page_title: Cornish engine valve gear
section_title: Assembled valve gear and its sequence
hierarchical_section_title: Cornish engine valve gear / Assembled valve gear and its sequence
caption_attribution_description: English: Valve gear for the 55 inch Cornish Engine at Battersea waterworks made by Harvey and CO. of Hayle. Some parts are labelled, but for clarity not all versions of all parts have been indicated. The assembly is shown with all the valves set in their closed position. The blue arrows show the direction they would turn to open. SA: Steam Arbor, QA: Equilibrium Arbor, XA: Exhaust Arbor, H: Handles, LQ: Locking Quadrants, PR: Plug Rod, T:Tappets, W: Attachment points for weights. Based upon “Valve gear, 55-inch Cornish engine, Battersea Waterworks”, The Engineer, Feb 22 1867, page 154.
context_page_description: Valve gear opens and closes valves in the correct order. In rotating engines valve timings can be driven by eccentrics or cranks, but in non-rotative beam engines these options are not available. In the Cornish engine valves are driven either manually or through ‘plug rods’ and tappets driven from the beam. This permits the insertions of delays at various points in the cycle, allowing a Cornish Engine to vary from one stroke in ten minutes, to ten or more strokes in one minute, but also leads to some less familiar components when compared with rotative engines.
context_section_description: The above components can be assembled into a frame allowing the complete engine to be run either manually or automatically. The three arbor form is relatively fixed but, as can be seen from some of the images, there are variations in the position of the various components along them, the number of both plug rods and cataracts, and the level of ornamentation applied. 
  	The Cornish engine has two distinct parts to its cycle – the indoors stroke, where the piston descends (usually - the Cruquius engine mentioned below being one of the exceptions) from the top of the cylinder, and the outdoor stroke where it is moving upwards. On the indoor stroke the plug rod needs to close the steam and exhaust valves, and on the rising outdoor stroke, the equilibrium valve.  Thus the first two valves are constructed to be closed by moving their handles down and the equilibrium valve is closed upwards. 
  The following table based on Pole and Winton shows a complete cycle of a Cornish Engine, with the various actions taken by the valves. It also shows the positions of the handles to simplify comparison with videos of such engines. The valves which are open are highlighted.
caption: Valve gear for the 55 inch Engine at Battersea. For clarity only some parts are labelled. All the valves are shown in their closed position, the blue arrows show the direction they would turn to open. SA: Steam Arbor, QA: Equilibrium Arbor, XA: Exhaust Arbor, H: Handles, LQ: Locking Quadrants, PR: Plug Rod, T:Tappets, W: Attachment points for weights.
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