by Allen Hazen
The sort of motor mounting in the truck that is standard in American locomotives is called "nose suspended": the traction motor is hung between the axle and the truck frame (& I can't remember which side is called the "nose" of the motor!). This is probably the simplest way to arrange things, and (partly because of that) the cheapest. Disadvantage is that a fair portion (like, maybe: half) the weight of the motor is directly suspended from the axle with no intervening springs: it is "unsprung mass". As such, it produces (what in steam locomotives was called) "dynamic augment" whenever the axle and motor move up and down (try to imagine what happens inside the truck when the locomotive goes over a low rail joint): the momentary force is greatly increased over the continuous force of gravity on the wheelset axle and motor. Bad for ride quality, bad for track structure.
Alternative mountings reduce the unspring mass. In a quill drive (as on the GG-1 electric of blessed memory) the motors were mounted entirely on the truck frame: there were spring connections between the quill and the wheel so the motors themselves didn't have to move up and down when the axle did. Another alternative (that has been used on some high-speed European passenger power, I think) is to have the traction motors inside the carbody, connected by cardan shafts to a gear-box on the axle: gear box plus half of cardan shaft is a lot less unsprung mass than gears and half of traction motor!
Dynamic augment is more severe at higher speeds. In practice the more expensive, lower unsprung weight, options seem to be chosen for very high speed (well over 100mph) passenger power, not for locomotives intended for use a freight train speeds.
There is also a general principle of engineering: don't try TOO many new ideas at once! BNSF and GE have come up with a locomotive which, except for the four-motor idea (& related control and adhesion problems) uses the same technology as existing power. Including conventional nose-suspended traction motors.
Alternative mountings reduce the unspring mass. In a quill drive (as on the GG-1 electric of blessed memory) the motors were mounted entirely on the truck frame: there were spring connections between the quill and the wheel so the motors themselves didn't have to move up and down when the axle did. Another alternative (that has been used on some high-speed European passenger power, I think) is to have the traction motors inside the carbody, connected by cardan shafts to a gear-box on the axle: gear box plus half of cardan shaft is a lot less unsprung mass than gears and half of traction motor!
Dynamic augment is more severe at higher speeds. In practice the more expensive, lower unsprung weight, options seem to be chosen for very high speed (well over 100mph) passenger power, not for locomotives intended for use a freight train speeds.
There is also a general principle of engineering: don't try TOO many new ideas at once! BNSF and GE have come up with a locomotive which, except for the four-motor idea (& related control and adhesion problems) uses the same technology as existing power. Including conventional nose-suspended traction motors.