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Poppet-Type Four-Way Valves
Figure 6-30 shows atypical four-way, poppet-type directional
control valve. This is a manually operated
valve and consists of a group of conventional
spring-loaded poppets. The poppets are
enclosed in a common housing and are interconnected
by ducts to direct the flow of fluid in
the desired direction.
Figure 6-29.—Three-way, poppet-type, normally closed directional control
valve (pilot-operated).
The poppets are actuated by cams on a camshaft
(fig. 6-30). The camshaft is controlled by
the movement of the handle. The valve may be
operated by manually moving the handle, or, in
some cases, the handle may be connected by mechanical
linkage to a control handle which is located
in a convenient place for the operator some
distance from the valve.
The camshaft may be rotated to any one of
three positions (neutral and two working positions).
In the neutral position the camshaft lobes
are not contacting any of the poppets. This assures
that the poppet springs will hold all four poppets
firmly seated. With all poppets seated, there
is no fluid flow through the valve. This also blocks
the two cylinder ports; so when the valve is
in neutral, the fluid in the actuating unit is trapped.
Relief valves are installed in both working
lines to prevent overpressurization caused by
thermal expansion.
NOTE: In some versions of
this type of valve, the cam lobes
are designed so that the two return/exhaust
poppets are open when the valve is
in the neutral position. This compensates for thermal
expansion, because both working lines are open
to the return/exhaust when the valve is in the
neutral position.
The poppets are arranged so that rotation of the
camshaft will open the proper combination
of
poppets to direct the flow of fluid through the desired
working line to an actuating unit. At the same
time, fluid will be directed from the actuating
unit through the opposite working line, through
the valve, and back to the reservoir (hydraulic)
or exhausted to the atmosphere (pneumatic).
To stop rotation of the camshaft at an
exact position, a stop pin is
secured to the body and extends
through a cutout section of the camshaft flange.
This stop pin prevents overtravel by ensuring
that the camshaft stops rotating at the point
where the cam lobes have moved the poppets
the greatest distance from their seats and where
any further rotation would allow the poppets
to start returning to their seats. O-rings
are spaced at intervals along the length of
the shaft to prevent external leakage around the
ends of the shaft and internal leakage from one
of the valve chambers to another. The camshaft
has two lobes, or raised portions. The shape
of these lobes is such that when the shaft is
placed in the neutral position the lobes will not contact
any of the poppets.
When the handle is moved in either
direction from neutral, the
camshaft is rotated. This rotates
Figure 6-30.—Cutaway view of poppet-type, four-way directional control
valve.
the lobes, which unseat one pressure poppet and one
return/exhaust poppet (fig. 6-31). The valve is
now in the working position. Fluid under pressure,
entering the pressure port, flows through the
vertical fluid passages in both pressure poppets seats.
Since only one pressure poppet, IN (2), is unseated
by the cam lobe, the fluid flows past the open
poppet to the inside of the poppet seat. From there
it flows through the diagonal passages, out one
cylinder port, C2, and to the actuating unit. Return
fluid from the actuating unit enters the other
cylinder port, C1. It then flows through the corresponding
fluid passage, past the unseated return
poppet, OUT (1), through the vertical fluid passages,
and out the return/exhaust port. When the
camshaft is rotated in the opposite direction to
the neutral position, the two poppets seat and the
flow stops. When the camshaft is further rotated
in this direction until the stop pins hits, the
opposite pressure and return poppets are unseated.
This reverses the flow in the working lines,
causing the actuating unit to move in the opposite
direction.
Rotary Spool Valve
Four-way directional control valves of this type
are frequently used as pilot valves to direct flow
to and from other valves (fig. 6-32). Fluid is
directed from one source of supply through the rotary
valve to another directional control valve, where
it positions the valve to direct flow from another
source to one side of an actuating unit. Fluid
from the other end of the main valve flows through
a return line, through the rotary valve to
the return or exhaust port.
The principal parts of a rotary spool directional control
valve are shown in figure 6-22.
Figure 6-31.—Working view of a poppet-type, four-way
directional
control valve.
Figure 6-32.—Sliding spool valve controlled by a rotary spool valve.
Figure 6-33 shows the operation of a rotary spool valve.
Views A and C show the valve in a position to
deliver fluid to another valve, while view B shows
the valve in the neutral position, with all passages
through the valve blocked. Rotary
spool valves can be operated manually, electrically,
or by fluid pressure.
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