iz PUMPS 509 
Next consider a double-acting piston pump. Here the delivery from 
one side of the piston takes place at the same time as the suction on the 
other side, and the variation in the rate of delivery through the delivery 
valves for one revolution of the crank is as shown at (4), Fig. 817, on a 
time base. 
The student should have no difficulty in constructing the rate of 
Fia, 817. 
delivery diagrams for cases where there are two or more plungers or 
pistons driven from the same crank shaft through cranks making known 
angles with one another, there being one delivery pipe for all. 
The resultant rate of delivery curve for a three-plunger pump is 
_ shown at (c), Fig. 817. The plungers are supposed to be all of the same 
_ size, and to be driven through cranks A, B, and C, which make angles of 
120° with one another. 
At (d), Fig. 817, is shown the resultant rate of delivery curve for a 
double-acting piston pump having two pistons driven through two cranks 
_ Dand E, which are at right angles to one another. The displacements of 
the pistons per stroke are assumed to be equal. 
441. Direct Driven Steam Pumps.—The shocks and irregularity in 
delivery which are almost inseparable from crank-driven pumps are to a 
large extent obviated in pumps in which a water piston is driven direct 
from a steam piston. In the latter type of pump the steam and water 
pistons are at opposite ends of the same piston-rod, and there is no fly- 
wheel and no crank shaft. The motion of the pistons being controlled 
mainly by the steam and water pressures, the pistons can more readily 
follow the moving water, the velocities of the pistons and water adapting 
themselves to one another without shock. 
In a pump the head of water in the delivery pipe is usually constant, 
and therefore when the water is in motion with fairly uniform velocity 
the resistance is fairly uniform. Hence the pressure of the steam on the 
steam piston must not vary to any great extent, unless means are adopted 
to store up energy during one part of the stroke and restore it during 
another. A fly-wheel will do this effectively, permitting the steam to be 
