>> > And speaking of "ya ever look close there", did you ever watch a jack >> > get momentarily pulled backwards (squashing the jack button felt) >> > which the cycle first starts up? > 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. 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. > >I'm not sure that this is the explanation for the effect, or whether >your observation regarding the spread contains the answer. 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. 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???? >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. ------------------------------------- >Bill Ballard RPT 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, 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? 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. Oh well, sorry to butt in. Just pondering the physics a little. Ron N
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