---------------------- multipart/alternative attachment I tested my pins in a vise and they still broke at the becket hole, which is where they break in standard pins. If there were no becket hole, I presume they would break at the transition, but at a higher applied force. When some of my pins are produced with a larger transition, I'll test them as well. I tested them with the tops of the vise jaws tightened at roughly 5/16" below the transition shoulder and also at roughly 1/16" below the shoulder to simulate the situation in both closed face and open face pin blocks. I used an old extension lever just in case the lever broke instead of the pin. Got my 24-year-old son to do the pulling. I figure the tip on the lever is plenty tight now. I have no idea how much force was used because I didn't want to break my torque wrench. Paul Mike and Jane Spalding wrote: > Paul and Terry, Whenever there is an abrupt change in size/shape, > there will be a stress concentration. This is what you've got at the > inside corner where the 1/0 upper portion of your pin meets the 2/0 or > larger lower portion. Picture a pin of uniform diameter, with a > straight line drawn along it. Now twist the pin, look at the line: > it's a uniform spiral, like a barber pole, or a candy cane. Now take > the stepped pin, draw the straight line along the lower portion, in > towards the center along the step, up along the upper portion. Twist > the pin, look at the line. Still generally a spiral, but: The spiral > on the upper (smaller diameter) section is faster than the spiral on > the lower portion. Where the upper spiral meets the step, there's a > distortion in the spiral which is your stress concentration. Stress > concentrations can be minimized by radiusing the inside corner, by > optimizing feeds and speeds in the lathe, and polishing the radius > after machining. Best case, I would guess Paul's pins still have a 20% > to 30% stress concentration factor at the step. Hope this helps Mike > Spalding > > ----- Original Message ----- > From: larudee@pacbell.net > To: pianotech@ptg.org > Sent: Sunday, January 27, 2002 12:19 PM > Subject: Re: Tuning Pin Size > Terry, > > I am not an engineer, but I consulted one while researching > for my patent, and that's how it was explaiened to me. The > problem is that when you twist or flex the top portion, the > bottom portion doesn't twist or flex as much. If you twist > or flex the pin that is 2" in diameter in the base and .276" > in the top portion to the breaking point, it's going to > break at the transition point every time. If your argument > were correct, it would break randomly at any point along the > top portion. Are there any engineers who would care to > elaborate? Carl? > > Paul > > Farrell wrote: > > > I'm trying to understand this. Let's say we have a 0.276 > > -in. dia. tuning pin that is 2-in. long. Let's say it has > > a shear strength of 300 inch-pounds. Meaning of course if > > you install the pin in a new Baldwin, put a tuning hammer > > on it (or a torque wrench) and try to turn it, when you > > get to a shear force of 300 inch-pounds, it will shear > > into two pieces - leaving one piece in your tuning lever > > tip and the other in the block. Now take a similar pin, > > but make it 6 inches long. Do the same things, and it > > should shear at 300 inch-pounds of torque. Length should > > not matter (you will of course get more twist with the > > longer pin before it shears). Now take a 0.286-in. dia. > > tuning pin that is 2-in. long. Let's say it has a shear > > strength of 350 inch-pounds. Do the same things to it and > > it will shear at a torque of 350 inch-pounds. Now take a > > pin with a bottom of 0.286-in. dia. and a top of 0.276-in. > > dia. Put it in that same nasty Baldwin block - or a strong > > vice - or whatever - just so it doesn't move - at it will > > shear at a torque of 300 inch-pounds. Now take a pin with > > a bottom of 2-in. dia. and a top of 0.276-in. dia. Put it > > in that same nasty Baldwin block - or a strong vice - or > > whatever - just so it doesn't move - at it will shear at a > > torque of 300 inch-pounds. The larger base would act just > > like the pinblock with the constant diameter 0.276-in pin > > in it. They would both shear at 300 inch-pounds. Or so it > > would seem to me. Concentrating shear forces? How does it > > do that? Terry Farrell > > > > ----- Original Message ----- > > From: larudee@pacbell.net > > To: pianotech@ptg.org > > Sent: Sunday, January 27, 2002 11:27 AM > > Subject: Re: Tuning Pin Size > > Terry, > > > > All of what you mention affects shearing, but > > the bottom portion also affects it by > > concentrating the shear forces at the point > > where the diameter changes. In other words, the > > greater the difference, the more torsion and > > flex will end at that point and the less those > > forces will be distributed thoughout the pin. > > > > Paul > > > > Farrell wrote: > > > > > "The larger the size difference between the > > > two portions, the greater the risk." Why would > > > that be? I should think the point at which a > > > pin would shear would depend entirely on the > > > metal composition (let's assume this is > > > constant), its diameter, and the tightness of > > > the pin/block fit (torque). As you make any pin > > > size fit tighter in the block, it will get > > > closer to its shear point. As you make any pin > > > smaller in diameter, you will move toward a > > > lower shear point. Diameter and torque - I > > > think that is all. Why would the diameter > > > contrast between the top and bottom portion > > > affect its shear strength? Is there something > > > about the machining process? Or do you mean (by > > > the above quote): 'The smaller the diameter of > > > the top portion of the pin, the greater the > > > risk of shearing' (because, of course, the > > > smaller diameter pin will have a lower shear > > > strength, and will shear at a lower pin > > > torque). How would the diameter of the bottom > > > portion of the pin affect the shear strength? I > > > am assuming that the rebuilder will > > > drill/ream/whatever the hole to a proper > > > diameter for the diameter of the pin bottom > > > portion. Terry Farrell > > ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/28/d4/93/99/attachment.htm ---------------------- multipart/alternative attachment--
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