Gear ratios: what do they mean?

[dtrainBNSF1]

I've recently done some reading up on F7s and found that although many railroads mechanically owned the same loco, each railroad had its own gearing to allow the locomotives to achieve certain top speeds. Southern Pacific, for example, had their F units geared at 62:15 for freight operations with a top speed of 70mph, 60:12 for passenger service with a top speed of 79mph (overspeed set at 83mph),65:12 (55mph) or 61:16 (71mph).

What do all of these ratios mean and how do they equate to a loco's top speed? Do they affect the amount of tractive effort a unit produces at certain speeds?

[mrennie]

I've recently done some reading up on F7s and found that although many railroads mechanically owned the same loco, each railroad had its own gearing to allow the locomotives to achieve certain top speeds. Southern Pacific, for example, had their F units geared at 62:15 for freight operations with a top speed of 70mph, 60:12 for passenger service with a top speed of 79mph (overspeed set at 83mph),65:12 (55mph) or 61:16 (71mph).

What do all of these ratios mean and how do they equate to a loco's top speed? Do they affect the amount of tractive effort a unit produces at certain speeds?

I think it's the gearing that couples the shaft of the diesel engine to the rotor shaft of the generator (the one with the "excited" rotor windings, that rotates past the stator windings in order to generate the electrical current that goes to the traction motors). It's not like the gears in a car. The ratio is fixed and you don't change gears.

EDI: Yes, I did Electrical Engineering (and Electronics) at uni ;)

[RAILSOHIO]

Example: 62 is the number of teeth on the axle, 15 the number of teeth (cogs) on the traction motor. The engine/generator drive ratio is fixed.

[mrennie]

Example: 62 is the number of teeth on the axle, 15 the number of teeth (cogs) on the traction motor. The engine/generator drive ratio is fixed.

Ah, OK, makes sense.

I was still thinking in turbine mode

(the turbine shaft is connected to the generator rotor shaft through reduction gears).

[Chacal]

Yup, it's traction motor vs axle.

[tbundy1982]

I think this is how this works. Please correct me if I'm wrong!

Put simply, the lower the top speed, the more tractive effort.

The top speed is determined by what rpm the traction motor will blow apart. Since the gears on the traction motor and axle are fixed, even free-wheeling over-speed will damage the traction motor. You could damage a switch engine that has 45mph gearing by pulling it dead-in-consist at 60mph.

The lower the top speed rating, the higher the tractive effort before wheelslip. The more revolutions the traction motor can turn before the axle makes one revolution, the lower the maximum top speed and the higher the starting force, assuming the locomotive weight remained constant. I can't tell you the physics of it, but it's something I've gathered from reading and research.

It's kind of like a automobile with a manual transmission. You can start in 1st gear and the takeoff is effortless but you're only going to get to 10-15mph before the engine begins to race. You can also start in 2nd gear, but the takeoff is labored, if it doesn't flat out kill the engine. You can, however, go twice as fast in second gear. If you were to pull a trailer, the difficulty starting in 2nd would be much more pronounced.

I'm guessing a similar concept applies with a steam engine, though the mechanics of the "gear ratio" look much different. Small wheeled engines can start a heavier consist but are limited in speed. Large drivered steamers could really speed, but they would slip and have difficulty starting heavy trains.


Tyler