested the next.</div> <div><br></div> <div>I am inclined to suspect that the biggest problem in the killer area is that the ribs are collapsing under the extreme down bearing loads to which they are typically loaded. 24 Kg is one hell of a weight to put on a single standard rib, even if it is only 560 mm long. Furthermore, most manufacturers reduce the rib sectional sizes in the treble. Typically, the rib under the first capo section might be around 20 mm X 20 mm. Ribs of these dimensions have absolutely no hope of any service life if they are subjected to 24 Kg or more, which is often the case.</div> <div><br></div> <div>Best,</div> <div>Ron O.</div> <x-sigsep><pre>-- </pre></x-sigsep> <div><font size="-2" color="#000000"><br> </font><font face="Times New Roman" size="+1" color="#0000FF"><b>OVERS PIANOS - SYDNEY</b></font></div> <div><font color="#000000"><i>    </i></font><font color="#0000FF"><i> Grand Piano Manufacturers</i></font><font color="#000000"><br>      _______________________</font></div> <div><font size="-2" color="#000000"><br> </font><font color="#000000">Web http://overspianos.com.au<br> mailto:ron@overspianos.com.au<br>      _______________________</font></div> </body> </html>