Gear Ratio Question

[Anonymous]

I have a number of questions about diesel-electric locomotive gearing:

1.) When a locomotive has a gear ratio given, what do the numbers represent? I'm guessing that they are the teeth count for the gear, but am usually wrong about such things. Also, is the first number for the gear mounted to the cranksahft, and the second for the gear on the alternator shaft, or is it the other way around?

2.) What is considered a "high" gear ratio, and what is considered a "low" one?

3.) What shape are the teeth on the gear (i.e. are they parallel to the axis, diagonal, v-shaped, etc.)?

4.) Is the correct term for the electricity-generating-device, alternator, generator, or dynamo? And for the internal combustion engine, is the correct term, engine or prime mover?

5.) In diesel-mechanical and diesel-hydraulic transmissions, what are common gear ratios?

I'm sorry to ask these rather basic questions, but I don't know much about this.

Any help you could provide would be most greatly appreciated, and I thank you very much for it.

Most sincerely yours,
Daniel Parks

[Randy Stahl]

Those are great questions!
1)yep...gear teeth on the traction motors 62:15 is a common one on an EMD 62 teeth on the bull gear and 15 on the pinion.
2) a high gear ratio means more teeth on the pinion . ie.bigger pinion
3)straight cut gears on freight locomotives.
4)some locomotives have generators (usually older ones) some locomotives have alternators . A generator makes direct current and is directly used by the motors. An alternator makes alternating current that must be rectified before it goes to the motors. Some times it is inverted back into AC for AC traction motors. Prime mover is probably the best, some times a locomotive will have more than 1 diesel, like a small one for hotel power on passenger trains or a hot start engine to keep it warm on cold days.
5) not a clue.
Randy

[Anonymous]

Dear Mr. Hemphill and Mr. Stahl,
Thank you very much for answering those questions. I have some new questions as well:

1.) How are the traction motors mounted to the truck so that they move with the truck around curves?

2.) Could you please explain why AC traction is sometimes preffered to DC?

Most gratefully yours,
Daniel Parks

[dehusman]

The traction motors are in the truck, mounted with the shaft parallel to the axle. The whole truck pivots when the engine goes around the curve. The traction motors are rigidly attached so they stay parallel to the axles.

AC traction provides better wheel slip control allowing for better traction at low speeds.

Dave H.

[Randy Stahl]

Axle hung, nose supported truck assemblys have been around since Frank Sprague invented them for his trolley cars in the 1880s.
A.C. traction motors also fit a more powerful motor in the same small space.
Randy

[Overmod]

A little more trivia:

The tooth ratio IS the gear ratio, just in case that isn't entirely clear; the reason for the weird ratios is that the traction motor-to-axle spacing is fixed, which constrains the size of gears that can 'match' inside the gearcase, and the need to have an integral number of matching teeth on both gears controls what the sizes can be. We have similar problems in watchmaking and MEMS, even though the scale is much, much smaller!

The teeth (correct me if I'm wrong, but I don't think so) are plain straight-hobbed, with no bevel, hypoid, or herringbone characteristics (note that because the traction motor is parallel to the axle there is no end-thrust -- ship reduction gears use herringbones because they inherently control substantial thrust, but these gears are considerably more difficult both to 'cut' and maintain than straight ones. The contour of the teeth, I believe, is a modified cycloid (I have come across a good Web article on gearing design, and can give people the URL when I find it, if they e-mail me) -- the point of this is that over the entire range of tooth contact, the driving tooth rolls rather than slides on the driven tooth, reducing wear, friction, etc.

dehusman knows what he's talking about but didn't describe it quite well enough for you to visualize a key point. The traction motor is connected rigidly to the *axle*, via the gearcase supports, on American power, and is essentially 'hinged' at the back end, where it joins the truck frame; this permits the suspension and equalization to work. Disadvantage is that some of the traction-motor weight is unsprung. I have always heard this method of mounting called "wheelbarrow suspension", which is what its inventor (Frank Sprague) called it. As you might imagine, there needs to be some freedom at the truck-frame-to-motor attachment to ensure that the springing works properly.

Radial trucks (imho The Wave Of The Future [cue theremin music]) do things a bit differently, because the outer axles pivot relative to the center one, and are joined by some form of linkage. Again, in American practice the traction motors follow the axles, and this implies that the outer-axle motors pivot along with the axle and carriers. There is a pretty good diagram with pictures of the Henschel Flexi-Float 'bogie' on the Web; I still live in hope of seeing a good exploded or isometric view of a HTCR-II production truck for comparison (look at the patent drawings for GE and EMD to get some idea of how the things work).

