The Bancroft Mill engine is a cross compound Corliss valve condensing steam. It runs at 68rpm controlled by a Porter Governor and a Lumb Regulator acting on the high pressure cylinder valves, employing Dobson trip gear. The high pressure cylinder James has a bore of 17 inches with a stroke of 4 feet. The low pressure cylinder Mary Jane has a 34 inch bore and a stroke of 4 feet.

Cross compound engines are so named because the cylinders and cranks are on either side of the flywheel and the steam crosses from the high to the low pressure cylinder, as compared to the Bradley Tandem Compound which is "in line", the cylinders being one behind the other.

Steam from the boiler at 50 lbs per square inch (originally 160psi) first expands in the high pressure cylinder, James. There is still some energy in the steam, so it is passed across to the low pressure cylinder Mary Jane. The engine is designed and set up so that each cylinder contributes and equal amount of power. The exhaust steam is then passed to the condenser in the basement to be turned back into re-usable water.

This mill engine was installed, and first started service in 1920, and ran a continuous reliable service through to 1978 when the mill was finally closed. In its heyday the mill produced 200,000 yards of cloth in a week from this one engine.

Engine maker William Roberts and Son, Nelson
Type Two cylinder cross compound
Power 500/600 ihp
Year 1920
Original steam pressure 160 psi
Present steam pressure 50 psi
Valves Corliss
Regulator James Lumb
High pressure cylinder diameter 17 inches
Low pressure cylinder diameter 34 inches
Stroke of both 48 inches
Flywheel diameter 16 feet
Flywheel weight 30 tons
Speed 68 rpm

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