Holcroft valve gear

It varied from the Gresley method of operation by using the combination lever assembly instead of the valve spindles to drive the middle cylinder of a three-cylinder design.

This had operational advantages over Gresley's design, namely eliminating the problems of flexure, bush wear and the influence of heat in the valve spindles.

Somewhat later, Nigel Gresley of the Great Northern Railway had designed and patented his own application of conjugated valve gear (late 1916) which required further development to enable it to be applied to three cylinders of differing inclination.

It proved impractical to locate the mechanism behind the cylinders on this design due to the leading coupled wheel fouling the area required for the gear.

This made it necessary to revise the design with side rods to transmit the motion forward to the lever mechanism mounted near the front of the frames.

[3] Harold Holcroft was close friend of Curly Lawrence, the pioneer designer of small live steam passenger hauling model locomotives.

South Eastern & Chatham Railway official photograph of Maunsell N1 prototype No. 822, taken at Ashford Works in 1922. The lever running forward across the outside cylinder is part of the Holcroft valve gear which drives the valve of the inside cylinder