Hi Nick Nice clear post....AS usual. Thanks Dale -----Original Message----- From: Nick Gravagne <gravagnegang at att.net> To: pianotech at ptg.org Sent: Thu, 29 Jan 2009 10:19 am Subject: Re: [pianotech] FW: laminated ribs again Whether you shuffle the deck or not, lam-ribs are most likely stiffer than the inherent wood species simply by dint of the many glue joints, especially if there are many such joints. Glulams in building construction are renowned for this and such built up beams can span long distances using lighter-weight members with minimal need for intermediate supports. Yet, I wouldn’t guess that the E factor (elasticity in psi) is noticeably higher in soundboard ribs; negligible in fact. Stiffness, in and of itself, is a meaningless goal: it is only relative to the manifold goal of creating a soundboard spring and balancing impedance issues. In most building design and construction stiffness is usually a top priority as relatively large deflections under dead and live loads are undesirable. Piano soundboards require a measurable deflection under bearing in order to satisfy the conditions of spring-building per the classical physics of potential and kinetic energy and work. Crude shop tests, the likes of which we are all prone to make, will not yield much useful data except perhaps in a broad way. Now having said that, lam-ribs fashioned by pressing into a curved caul exhibit several notable differences from ordinary ribs. Number 1: strictly practical, cutti ng a crown into a rib is not required as the rib-lam pressing creates the crown. True, there is other work involved in gluing up the lams and then processing, but I find rib-lams easier and more fun. Also, with rib-lams wood grain is not removed as in crown cutting, rather it is maintained in length (to whatever degree it was pure to begin with) along with the other lams. Number 2: more importantly there is a world of difference in the mechanical behavior between a rib-lam and an ordinary rib. A rib-lam, what I also refer to as a “leaf-spring rib”, has built into it opposing forces of tension and compression, not unlike that of a pressed hammer. The bending stresses and strains introduced into the lams are no where near the serious limit of fiber stress. Still, these forces, for practical purposes, are forever present in the rib, notwithstanding stress-set, creep and later plastic deformation over many years, which would account for a perhaps ½ of the initial set when newly pressed (don’t confuse this to mean half the crown will disappear). Number 3: Rib-lams are related to true arches whereas ordinary ribs, whether crown cut or not, operate solely as beams or girders. The only true arching that takes place with ordinary ribs, flat or crowned to begin with, occurs during panel compression in end-use ambient conditions. Flat ribs pressed in crowned presses have introduced into them the same stress conditions as rib-lams (the holding effect only taking place at the rib-panel interface), and this before any panel compression later on increases that arch even further. True arching, however it is attained, is always a good thing from perspectives of strength-to-weight ratios, of manageable stiffness, and of the “spring” we should be building into our soundboards. Much more to be said RE arches: boundary effects at the rim and deflection values vs. beams, but space is limited. Number 4: More mass in the treble is also a relative concept as it relates to impedance. Recall that impedance is neutral, like many concepts in physics and engineering. Impedance (Z) represents the overall opposition to soundboard system movement as transmitted by string vibrations. Ignoring internal friction and air pressure, the two prominent impeding factors are mass and stiffness; increasing mass always increases stiffness, but increasing stiffness does not increase mass. Power output (initial loudness or pop) is at odds with sustain. Balancing these two forces begins with a grasp of science, but is ultimately determined by artistic ears, and this is subjective to a degree. The theory of Z tells us that mass “traps” higher and highest frequencies; these can find no voice as they come up against an immovable wall. This relationship is expressed within a larger mathematical compass when Z increases as f x m (frequency times mass) and Z is said to be “mass-dominated”. On the other hand, as frequencies trend lower, Z again becomes large when Z=2 0changes as k/f (stiffness divided by frequency) and Z is now “stiffness-dominated”. The theory suggests that care should be taken to control mass in the treble, and care should be taken to reduce serious stiffness in the bass and tenor. In the final analysis our ears must be the judge and guide us as to the next project, especially as same model piano comes into the shop. Nick Nick Gravagne, RPT Piano Technicians Guild Member Society Manufacturing Engineers Voice Mail 928-476-4143 From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of David Love Sent: Wednesday, January 28, 2009 7:06 PM To: pianotech at ptg.org Subject: Re: [pianotech] FW: laminated ribs again It’s important if you are doing RC&S boards where the rib provides the support without help from panel compression. If you base deflection and stiffness characteristics on the ribs then they need to be predictable. A good rib maker should be able to slice and shuffle to get pretty good uniformity where the variations will be pretty minimal. David Love www.davidlovepianos.com From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of erwinspiano at aol.com Sent: Wednesday, January 28, 2009 5:51 PM To: pianotech at ptg.org Subject: Re: [pianotech] FW: laminated ribs again Still ,now mater how you slice the deck, shuffle the cards or glue up the ribs. There will be some variations. Just not as many. How to measure=2 0that & what to do with it is another matter. My point is I think the tendency is to look at these things with a microscope when a magnifying glass will do. Know what I mean? Dale I think it’s not so much an issue of stability but rather of tossing a salad of different lots so that there is a greater likelihood of blended uniformity from rib to rib. If you just cut all the plies from one piece of wood and then glue them back together that would defeat the purpose of lamination. One needs to cut the plies from a number of different planks, shuffle the deck and them glue them up. That insures greater uniformity and thus predictability (the real goal) between ribs. I do wonder, however, since added mass in the treble seems to be a goal, why one wouldn’t use a heavier wood or better yet a graduated blend of differen t woods to make up the laminations so that you could gradually increase the mass coefficient as you ascended through the treble. Rather than just switching the type of wood, as has been done in the past, you could simply gradually add one, two, three plies of a heavier would in each consecutive rib as you moved into the treble. David Love www.davidlovepianos.com Hi Marco I have used laminated ribs infrequently. Although I agree with the big boss as to a more stable Modulus of=2 0elasticity=2 0they can still vary a great deal depending on the grain density/wood density you laminate with. Laminating two sets of identical Stwy B ribs using wider grain wood on one & very tight grain on the other, indicated a definite and marked difference in the strength of those two sets. The denser grain was a stiffer set of ribs. I believe it important to make note of this difference & make an informed decision about stiffness parameters. Ie. how deep/wide/crowned the rib should be...cutouts. The next best thing to Laminated ribs is good set of stiff Sitka ribs cut from the same cant of wood, which, will yield a more even M.E. than ribs selected willy nilly as the to grain count. Wood density/stiffness will be more uniform. I know it's always something. I'm open to hear more comment on this. Dale Which stars will make the biggest headlines in 2009? Get Hollywood news, celebrity photos and more with the PopEater Toolbar. -------------- next part -------------- An HTML attachment was scrubbed... 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