[Farmall] Tachometer

[soffiler at myeastern.com]

From: "tomehrkam at houston.rr.com" <tomehrkam at houston.rr.com>

> I have used strobe lights to measure rpm. My company has
> one with a calibrated dial. You illuminate the shaft/
> pully etc with the strobe and adjust the frequency so the
> shaft appears to stop. You can read the RPM off the dial.

I've used the strobe-type tach also.  One caveat:  you can
be off by a factor of 2, 3, 4, or more if you aren't
careful.  Basically the strobe flashes to "stop" the motion
of the spinning object in question.  But if it only flashes
every OTHER rotation, it looks exactly the same to you, and
in that case your measured RPM is half what it should be. 
Could go 1/3, 1/4...  The easy way to avoid this is to tune
the tach so an individual feature of the spinning object
appears DOUBLED.  The strobe is now flashing TWICE per
revolution.  Then cut that reading in half, observe and
confirm you've got it right.

Steve O.