• Traction motor gear ratio miscellanea

  • Discussion of General Electric locomotive technology. Current official information can be found here: www.getransportation.com.
Discussion of General Electric locomotive technology. Current official information can be found here: www.getransportation.com.

Moderators: MEC407, AMTK84

  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.

(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.
  by Allen Hazen
The basic story about gear ratios with GE's 752 motor is well-known. For freight locomotives (65 mph top speed until the 1960s, 70 mph thereafter), a gear ratio of 74:18 (Note: 92 teeth) was standard from the 752's introduction until the 752AF was announced in about 1980: the 752AF was used in, e.g., the B36-7 locomotive with GE's "Sentry" wheelslip control system, and had, as the standard freight gear ratio, 83:20 (Note: 103 teeth), GE stating at the time that this was better for the higher power ratings of their new locomotives. (There were also some lower gear ratios used with the 752 motor for higher-speed locomotive applications: passenger units and many high-horsepower U-Boats -- we'll come to these...)

1. I guess I can see why using smaller and more numerous teeth is better if you want to transit a very high horsepower from the motor to the wheels. The big stress on a gear tooth comes as it engages: suddenly, from moving freely with no applied loading, as the tooth reaches the point where it meshes with the other gear, it gets the full force applied to it. Using smaller, but more numerous, teeth makes this shock a bit smaller. Or-- I'm not an engineer, just a railroad enthusiasts with an interest in technological history -- am I missing something?
2. The 74:18 gear ratio was even used with switchers: the 1,000 hp Alco T-6 used this ratio. My guess is that this ratio and the 752 motor combined to give a continuous tractive effort as high as the wheel-slip control systems of the time were capable of dealing with, so there was no point to going to a higher ratio for low-speed applications. I doubt that T-6 units operated at 65 mph very often!
QUERY: does anyone know of a higher gear ratio being used with the 752 motor?

But the 74:18 ratio is actually older than the 752 motor! The 752 motor was apparently (I think this came out in the discussions on the "PA-1 Traction Motors" thread on the Railroad.net "Alco" forum a few years ago) a development of the 726 motor, which dates back to the late 1930s. The first Alco FA-1 and RS-2 locomotives, in 1946 (well... the first FA-1 had apparently been constructed in late 1945, but they didn't get to their purchaser -- the GM&O -- until 1946: see"244A engine" thread on the Alco forum), had 726 motors with the 74:18 gearing.(1) Before that... The 726 motor was an option on Alco's pre-WW II Dl-109 locomotive (the "Needle-nose passenger units). Supposedly the standard motor for this (series of) model(s) was the 730, but that was recommended for exclusive passenger service: units in dual service (like the New Haven's: over 4/5 of the total built) got the 726. And here the situation with gear ratios and tooth sizes gets murky: (2)
-------With the 730 motor, the locomotive was available with gearing for 120 mph top speed (58:25) or 100 mph (61:22). Note that both of these total 83 teeth.
-------Still with the 730 motor, there was also an 80 mph option with 71:21 gearing. Note that this one comes to 92 teeth, so apparently the same as the tooth size used on post-war 65 mph units.
((((Remark: That last bit assumes that the size-- well, the axle-center to motor-center distance-- of the 730 was the same as that of the 726. An assumption I will make until shown evidence to the contrary!))))
-------On the other hand... An 80 mph top speed was available with the 726 motor with 64:19 gearing -- 83 teeth.
Your guess is as good as mine why GE's engineers thought finer teeth were needed for the lower speed with the 730 motor when the 726 motor, for the same top speed,could use the tooth size the 730 used for higher speeds.
(((QUESTION: Do you maybe have a better guess than mine?)))

Postwar, the 83-tooth ratios continued to be used for higher speeds. Alco's PA passenger unit was available with 752 motors geared 58:25, 60:23, 61:22, 62:21, and 64:19-- all 83-tooth ratios(3). And a bit later Alco offered its FA-2 ("freight and passenger locomotive") and RS-3 geared for 65 mph with 74:18, but also for 75 mph with 65:18 and for 80 mph with 62:21-- 92 teeth for the basic freight gearing, 83 for the higher speeds(4).

Next installment will look at ratios used on GE diesels in the 1960s and later.

