Del, Thank you for the take on this problem. I did try setting the nose bolt a little low and pulling the strut down to try and close the crack. It didn't help. I have found it's difficult to ever close a cast crack because of broken particles in the crack itself. John Dewey metioned that when he stopped by the other day. Before I weld the plate on the strut I will pull the strut down a bit and also plan on having some string tension. Welds will all be in the horizontal plane and will provide a lot more support to the gusset. Delwin D Fandrich wrote: > > My, what a bucket of snakes we've opened here. (I've never been bothered by > buckets of worm -- they just kind of lie there squirming around. Snakes, on > the other hand...) As may be, I'll warn you in advance...if you get bored > easily delete this post immediately on to the next one. > > The crack described by Richard is on the top of the longitudinal strut > running roughly parallel to and above the highest string in the bass > section. If I read the description of this crack correctly, it is in the > strut passing over the tenor strings. The crack started at top of this > strut and runs down into the body of the strut. > > OK. Baldwin does not use any type of horn to transfer any of the string > load to the belly structure so this places all of the stress on the > longitudinals. These struts run well over the line of stress, i.e., the > centerline of the strings. This places the lowest portion of these struts > in compression -- gray iron's best feature is its compression strength, and > even there it is none too high -- and the top portion in tension and grey > iron has very poor tensile strength. By that I mean it is VERY weak in > tension. One of the functions of the nosebolt is to keep the struts from > bowing up and allowing excessive tension from developing in the top portion > of the longitudinal. If the nosebolt is/was loose enough to allow the strut > to bow upward, this would allow greater tension to develop at the top of the > strut than the gray iron is capable of withstanding and a crack develops. I > wouldn't recommend it, but in all probability if the nosebolt could be > adjusted so that the strut is brought back down enough to close the crack > the piano probably could be tuned without further problems. Yes, it would > make me nervous but I think it could be done. > > How to repair this problem. I have no experience with the 'Lock-N-Stitch' > process. I have read quite a lot of promotional literature produced by the > company making the things, but I have not actually used the process. So > anything I have to say about it is based strictly on my observation of the > components and my experience with gray iron. > > The Lock-N-Stitch' process involves drilling and inserting a plug that is > held in place by the geometry of the threads on the plug. In the end any > tension placed on the member in question is going to be held by the ability > of the threads to bite into the existing iron and hold it in tension. I > personally would not trust it to hold in this situation. I suspect the > process would work well in the repair of a flat panel that is not held under > high tension over a long period of time. > > I have had good experience with having gray iron welded. The shrinkage rate > of gray iron from its molten state to its frozen -- solid -- state is closer > to 1% than it is to 6%. The patternmakers rule is about 1010 mm for a 1000 > mm rule, or about 1/8" per foot. This is significant in a one meter long > longitudinal but not in a 20 mm weld. I am also not enough of a > metallurgist to explain exactly why gray iron is so problematic to weld, but > the folks I have consulted have told me it has more to do with the amount of > carbon in the iron than with the shrinkage of the iron. Iron structures > with less carbon -- malleable iron, for example -- can be easily welded even > though it shrinks at roughly the same rate as does gray iron. I learned > long ago to use the word 'gray' when describing the type of iron used in > piano plates. These guys work with a lot of iron castings but few of them > use anywhere close to the amount of carbon in them that is used in piano > plate castings. > > So, how to repair Richard's Baldwin. It sounds like it is pretty much all > done and it will probably hold. Personally, I would not have used the > Lock-N-Stitch process in this repair. I would have veed out the crack, had > it welded with one of those nice new welding rods that are supposed to work > well with high-carbon gray iron and made sure the nose bolt mounting was > solid and that the bolt itself was properly adjusted. His idea of sistering > the bar with steel plate is probably good insurance. If anything, I would > have pre-loaded the strut by pulling the nosebolt down just a bit to reduce > the development of tension along the top of the strut. I would then check > the stringing scale to be sure someone didn't get in there and raise the > scale tensions significantly and tune the sucker. After all of this, if I > still had any reservations about my repair holding, I would fabricate some > type of 'horn' that I would mount below the plate to couple some of the > stress from the strings directly to the bellyrail/rim brace assembly. The > Baldwin has a fairly sturdy structure down there and I could probably > transfer a substantial portion of the load to this portion of the framework > without introducing any other problems. > > One last comment about nosebolts. They are used in several ways depending > on how the piano structure and plate are designed. > > As Ron pointed out they are often used to mass-couple the plate to the wood > structure of the piano. The most obvious example of this is the bell and > bolt arrangement used in the larger Steinway pianos. They do raise the > impedance of the plate and reduce the mechanical losses to the plate. I.e., > they aid sustain slightly. > > They are also used to stabilize the plate. As noted earlier, gray iron > works better in compression than it does in tension. The nosebolt is often > used to keep struts from bowing upward and placing the top of a longitudinal > strut in tension. This is the purpose of the nosebolts going through the #1 > bass longitudinal strut -- the long strut running along the bass side of the > lowest bass string. Without the nosebolt in place these struts do have a > lot of flex and can easily break. (Another way to stabilize these long > struts is to give them a strong inverted 'T' shape.) > > And, yes, they are used to control plate ringing. It is often more > economical and practical to design a nosebolt system to control ringing and > to stabilize the plate than it is to simply add more mass. Simply adding > mass to a gray iron casting does not always add strength. In fact, it > sometimes will weaken the overall casting. Complex gray iron castings cool > at uneven rates. When a very massive section is next to a less massive > section a very strong internal stress will be present in the finished > casting. Sometimes this stress can be great enough to cause the casting to > break even before the intended load is placed on it. A more practical > method is to reduce the sections to their practical minimum and use a > well-placed system of nosebolts to control both flex and ringing. This, by > the way, was the approach taken by the designers of most of the Steinway > plates. They are reasonably light weight, do not ring appreciably in > practice and are quite stable. You can't ask for much more than that from a > piano plate. > > Regards, > > Del > Delwin D Fandrich > Piano Designer & Builder > Hoquiam, Washington USA > E.mail: pianobuilders@olynet.com > Web Site: http://pianobuilders.olynet.com/
This PTG archive page provided courtesy of Moy Piano Service, LLC