[Jukebox-list] Rockola Tempo2 Speed

[Ray Finch]

I thought the size of the idler wheel would make a difference, but 
thinking it through I see that it does not.

Leaving the math aside, think of it this way:

If you think about the outer surface of the idler wheel, as the motor 
shaft is turning there is a certain amount of rubber that has to pass by. 
With the motor speed being constant, the larger the idler wheel more 
rubber that passes by and the slower the wheel turns.  So far so good, but 
that's where people get hung up.

Although a larger wheel will indeed turn slower (because it is bigger and 
so has more rubber that has to pass by) the actual _surface speed_ of 
where the rubber wheel contacts the motor shaft can't change - if it did 
then there would be slippage.  Since the wheel if fed by the size of the 
motor shaft, the _surface speed_ of the wheel will be the same no matter 
what size the wheel is.

It's a bit like trying to explain why the middle of a record turns slower 
than the outside edge when the whole record is turning one speed.

In order for the physics to work out, a larger wheel has to move slower to 
cover more rubber going by on its outside surface.  By the same token, if 
you have a smaller wheel, it will turn faster *only* because there is less 
rubber going by on its outside surface.  None the less, the outside 
_surface_ of the wheel is will still be the same speed regardless.

So, since the surface speed of the wheel is always the same and that 
surface is in contact with both the motor shaft and the turntable it 
doesn't matter what size the idler wheel is or what speed it "appears" to 
be going as the surface speed does not change. The mechanical 
advantage does _not_ come from the size of the idler wheel, but rather 
from the size of the turntable and the size of the motor shaft.

2 cents,

Ray Finch




On Mon, 5 Nov 2007, Ron Rich wrote:

> Bob,
>  I gave up long ago trying to 'splain" that the wheel size is not relevent--no one seems to ever believe it !  Ron Rich
>
> Bob Ellingson <bobe at halted.com> wrote:
>  At 02:52 AM 11/5/2007 -0500, you wrote:
>> Bill, keep it simple. If the tape increased your speed to the correct
>> speed, that tells you that the ratio is slightly off between either the
>> motor end piece which engages to the idler wheel, or the idler wheel is
>> slightly too small.
>
> The size of the idler wheel will not matter! If you calculate
> the ratio of the idler wheel to the motor spindle, and then
> the ratio of the turntable to the idler, you will see that it
> cancels out:
>
> Spindle Idler Spindle
> -------- X ---------- = --------- = Speed
> Idler Turntable Turntable
>
> The problem has to be friction, slippage, or mis-match
> of the spindle-to-turntable size. This isn't in a 50 Hz
> area, is it? Maybe the spring that enlarges the spindle
> for use in foreign areas has fallen off?
>
> --Bob
> =======================================================================
> Bob Ellingson bobe at halted.com
> Halted Specialties Co., Inc. http://www.halted.com
> 3500 Ryder St. (408) 732-1573
> Santa Clara, Calif. 95051 USA (408) 732-6428 (FAX)
>
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