Re: ATM Building a clutch

["Dave Pitou" <dpitou@ix.netcom.com>]

Hi Scott,

Teflon clutches work great as long as the required torque isn't too high. If
you try to up the torque by using more force the teflon just cold flows and
you don't get much of an increase.

The friction force in a clutch is Mu*N where Mu is the coef. of friction
(~0.04 for teflon on smooth steel) and N is the force normal
(perpendicular ) the the clutch surface. The normal force in a cone clutch
is the sine of the clutch angle times the axial force. You can use these
equations to calc. the cone angle given the required torque and available
axial force, but if you can get enough axial force I suspect any angle can
be made to work. With a shallow taper the teflon will need to be accurately
machined (tough to do) in order to hold TIR to a small value.

The steel can be either fine ground or polished. If it's course ground
you'll get more friction, but you'll also get teflon dust all over.

Dave


----- Original Message -----
From: "Scott Ewart" <scott.e@quicksurfer.com>
To: "ATM list" <atm@shore.net>
Sent: Saturday, August 24, 2002 10:16 PM
Subject: ATM Building a clutch


>
> 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
>
>