Milwaukee Road class A2
The class A2 designated the first Atlantics on the Milwaukee Road to have a Vauclain-type compound engine. This means that there was a high and a low pressure cylinder on each side, which were connected to a common crosshead. The trailing axle had special inside bearings developed by J.H. DeVoy which were part of a large single casting.
Nine A2 and 15 A2a were built by Baldwin in 1901 and 1902 respectively. The five A2b had drivers whose diameter had been increased to 85 inches. While these were built in the railroad's own workshops at Milwaukee, the twelve A2cs were built at Baldwin again. These were all scrapped between 1927 and 1930.
An exception were two locomotives which were built in 1907. These had what was known in the USA as a “balanced compound” drive, which had separate inside and outside cylinders. These locomotives were rebuilt in 1938 to class A-4a with simple expansion and remained in service until 1951.
Technical data
General
| Property | A2 | A2a | A2c | A2 balanced compound |
|---|---|---|---|---|
| Built | 1901 | 1902 | 1908 | 1907 |
| Manufacturers | Baldwin | Milwaukee | ||
| Wheel arrangement | 4-4-2 (Atlantic) | |||
| Gauge | 1,435 mm (Standard gauge) | |||
Dimensions and weights
| Property | A2 | A2a | A2c | A2 balanced compound |
|---|---|---|---|---|
| Length | 21.77 m | 22.86 m | ||
| Wheelbase | 8.52 m | 8.90 m | 9.80 m | |
| Total wheelbase | 18.30 m | 18.53 m | 18.62 m | 19.32 m |
| Service mass | 80.3 t | 95.4 t | 92.6 t | |
| Adhesion mass | 41.7 t | 49.3 t | 48.1 t | |
| Total mass | 139.9 t | 155.6 t | 153.4 t | |
| Rigid wheelbase | 2.21 m | 2.24 m | 2.29 m | |
| Water capacity | 26,498 l | |||
| Fuel capacity | 9.1 t (coal) | |||
Boiler
| Property | A2 | A2a | A2c | A2 balanced compound |
|---|---|---|---|---|
| Grate area | 4.339 m² | 4.34 m² | 4.181 m² | 4.255 m² |
| Firebox surface | 19.231 m² | 16.072 m² | 19.881 m² | 15.608 m² |
| Tube heating surface | 279.453 m² | 276.294 m² | 279.453 m² | |
| Evaporative heating surface | 298.684 m² | 295.525 m² | 296.175 m² | 295.06 m² |
| Total heating surface | 298.7 m² | 295.5 m² | 296.2 m² | 295.1 m² |
Power plant
| Property | A2 | A2a | A2c | A2 balanced compound |
|---|---|---|---|---|
| Driver diameter | 2,134 mm | 2,140 mm | 2,159 mm | |
| Boiler pressure | 13.8 bar | 15.2 bar | ||
| Expansion type | compound | |||
| Cylinders | four, HP: 381 x 711 mm and LP: 635 x 711 mm | |||
Power
| Property | A2 | A2a | A2c | A2 balanced compound |
|---|---|---|---|---|
| Power source | steam | |||
| Estimated power | 1,230 kW (1,672 PS) | 1,193 kW (1,622 PS) | 1,230 kW (1,672 PS) | 1,193 kW (1,622 PS) |
| Starting effort | 83.4 kN | 83.2 kN | 90.7 kN | |
| Starting effort simple | 100.1 kN | 99.8 kN | 108.8 kN | |
Calculated values
| Property | A2 | A2a | A2c | A2 balanced compound |
|---|---|---|---|---|
| Factor of adhesion | 1 : 4.91 | 1 : 4.92 | 1 : 5.34 | 1 : 5.20 |
| Optimal speed | 90 km/h | 89 km/h | 84 km/h | 80 km/h |
| Power-to-weight ratio | 15.3 kW/t | 14.9 kW/t | 12.9 kW/t | |
| Heating Surface Equivalent | 394.8 m² | 375.9 m² | 395.6 m² | 373.1 m² |
| Cylinder volume/HSE | 1 : 981.15 | 1 : 1,030.63 | 1 : 979.32 | 1 : 1,038.32 |
| Adhesive weight/Total | 1 : 1.92 | 1 : 1.93 | ||
| Tractive force/Heating surface | 1 : 5.83 | 1 : 5.88 | 1 : 6.39 | 1 : 6.42 |
| Grate area/Heating surface | 1 : 68.84 | 1 : 68.09 | 1 : 70.84 | 1 : 69.34 |
| Firebox/Tube surface | 1 : 14.53 | 1 : 17.39 | 1 : 13.90 | 1 : 17.90 |
