Part 37 – Slide and Piston Valves

Under Stirling the steam chests (with their vertical slide valves) were cast integral with the cylinders.  The castings were usually located between the main frames, but on the 4-2-2s the castings were straddled by the frames, with the cylinders on the outside and the steam chests (containing the slide valves) on the inside. Stirling had been forced into this situation because of their 8ft diameter driving wheels, as otherwise the boiler barrel would have been pitched unacceptably high to clear the (inside) connecting rod cranks at top centre.  Now the boiler only had to clear the eccentrics for the (inside) valve gear.  (When larger diameter boilers were introduced in 1880, the pitch of the boiler was raised a further ¾in.).  The clearance was tight, so much so that the lagging had to be cut away to clear the inner eccentric straps, see lower illustration on p. 55 of THE STIRLING SINGLES (David & Charles, 1965).  However, it is possible this treatment was quite normal practice for the earlier engines from 1880, as perhaps the Works did not bother to raise the height on existing engines, but chose instead to cut away the lagging to provide the clearance.

Changes were made under Ivatt.  As far as his two-cylinder designs were concerned, only his 4-4-2s (which were perhaps based on the Stirling 4-2-2s rather than on his own 4-4-0s) had outside cylinders.  Ivatt introduced horizontal balanced slide valves above the cylinders for his 0-8-0s which had large diameter cylinders, leaving insufficient room for vertical balanced slide valves between them.  Over the next few years there was a mix of slide, balanced slide and piston valves, which was not straightforward.  The list below covers the various classes (mostly with Stephenson valve gear), showing the various combinations. In general, the position with engines having Walschaerts valve gear was straight-forward, as the valve gear, cylinders and steam chest (with piston valves) were (normally) outside the frames.

SV Valves; BSV Balanced Slide Valves; PV Piston Valves

LP Low Pressure; HP High Pressure

(b) First Doncaster drawing specifically marked Balanced Slide Valve

(v) SV/BSV Vertical were positioned between the inside cylinders;

                Above were positioned above the cylinders;
               Alongside were positioned adjacent to the outside cylinders, but between the frames

(w) Walschaerts valve gear (otherwise Stephenson valve gear)