LOCOMOTIVE ENGINES.
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example, and in both examples the valves are frictionless in theact of lifting, which is the main thing to be studied in safety-valvearrangements. To carry this more fully into effect the ball safetyvalve has been designed, fittedwith a lever and spring balance.
All the bearings are of the ball-and-socket description; thus free-dom of action is secured, whilethe tendency to corrode or stickis rendered nil. The ball is gen-erally cast in gun metal, and theseating of brass. This valve hasthe advantage that the ball maybe turned into any position tosuit the wear.
Various forms of direct spring-loaded valves have been success-fully adopted; one form consist-ing of a large plate 12 inches indiameter, loaded with a number of volute springs,formly around the centre of the valve, the bearing surface of thesprings resting in cups on the valve,while the large flat surface at thetop bearing on the covering platerenders the action quite true, andas there are no spindles connectedto the outside springs there is noliability to jam. So large a valveis not really required for the loco-motive boiler; but the principlecan be carried out with one smallvalve having a central volutespring.
Although there may be a differ-ence of opinion as to the propor-tion which the working pressureof a boiler should bear to its ab-solute strength, it will doubtless be admitted that the greater the cer-tainty of a safety valve opening at the required pressure, and thegreater its ränge of lift under a given excess of pressure, the less will
Fig. 423.—Ball Safety Valvewith Lever and Balance.a, Ball valve. b, Seat for valve. c, Lever.d, Stud. e, Salter spring balance.
arranged uni-