The Bancroft Mill engine is a cross compound Corliss valve condensing steam. Speed is 68rpm controlled by a Porter type governor and a Lumb regulator acting on the high pressure cylinder valves' Dobson type trip gear is fitted. 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 160lb psi) is first expanded in the high pressure cylinder, and "James" turns the flywheel over. There is still more energy in the steam, so it is passed across to the low pressure cylinde "Mary Jane", the same steam is re-used to give the flywheel a further turn. 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||Twin cylinder cross compound|
|Original steam pressure||160 psi|
|Present steam pressure||50 psi|
|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|