One last bit of data for the 726 motor: ATSF's HH-1000 switchers had 726 motors with 68:15 gearing, another 83 tooth ratio(5).

(1) Kirkland's "The Diesel Builders: Volume II Alco." Recently I have seen confirming evidence: I have been shown an Alco data card for GM&O 706, one of the first FA-1 built. It lists the gear RATIO as 4.11. I think that's probably rounded off from 4.11111111... (repeating decimal, 4 1/9), the ratio for 74:18.
(2) Ibid.
(3) Alco TP-400, operation and service manual for PA-1, FA-1 and RS-2 (at the end of the "diesel engine" section), available on-line at George Elwood's "Fallen Flags" railroad image site.
(4) Alco TP-800, operation and service manual for FA-2, RS-3 and RSD-4 ("general data" section this time), also available from George Elwood's marvelous site.
(5) Santa Fe roster linked to in my previous post.
  by MEC407
Allen Hazen wrote:I doubt that T-6 units operated at 65 mph very often!

I concur... but what an amazing sight that would have been! :-D
  by Allen Hazen
(((Sorry for delay in posting third part!)))

The story so far: GE appears to have introduced the tooth size that produces 92 teeth total for the 80 mph re-gearing of the 730 motors on some Rock island Dl-10?. It was then adopted for the 65 mph freight gearing -- 74:18 -- on post-WW II units with the 726 and 752 motors: it continued in use for this (by the 1960s thought of as having a top speed of 70 mph) into the Dash-7 era. For higher speeds, however, they stuck -- at least into the 1950s -- with the larger tooth-size that gave ratios with a total of 83 teeth.

(QUESTION: Anybody know what the gear ratio used -- with 752F1 motors, a subtype I don't recall seeing on Alco or GE diesel units -- on the New Haven's EP-5 passenger electrics of 1954? Several sources agree that they were geared for a 90 mph top speed, but don't say what the actual gear ratio was. 62:21 was given as the ratio for 91 mph top speed on Alco PA-1 locomotives, but ***MAYBE MAYBE*** GE took the occasion of building a locomotive rated at 667 hp per axle to introduce something new!)

At some point GE introduced a new tooth size that gave ratios totalling 103 teeth. Initially this seems to have been used for higher speed gearings, on high horsepower 4-axle U-series types and 6-axle U-series types intended for passenger service.
---U30B for the Illinois Central and Louisville & Nashville -- IC's, at least, date from the first year of U30B production-- and Conrail's U36B-- built in December 1974-- all had 81:22 gearing and 75 mph top speeds.(1) (F.w.i.w., the Conrail units had a 12 mph continuous speed.)
---Santa Fe's U30CG cowl passenger U-boats had 77:26. (2) Assuming that the limit is traction motor r.p.m., and that 74:18 gives 70 mph, this would give a 97 mph top speed.

ANOMALY: Santa Fe's U23B were built with 79:24 gearing (87 mph top speed on the assumptions of the last paragraph), but were later regeared to 74:18. (What was Santa Fe dreaming about when they ordered intermediate road switchers geared for 87 mph?)

Finally, in about 1979 or 1980, GE introduced an upgraded motor, the 752AF, for use in combination with a new wheelslip control system, on high horsepower locomotives, and, to better cope with the higher power, started to use the 103 tooth size for the basic freight gearing: 83:20 (ratio 4.15) replacing 74:18 (ratio 4.11).

GENERAL QUESTION: Why did GE go to the smaller teeth for high speed gearings, when they had earlier been happy with larger teeth for these speeds? And why did they continue to use the intermediate tooth size for over a decade after this on basic freight units?

NEW INFORMATION (2): Santa Fe was a big purchaser of Dash-7 units. Early B23-7 and C30-7 had the "traditional" 74:18 gear ratio, but later Santa Fe orders of both types had 83:20.
QUESTION: did this just mean they used the new gear ratio with the earlier motor sub-type, or did late Santa Fe B23-7 and C30-7 have 752AF motors and/or Sentry wheelslip control?

And Santa Fe's U36C units were delivered with 74:18 gearing, but were later converted to 83:20 (2).
QUESTION: Was this change made when the units were rebuilt as "SF30C"? And does it betoken a change to the more robust traction motor subtype and the new adhesion control?

