A beginner’s introduction to Walschaerts Valve Gear – Robin Gibbons

Walschaerts valve gear

Our speaker was our Branch chairman Robin Gibbons.

The talk started with some basic concepts including the piston / connecting rod motion, the contrast between a double-acting steam loco cylinder and a single-acting internal combustion engine, the anatomy of the piston valve and cylinder, lap and lead, the benefit of using expansion of the steam, indicator diagrams, valve events and of course Walschaerts valve gear itself.

Issues such as angularity / obliquity and its effect on the geometry of the motion, the velocity of the piston, and the direct relationship of power to steam throughput were all introduced.

Lap was explained at some length and Robin made reference to the arguments for and against long lap, and he showed an interesting table of port openings for various classes of loco at 25% cut-off, compared inter alia with their laps, based on the table in Cook’s “LMS Locomotive Design and Construction” (published by the RCTS). Some time was spent studying valve events and we had a brief look at a “Reuleaux” diagram which illustrated the valve movement for 66% and 25% cut-offs for a Gresley D49.

Robin explained the actual Walschaerts valve gear by separating it into its two components: the lap and lead mechanism and the cut-off mechanism. Robin used Dockstader’s valve gear simulation model (freely available on the web) to illustrate the combined motion.

The talk ended with a look at actual valve events for a Britannia and, in comparison, the Stephenson-geared Black 5 44767, and a look at the indicator cards from the BR Performance and Efficiency Tests for a Modified Hall and a V2. The latter showed very asymmetric events for the inside cylinder and Robin played a short excerpt of a recording of a V2 on the Waverley route providing a good example of the well-known syncopated exhaust. We finished off with a brief look at the Gresley conjugated gear.

We had an audience of around 24 people.