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 CHAPTER IX SETTING THE VALVES Object of Valve-Setting. The movement of the slide valve with the definitions of “lap,” “ lead,” and “travel” of the valve, together with an explanation of the positions of the piston at which the steam is cut off, the exhaust opens, and compression of the steam begins, are explained in The Steam Railway Locomotive and in Steam Engine Valves and Valve Gears. The distribution of the steam, and the location of the essential points of cut-off, release, etc., are settled in the drawing office when the engine is designed, but in the erection of the valve gear small inequalities and differences occur. The object of valve setting is to rectify these differences, more especially with a view to producing as nearly as possible an equal distribution of the steam at the front and the back ends of the cylinders. The rectifications are made by slight alterations to the lengths of the valve spindles or eccentric rods.

It should, however, be pointed out that it is impossible to obtain exactly equal distribution of steam at the two ends of the cylinder, owing to the effect of the angular obliquity of the connecting rod. If the connecting rod moved Rh