In the preceding work, the completed plates were used to assist the student in developing the layout drawings for other parts of the pump. In this Section, Plates K and L„ being given in full detail, offer a good start for the development of the water cylinder, which is the purpose of Plate I. As before, work should begin at the inside and progress outwards. Thus the piston rod with its nut should be drawn first, the hub of the plunger built around it, then the plunger barrel, the bushing, and ring to clamp the bushing. The limits of the plunger travel should be sketched in, and the valve outline shown, in order to determine clearances. The progress of Plate I is on exactly the same basis as that stated in detail for the steam cylinder layout; hence it need not be repeated.
The points controlling the design of the water end must, however, be studied to enable the student to work intelligently. The fit of the rod into the plunger hub is loose, 1/16 -inch play being allowed, in order to permit the plunger to be guided solely by its bushing, and thus be independent of any change of alignment of the piston rod.
The relative length of plunger and bushing should allow the end of the plunger to overrun the edge of the bushing at the termination of the stroke, to prevent the formation of a shoulder. The bushing is made of brass because of the better bearing of the two dissimilar metals, brass, and iron. Of course there is no lubrication except the water, and the dissimilar metals tend to "cut" less than if both were alike. The brass bushing also prevents the plunger from "rusting in" in case of long periods of disuse. The bushing being of expensive material is made as light as possible, hence it has no stiffness of its own. Therefore, it is reinforced by a deep cast-iron ring, which also takes the bolts and clamps the bushing tightly to its ground seat. These stud bolts are usually made of "tobin bronze." a rust-proof material, possessing strength almost as great as that of steel. This arrangement permits ready removal of the bushing when necessary.
As the parts of the common pump valve illustrated in detail on Plate L must be often replaced during service of the pump, provision must be made for unscrewing the stem and substituting a new one. This must be done through the hand holes provided on the cylinder. The lower valve deck must be located so that the inner valves when unscrewed will not strike the clamp ring. As shown in Plate I, the clearance is pretty small, almost too small, but as it affects only two valves, it will probably cause no inconvenience. No hand holes are necessary for the end chambers, as access to the valves is had by removing the outer heads.
The upper deck may be placed at a height giving sufficient clearance to allow the upper nuts of the clamp ring to be unscrewed with a socket wrench from the end of the pump. These decks are subjected to a severe pounding from the pulsations of the pump, and should be amply strong; 1¾ inches is deemed thick enough for this case.
The middle transverse wall may be 1½ inches thick and the middle longitudinal wall a little thinner, about 1¼ inches. With high pressures these walls, being flat surfaces and the valve decks likewise, are likely to fracture under the heavy pounding. To avoid making them excessively heavy they are often strongly ribbed, either on the inside or outside, usually the former.
The curving side walls are of better form to withstand pressure, and need not be as thick, 1 inch being sufficient. This can be decreased to ¾ inch in the suction passage below the deck, where little pressure exists.
The outer head is also considered strong enough at 1 inch thickness, on account of its curved shape. It requires 7/8-inch studs. Studs are preferred to tap bolts in this case, as in all other similar cases, on account of the frequent unscrewing of the nuts for purpose of removal. One or two unscrewings of a tap bolt in cast iron will destroy the tightness of the thread, while the stud, being steel, stands the wear better.
The valve seats are. taper screwed into the deck; they are sometimes forced in on a plain taper fit. They are located as closely as strength of the deck between the holes will permit. It is not well to place the edge of the valve closer than ½ inch from the cylinder walls. The valve holes in the lower deck should be in line, or nearly so, with the holes in the upper deck, in order to allow the shank of the mill to pass through when milling the lower holes.
The suction opening is 7 inches in diameter, 12½-inch flange, 10½-inch bolt circle, ¾-inch tapped holes.
By means of the hand hole at the end of the suction passage, any dirt which may have been brought in through the suction pipe may be removed.
With the foregoing discussion the student should be able to produce Plate I, which is the preliminary step to the detail drawing of the water cylinder as shown on Plate J.