Diesel trucks

Three axle trucks were intended to spread weight over a longer wheelbase on some diesels, like the EMD SD series, the E,  ALCo PA and FM "Erie-built" type passenger units. These 6 wheel trucks only had two raction motors in them. The center axle was an idler. Spreading weight was important for light trackage as well as for restrictive bridge ladings.

Some heavy transfer and freight type diesels had three traction motors in their trucks, driving all axles. Some Baldwin units come to mind as well as huge, double-ended Lima-Hamilton transfer diesels. Here, the center axle is off center in the truck because of the additonal traction motor. Transfer service involved moving long, heavy frieghts between the major yards of different railroads for interchange.

Truck designs had speed limitations. The ALCo Blundt and EMD "AAR" trucks in switchters were generally limited to 35 or 40 mph. The 4 wheel trucks seen under EMD F and GP type units and those found under ALCo F, RS and Baldwin F as well as Fairbanks Morse C-liners and road switchers could be run at speeds of 60 MPH and over. However this is just a simple generalization.  Builder manuals and railroad rules determined the maximum speed permitted for each type of locomotive.

One correction...the EMD "SD" series were C-C trucks, not A-1-A trucks as on the E-units. In other words, ALL axles are powered on an EMD SD unit.

I am not as familiar with the others you mentioned, Earl.

Neither EMD nor GE has offered/produced 4-axle diesel electric freight locomotives for many, many years (maybe 20?). With the development of the EMD Radial (stearable) 3-axle truck, and GE's subsequent "stearable truck", designs the 6-axle "SD" type units are now capable of going around sharp curves, where GP 4-axle units previously where required. A perfect example is the Alaska RR, who could NOT utalize 6-axle "SD" units prior to the development of the HTC-R EMD radial truck design, due to their curvy track.

Hot: Never have heard the term "steerable truck." How does it work? Some sort of lateral motion device? 

Thanks!  

SJC,

NOT with lateral motion devices. The EMD "Radial Truck" works off a huge "center pin" from the underframe, which had a VERY large "arm" which controls the axle "stearing arms" in the HTC-R truck frame assembly. It is a bit difficult to describe, but as I recall there was some nice technical information on the EMD website, and Railfan & Railroad Magazine did an nice article on it, back when the 70 series locomotives came out (1995?). Yes, this has been around for a LONG time!

I don't know how the "stearable truck" design works on GE units.

The South African Railways had "inter-bogie control" more than 30 years ago on their diesels, which involved "stearing arms" between the front truck (bogie) and the rear truck (bogie), underneath the fuel tank.

Thanks for the info. I MAY just have that issue of R&R down on the train room bookshelves. If not, I'll have to go hunt it down. I will also check out the EMD site when I get a chance. 

Didn't know they had been around so long. I thought they were a new concept. 

Very simply...more pulling power.

And lighter axle loadings.

quote:

Three axle trucks were intended to spread weight over a longer wheelbase on some diesels, like the EMD SD series,

Comparing the two:

      Total Weight     Wt./Axle

GP9 247,000 lbs. = 61,750 lbs.

SD9 368,000 lbs. = 61,333 lbs.

GP40 277,000 lbs. = 69,250 lbs.

SD40 396,000 lbs. = 66,000 lbs.

GP60 287,000 lbs. = 71,875 lbs.

SD60 390,000 lbs. = 65,000 lbs.