This is a multi-part message in MIME format. ---------------------- multipart/alternative attachment Comments below - I need some clarification to address these issues. Terry Farrell ----- Original Message -----=20 From: Richard Brekne=20 To: Pianotech=20 Sent: Monday, October 06, 2003 9:42 AM Subject: Re: RC vs CC again Terry... et al=20 I wanted to get back to you on this with a better explaintion of what = bothers me about this position you take below.=20 Consider the following.=20 When ever a board is bent the, convex side will be stretched relative = to the concave side. Correct - I'm assuming we are talking about one solid board here - not = a soundboard - but rather, let's say, a one-inch by one-inch by = three-foot solid piece of wood. This is the same regardless of the over all compression level found = in the board. I think so, not quite sure what you mean by the "over all compression = level found in the board." Pressing downwards on this curved board will tend to have the = opposite effect... ie stretch the concave side and compress the convex = side.=20 If the board is curved and it is the same board - presumably had = originally been straight - then some force is holding it in its curved = configuration. What is that force? Is it trapped at the ends? Is it = glued to a panel that has expanded with increased moisture content? But = yes, if you have this curved board the net change in tension or = compression will be to stretch the concave side (decrease compression - = however, being that it was straight to begin with, it was under some = amount of compression in its curved state, so when you press down that = state of compression lessened - but it is still compressed relative to = its state when straight). Same deal with the convex side. Ribs glued to the concave side will fight any tendency of the panel to = stretch on its concave side. I'm not sure exactly what is being described here. "Ribs glued to the = concave side" of what? What kind of ribs? Thats what causes them to bend in the process of crown formation in = the first place. Downbearing applied to the convex side of the panel = will tend to compress the convex side, and stretch the concave side... = but the ribs resist stretching of the concave side.=20 I don't think it is correct to say that the "ribs resist stretching of = the concave side." If I am understanding you correctly you are = describing a CC crowned soundboard above. In that case, the concave side = of the bent rib is in a state of compression and is trying to expand on = its own to straighten back out. So as stated above, when you apply = downbearing, yes, the concave side of the rib stretches, but only with = reference to its curved compressed state - it doesn't stretch relative = to when it was straight. I think it is more appropriate and more clear = to state that when downbearing is applied, the concave side of the rib = experiences a reduction of its compressed state. So how does that equate to=20 "As the crown lessens under increasing load, the ribs will pull = downward a bit less, but only because they are being bent upwards less = by the panel."?=20 See above. Seems to me the ribs will be even more stressed by down bearing...erg = they support crown against applied downbearing.=20 Yes ???=20 To the best of my knowledge, and I think I am thinking correct physics = here, no. In their bent state, the originally straight ribs are always = trying to straighten out, i.e. pull the panel down, but the panel's = physical properties are able to resist some compression and not allow = the rib to flatten. When you apply downbearing, the rib becomes less = stressed because it is closer to a relaxed, natural, state. The panel = experiences increased compression with increased downbearing. RicB=20 Farrell wrote:=20 Richard wrote: "This assertion that ribs don't support the bearing = load also is bothersome. Why=20 wouldn't the ribs in a CC board support both crown and bearing... = just because the=20 panels compression forces the ribs to bend?" When flat ribs are = glued to a very dry flat panel, no force vectors exist in the system. As = the panel takes on moisture and the ribs bend, the ribs will always be = trying to pull the panel flat. Period. If that assembly has crown, the = ribs will always try to go straight by pulling down on the panel. The = compression forces in the panel are the only thing that has provided the = force necessary to bend the rib into a crown. Whether or not you ever = load that soundboard on top, the ribs will always be fighting the panel = compression to go flat. When you do apply a load to the top of the = soundboard, the compression on the panel will increase, but the ribs = will do nothing other than still try to go flat. As the crown lessens = under increasing load, the ribs will pull downward a bit less, but only = because they are being bent upwards less by the panel. Terry Farrell=20 --=20 Richard Brekne=20 RPT, N.P.T.F.=20 UiB, Bergen, Norway=20 mailto:rbrekne@broadpark.no=20 http://home.broadpark.no/~rbrekne/ricmain.html=20 http://www.hf.uib.no/grieg/personer/cv_RB.html=20 =20 ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/db/85/ab/90/attachment.htm ---------------------- multipart/alternative attachment--
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