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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.
6-21
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