520

MODERN STEAM PRACTICE.

with a strengthening rib between the boss and the outside ring.The pins for the eccentric rods are the same diameter as the slide-valve rod. The rods are forked at the link end, with a T end forbolting to the gun-metal eccentric strap; they are made quite flat,the area at the smallest end equals the area of the slide-valve rod,and for every foot in length add about inch to the breadth ofthe rod. The bolts for the T ends are the same diameter as thosefor the strap. The slotted link is generally adopted; the distancebetween the centres of the eccentric rods on the link equals threetimes the throw of the eccentric. The holes for receiving the pinsare fitted with gun-metal bushes cast hard, and adjusted with setscrews, screwed into the lugs on the link. The sectional area of theslide-block pin is found by multiplying the slide-valve rod area byi‘ 3 - The pin is conical, and fitted with a tapered ferrule of Steel,which is split, having set screws and nuts on pin for adjusting thewear. The slide block is of brass, with a loose side flange securedwith screws; the length of the block equals twice the diameter ofthe pin. The width of the link should be about one-sixth morethan the diameter of the slide rod; the sectional area equals thatof the slide-block pin. The sliding block or crosshead to which thepin is secured is generally a flat bar, moving in brass guides, thecrosshead having a lug forged on for taking the slide-valve rod.The point of Suspension on the link is midway between the eccentricrod pins, on the radius line of the link, a cross bar and pin beingrivetted to the link at that point The Suspension rods are foundby dividing the valve-rod area by r6, which gives the area of thepin in square inches; their diameter at ends equals the diameterof the pin. The Suspension rods have brass bushes fitted in theeyes.

Power required to move the slide valve. — The power requiredto move the slide valve depends on the size of the valve and amountof pressure on the back of it, to which is added the weight of thevalve in lbs. One-sixth of the total may be considered the forcein lbs. that is required to move the valve when under full pressure(i.e. steam and vacuum). In practice, for an area of 800 squareinches of valve surface on the back of valve, and when provision ismade with rings on the back to relieve the valve from the steampressure, a power of 7 to i is allowed; with a steam pressure from20 to 30 lbs. per square inch, supposing the valve’s area was 4000square inches, more of course would be needed, the area being five