>>When I think of it, I can see only one ideal situation, that is when the hammer is perpendicular to the string at impact time, and the shank is closest to parallel to the string at that time. In this condition, the energy transfer from the hammer to the string seems maximized to me. >From an engineering perspective (at least, this engineer's perspective), maximum energy transfer of the rotating mass of the hammer would dictate that the hammer be perpendicular to a line from the tip of the hammer to the center pin. That's not likely to happen given the physical constraints we find ourselves with. If the shank were pushing the hammer against the string then the hammer would need to be perpendicular to the shank. But it isn't. It is the kinetic energy of the entire rotating mass that is slamming the hammer against the string. So the maximum force line will be perpendicular from the point of impact to the center of rotation. The other thing to maximize energy transfer is to make the above perpendicular line to be in line with the center of percussion, http://en.wikipedia.org/wiki/Center_of_percussion. This is the sweet spot on a bat when playing baseball. If you hit the ball on this spot all of the bat's rotating energy goes into ball. If you don't hit the sweet spot, some of the bat's rotating energy comes back into your hands and you feel the sting. The woody sound one hears in the treble is a result of the hammer not being in line with the sweet spot. Some of the rotational energy is being absorbed by the shank and flange causing the wood to vibrate. Thinning the shanks moves the center of percussion further out towards the hammer which reduces the vibrational woody sound. One could toy with adding weights to the back side of the hammer to move the cp further out to get an even better alignment of the hammer with the cp. I would think it also be important for the hammer to be perpendicular to the strings at impact. Now, if we can get Del to design a piano to make all that happen at once, we'd have a harmonic convergence and world peace. ;-) Dean Dean May cell 812.239.3359 PianoRebuilders.com 812.235.5272 Terre Haute IN 47802 -----Original Message----- From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of Stéphane Collin Sent: Friday, June 20, 2008 11:14 PM To: 'Pianotech List' Subject: RE: Bechstein B hammer rake / more thoughts Hi Ric. When I think of it, I can see only one ideal situation, that is when the hammer is perpendicular to the string at impact time, and the shank is closest to parallel to the string at that time. In this condition, the energy transfer from the hammer to the string seems maximized to me. If the shank is not parallel to the string, there is a portion of the hammer movement that will not be normal to the string plane (but parallel instead), and that movement will spend energy in friction (of the hammer against the string), and the rebound of the hammer will also be slightly impeded because the hammer will have to reverse the direction of that portion of movement too. I fail to understand the importance of the shank being horizontal. What are the ideas of Bob Hohf about that ? The trigonometry is fine, but you could add an analysis of what happens with the measurement errors that you do when measuring - String height over keybed (I don't seem to be able to do this measure with less than 0.5 mm error) - Rounding this value for all notes between two samples you measured (typically at each side of each section), so if for example you measure 231 mm at the right of the treble section and 230 mm at the left of the alto section, all notes in between will have an additional error of 0 to 0.5 mm - Center pin over keybed : here you have two measures and one approximation : the approximation is that when you measure any point on the stack over the keybed, you do this with the action out of the piano (I suppose, or do you have a better way ?) and so the action rests on another surface against which you do the measure, but is the action frame sitting exactly the same way ? I find it difficult to avoid another 0.3 mm error here. Also you don't measure directly the center pin height (or do you ? but how ?) you measure one accessible point at the top of the stack, then measure the vertical distance between that point and the center pin, and here again errors sum up - yet another rounding error for all notes between the extremes, if there happens to be a slight difference in the measurement of the first bass center pin height and the last treble one - angle between string plane and keybed (I can't even figure out how to measure this accurately. My best take up till now was to throw a ping pong ball from 20 cm over the strike line and measure where it falls down after its first rebound, but this assumes that the keybed is horizontal, which I can grosso figure out with a bubble gage) - again approximations across the scale - and finally the error in hammer bore itself This is where the sample hammer with known bore distance trial in situ shortcuts a whole bunch of accumulated errors (in the trigonometry formulas, any measurement you have done will multiply even further all the errors), as there is only one approximation, that is the perpendicular condition of the hammer with the string plane, which is quite easy to measure with a piece of straight wood with a nail put in it at straight angle, which you put on the strings with the nail facing the ground. And if the hammer is perpendicular to the shank, then necessarily, the shank will be parallel to the string plane. But this tells nothing about the horizontality of the shank, and I fail to understand why I should care. So I don't understand either the rake thing, short of trying to float for all the said approximations when it is time to have the actual end user hammer as perpendicular to the actual string as practically possible. What do you think ? Best regards. Stéphane Collin. -----Original Message----- From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of Richard Brekne Sent: samedi 21 juin 2008 0:38 To: pianotech at ptg.org Subject: Bechstein B hammer rake / more thoughts Hi agin "The most clever comment about that came from John Delacour, who considered that the distance between shank center pin and hammer molding center was fixed by design, which makes sense in the way the back checks for example are also at a fixed place offering the exact catch angle if there is such a thing" Been thinking a bit more about all this and have the following observations. I've assumed its generally accepted that the shank should be in horizontal position when the hammer hits the string. If this is so, and if one insists that the hammer should be perpendicular to the string at impact as well, it seems to me that it follows that both the actual hammer bore length and the distance out on the shank the hammer (center of molding) is given by the position of the strike line and the angle the string is off horizontal. Actually boils down to 8th-9th grade basic triangle trig. The horizontal line out to the normal up to the strike point becomes the adjacent line to the angle between the string plane and the horizontal, and the line parallel to the string plane out to this same normal becomes the hypotenuse. The rest is then given. If one on the other hand accepts the design parameter the factory gives for hammer shank center to center molding distance and insist on a hammer to string perpendicular relationship, then one accepts the eventuality that the shank could be off horizontal.... perhaps significantly sometimes at impact. Bob Hohf's article on action elevations comes to mind when it comes to the desirability of a horizontal shank at impact. Does anyone have any comments about just how horizontal the shank should be at impact ? Cheers RicB
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