Don wrote: > > The capstan is supposed to be "involute". i.e. it > doesn't slide at all--it rolls or rocks on the > whippen. > A common misperception. It does slide. The involute curve is the standard gear tooth profile. The important quality of the involute curve, is that it transmits uniform angular velocity. So that as the driving gear rotates at constant RPM, the driven gear also rotates at constant RPM, with minimum noise and vibration. But they do slide. Make your self a paper cutout and try it. It's also interesting to consider the angle of gear teeth. Standard angles for spur gears are 14 and 20 degrees. 14 degree teeth are smoother and quieter, and have a longer wear life because at a given load they have lower contact stress. In fact, zero angle or negative angle would be even better, from a contact stress point of view, but with small or negative angles there is a geometry problem known as "undercutting", where the base of the tooth ends up narrower than the contact area, and is prone to fracture. 20 degree teeth are a compromise, used for pinions with very few teeth to prevent undercutting. So does a capstain slide on the wippen cushion felt, except for the instant that the contact point passes through the "magic line". Before and afer that point, the Overs capstain slides the least, standard vertical capstan slides a little more, and the Steinway backwards leaning capstan slides a little more than that. more than you wanted to know, and it still does not explain why S&S might have done that. Mike
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