In the mid 1920s many western railroads were using the 2-10-2 and 2-8-8-0 type locomotives for mainline freight trains. These locomotives were getting the job done but were limited to about 20 mph. The Union Pacific Railroad experimented with a three-cylinder 4-10-2 in 1925 and actually bought ten of them. These 4-10-2 locomotives were capable of slightly greater speed while being capable of pulling about 20% more tonnage.
The UP wanted more and approached the American Locomotive Company with a design for a 4-12-2 and received the first of this new design in 1926. The locomotive had more power and rode very well at higher speeds. With the arrival of this new wheel arrangement, came the need for a new name. The 4-12-2 was dubbed the 'Union Pacific' type and it namesake railroad was the only North American railroad to use it.
The American Locomotive Company built eighty-eight of these fast freight locomotives for the Union Pacific Railroad. They were three-cylinder locomotives with the third cylinder located in the center of the steam chest. The third cylinder drove an eccentric in the axle of the second pair of drivers. The fourth set of drivers were flangeless, but it was later determined that this was not necessary and a small flange was added to these drivers.
The UP ran these locomotives at 50 mph hauling 120 car trains.
|Number of Locomotives by Builder|
|Railroad||ALCO||Baldwin||Lima||Other||From Other RRs|
There is an extremely accurately detailed model of the UP 4-12-2 available for general viewing at the Schenectady Museum, in Schenectady, NY. The model is in a section of the museum called the Schenectady Heritage Area. The model provides an excellent view of the lateral motion devices fitted to the first and last driving axles, spring rigging, main and side rod designs, cylinder configurations, valve gear arrangements, etc. The model is covered by a clear plexi-glass cover which allows for viewing but no touching.
In summary, the 9000's designers wanted to avoid having to put a crank in the axle of driver number 1. So they did the following things:
Because ALCO placed the center cylinder on the UP 9000s at an incline of 9½°, some have said that it would be necessary to set the crank pins at 120°-129½°-110½° to preserve the torque distribution and to keep the system balanced. This was not the case with the 9000s. The one inch shorter stroke of the center cylinder and valve event (timing) dimensional settings allowed for the center cylinder inclination.
This assertion has been confirmed by Joe Davenport of UP's Mechanical Department as well as by certain ALCO documents. Additionally, ALCO had repeatedly referred to the use of 120° crank spacing in their literature devoted to their effort toward sales of three-cylinder power. Finally, a Union Pacific drawing, number 112CA27929, dated May 6, 1941, titled "Axle--Key Location" confirmed the 120° spacing. The intention of this drawing was to provide machinists with data permitting proper location of the keyways necessary to affix driving wheel center castings to their respective axles. Sure enough, all three pins are clearly displayed and specifically noted as 120 degrees apart. Of course, only the 2nd main axle actually had three pins since that is where the inside cylinder was connected. Pins on the other five axles were also of necessity spaced at 120° and thus drawings for these engines can take your eyes a moment to adjust to given the usual propensity for pin spacing at 90 degrees with two-cylinder engines.
It is suspected that one of the reasons the Baldwin #60000 used a 90°-135°-135° configuration had to do with it being a compound locomotive, with the center cylinder receiving high-pressure boiler steam, and the outer cylinders receiving the low-pressure steam from the center cylinder. And according to John Bush, the locomotive had "a very interesting" outside valve gear design. The original valve gear used a conjugated assembly that synthesized inner cylinder valve gear motion from the outer valve gear on either side of the locomotive.
UP 9000 driver "quartering" information graciously provided by John E. Bush.
UP 9000 exhaust information graciously provided by John E. Bush.