Mark is correct in calling the main generator the main generator -- although individual makers or railroads may have their own particular name for it, "traction alternator" being one you'll find commonly used for straight-AC "generation". In the engineering terminology I grew up with, a "generator" was assumed to be a DC machine, and an "alternator" was an AC one -- this distinction would be carried into whatever the 'electricity producer' on the locomotive was. Now, keep in mind that there are other 'generators' driven along with the main one -- Randy can give you a comprehensive list grouped by locomotive type and service -- with one being of particular importance. Typical large generators or alternators can't 'start' themselves: they need a source of initial current to build up their fields to the point they can start extracting electrical current from shaft torque/horsepower. That's generally (no pun intended) arranged via a small generator called an 'exciter', which in my experience uses permanent magnets for the field and hence is self-starting when spun.

AC traction has many other advantages beside high TE at low speed (although that's the principal advantage for most American railroads in current service). The motors are cheaper, lighter, require no brushes, and their armatures can be spun much more quickly than the wound DC type. They can also have less difficulty with back (or counter) EMF, when induction motors are used, because there is no inherent tendency to 'generate' electricity that is, following Lenz' law, opposing the electricity being fed to power the motor, as there is with a brush-commutated DC motor with fixed field strength.

Part of the issue with 'engine' and 'prime mover' is that there is a diesel motor and also traction motors; I believe the usual convention among railroaders is that when you say "motor" you mean traction motors, as in "six-motor unit". One of the reasons, IIRC, that EMD tried to use the term 'unit' for a locomotive is that it precludes calling the thing an "engine" (as in 'steam engine') as synonym for locomotive, and thereby confusing the diesel engine with the diesel engine, if you get my point... calling the powerplant a 'prime mover' neatly solves this, but I always thought that term was just the faintest bit 'affected' -- something a whiz kid, not a railroader, would use. Not that I have any better suggestions!

Finally: the actual axle ratios of many diesel-mechanical units aren't of primary significance, because the transmissions themselves can have multiple speeds. Think of this as the transmission ratios in your car, multiplied by the ratio of your rear end (e.g. 3.73 or 4.10 -- these again referring specifically to the tooth count on ring and pinion, but suppressing the actual numbers because a numerical ratio is easier to use in some calculations). The limitation of wheel gearing in most DC locomotives is imposed by the motors long before you start to get into implicit problems with the gears per se (I'll gleefully correct this if not true in any circumstances) -- I wouldn't think that the kind of gearing used in diesel-hydraulics is significant as it is for diesel-electrics.

I'm deeply impressed that there are people who can remember 'oil-engine' being used to describe a true Diesel -- I thought that term pretty much went out with the Ingersoll-Rands and others 'of that ilk', which would make those old heads rather old indeed!

[Anonymous]

QUOTE: 62:15 is nominally 4:1, a common ratio. It gives you 70 mph maximum speed for a D77 traction motor.

Actually, that's not accurate; 62:15 gears yield a 65mph maximum speed, not 70mph. If you want 70mph maximum, you'd need 61:16 gears.

http://www.willtrens.hpgplus.com.br/Manual%20SD40-2/EMD%20SD-40-2%20Operator%20Manual.htm

[jchnhtfd]

Odd point of trivia to add to an otherwise almost overwhelmingly complete description... one will never find that the gear ratio (tooth ratio) is reducible to a small integer. That is, it will always be something weird, like 62:15, or 61:16; never something like 60:15 or 64:16. Why? So that the same set of teeth meet as seldom as possible. This distributes wear on the gears as evenly as possible, and thus makes them last longer.

Other miscellany -- the wheelbarrow motor mounting is much the simplest and most reliable, and the unsprung weight isn't that much of a hassle. In the old days, though, some electric motors -- most noticeably the GG1 -- used other drive schemes, in which the motor was independent of the axle (the GG1 used a quill drive, which I can explain if anyone is interested though I can't think why...) or in which part of the motor was, such as the GE 'bipolar' designs (notably Milwaukee Road).

[Modelcar]

....With the discussion of ''straight cut gears" in traction motor gearing can I assume they can be described as "spur cut gears".....and on the mounting of the traction motor housing to the truck...can we use the term "torque reaction mount"....that might describe it.....Also generally straight cut gears are ones that make more "singing" or plain old noise.....as opposed to helical cut gears.

