the regulator ball and provides the pump with an
unrestricted fluid flow back to the reservoir.
In figure 10-19 the regulator ball is unseated. When
this happens, pressure drops immediately. Now the
importance of the check valve can be seen. With the
sudden reduction in pressure, the check valve snaps
shut; and the fluid trapped in the system line continues
to hold the regulator piston in the raised position. This
trapped fluid also maintains pressure on the system until
the mechanism actuates or is relieved by leakage, either
of which can cause the regulator to cut-in.
Hydraulic systems using a constant volume pump
require a pressure regulator valve; those using a variable
volume pump do not.
The purpose of a selector valve is to control
direction of fluid flow; this, in turn, controls
operation or direction of the mechanism. Although all
selector valves share the common purpose of controlling
the direction of fluid flow, they vary considerably in
physical characteristics and operation.
The valving element of these units may be one of
three types: the poppet type, in which a piston or ball
moves on and off a seat; the rotary sped type, in which
the spool rotates about its axis; or the sliding spool type,
in which the spool slides axially in a bore. Selector
valves may be actuated mechanically, manually,
electrically, hydraulically, or pneumatically.
POPPET VALVE. Figure 10-20 illustrates the
operation of a simple poppet valve. The valve consists
primarily of a movable poppet that closes against a valve
seat. In the closed position, fluid pressure on the inlet
side tends to hold the valve tightly closed. A small
amount of movement from a force applied to the top of
the poppet stem opens the poppet and allows fluid to
flow through the valve.
The poppet, usually made of steel, fits into the center
bore of the seat. The seating surfaces of the poppet and
the seat are lapped or closely machined, so the center
bore will be sealed when the poppet is seated. The action
of the poppet is similar to the valves of an automobile
engine. An O-ring seal is usually installed on the stem
of the poppet to prevent leakage past this portion of the
housing. In most valves the poppet is held in the seated
position by a spring. The number of poppets in a
particular valve depends upon the design and purpose
of the valve.
ROTARY SPOOL VALVE. The rotary spool type
of directional control valve has a round core with one or
more passages or recesses in it. The core is mounted
within a stationary sleeve (fig. 10-21). As the core is
rotated (generally by a hand lever or a knob) within the
stationary sleeve, the passages or recesses connect or
block the ports in the sleeve. The ports in the sleeve are
connected to the appropriate pressure, working and
return lines of the fluid power system.
SLIDING SPOOL VALVE. The sliding spool
valve is probably the most common type of valving
element used in directional control valves. The
operation of a simple sliding spool directional control
valve is illustrated in figure 10-22. The valve is so
named because the shape of the valving element
resembles that of a spool and because the valving
element slides back and forth to block and uncover ports
in the housing.
The valve is shown in neutral position (no fluid
flow); but by moving the spool valve to the left position,
Figure 10-20.-The basic operation of a simple poppet valve.