New Holland CX720 Repair Manual page 2479

Control valve - Static description (C.20.F.13 - C.30.A.20)
System in Neutral
Charge pump oil flows to both cavities (20), from port (21), oil also flows from port (21), through the check valve orifice
(22), to the central land of servo spool valve (28).
Charge pressure oil flows equally from cavity (21), through the orifices (A) and (B), to cavity (23), to the case drain.
As the electric solenoid (24), is not energized oil flow from cavities (20), into cavity (23), is the same, so the pressure
in cavities (25) and (27), will be equal and the spool assembly will remain in neutral.
Servo Solenoid Valve 10a Energized
Anchor (29), of the servo solenoid valve (10a) moves to the left when the solenoid is energized. The movement of
anchor (29), is proportional to the current through the solenoid dependant on the position of the hydrostatic lever.
Oil flow from the left hand cavity (20), to case drain cavity (23), will decrease and the flow from the right hand cavity
(20), to cavity (23), will increase. The pressure in cavity (25), on the left side of the servo spool (26), is higher than the
pressure in cavity (27), on the right hand side of the spool (28), causing the spools (26) and (28), to move to the right.
The charge pump oil can now flow from the center of spool (28), through the check valve orifice (22), to the hydrostatic
pump servo cylinder through orifice (30), and port (31), causing the pump swash plate to move.
As the oil flow increases to the servo cylinder through port (31), the displaced oil on the opposite servo cylinder will
flow back through port (32), and orifice (33), past the spool (28), through to the case drain through orifice (34), and
port (35).
NOTE: When the opposite direction of travel is selected spools (26), and (28), will move to the left. The feed pressure
to the servo cylinder will be through orifice (33), and port (32). Displaced oil from the opposite cylinder will flow to
case drain through port (36), and orifice (37).
As the swash plate moves the mechanical feed back linkage will move to the right compressing its internal spring
when the spring force is equal to the pressure in cavity (25), spool (28), will move back to neutral holding the swash
plate in its new position.
Any movement of the swash plate or hydrostatic control lever will affect the balance of spring force and cavity pressure.
The spool (28), will then move to a position where the balance is restored
If a shock load occurs in the system it will be felt by the pump swash plate and will create an increase in pressure in
the servo cylinder lines and servo valve. To protect the servo valve from the increased pressure in the system the
check valve (10c) will open connecting the servo cylinder lined directly to the charge pressure circuit. In this circuit
the pressure is limited by the charge pressure relief valve.
10.
10a.
10b.
10c.
20.
21.
22.
23.
24.
25.
26.
27.
POWER TRAIN - TRANSMISSION Hydrostatic
Servo Valve Assembly
Servo Solenoid Valve
Orifice Check Valve
Servo Valve Spool
Oil Cavity
Oil Feed Port
Check Valve Orifice
Case Drain Cavity
Electric Proportional
Solenoid
Spool Cavity
Spool
Spool Cavity
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
2 4/7/2006
C.20.F / 27
Spool
Anchor
Servo Cylinder Port
Orifice
Servo Cylinder Port
Orifice
Orifice
Case Drain Port
Case Drain Port
Orifice
Swash Plate Assembly
(Feed Back)