Eastern Railroad Discussion > SD9 questions

Just for discussion :

Why didnt the SD9 seem to catch on as well as the GP9 did? I not meaning just 6 axles over 4, but the RR's have reasons for everything, and I am wondering why they all seemed to move toward 6 axles during the late 60's anyway.

Was it just to little HP for what they needed? Same HP as GP but more feet to get traction.....

Thanks in advance.

Weight and because of this the GP-9 was more versatile.

FDFrisch

There are several reasons why the SD7 and SD9 were not as popular as the GP7 and GP9. Weight really wasn’t much of an issue for Class-1 roads, since even an SD9 was lighter than many large steam locomotives. Price was. The SD model was much more expensive than the 4-axle unit. Plus, tractive effort verses speed was an issue. The extra axles gave better tractive effort to start a train, but did little to help a train move fast, which is what many railroads were seeking during the first generation of the diesels. Also, with such low horse power to start with (less that 1800hp), a 4-axle model had enough tractive effort to get going without slipping, in most cases. Unit reduction wasn’t an issue until the 1960s and 1970s, so long strings of versatile 4 axle models worked well. Of course some railroads saw the need for lots of tractive effort and low speeds. That’s where the SD7 and SD9 shined… and it laid the groundwork for future 6 axle models.

Galen Wright
Lynchburg, VA

Basically, Geeps were for faster and lighter while SDs were more for heavier and slower. The SDs were more adept for 'drag' type operations where strength was needed over speed, like moving iron ore and other heavy commodities.

As the 1960s came to be... a battle between EMD, Alco, and GE was in full swing towards the goal of more horsepower. Why? Mostly for unit reduction reasons, and the ability to move larger trains faster. Fuel efficiency wasn’t yet a major worry at that time.

As horsepower grew past 2,000hp, a major problem developed for all three builders. The ability to generate and apply that power to the rails was becoming “slippery”. With a 4-axle locomotive, more horsepower was applied to each traction motor than with a 6-axle unit of the same horsepower rating. The technology was not yet in place to adequately control the power to the traction motors in a high horsepower unit, and eliminate undue slipping. As a stop gap measure, the builders found that they could optimize more horsepower on a 6-axle unit than with a 4-axle unit. Hence, why EMD offered a 2,400hp SD24 but only was able to squeak out 2,250hp from its GP30.

As the horsepower race heated up in the mid-1960s EMD was losing the battle. So they responded with the release of its 3,600hp SD45. At this time there were two reasons this unit was only available in a 6-axle model. One, the 20 cylinder power plant couldn’t fit on a 4-axle locomotive frame. Second, the ability to control the horsepower was almost impossible. EMD was having enough slipping problems from its GP40 model, which was just 3,000hp.

Many roads found that past the 3,000hp mark, the 6-axle units worked well in a variety of services. SCL bought 3,600hp SD45s and SD45-2s specifically for high-speed service. Past the 3,000hp threshold, not only were 6-axles good for getting a train going, but with high horsepower it could move fast. SCL had a many problems with the 4-axle, 3,600hp U36B slipping at train startup or on grades.

Galen Wright
Lynchburg, VA

Why, during all the struggles for traction, did the loco makers eliminate sand on all but the lead axles? Steam locomotives were equipped with large sand domes and sand to all driving axles to start the big trains. The later emd sand systems removed all finesse and were an open/closed proposition. It seems to me another operating tool was given up.

Keep in mind, GP stood for "general purpose", SD for "special duty", and that's how these units were viewed in the 1950's context, as several posters have noted.