[R-390] R-390A Cosmos PTO has erratic linearity

[Bill Hawkins]

Well, 200 parts in 2 million is 0.01% accuracy. I'd say that was amazing
for a mechanical, analog device.

The answer to your question may be in the Collins Engineering Report, if
it states the accuracy for the PTO.
I seem to remember +/- 500 cycles, but you should find that in print.

The age of digital calculators does make one think that all of the
digits ought to be accurate, with no regard for percent error.

Kind regards,
Bill Hawkins

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Larry H
Sent: Saturday, June 14, 2014 4:16 AM

I'm restoring a Stewart Warner R390A with a cosmos pto type 136-1.
Although I've had to fix a number of problems on the R390A, it looks and
operates well now except for 1 issue (or I think it's an issue). I've
graphed the linearity of the pto numerous times every 25 KC, and it
tracks mostly +/- 100 - 200 cycles (with a few exceptions up to 300
cycles), but after adjusting the linearity bad spots numerous times,
different ones keep popping up.  IE, the measurement repeatability is
poor, but still mostly with in +/- 300 cycles.  This is after letting it
warm up for 2 hours.  The frequency stability is very good.  Also, if I
graph the pto linearity, use it for a few minutes, and graph the
linearity again (without making any adjustments to the pto) its
different than the previous graph.  Everything inside the pto seems
tight and the gear train to the readout is tight (IE: very little
slack). I have cleaned the pto shaft grounding contact and checked  the
contact pressure.  The endpoint also varies about +/- 100 cycles. I have
the prescribed spacing in the oldham coupler (its in very good
condition) and it is ligned up very closely with the KC shaft and the
spring is installed correctly. The gear train and the pto operate very
smoothly.

Should this be better or is this normal?  Any suggestions would be most
welcome.  Thanks.

Regards, Larry