That's absolutely correct, which leads to the next step of deciding how to
actually accomplish this "friction spin" process, and how to start & stop
the contraption since the mass (wieght) of the 12" disk is a consideration,
as well as just what sort of load you're planning to put on this disc (since
that's going to cause slippage, etc.) A good starting place would be to put
some sort of rubber rim on your 12" disc, and just lay the shaft against it.
If this works for your application, great, but don't be surprised if it
doesn't. In all probablilty, you're probably going to have to use a v-belt
arrangement and a jackshaft (idler pulley arangement) to get the necessary
torque transmittal in conjunction with the rather substantial speed
reduction you need (dropping 800 rpm to 30 rpm).

BJT


"Paul Kierstead" wrote in message
...
Pete Macke wrote:
If a shaft is turning at about 800rpm and you want to use a drive wheel
to
friction spin a 12" disk at 30rpm what size should the drive wheel be?

Have you seen the carnage when such questions were asked previously? Are
you /mad/?

Ok, assuming the drive wheel was at the very perimiter of the 12" wheel,
the size would be 12*(30/800) or about 0.45", ballpark 1/2" category
unless it is rocket science, in which case I recommend other means...

PK