98

MODERN STEAM PRACTICE.

relieved of much of the steam-pressure. A small pipe is introducedthrough the valve-casing cover, in connection with the eduction-pipeon the cylinder, thus any slight leakage of steam between the ringin the recess and the valve is taken over into the condenser whenso fitted.

Other engineers have constructed the valve as a hollow frame,having merely sides; the back in this arrangement being fittedwith a narrow piston-ring edge, having springs to keep the valveto the face on the cylinder, and also to press the piston-ring againstthe back of the valve-casing or the cover. This is certainly arefinement, and so long as the rubbing surfaces remain steam-tight,the plan is to be commended, as it is impossible such a valve, un'derany circumstances, can have any more back-pressure than merelythe rubbing surface that is not covered by the piston-ring; but inthe event of leakage between the rubbing surfaces, the plan is not atall to be desired, as the vacuum would be impaired, and great wasteof steam occur. Therefore, when it can be conveniently applied,the plain ring System appears best, screwing the ring- against theback of the valve, as such a plan can be adjusted at any time with-out breaking a joint, the set bolts being simply screwed into tappedholes in the valve-casing cover; it must be borne in mind, however,that the valve-gear must be proportioned to meet the full pressureon the valve, as at times the best arrangements will get out ofOrder. When cast-iron surfaces are adopted, one-sixth of the totalpressure on the valve may be taken for calculating the strength ofthe valve-rod and adjuncts, that is to say, if the faces are in goodworking order, and the lubrication properly attended to.

The position of the valve next Claims attention. In all caseswhere practicable, it should be placed on its edge, so as to drainor run off moisture or water that may collect when the engine is notworking, a small valve being fitted to the casing for running it off;thus the faces are kept as dry as possible, preventing the corrosionthat would otherwise set in.

The reciprocating motion imparted to the valve is usually obtainedby means of a simple eccentric, although cam arrangements at timesfind favour. The eccentric wheel or sheave has both a rotatory andreciprocating motion, its action is somewhat the same as a pin re-volving around a fixed centre, such as the main crank-pin of thesteam-engine; in fact, a plain crank and pin is offen used instead ofan eccentric; but when the line of eccentric or valve-rod cuts across