Hi Ric I've been gone this week. First, transporting the little redesigned Stwy O to Alameda for the class, then teaching the class & visiting with friends, returning piano etc,etc add exhaustion. It's Friday & I'm glad so I havent' been ignoring this Hi Dale This argument about ribs not supporting crown has bothered me from the first moment I heard it. And after reading Nossamans well written article in the latest Journal I think I know why. Ok, nobody questions that in a CC board ribs do not provide beam support for the load. But that's not quite the same thing as saying they don't support load in a different fashion. Hey they add something . If that wasn't true every C. C. Board wudda caved on inception. The same thing goes for the crown argument. And that's where Ron's article comes in. I'm not sure I know which article this is. I'll find it. what month? About 2 years ago I posted a couple threads with some drawings trying to explain why I thought the ribs in a CC board had similarities to a cable in the sense that they attempt to constrain the board from expanding... so the panel has to bend instead. That very resistance to the panels expansion is every bit as much a load support but in an entirely different way. What the kicker back then was, was that I tried to argue that the ribs strain (note the word usage) against the expansion forces from the panel. I was told then that no.. the ribs don't strain.... they simply bend against their better nature. As I read the other posts on " Strain" & I have to see the ribs being pulled up from there flat orinetation by panel expansion as ......strain? If some one bends me out of shape I feel ....strained Enter Ron's article disclaiming the buttress arch. In that article he shows by experiment that the top half of the ribs not only bend, but they expand lengthwise. That expansion is critical to his whole argumentation (which by the way made perfect sense to me). But that same rib expansion shows conclusively that the ribs do strain, and significantly so against the expanding panel. The strain is being initiated by the panel expansion If you stop to think about it this only makes sense. If the ribs can not strain lengthwise at all, then neither could the panel crown, yet if they strained equally through their height then they would not constrain the panel at all. It is because they DO strain ... more on top and increasingly less towards the bottom combined with the panels compression that crown and crown strength occur. And it doesn't really seem to me to be so much a stretch of the mind to imagine mathematical explanations for all this that would fit very nicely into design thinking. The height and width of ribs don't add up to combine in a kind of beam strength / mass relationship... but rather a kind of strain strength / mass one. I think I follow you. Frankly it helps me to think the whole assembly as plywood One thing is clear about load support in CC boards. The more you push on it, the more it resists... until its overloaded of course. But until that point there is definitely load support and the ribs are definitely part of that... just not in the sense of beams. This is true but the ribs are adding stiffness & mass & mass is a often neglected piece of the discussion at times. Ie. I've heard many C.C. board with little crown or bearing that really sounded to my ear,very good, enjoyed them greatly. So what's making them work? rib stiffness/ panel assembly stiffness & being glued as an assembly to the rim adds stiffness. When I load up a RC & partially panel supported board, there can be a very powerful strong non-linear resistence during the pre stress bearing set up. As I pound the board down, slipping a wooden wedge between the central plate strut, the resistence to further deflection stops at about 3 to 4 mm even if I continue to pound really hard with fist on bridge. It feels like I'm pounding on a gym floor at that point. The stiffness in that develops in a Sitka panel at 5.5 to 6.0 E.M.C. can exert a lot of stiffness factor which really quite amazing. & So is a purelay C.C. panel at 4% as well. I got to thinkling about how strong spruce expansion is when considering ,that a thoroughly dried C.C. a panel can bend 1 2 or more ribs for really quite along time and sound quite good. This is amzing. The longevity issues & reliability issues of C.C. boards are always the big question mark. Ie the G-2 Yamaha I wrote about thinning the panel on is a C.C. board but with sitka spruce ribs. Yikes How long can that board convince those stiff suckers from sucking the life out of the crown, & it's only 29 years old. In 20 years it may be ready for the dumper, as are many mass produced pianos. In my boards I know panel expansion gives some non linear strength but I love the way they sound. I've got a pretty stiff rib set under there as well. I'm wondering if the non linear panel qualities are noticed or utilized more predominately in the large bottom end of the panel. I've been considering that the shorter ribs in the treble end, Say the last 5 ribs, are getting short enough to perhaps add beam strength as they don't crown much Even under panel compression a flat rib doesn't normally show much residual crown when strung up & the noticeably shorter spans the board makes from belly rail to rim also adds a bit of stiffness but it seems to me that it is the whole of the structure itself which is seen as a mystery as to how to design one from an engeneering standpoint & understanding it. Hey it works, we can't argue with that...but how well, how long....this we can debate till were blue in de face. Dale Cheers RicB ------------------ Dale, Those ribs were originally built into a CC board. How can a compression crowned board get mechanical support from the rib scale, however "good" the scale looks? The ribs in CC boards resist the crown that panel compression is trying to form and maintain, and just put more compression load on the panel. -----Ron No, I get all that Ron, but if the ribs are built significantly taller & of stiffer material by design then more panel compression can be taken out of the equation. Another thought is, & I've witnessed this quite a few times is, that Stwy A's (1 & 2"s) in general can produce a pretty wonderful sound even with a flat or flattish board providing there is some small but consistent bearing load still intact so in this case it would seem that there are enough impedance factors about the rib scale to make the system work rather well. Ok maybe a freak of nature but it happens fairly frequently. About 5 years back I had such an long A I was going to resell it. It had Steinway hammers which were quite soft & made it sound really good. It was hard to imagine that a new board would make it sound much better. It truly sounded glorious but it was a spec job & I don't sell old boards very often. I didn't do any thing to the action until later for a really good A b comparison. So I built a board with the same number of ribs making them crowned at about 60 ft. Made em taller but not much. Used sugar pine in the bottom & yellow pine in the top. The sound was cleaner and the sustain was about the same which was awesome. It just had it!! I attribute much of this to the original basic rib scale design. Something was working or several things were. Do you see what I'm saying? I'll crunch some numbers & see what I got. I greatly Appreciate the design sharing & information swap. Thanks Dale It's an entirely different system. Do a bearing load analysis on the ribs as load carrying beams and see what the numbers say. They'll say that the rib scale isn't adequate to support bearing without substantial panel compression support. -------------- next part -------------- An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/20060407/5ccdae38/attachment-0001.html
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