"SD" vs. "GP"

[J Campbell]

Lets see if I can make this understandable...

What makes the SD's so special?  In other words, aside from the obvious (trucks, etc.), what are the differences between EMD's General Purpose engines and their Special Duty engines?

Why were the SD's labeled as special duty?

[steinjr]

Lets see if I can make this understandable...

What makes the SD's so special?  In other words, aside from the obvious (trucks, etc.), what are the differences between EMD's General Purpose engines and their Special Duty engines?

Why were the SD's labeled as special duty?

SD's were intended for branch lines, ie lighter rails. More axles, lower ground pressure per axel.

 Smile,
 Stein

 

 

[ndbprr]

Because four wheel trucks were the standard when the designation was started so they had to distinguish somehow between standard ( General purpose) and nonstandard (special purpose).

[wjstix]

GP's are "General Purpose" engines, in some ways a road-switcher version of an F-unit. Railroads bought them for general freight train service, like way-freights and fast freights, though some railroads bought them for switching work or even passenger trains.

SD's with their 6 powered axles were I believe designed more for slower speed 'drag freight' kind of operations....iron ore trains (like DM&IR's SD-9's) or coal trains.

[cuyama]

What makes the SD's so special?  In other words, aside from the obvious (trucks, etc.), what are the differences between EMD's General Purpose engines and their Special Duty engines?

Why were the SD's labeled as special duty?

The name was pirmarily marketing, I think ... after all, if you've just released a locomotive labeled "General Purpose" (GP) that has two axles per truck in 1949, there must be some "Special" reason to now release a locomotive with three axles per truck in 1951. In fact, the additional axles were touted with a couple of benefits, at least: lower per-axle weight loading for branchline and other light rail; and more traction for the same (or more) horsepower through more wheels on the rail. In many other ways, the GPs and SDs were similar, especially in the earlier years. Not to mention that competitive builders had already released 6-axle roadswitchers, so EMD was playing catch-up to a degree.

http://www.emdiesels.com/lms/en/company/history/index.htm

The history of the first SD

http://www.steamlocomotive.info/F92003.cfm

Byron
Model RR Blog

 

[Autobus Prime]

SD's were intended for branch lines, ie lighter rails. More axles, lower ground pressure per axel.

SD's with their 6 powered axles were I believe designed more for slower speed 'drag freight' kind of operations....iron ore trains (like DM&IR's SD-9's) or coal trains.

Folks:

According to "EMD's History in Models" in the Oct 1972 MR, both.  You could either order an SD light, with small fuel tank and light weight spread over six axles, to reduce axleload, or take advantage of the extra axles by ordering a loco ballasted up to normal axleloads.  The heavier total weight would give the advantage in drag service. Friction depends on weight, so a heavier loco could apply more power to the track at low speeds than a lighter one of the same power.

[ndbprr]

The PRR ordered the two heaviest SD engines ever made (SD-7s) and assigned them to the Madison incline in Indiana for getting loads up and down the escarpment down to the Ohio river.  How steep was it?  5.98% comes to mind.  Steam engines were only allowed to back down the hill to keep water on the crown sheet.  If you see pictures of the PRR engines the side sill is about double normal thickness and they were equipped with rail washers and no MU capability.  When two engines were needed so were two crews.

[Railway Man]

SD's were intended for branch lines, ie lighter rails. More axles, lower ground pressure per axel.

SD's with their 6 powered axles were I believe designed more for slower speed 'drag freight' kind of operations....iron ore trains (like DM&IR's SD-9's) or coal trains.

Folks:

According to "EMD's History in Models" in the Oct 1972 MR, both.  You could either order an SD light, with small fuel tank and light weight spread over six axles, to reduce axleload, or take advantage of the extra axles by ordering a loco ballasted up to normal axleloads.  The heavier total weight would give the advantage in drag service. Friction depends on weight, so a heavier loco could apply more power to the track at low speeds than a lighter one of the same power.

That's correct, and I'm glad to see someone has it right!  There were two primary market segments -- one actually a niche -- for the SD7.  The primary market was for heavy tonnage, drag-freight service, where the SD with 50% more adhesion had 50% more tractive effort than the equivalent GP, albeit at half the speed (6 mph instead of 11 mph -- there is no free lunch).  The minor market niche was for granger railroads with light rail and weak bridges that wanted a locomotive with 50% less weight per axle than a standard GP, but still have the same tractive effort as the GP.

The preponderance of SD7s and SD9s sold were standard weight or ballasted above standard weight and used in drag freight service.  Almost all of the lightweights have vanished into the steel mill scrap ladle as their narrow role has disappeared -- most of the light-rail branch lines they once tip-toed down are long ago abandoned. 

RWM