This is a multi-part message in MIME format. ---------------------- multipart/alternative attachment A stress concentrator is as its name says, anything that will concentrate a stress load. It is what enables us to cut glass, for example. The scratch in the glass acts as a stress concentrator. When a bending load is placed on the glass it is transfered very uniformly accross the entire surface of the glass because it is so smooth, unless there is some kind of flaw, like a scratch. There the outer stress lines will "flow" under the scratch, combining with the inner stress lines, which produces a higher stress load. And that is where it cracks. A keyway in a rotating shaft is another stress concentrator. It will nearly always be the point of failure if such a shaft breaks. Think of a shaft projecting from a steel plate. As a load is applied to the end of the shaft the stress must be transfered to the plate. The stress lines will concentrate right at the joint of the two members as they bend around the corner. If you were to make a larger weld area, or even add a circular boss around the shaft to distribute the load, the shaft would carry a much higher load before breaking. Anytime you make a mechanical connection there is a stress concentrator at the outer perimeter of that connection. Making that contact area larger with more perimeter is a way of reducing that. So the washer gives both a larger surface contact area for distributing the load plus a bigger perimeter for reducing the concentration of stress. Since cast iron has relatively low tensile/shear strength it is particularly susceptible to stress concentrators. That is why everywhere you look on the piano frame you will see no sharp corners. They use plenty of radius. Also look at the struts; they do not abruptly end, they taper. None of this is to say that your plates will crack if you don't use a washer. But in designing heavy equipment we would never allow a bolt head or nut to rest directly on cast iron. We would use a hardened flat washer. It's just what I am used to doing. Of course what I designed was subjected to very high stress loads typically. Hope this helps. Dean -----Original Message----- From: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]On Behalf Of Terry Sent: Saturday, January 08, 2005 2:40 PM To: Pianotech Subject: Re: seeking wood screws Hey, ya learn sumptin' new every day. Please tell me more. How does the bare nut act as a stress concentrator (and what exactly is a stress concentrator). How does a flat washer help distribute the load (if the nut was only touching the plate in one spot, it seems that even with a washer in there it would still do the same)? I know sometimes I will pose a question facetiously, but as I do not have any background in mechanical engineering, I am genuinely curious to hear more on this. Thanks. Terry Farrell ----- Original Message ----- From: Dean May To: Pianotech Sent: Saturday, January 08, 2005 12:58 PM Subject: RE: seeking wood screws It's a throwback to my days as a design engineer. It is bad practice to place any kind of stress concentrator against cast iron. The washer is a little insurance to distribute the load, though ideally it should be hardened to maximize its effectiveness. Thanks for the tip on the hole. Dean -----Original Message----- From: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]On Behalf Of Terry Sent: Saturday, January 08, 2005 11:34 AM To: Pianotech Subject: Re: seeking wood screws Why do you put a washer on top of the up-side-down lock nut that goes on the backside of the plate - assuming of course that the nut has enough horizontal surface area to mate with the plate (and not want to fall through the hole)? I would just be afraid of a potential rattle/buzz if one of the assemblies got a little loose (I realize that none should be). I just haven't found that to be needed. I do put a washer on the top though - but mostly to protect the plate finish while tightening the acorn nut on top. 5/16-inch hole gives me a nice very snug fit. Terry Farrell All this talk of screw sources has had me browsing some of these sites. Very cool. I've always tried to maintain an organized supply of various screw types and sizes which has been a huge asset. But it is not always easy to get big boxes of the sizes you want any more. Most places only sell the little 15 piece plastic packages (very expensive way to buy screws). Anyway, I have been wanting to do the plate suspension trick for adjusting downbearing. Del spoke of it at the recent Indy conference. This involves threading socket cap screws (headless bolts with recessed allen socket) into the inner rim, running down a lock nut upside down with a washer on top (a sufficient clearance well counterbored into the soundboard), the plate lowered onto the washer, a chrome washer and acorn nut on top. The height of the plate can then be easily raised or lowered by removing the acorn nut and turning the stud with and allen wrench. Attached is what I ordered. I got the hanger screws just for fun. If the machine thread doesn't work well, which Del said it would thread right into the wood, I can always fall back on the hanger screws with their lag thread. Does anyone have a recommended hole size to drill the rim for the 3/8" machine thread to screw into? Blessings, Dean ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... 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