. Locomotive text for engineers and firemen; a complete treatise on the engine, electric head-light and standard code of train rules . PLATE Cone American Ba/ancera/ve. The hole D is drilled through the top of the valveto allow any steam passing by the strips or rings toescape to the exhaust, without destroying the balancefeature of the valve. The Allen Ported Valve. With the plain slide-valve it has been found difficult in short cut-offs to getthe full steam-chest pressure at the beginning of thestroke without excessive lead, and to overcome thisdifficulty the Allen ported valve (Pl
. Locomotive text for engineers and firemen; a complete treatise on the engine, electric head-light and standard code of train rules . PLATE Cone American Ba/ancera/ve. The hole D is drilled through the top of the valveto allow any steam passing by the strips or rings toescape to the exhaust, without destroying the balancefeature of the valve. The Allen Ported Valve. With the plain slide-valve it has been found difficult in short cut-offs to getthe full steam-chest pressure at the beginning of thestroke without excessive lead, and to overcome thisdifficulty the Allen ported valve (Plate 57) was PLATE 57. This valve has a supplementary port AA supplyingsteam to one steam port from both sides of the valve atthe same time, thus giving double port opening forthe same travel of the valve, and an increase of steampressure in the cylinder. This valve has given excellent i8o VALVE MOTION. results, as it admits a good supply of steam at a highinitial cylinder pressure with short valve travel, and it isespecially adapted to high speed engines working inshort cut-offs. This valve is of the D type. PISTON VALVE. The piston valve derives its name from its con-struction. It consists of two pistons PP, connected bya stem which is usually hollow (Plate 58). The pis-tons have two or more packing rings which formthe exhaust and steam edges of the valve. Packing ringsI and 2 form the steam edges and rings 3 and 4 the ex-haust edges.
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