This topic has interested / confounded me for years, and it's been frustrating particularly because my engineering background, which included a stint in the gear manufacturing industry, should allow me to make sense of it. First, thank you Nick for the link to Wikepedia. Look carefully at the animation, and read the accompanying text. Notice that the reason for using the involute in gears is not to eliminate sliding friction - sliding friction and wear is listed as the main drawback of involute gears, hence the need for effective lubrication on gears. The involute's strength is that it transmits angular motion very smoothly. And it turns out that, as you work with the pressure angle and other parameters to optimize smoothness, tooth strength, and power ratings, you end up increasing the amount of sliding. Also, note the definition of "involute": "the spiraling curve traced by the end of an imaginary taut string unwinding itself from that stationary circle". Does anybody really believe that the top of the capstain, or the heel of the wippen, are shaped like true involutes? But they do share one similarity with involute gear teeth: They slide. When Ron Overs unveiled his new action geometry in Reno, I was certain he had discovered and corrected a major flaw in piano action design. While standard parts don't allow us to go as far as Ron did, we could at least convert angled capstain actions to vertical capstains. So why, when I did just that (S&S L) did it make no improvement, possibly made it worse?? I like Dale's explanation: *The broad faced angled capstan actually has an accelerating effect when positioned properly. At rest, the key side edge of the Broad faced capstan touches on the wippen felt edge furthest from the wippen center pin. As the capstan/wippen moves thru its rotation the capstan moves upwards & rotates to the middle of the capstan & then to the rear edge. SO, as it continues thru its arc, the point at which the wippen is lifted moves to the rear edge of the capstan & closer to the wippen pin thereby accelerating the key/action motion. Kind of an automatic transmission affect.Things move faster without shifting. *If sliding friction is unimportant (especially if it's minimized by polishing the cap and teflonning the wip heel), and if the change in leverage during the stroke works as Dale describes, that would explain my experience, and also the results of the "what happens if..?" experiments. Lots of good new information this morning, need time to digest it. Mike Nick Gravagne wrote: > > Right Jon, > > The “standard” capstan-to-whip-heal-interface motion of the parts > (beginning from rest) follows this pattern: > > 1) Slide with friction, 2) then roll at magic line with no friction, > 3) and finishes with slide-friction. The indentation in a veteran whip > cushion should reveal an oval shape. > > The involute slide path, supposedly described at the interface of a > tipped capstan and sloping whip heal, should roll through the complete > path, hence no friction (effectively). I have understood this to be > the case for many years, but have never verified it for myself, > although I have no reason to doubt Chris Robinson (I also took that > class many years ago). > > The rolling condition obtains since the force line, or line of action, > common to both the capstan and the heal runs along a tangent common to > both surfaces. Said another way, “both contacting surfaces are always > perpendicular to the plane of contact.” Relative to gears, this > condition exists as the gear teeth mesh; the teeth roll on each other > without the immense friction and wear which would otherwise exist. > > Check out this link: http://en.wikipedia.org/wiki/Involute_gear -- has > a neat animation. > > /*/Nick Gravagne, RPT/*/ > > /*/Piano Technicians Guild/*/ > > /*/Member Society Manufacturing Engineers/*/ > > /*/Voice Mail 928-476-4143/*/ > > ------------------------------------------------------------------------ > > *From:* pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] > *On Behalf Of *Jon Page > *Sent:* Saturday, March 14, 2009 5:26 PM > *To:* pianotech at ptg.org > *Subject:* Re: [pianotech] key position at rest > > >Why they angled them backwards I'm still unsure of. > > The interaction between the angled capstan and angled cushion > > is called an involute gear (Chris Robinson stated this in a class > > many years ago). > > -- > > > Regards, > > Jon Page >
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