by

**Allen Hazen**This is the first of what I intend to be perhaps three posts: information, generalities, puzzlements...

First off. I've found a new "resource" for locomotive information:

http://www.atsfrr.com/resources/Crosset ... el_roster/" onclick="window.open(this.href);return false;

This is a detailed roster of Santa Fe diesel locomotives-- not yet complete, but seems to be getting updated with some regularity. Pages on some, though not all, locomotive models give traction motor data, including gear ratios. Enjoy!

Generality: The size of the teeth in the gearing of a traction motor is -- for a given basic motor type -- inversely proportional to the SUM of the two numbers in the ratio. (Simple geometry: the gear on the motor's rotor and the gear on the driving axle can both be thought of as circles: the sum of their radii is the distance between the axle and the motor's rotating bit, and so is constant for a given motor design. It is, therefore, independent of the gear ratio: to get a higher gear ratio (for a lower locomotive speed) you increase the size of the gear on the axle, but decrease the size of the motor gear by the same amount. The total of the circumferences of the two circles is likewise constant: 2 x Pi x axle-to-motor-axis. And the total of the number of teeth on the two gears, times the size (distance between one tooth and the next) of a tooth, is just the sum of the two circumferences.)

Now, builders of traction motors often -- not always, as we shall see, but often -- like to use the same tooth-size with several different gear ratios. Example: an off-brand locomotive builder (which I shall not name onthis form (Grin!)) marketed its F-7 locomotive between 1979 and 1953 with eight options for gear ratio... all with a total of 77 teeth.(*)(1) Closer to home, the Fairbanks-Morse "Erie-built" locomotive of the late 1940s was offered with three different gear ratios for its GE 746 motors, all having 87 teeth.(2) But-- perhaps not surprisingly, given its long history-- the story with the GE 752 motor, used on Alco locomotives starting in 1947 and on GE mainline diesels through the Dash-9 and ES44DC, is... more complex.

To be continued...

(*) This un-named company based its traction motor design on the GE 716, which it had used on its early locomotives before trying to make its own electrical stuff. The only gear ratio I have to hand for a 716 motor (3) is 52:25 -- 77 teeth total. So it looks as if this company's design choice was just copying what they had bought from GE in the 1930s.

--

Sources:

(1) F-7 operator's manual. (Original by the manufacturer: I have a facsimile from the ?? 1970s ??.

(2) Erie-built description in John S. Kirkland's "The Diesel Builders, Volume I: Fairbanks-Morse and Lima."

(3) In a description of the "power cars" built by anonymous for the CB&Q's Zephyr articulated trains.

First off. I've found a new "resource" for locomotive information:

http://www.atsfrr.com/resources/Crosset ... el_roster/" onclick="window.open(this.href);return false;

This is a detailed roster of Santa Fe diesel locomotives-- not yet complete, but seems to be getting updated with some regularity. Pages on some, though not all, locomotive models give traction motor data, including gear ratios. Enjoy!

Generality: The size of the teeth in the gearing of a traction motor is -- for a given basic motor type -- inversely proportional to the SUM of the two numbers in the ratio. (Simple geometry: the gear on the motor's rotor and the gear on the driving axle can both be thought of as circles: the sum of their radii is the distance between the axle and the motor's rotating bit, and so is constant for a given motor design. It is, therefore, independent of the gear ratio: to get a higher gear ratio (for a lower locomotive speed) you increase the size of the gear on the axle, but decrease the size of the motor gear by the same amount. The total of the circumferences of the two circles is likewise constant: 2 x Pi x axle-to-motor-axis. And the total of the number of teeth on the two gears, times the size (distance between one tooth and the next) of a tooth, is just the sum of the two circumferences.)

Now, builders of traction motors often -- not always, as we shall see, but often -- like to use the same tooth-size with several different gear ratios. Example: an off-brand locomotive builder (which I shall not name onthis form (Grin!)) marketed its F-7 locomotive between 1979 and 1953 with eight options for gear ratio... all with a total of 77 teeth.(*)(1) Closer to home, the Fairbanks-Morse "Erie-built" locomotive of the late 1940s was offered with three different gear ratios for its GE 746 motors, all having 87 teeth.(2) But-- perhaps not surprisingly, given its long history-- the story with the GE 752 motor, used on Alco locomotives starting in 1947 and on GE mainline diesels through the Dash-9 and ES44DC, is... more complex.

To be continued...

(*) This un-named company based its traction motor design on the GE 716, which it had used on its early locomotives before trying to make its own electrical stuff. The only gear ratio I have to hand for a 716 motor (3) is 52:25 -- 77 teeth total. So it looks as if this company's design choice was just copying what they had bought from GE in the 1930s.

--

Sources:

(1) F-7 operator's manual. (Original by the manufacturer: I have a facsimile from the ?? 1970s ??.

(2) Erie-built description in John S. Kirkland's "The Diesel Builders, Volume I: Fairbanks-Morse and Lima."

(3) In a description of the "power cars" built by anonymous for the CB&Q's Zephyr articulated trains.