Bill Ballard, Ron N and list, Bill wrote; >I've always assumed that the sliding >friction between the top of the jack and the knuckle had a static >phase, one which at the start locked the jack top to the knuckle, and >which was not transformed into a sliding friction until the point >where the potential energy store in the compression of the jack >adjusting button felt finally exceeded the force of static friction >at this spot. A very nice description Bill. Furthermore, the nature of the jack motion you describe is the primary reason why the jack stop cushion must be quite thick and of a soft grade of felt. When key depression begins, there is a noticeable backward movement of the jack tip (away from the hammer center) before the onset of jack/knuckle sliding friction. >Remember that while the contact point on both arcs (the >knuckle's and the jack top's) are both heading in the same direction, >the horizontal component (sliding motion) is greater in the knuckle's >arc, and it is also increasing. No wonder the jack is not inclined >(no pun intended) to immediately break free of this static friction >grip with the knuckle. The arcs of motion will only be travelling in the same direction at that instant they pass through the line-of-centers - when jack/knuckle friction will be at a minimum. On either side of the line-of-centers friction will increase proportional to the lever-contact's displacement from the line. Ron Nossaman responded to Bill's post; >I'd say your explanation is right. The described arcs are intersecting, >rather than tangent, and the radii of both arcs (wippen flange center to >jack top, and hammer flange center to knuckle contact point) are decreasing >as the jack rises. Exactly. And the rate of of the radii change is proportional to the friction. >I don't understand why widening the spread would change >this. Seems like that just tilts the jack forward more and makes the wippen >arc radius longer. Wouldn't that make it worse???? Most probably. Bill Ballard continued; >It would seem quite likely that this effect could still occur even in an >action in which axes and contact points had been properly dealt with, >such as in Ron's action. It certainly does, but to a lesser degree by virtue of the reduced contacting-surface offset from the line-of-centers. Ron N responded to Bill's above statement; >It should, but since these described arcs in his action are so much more >nearly tangent (as near as is geometrically possible) the effect would be >minimal. Then again... Hey Ron Yes? >in an action like yours, if the jack was >backstopped by a relatively light, short range spring instead of a >relatively firm felt, wouldn't the jack ride the knuckle through the entire >range of movement, deflecting and rolling rather than sliding, and produce >very nearly zero friction until letoff? It probably would, but I suspect that the load on the spring might still cause a counteracting load to be placed on the knuckle (as it also will with the felt), which would oppose the rise of the hammer shank. >I know you've already got very >nearly zero friction there >but I was wondering if you or anyone else has >tried this, or seen it anywhere? The closer the contact point to the >hammer/wippen center line, the better the result, and the less it would be >necessary. Its interesting that you should ponder over further possibilities. While I had a couple of other ideas lurking around in my head (not the particular one you mentioned), I opted for a more standard layout (ie. incorporating a standard knuckle in the form of a roller) which might cause less controversy in the technical community. >Oh well, sorry to butt in. Just pondering the physics a little. You both (Ron N and Bill) seem to be pretty much on the mark - your pondering was welcome. It was good to read your contributions at the end of my Monday here in Sydney. I'm looking forward to some of the Reno bar-room discussions already. Regards, Ron Overs -- Overs Pianos Sydney Australia ________________________ Web site: http://www.overspianos.com.au Email: ron@overspianos.com.au ________________________
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