The Roberts Engine
The Roberts engine at Bancroft is a two cylinder horizontal cross-compound engine. ‘Compound’ means that the steam first passes through the high pressure cylinder (James) and then the low pressure cylinder (Mary Jane) to extract as much energy from the steam as possible. Efficiency is further improved by a condenser in the cellar which condenses the exhaust steam creating a vacuum which increases the effective pressure across the low pressure cylinder.
| Engine Builder | William Roberts & Son, Nelson |
| Type | Two cylinder cross compound |
| Power | 500hp as ordered, though known to have delivered 600hp |
| Date Commissioned | 1920 |
| Original/Current Steam Pressure | 160 / 50 psi |
| Valves | Corliss |
| Trip Gear | Dobson |
| Governor | Porter |
| Regulator | Lumb |
| HP Cylinder Bore | 17” |
| LP Cylinder Bore | 34” |
| Stroke | 48” |
| Flywheel Speed | 68rpm |
| Line shaft speed | 136rpm |
| Flywheel diameter | 16 feet |
| Flywheel weight | 30 tons |
There is more information about the Mill in the Guide Book available in the Gift Shop or at Reception.
The only other existing engine made by William Roberts is ‘Peace’, a tandem compound at Queen St Mill, Burnley.