_a._ Cushion-release valves regulating the amount of cushion steam
retained at ends of stroke must be provided.
_b._ The cushion release must be through an independent port as shown
in Figs. 2 and 3, so located as to positively retain a certain amount
of cushion steam.
The old form of cushion release through bridge between ports is
not acceptable. This form while leading into the exhaust passage
as formerly, differs by starting from a small independent port
(about 1/2-inch wide × 2-1/2 inches long) through the cylinder wall,
located about 3/8 or 1/2 inch back from the cylinder head. (The
exact position for affording the best action has to be determined by
experiment with each different make of pump, as it depends somewhat
on the extent of clearance space and on the point of closure
of exhaust by piston and somewhat on the weight of reciprocating
parts.)
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
This style of cushion port makes the pump safer in case cushion valves
are unskillfully left open too wide and tends to prevent a pump from
pounding itself to pieces in case of a sudden release of load, as by a
break in suction or delivery mains, or by a temporary admission of air
to suction pipe.
[Illustration: FIG. 4.]
Pumps made with this form of cushion release, have given very
satisfactory results, and if the ports are properly located, there will
be no re-bound of piston.
_c._ Cushion valves must be always provided with hand-wheels marked as
per sketch, for the reason that very few men in charge of fire-pumps
are found to clearly understand or to remember their use.
The lettering must be very open, clear and distinct, not liable to be
obscured by grease and dirt, and of a permanent character.
It is desirable that spindle or wheel be so formed that a monkey
wrench can get a grip to open a jammed valve. Fig. 5 shows the stem
flattened for this purpose.
[Illustration: FIG. 5.]
_d._ The valve and stem of cushion valve must be in one piece without
any swivel joint.
Swivel joints are apt to come apart and make it impossible to operate
the valve.