ATM Building a clutch

["Scott Ewart" <scott.e@quicksurfer.com>]

Hi everyone,
    I need a clutch for the roller drive of my split ring.  I came up with
something that's adjustable, zero backlash, and I can make it myself on a
lathe.  The drive shaft will end with a tapered cone shape.  Then the roller
will have a matching taper on the inside with a layer of Teflon in between.
A bolt with a spring and washer will hold it together and provide the
adjustment.
    I have a question for a anyone with some engineering background.  I want
to try to get the taper angle and diameter of the friction surfaces right
the first time.  The max. O.D. of the roller is 1.5" and about 2" long.  I
figured I'll need maybe 75 oz-in. of torque to hold the scope in place while
changing eyepieces.  I'd want a range from 50 to 100 oz-in, maybe a little
more.  So what taper do you think will give me good adjustability?  The
diameter can be anywhere from about .5" to 1.3".  Where will I get the right
torque range with Teflon against stainless steel?  How polished should it
be?
    Thanks for any input.

Scott