Successor Valve Types
In the fully developed forms of the high-speed engine (from around 1900) though, expansion was controlled by governing the timing of a single valve, rather than a separate expansion valve. These led to further complex valve types such as poppet valves, often driven by cam-based valve gears rather than linkages.
Increasing use of superheating encouraged the replacement of slide valves with piston valves, as these were easier to lubricate at the increased operating temperatures. They also made it impractical to use secondary valves like the Meyer, running on the backs of the primary valves. Possibly the last new design to use a secondary valve as an expansion valve was the Midland Railway Paget locomotive, that used bronze sleeves as expansion valves around its cast iron rotary valves. This design was unsuccessful, owing to mechanical problems with differential thermal expansion of the two valve materials.
Read more about this topic: Expansion Valve (steam Engine)
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