9
c.
“
T
”
Single Acting Spool
-
“
T* spools direct flow
to one end of an operating cylinder only as in the example
of the lift mechanism on a fork-type truck. Return flow is
from the same end of operating cylinder and relies on
gravity or mechanical means. See Figure 6 for spool
position versus flow.
“
T
”
Single Acting Spool
B
A
P
T
MAN
MAN
4.
Flow Control and Relief Valve
General
- The partial flow by-pass system in the
CM2 and CM3 series valves makes use of a compound type
flow control and relief valve arrangement. By sensing the
pressure drop across an orifice at the entrance to the by-
pass, the valve acts as a flow control to limit flow through the
by-pass.
When a spool is completely shifted, the flow control
is inoperative and full pump volume is available to the
system. The control valve then functions as an overload
relief valve. System pressure is limited to a prescribed
maximum by the action of this valve.
a.
Flow Control
- Figure 7a shows the flow control
valve operation with the spool in neutral. Flow across the
by-pass orifice results in a pressure drop. The decreased
pressure is sensed at the spring end of the valve sub-assem-
bly through a sensing orifice. The slightly higher pressure at
the other end of the valve permits it to shift down, diverting
excess flow to the tank passage. With less than rated flow,
shown on the installation drawing, there would be insufficient
pressure drop across the by-pass orifice and the flow control
valve would return to the closed position. Since the control
valve is held closed by the large spring and all flow would be
through the by-pass passage.
b.
Relief Valve
- Operation of the relief valve feature
is shown in Figure 7b. Here an operating spool would be
shifted, porting fluid to the system and blocking the by-pass.
Figure 7b shows operation at less than the relief
valve setting. There is no flow over the by-pass orifice, so full
system pressure is sensed at the spring end of the control
valve, as well as the opposite end. The valve is thus hydrau-
lically balanced and the large spring holds it closed.
Figure 6.
Neutral
Out
Pressure
Tank Return Flow
A
“
T
”
spool in
“
OUT
”
position.
Pressure to
“
B
”
port.
“
A
”
port
blocked.
By-pass
blocked.
In
“
D
”
Spool
B
A
B
A
B
“
T
”
spool in
“
neutral
”
position.
“
B
“
port
blocked.
“
A
”
port blocked.
By-pass open.
“
T
”
spool in
“
IN
”
position.
“
B
”
port to tank.
“
A
”
port
blocked.
By-pass
blocked.
Tank
port
By-
pass
carry
over
port
Pressure port
A
B
Maximum pressure is determined by the setting of
the small spring inside the control valve assembly. When
system pressure is high enough to overcome this small spring,
the poppet is forced off its seat. (See Figure 7C). Fluid
immediately flows past the poppet to the tank passage. This
flow created a pressure drop across the sensing orifice and
the control valve is no longer hydraulically balanced. When
this pressure differential is great enough to overcome the large
spring, the valve shifts permitting flow to the tank passage.
5.
Check Valve
- Timing of the valve spools is such
that the cylinder port opens to pressure and tank before the
by-pass passage is completely blocked. To prevent return
flow from passing into the pressure passage, check valves
are provided in each operating section except the
“
B
”
section. The load is thus preventing from dropping.
6.
Detent
- The spool detent consists of a special end
cap with a spring loaded plunger. The plunger engages in a
groove in the spool extension and holds the spool in the
desired position. Detent parts are illustrated in the exploded
view in Figure 9a.
B
Figure 7.
Neutral
In
Pressure Drop
By-Pass Flow Control
Orifice Determines
Flow Rate
In
A
A
B
Control Valve
Sensing Orifice
Spring holds Relief Valve
Closed when Pressure is
equal on both ends.
Tank port
By-pass carry
over port
Pressure port
C
System Pressure
sensed in Spring Cham-
ber through Orifice
Pressure in Spring Chamber is lower
than System due to Pressure Drop
accross Sensing Orifice
Poppet forced
off Seat as
Relief Valve
setting is
reached.
Tank Return Flow
Pressure
A
B
A
B
Flow Control
and Relief Valve
Tank port
By-pass carry
over port
Pressure port
Pressure port
Tank port
By-pass carry
over port
