Wanted: TRS-80 PC-2 Pocket Computer Plotter pens

From: Peter Turnbull <pete_at_dunnington.u-net.com>
Date: Thu Apr 17 17:28:01 2003

On Apr 17, 7:20, Ethan Dicks wrote:

> What I could _really_ use is a source of the gear that fits directly
> on the X and Y motor shafts. I have tried going to local RC car and
> model train shops, but they don't carry gears. This thread has come
> up before, but I still don't know how to sit down and accurately
> determine the physical characteristics of these gears to the
> satsifaction of a mail-order gear vendor. I can mic the shaft, I
> can count teeth, but the rest eludes me.

There are two common ways of specifying gears. One is by diametral
pitch (DP), where you specify, in effect, the distance between teeth
(actually it's done by dividing the number of teeth by the pitch
diameter). The other, mostly used for metric gears, is by "module",
and is the reciprocal of DP, ie you divide the pitch diameter by the
number of teeth. Gears that mesh will have the same DP (and, of course
the same module).

The trick is to find the pitch diameter -- that's the "effective"
diameter of the gear wheel. If you're using DP, for normal gears the
amount of the tooth above and below the pitch diameter, called the
addendum and dedendum, is standardised, and it conveniently works out
such that if you can accurately measure the outside diameter (OD), and
you can count the number of teeth (t), you can easily work out the DP
without actually measuring the pitch diameter directly:

    OD = (t+2) / DP therefore DP = (t+2) / OD

and if you do need to know the pitch diameter, then since DP = t/p it's
just p = t/DP.

There's just one caveat: those formulae only apply to gears with a 20
degree pressure angle, and with more than 15 or so teeth (otherwise you
need to make some corrections as the angles get more extreme).

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Thu Apr 17 2003 - 17:28:01 BST

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