[Randy Stahl]

QUOTE: Originally posted by Modelcar

....With the discussion of ''straight cut gears" in traction motor gearing can I assume they can be described as "spur cut gears".....and on the mounting of the traction motor housing to the truck...can we use the term "torque reaction mount"....that might describe it.....Also generally straight cut gears are ones that make more "singing" or plain old noise.....as opposed to helical cut gears.

Yea their noisey gears... I wouln't have it any other way... they sound like an electric interurban.
Randy

[Modelcar]

....When you mention and I just read the "sound like an electric interurban"....the recollection of that sound bounced right back through my memory like it was yesterday....I could hear the cars on the street in Johnstown, Pa....and the sound so vivid you point out.....

[Randy Stahl]

QUOTE: Originally posted by Modelcar

....When you mention and I just read the "sound like an electric interurban"....the recollection of that sound bounced right back through my memory like it was yesterday....I could hear the cars on the street in Johnstown, Pa....and the sound so vivid you point out.....

Try a GP 40 it's the closest Iv'e found to a trolley car.
Randy

[Modelcar]

....Guess I've not really tuned into that sound on a Diesel unit...The gearing sound, that is....Funny, never crossed my mind. But on street cars and Interurban cars it is really noticeable.

[Randy Stahl]

You need to get em rolling and throttle down a bit to hear em , or really get em going... they'll start to sing.
Randy

[Modelcar]

...And there is much more ambient noise surrounding a train than an interurban car running by itself too....so gear whine would be more outstanding.

[Modelcar]

...One can point out the diffenence in an auto say, a 4-speed manual...older model T-10 transmission. Forward gears are all helical cut and quiet grears [if made properly], and reverse is spur gear arrangement and when one backs the vehicle in reverse the spur gears whine....

[Paul Milenkovic]

How does this work with DC motors on something like an Amtrak AEM-7 that goes 125 MPH, or on the original Metroliner MU cars that could do 160 in tests? These are all DC traction motor applications, so they must be geared really tall and have pretty weak initial starting torque.

I also understand that the PCC streetcar had motors mounted crosswise to the axles and attached to the sprung truck rather than axle hung, and the motors drove the wheels through a bevel gear -- not the traditional axle hung (wheel barrow style where the motors are parallel to the axles). The PCC streetcar arrangement looks a lot like the drive of an RDC, only it has electric instead of Diesel motors and the electric motors are attached to the truck, not attached to the car body and driving the truck through a long shaft. Something tells me that the CTA (Chicago Transit Authority) EL cars are really modified PCC cars and have this arrangement.

I read that the Japanese Bullet Train (Shinkansen) uses the PCC motor arrangement to reduce unsprung mass. The French also had that monomotor bogie where they had one big honking electric motor up in the car body driving the wheels through shafts much like the setup on the Diesel hydraulics, and that was the type of locomotive where they ran 200 MPH some long time ago in their original high-speed tests.

[M636C]

In 1986 after visiting Expo 86 in Vancouver, I went up to Tumbler Ridge, very indirectly, by road. We were hanging about looking for the locomotives near the mine, when we heard this strange noise from around the curve in the distance. In due course three GF6Cs arrived, running light, and we realised that what we were hearing was the gear and motor noise, normally drowned out by the engine (of an SD40-2, say, with similar trucks and motors). We decided that the noise was like a tramcar, but much louder and deeper.

It's sad to think that the GF6Cs are stored, and the BCR is just a subdivision of CN.

Peter

[Anonymous]

Dear dehusman, Randy Stahl, Overmod, xBNSFer, Mr. Hemphill, jchnhtfd, modelcar, Paul Milenkovic, and M636C,

Thank you all very much for your help. It is very greatly appreciated.

Most deeply gratefully yours,
Daniel Parks

[athelney]

QUOTE: Originally posted by M636C

In 1986 after visiting Expo 86 in Vancouver, I went up to Tumbler Ridge, very indirectly, by road. We were hanging about looking for the locomotives near the mine, when we heard this strange noise from around the curve in the distance. In due course three GF6Cs arrived, running light, and we realised that what we were hearing was the gear and motor noise, normally drowned out by the engine (of an SD40-2, say, with similar trucks and motors). We decided that the noise was like a tramcar, but much louder and deeper.

It's sad to think that the GF6Cs are stored, and the BCR is just a subdivision of CN.

Peter

One of the GF6C's # 6001 is at the rail museum in Prince George BC -- don't think they have any overhead wires to run it though.