(Small puzzle: There are PRR locomotive diagrams in the Pennsylvania section of George Elwood's site. They give the gear ratio of the E-44 as 63:20, with 752E5 motors, and of the E-44a as 61:22, with a top speed of 75mph. I think these are almost certainly errors, and that they should be 83:20 and 81:22. In which case they are the earliest applications I have seen evidence for of the 103-tooth tooth size. Since these units, built in the early 1960s, were much more powerful than contemporary diesels, this would confirm that GE's engineers thought the smaller teeth were better for absorbing greater power.)

(1) IC, L&N and Conrail locomotive documents fromGeorge Elwood's site. (Alas, all of Conrail's ex-NYC and ex-PC U30B and U33B had been retired before the locomotive book reproduced was prepared, so I don't have figures for them!) Build date of 12/74 for the Conrail U36B from Marre and Withers, "The Contemporary Diesel Spotter's Guide," 2008 edition.
(2) The Santa Fe website linked in my first post in this string.
  by EDM5970
I don't have any thoughts on gear ratios right now, but have to say that even on the best of track a T-6 won't get up to 65 MPH, at least not with 74:18 gearing. One T-6 operator once told me that they tend to poop out at around 40. The 752 motor is a 600 volt motor, with enormous lugging ability, making it great for switchers. But the T-6 only has three contactors, limiting your motor connections to full series (150 volts to each motor) and series-parallel (300 volts to the motors). This lower voltage limits the maximum motor RPM and thus the track speed, trailing load permitting. Field shunting does help a bit, but still won't make up for the lower voltage.
  by Allen Hazen
Thank you for the details! I wasn't thinking about the electrical aspects when I made my snarky remark about going fast in a T-6: I was just thinking about the general inadvisability of trying for high speeds in a switch engine with (except for one unit that was built with Hi-Ad trucks)switcher trucks.
Supposedly the change from "S" to "T" (for "transfer") in the model designation was justified by a transition control system that made the T-6 a bit better than previous switchers for road service: obviously Alco didn't think, at the end of the 1950s, that a 1,000 hp unit would have ENOUGH use as a road engine to justify provision for more elaborate transition!

(Alco used the 6-251 engine for a variety of export road switchers: New South Wales 48 class being the most noteworthy in Australia. These units would have had very different electrical systems from the T-6, however: they wouldn't have had big, U.S. style, 756 motors for example. And their trucks were designed for road service.)
  by Allen Hazen
One of the many, many, nice things on George Elwood's "Fallen Flags" site is the Pennsylvania Railroad's diagram book page for the T-6:
http://prr.railfan.net/diagrams/PRRdiag ... z=sm&fr=ge" onclick="window.open(this.href);return false;

It says the "maximum speed" is 60 mph-- but of course this is a maximum PERMISSIBLE speed for a locomotive with switcher trucks (the same maximum speed is shown for the S-2), and not a claim about the ATTAINABLE speed. (Grin!)
  by Allen Hazen
In Kirkland's Alco book, the brief section on the C-628 says it was offered with either 74:18 gearing or 81:22. I don't know if any were actually built with the (slightly) higher-speed ratio: the C-628 was offered with a steam generator for passenger service, but no U.S. railroad ordered this. But at least it shows that a 103-tooth ratio was OFFERED on early second-generation diesels. (Note that the C-628 was introduced at the end of 1963: about the end of the production period for the E-44, which I think -- see a few posts back -- had 103-tooth gear ratios.)
At which point I thought I'd see whether I could find a reference to non-74:18 gear ratios on C-C second generation Alcos and GEs (other than the Santa Fe's). George Elwood's "Fallen Flags" site has locomotive data for a few railroads.
--Louisville&Nashville had C-628, C-630, U25C and U28C: all with 74:18.
--Erie Lackawanna had U33C and U36C with 74:18, and (passenger service) U34CH with 80:23.
--Elwood's site has data (& diagrams) for some but not all Pennsylvania types. C628, C630, U28C and U30C all had 74:18.
--Illinois Central's C636 had 74:18. This is the only C636 information I have so far found.
  by Allen Hazen
Another data point. (Credit this to George Elwood-- I found the website via a link from the Western Pacific section of his "Fallen Flags" site.)
http://www.wplives.com/diagrams/locomot ... index.html" onclick="window.open(this.href);return false;
has Western Pacific locomotive diagrams from 1970, including three for the line's U30B. All had 81:22 gearing for a 75mph top speed. (And, while compiling technical details on U-boats, 751-759 had GTA-9 main generators and 760-769 had GTA-11.)
  by JayBee
Allen, GE switched to the higher number of teeth to cure the problem of stripped pinion gears. These were called "fine mesh" gearing. The idea was to have more surface area of the gear teeth in contact to reduce the pressure on any one tooth thereby reducing breakage of the pinion teeth. EMD did similarly when the 50-series locomotives were introduced, changing from the 62:15 gearing to the 70:17 gearing.
  by Allen Hazen
Yes, that's clear enough. What's puzzling is the timing of making that change for different speed ranges. GE started using the smaller teeth (103 teeth total in a ratio) in the 1960s for speeds higher than 70mph, but didn't adopt it as standard for 70mph units until the end of the 1970s. Earlier, however (Dl-10X and PA days) they seem to have thought that higher-speed units could use larger rather than smaller gear teeth.

