[GreenKeys] Model 15 timing

[david freeman]

While noodling through some problems on a new Model 15, I ran across an odd gear set in one of my three units. As I was working out the reductions on paper I cannot make sense of the basic timing, saying nothing about this odd machine. Can some one help me. I�ve assumed 45.45 baud, 7.5 bits/letter, and 6.06 letters/word. That gives me 22msec per bit which I seem to remember is correct, 60WPM and 363.6 operations/minute. Operations per minute is referred to in some of my Teletype manuals, and 364 is a figure shown for 60WPM machines. My question is, I have a 1800 RPM motor, and a reduction to the main shaft of 7teeth/30teeth equals 420 RPM. Shouldn�t this be 363.6 RPM? Does the machine use more than one revolution of the main shaft per letter, and if so how? 
OK at any rate, as I was swapping typing units, I found one of my machines has a different set of gears driving the keyboard. Two have 21 teeth driving 24 teeth, the odd ball has 24 teeth driving 26 teeth. The cams tripping the electrical contacts look identical, but a small difference would be hard to detect I imagine. So assuming the main shaft RPM (420) from above is correct the keyboard drum would turn at 367.5 RPM for the �standard� units, and 387.7 RPM for my oddball. 367.5 is pretty close to my 364 OPM ideal. What is the 387.7 unit? I�ve had it on the air and had good reports with it. Just a mystery I�d like to know the answer to.

Dave, WW8S

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