I am trying to find enough data to draw up a 7 1/4" gauge model of the Robert Stephenson locomotive "Lancashire Witch" of 1828. This was the real forerunner of the more famous Rocket and seems to be the first locomotive to use a variable steam cut off. This is the problem, the original drawings make no sense in that they show a pair of quadrant controls on top of the boiler equidistant about a vertical shaft within the boiler driven by a pair of bevel gears on the rear axle. Verbal descriptions state that this vertical shaft "operates a rotating plug valve; by means of this valve it is possible to obtain the expansive action of the steam during half the stroke of the piston".
The vertical shaft has to be central within the boiler to clear the two flues, if it goes to only one plug valve then two interlinked quadrants, presumably operating some form of pair of sleeves, one to each steam pipe, cannot both rotate around the centre of the same plug valve so I think that there must have been two such valves, one for each cylinder and connected by gears within a husing inside the boiler shell. For this to work, one quadrant would be on the centreline of the boiler (in line with the vertical shaft) and one would be offset to one side, such an arrangment is shown by a drawing prepared by two french engineers (Coste and Perdonnet, "Annales des Mines", 2nd series, 1829, Vol VI) who visited the locomotive but they also thought that there was only one valve.... although they did not get to see the inside workings.
One problem with the offset quadrant is that it would make the steam volume in the left and right hand steam pipes different which may lead to the engine running being a little irregular. The same drawing by the french engineers also shows the boiler crosshead feedpump being on the side with what would then be the largest steam volume so perhaps this was an attempt to offset any irregularity?
The whole thing is something of a mystery but was plainly a very important development in the history of the steam locomotive and so I would like to get it right. If anyone has anything that they can add to this from their own collections of information I would be very interested to hear.
The published information is mostly as per "A Century of Locomotive Building by Robert Stephenson and Co, 1823/1923".
