• SD50 vs. SD40-2

  • Discussion of Electro-Motive locomotive products and technology, past and present. Official web site can be found here: http://www.emdiesels.com/.
Discussion of Electro-Motive locomotive products and technology, past and present. Official web site can be found here: http://www.emdiesels.com/.

Moderator: GOLDEN-ARM

  by Narr8rdanny
What are the main differences, mechanically, between an SD50 and an SD40-2?
I just recently shot some drone video of trains that had one of each and noticed that the hood on the 40 is much shorter than on the 50.
I know the 50's had major problems after they were introduced, but were saved via a modification. Could someone give me the major difference here?
Thanks very much,

Danny Harmon
  by trainiac
I'm not an expert on the minutiae of the mechanical details between the two units, but the main reason for the longer carbody was the relocation of the dynamic brakes from above the prime mover to behind the cab, along with a stretched radiator section. The first SD50 prototypes were built on frames the same length as the SD40-2 and lacked the rearmost door of the engine compartment present on later units. By contrast, the GP50 and GP60 kept the dynamic brakes above the prime mover along with a shorter radiator section, so their hoods were not much longer than on the GP40-2.

Mechanically, the SD50 uprated the prime mover from 3000 to 3500-3600 hp. My understanding is that the higher engine speed associated with the horsepower increase (950 instead of 900 rpm) led to vibrations that negatively affected reliability. When the 3800 hp 710 engine was introduced, the speed was dropped back to 900 rpm until later in SD70 series production. The SD50 had quality-control issues with parts being outsourced or cheapened.

The SD50 also had reliability issues with the electrical system. Someone else could chime in with more info, but I believe the electrical system was halfway between the relays of the SD40-2 and the microprocessors of the SD60. It incorporated EMD's "Super Series" wheelslip control, which was Doppler radar-based.

With derating and a lot of work, an SD50 could be made to operated similarly to an SD40-2. EMD's ultimate solution was to replace it with the SD60, which solved most of the problems.
  by Allen Hazen
Various internal components were changed. The SD40-2 had a 645E engine (rated at 3000 hp); the SD50 replaced this with a 645F, rated at 3500 in early "50-series," and at 3600 in the later production. Don't ask me what all the changes made from one model of the 645 engine to the next were! I think one was that the crankshaft of the 645F had a very slightly elliptical (instead of round) cross section, the idea being that the tiny gap opened up on each revolution between the crankshaft and the piston-rod big end would pump lubrication oil.

Electrically, the SD40-2 had D-77 traction motors, the SD50 D-87. The new wheel slip control system changed the transition arrangements, and was happier with a higher capacity main generator: the SD40-2 had an AR-10 alternator, the SD50 an AR-15.

So: evolutionary changes, but definitely changes.

Visually, the change in the position of the dynamic brakes mentioned by Trainiac is probably the most dramatic (along with the elimination of the SD40-2 "back porch" because of the longer long hood). Slightly more subtle, the radiator air intakes high on the rear of the long hood are a bit larger (in their vertical dimension) than those on the SD40-2: the higher rating of the engine meant that more radiator air was needed to cool it.
  by Narr8rdanny
Thanks to both of you.
That's exactly what I was looking for. Hope you don't mind if I use some of this in a video description.

Thanks again,

Danny Harmon
  by Wayside
The SD50 wheel slip control system was not EMD's finest development. At low speed, it used a radar system, directed at the ground, to detect minute differences in wheel speed compared to ground speed, indicating wheel slip which was countered with load control reduction to eliminate the detected slippage. This was a stop-gap arrangement prior to the computer controls of later models (60 series), and it didn't always function as desired. I remember telling the Conrail CMO-Locomotive about SD50 synchronous wheel slips at higher speeds that I had personally experienced, and he was incredulous.
  by Engineer Spike
I have seen these strange SD50 wheel slips. One night I had a NS 50, and it was raining. The speed needle was jumping all over. I was afraid that an axle would over speed. I had this happen one fall on a SD40-2. The leaf goop made an axle break free. As luck would have it, it was the one tied to the speedometer, and also over speed. Unfortunately I got a resulting ove speed penalty brake, since I wasn’t able to catch it with the independent brake.