The expansion slide valve and governors
An improvement on the simple slide valve was the expansion slide valve, this was a two part valve, of which there were numerous designs, some manually adjusted and some automatically adjusted through a governor mechanism
The section above shows a manually adjusted version of the Meyer expansion slide valve, the main slide valve V works from an eccentric on the crankshaft and operates just as a simple slide valve, except that it has a smaller valve working on its back which controls the admission of steam through the main valve ports T into the portface of the cylinder. the subsidiary valve is also driven by an eccentric on the crankshaft and consists of two valves V1 and V2, which by rotating the handwheel W and shaft K adjust through a right and left hand thread and nuts so they can be set closer together or further apart. If they are set closer together more steam is admitted over a longer period of time, if adjusted further apart the steam is cut off earlier during the stroke and has more time to expand in the cylinder.
Some engines fitted with this sort of expansion slide valve were fitted with a mechanism where a gear replaced the manual handwheel W, this gear was linked through the governor so that the expansion valve could be automatically adjusted while the engine was running, this method was not very quick responding to changes of speed and load and so had a limited range of applications such as pumping and textiles. Where rapid changes of load and speed occurred on an engine so equipped an additional overspeed governor which throttled the steam supply would also be needed.
See also the Valvegear page.
Expansion Governors
A similar effect but with a much more rapid response (although lower levels of efficiency) resulted when the expansion slide valve was driven either from an eccentric where governor movement varied the throw of the eccentric (and thereby altered the range of movement on the expansion valve as above) see also the Robey and Richardson Governor.
Or as with the Hartnell expansion governor above, from a fixed eccentric via an expansion link (l) and die block (h), the position of which is governor controlled and either increases or decreases the travel of the expansion valve rod (k), admitting steam earlier or cutting it off earlier depending on speed or load (see also the description of the Richardson expansion governor)