Thanks for the bit about ... that other company's ... "50-series" units (introduced about the same time as the B36-7 which brought in the 752AF motor with 83:20 gearing). As a fan of GE locomotives I'm afraid I hadn't looked into what the competition did!
  by JayBee
Allen Hazen wrote:JayBee--
Yes, that's clear enough. What's puzzling is the timing of making that change for different speed ranges. GE started using the smaller teeth (103 teeth total in a ratio) in the 1960s for speeds higher than 70mph, but didn't adopt it as standard for 70mph units until the end of the 1970s. Earlier, however (Dl-10X and PA days) they seem to have thought that higher-speed units could use larger rather than smaller gear teeth.

Thanks for the bit about ... that other company's ... "50-series" units (introduced about the same time as the B36-7 which brought in the 752AF motor with 83:20 gearing). As a fan of GE locomotives I'm afraid I hadn't looked into what the competition did!
Allen, I would look into the relationship between Motor(electric) rotational speed and and wheel speed(track speed). Gearing and wheel diameter affect torque and hence Tractive Effort(Drawbar pull). But Diameter of the Bull and Pinion Gears, along with Wheel Diameter affect Traction Motor RPMs. GE may have wanted a larger diameter pinion gear, and earlier 103 tooth gear sets may not have been true fine mesh types.
  by Allen Hazen
It's time for some more traction motor / gear ratio trivia!

While looking for something else, I spotted (in the EP-5 article in issue #42 of "Extra 2200 South") a gear ratio for the New Haven's EP-5 rectifier electric passenger locomotives: 63:20, for 90 mph. Another instance of the "83-tooth-total" tooth size: GE evidently thought the teeth were robust enough for use on what was, for the time, a very high-horsepower locomotive (though one used in passenger service, so probably not as hard on its gear teeth as a freight unit would be).

The EP-5 had 752F1 motors, and in case you were wondering why you hadn't seen references to the 752F1 elsewhere, a GE document that came into my possession says this form was special for the New Haven rectifier locomotives (and that it is similar to the 752E1 "except for ground brush holder and a flash ring").
  by NorthWest
Interesting. I wonder if the traction motors were replaced with standard 752s on PC's freight rebuilds. Does anyone know?
  by Allen Hazen
Several posts up, I said I thought the gear ratios given for PRR E-44 electric locomotives on the PRR diagrams were probably wrong, and that they probably had 103-tooth ratios. I'd like to take that back: I just noticed, in a GE list of traction motor types, that the 752N1 (used on uprated, E44A, units) is shown as having an 83-tooth ratio. So I now have confirmation of what it says on the PRR diagrams.
…So my current guess is that the E44 fleet all had 83-tooth ratios (like the New Haven EP-5). Why, I don't know: the ratios (61-22 and 60-23, I think) suggest higher speeds, but these locomotives were designed for freight service and the PRR diagrams show tham as having a 75mph maximum speed.
(Hmm… I seem to recall that the E44 had the reputation of being noisy, because of their constantly running cooling fans. Given the power that their motors were expected to soak up, maybe they needed, and had, super-sized traction motor blowers?)