[GreenKeys] Model 19 gears

[Jim Haynes]

On Tue, 24 Jul 2018, Kevin H. Phillips wrote:

> The pinion gear on the motor appears to be #74919LS.  I can't see the numbers

The number on the driven main shaft gear is unfortunately not readable
except by taking the gear off and looking at the inside.  Why they did
it that way?

The standard gear set for 60 wpm is 74912 and 74913 for 1800 rpm motor, 
and all the other numbers on the charts I have are somewhat higher part
numbers.  That suggests the 74919 is contemporary with the early ones.

Here's what you can do.  Count the teeth on the gears.  For 74912 and 
74913 they are 7 and 30 teeth, respectively.  Since the motor runs at
1800 RPM that gives a receiving shaft speed of 420 RPM.  Which is exactly
7 per second.  The receiving shaft has to go around faster than the
signaling speed so that the selector has time to stop between characters.

Another way that doesn't involve counting teeth is to turn the motor by
hand and see how many revolutions of the motor are required to have the
receiving shaft go around once, then calculate the receiving shaft speed
from the motor speed of 1800 RPM.

For 7.42 unit code the character length is 7.42 times 22 milliseconds, or
about .1632 seconds per character (at 60 wpm) which is about 6.13
characters per second or 367.64 characers per minute, and is commonly
stated to be 368 operation per minute.  To the receiving shaft is running
1.14 times as fast as the character rate which is the speed of the 
keyboard shaft.

Western Union preferred 7.0 unit code at 45.45 baud, so that gives .154
seconds per character or 6.5 characters per second or 389.6 operations
per minute.  Which does not entail running the receiving shaft any faster
than for 7.42 code because there is still plenty of time for the
selector to stop.  Hence the same gear set works for either 7.42 or 7.00
code at nominal 60 wpm or actual 45.45 baud.

Now if you calculate the receiving shaft speed for the gears you have
and compare that with 420 rpm you can see whether the machine is geared
for faster or slower than nominal 60 wpm or 45.45 baud, and about how
fast it might be.

We know that 50 baud is standard in many parts of the world, but I for
sure don't know if 50 baud Model 15/19 machines were being made at the
same time as 45.45 baud ones.