Originally placed into service as Mallet Consolidations (MC), several classes of 2-8-8-2 cab-forwards were simpled and superheated, being redesignated Articulated Consolidations (but still called Mallets). The first of the updates resulted in a modest overhaul (see Locobase 9295) that clearly didn't satisfy and a new layout was adopted. The new program was undertaken at the same time later AC classes were being produced by Baldwin. Locobase doesn't know which influenced which, but both major upgrades (modifications and new-builds) included the Type E superheater and its larger surface area.
Compared to the later ACs, however, the MC conversions were smaller engines with less capacious grates and fireboxes. The tubes were longer than those of the later AC class engines, as was the combustion chamber.
17 MC-1 (Locobase 339) & MC-2s, 11 MC-4s (Locobase 3358) , and all 20 MC-6s (Locobase 8724) were rebuilt, including the 4000-4001, which were conventional-cab locomotives and did not operate in the Sierras, and the 4041, which was the first superheated and had different data; see Locobase 9296
Locobase 9296 describes the general course of simpling and superheating that was applied to the several Mallet Consolidation classes in 1928-1927. These all had Type E superheaters and the entire installation represented a substantial makeover.
One year earlier, however, the Espee had begun the project with the less-ambitious update shown here that used a Type A superheater, shorter tubes, and no combustion chamber. It seems clear that the railroad concluded that this was an insufficiently powerful update and turned to a much different layout.
Conventional Mallet compound predecessors of later "cab-forward" Espee articulateds. As US builders and railroads began adopting superheaters, several variations appeared before the Schmidt smokebox superheater became the preferred installation. Among the early designs was a Baldwin set up that placed a relatively large feedwater heater (1,221 sq ft/113.45 sq m) between two sections of boiler. Its reheater design was an apparatus that lined the inside of the smokebox. Steam exhausted from the HP cylinders was circulated through the 655 sq ft (60.85 sq m)of the reheater before being directed to the LP cylinders.
The design was modestly successful as a heavy freight pusher, but its conventional layout allowed nearly lethal exhaust to swirl into the cab as the engine traveled through the SP's many tunnels and snowsheds. To combat this oppressive condition, the two were later rebuilt as simple articulateds with the cab-forward design.
33, p. 306. MC-2s followed the MC-1s in production seven months later in November 1909 as works numbers were 34019, 34043-34044, 34046-34047, 34063-34067, 34093-34097.
MC-4s (4017-4028) followed in April 1911 (works numbers 36490-36493) and May (36524-36527, 36614-36616, 36634), and MC-6 (4029-4043) were produced in October 1912 as works works numbers 38523-38534 and 38711-38713 in December 1912.
Like the MC-1, this class had a small (655 sq ft/60.85 sq m) reheater for the LP cylinders, but a larger "pre-heater", which Baldwin called a feed water heater, that used the same tube layout as the longer firetubes but heated water with exhaust gas. Each of these tubes was 63" (1,600 mm) long and together comprised 1,221 sq ft (113.43 sq m) . The combustion chamber lay between the firebox and the feed water heater. The four 15" (381 mm) piston valves had removable bushings.
Later the MCs (Mallet Consolidations) became AC (Articulated Consolidations) when they were simpled in the 1920s. See Locobase 9626.
Locobase 329 describes the conventional-cab MC-1s, which introduced the Baldwin feedwater heater as a separate section between two sections of boiler and a Baldwin reheater (of HP exhaust steam). To overcome exhaust-fume poisoning, the builder turned the next group of engines around. As the "cab-forward" or "back-up" MC-2 & MC-4, those articulateds retained both features.
The MC-6 class also had the 1,221-sq ft (113.43 sq ft) heater (area not included in the evaporative heating surface area) that used the same tube layout as the longer firetubes but heated water with exhaust gas. The builder discarded Baldwin's reheater for a Schmidt superheater, however, which would prove to be the preferred form of reheating steam. All four cylinders received steam through 15" (381 mm) piston valves.
Later the MCs (Mallet Consolidations) became AC (Articulated Consolidations) when they were simpled in the 1920s.
|Specifications by Steve Llanso of Sweat House Media|
|Railroad||Southern Pacific (SP)||Southern Pacific (SP)||Southern Pacific (SP)||Southern Pacific (SP)||Southern Pacific (SP)|
|Number in Class||47||1||2||27||15|
|Locomotive Length and Weight|
|Driver Wheelbase (ft)||15||15||15||15||15|
|Engine Wheelbase (ft)||56.59||56.58||56.58||56.59||56.59|
|Ratio of driving wheelbase to overall engine wheebase||0.27||0.27||0.27||0.27||0.27|
|Overall Wheelbase (engine & tender) (ft)||90.33||90.33||83.50||90.33||90.33|
|Axle Loading (Maximum Weight per Axle) (lbs)||50,260||51,200|
|Weight on Drivers (lbs)||440,800||440,800||394,145||398,000||400,700|
|Engine Weight (lbs)||481,200||481,200||425,900||433,600||435,800|
|Tender Loaded Weight (lbs)||180,200||179,400||184,660|
|Total Engine and Tender Weight (lbs)||481,200||481,200||606,100||613,000||620,460|
|Tender Water Capacity (gals)||10,000||10,000||10,800||10,000||10,000|
|Tender Fuel Capacity (oil/coal) (gals/tons)||3200||3200||3420||3200||3200|
|Minimum weight of rail (calculated) (lb/yd)||92||92||82||83||83|
|Geometry Relating to Tractive Effort|
|Driver Diameter (in)||57||57||57||57||57|
|Boiler Pressure (psi)||210||210||200||200||200|
|High Pressure Cylinders (dia x stroke) (in)||22" x 30" (4)||22" x 30" (4)||26" x 30"||26" x 30"||26" x 30"|
|Low Pressure Cylinders (dia x stroke) (in)||40" x 30"||40" x 30"||40" x 30"|
|Tractive Effort (lbs)||90,941||90,941||85,039||85,039||85,039|
|Factor of Adhesion (Weight on Drivers/Tractive Effort)||4.85||4.85||4.63||4.68||4.71|
|Firebox Area (sq ft)||362||240||232||240||240|
|Grate Area (sq ft)||68.40||68.40||68.40||68.40||68.40|
|Evaporative Heating Surface (sq ft)||4876||4417||5173||4941||4177|
|Superheating Surface (sq ft)||2150||839||655||839|
|Combined Heating Surface (sq ft)||7026||5256||5828||4941||5016|
|Evaporative Heating Surface/Cylinder Volume||184.71||167.32||280.61||268.02||226.58|
|Computations Relating to Power Output (More Information)|
|Robert LeMassena's Power Computation||14,364||14,364||13,680||13,680||13,680|
|Same as above plus superheater percentage||18,817||16,662||15,185||13,680||16,006|
|Same as above but substitute firebox area for grate area||99,586||58,464||51,504||48,000||56,160|