Capstan/wippen heel intersection

[Richard Brekne]

Hi there Clark... this one caught me a bit off guard and I had to delve in and
check out this and that and see if I could make sense of it... I do see the point
here, and I would think off hand that this would apply much more to the jack /
knuckle / whippen contact sequence then the capstan / whippen cushion.

Even so... the closer these two points are to behaveing as a pivot.. the less
friction and perhaps better delivery of present leverage... or what ?? That the
pivot is unobtainable doesnt change that getting as close as one can improves
efficiency ??



Clark wrote:

> Hi,
>
> Just a quick response to a line in Richard Brekne's post:
>
> > From a friction standpoint, and I believe from a leverage standpoint.. the
> > capstan /cushion should act as nearly like to a pivot as is possible.
>
> The intersection is less like to pivots than gear or cam action. In
> order for one to drive the other the respective vectors of velocity
> _commonly tangent to the capstan and heel at the points of contact_ are
> equal in magnitude and direction; however, their motion is angular and
> the _actual_ instantaneous velocities are perpendicular to the lines
> from these points to the respective action centers. The difference
> between the pairs of actual velocities is the instantaneous velocity of
> sliding between the profiles, and which should be smallest around the
> line between the two centers. An interesting note, though:
>
> > The energy lost in friction at the teeth [action part contact profiles]
> > and at the bearings is...less during recess [above the line between
> > centers] than during approach [below it]...During approach the teeth [parts],
> > while sliding on each other, are pushing into mesh, while during recess
> > they are drawing out of mesh. Gears [actions] would therefore be more
> > efficient and durable if the action were confined to recess.
>
>         p236. Albert, C.D. & F.S. Rogers. "Kinematics of Machinery"
>
> The path of the point of contact is a function of the profiles of the
> parts, compounded a little in this case by nonlinearities of heel
> padding and compression over time (oh, and the deformation of keys under
> load); for one, involute profiles have been favored for gear teeth
> because they can tolerate changing geometry. Anyways, a more efficient
> action also is a more durable action.
>
> Regards,
>
> Clark

--
Richard Brekne
RPT, N.P.T.F.
Bergen, Norway