Joy's valve gear

I have rebuilt (if you'll pardon the expression) my Bulleid valve gear model to include all three sets of motion and the triple-throw crank that drives them.

 

Lyd actually has a modified version of Joy's gear that eliminates the anchor link and has a return crank instead, as did the original L&B locos. Not exactly sure why this is, possibly with a relatively long stroke and small wheels the anchor link would be out of gauge at the bottom of its travel. I believe that some of Webb's compounds also had this modified version, but upside down as the valves were below the outside cylinders, again I'm not sure exactly why, and I don't know which locomotives this applied to but I think the tank engines. At least on Lyd the valve gear is visible in action. The modified version was not covered by David Joy's patent.

 

A check in Ahrons shows that Webb used the modified Joy gear for the outside cylinders of some of the Experiment class (his first 3-cylinder compound 2-2-2-0's) but these were later converted to standard, and also on his 4-2-2-0 tank, both upside down as the valves were below the cylinders. Outside valve gear was most uncommon in those days, but he had to use it for the 2-2-2-0's and the tanks as the fireboxes were in the way of inside gear. When the fireboxes were not in the way in the 2-2-2-2's and (3-cylinder compound) 0-8-0's he used the normal arrangement for outside cylinders in Victorian days with the valves between the frames and inside Stephenson gear; these locomotives had slip eccentrics for the middle low pressure valve. For LNWR locomotives with two inside cylinders the main selling point for Joy's gear was that eccentrics were unnecessary and this left the middle of the driving axle clear for an additional main bearing.