New Holland CX720 Repair Manual page 2465

POWER TRAIN - TRANSMISSION Hydrostatic
TRANSMISSION Hydrostatic - Dynamic description (C.20.F -
C.30.A.10)
The CX Combine Hydrostatic System contains a reservoir, Charge Pump, Hydrostatic Pump, Brake Manifold, Drive
Motor and optional powered steering axle.
The hydrostatic pump assembly incorporates the charge pump and brake manifold and is connected to and driven
from the engine gearbox.
Neutral
With the engine running and the hydrostatic lever in neutral the charge pump receives oil from the reservoir (1) through
a 100 micron filter screen (2), into the charge pump (3) The oil is then pumped through filter (5) to the multi function
valves (8) and (9). The charge pressure will open the check valves (8a) and (9a), allowing charge pressure to fill the
hydrostatic pump (4) high pressure lines (11) and (12), brake manifold (15), and drive motor (16). Charge pressure
oil will also fill the servo valve assembly (10), and servo cylinders (4a).
The hydrostatic pump swash plate stands square to the input shaft. With no axial movement of the pistons there is
no flow of high pressure oil in the high pressure line (11).
When the charge pump pressure in the system increases to 34 bar (493 psi) the charge pressure relief valve (7) will
open and oil will flow into the hydrostatic pump casing through the thermostatically controlled oil cooler bypass valve
(17), to the reservoir (1).
If the oil temperature is below 49 °C(120 °F) the oil will flow directly to the reservoir (1) , when the oil temperature
is above 49 °C ( 120 °F) the thermostatically controlled oil cooler bypass valve (17), will move diverting the oil flow
through the oil cooler (18).
Forward
When the hydrostatic lever is moved forward the servo solenoid will move the servo solenoid valve (10a), proportional
to the position of the hydrostatic lever. Charge pressure oil can then flow to the forward side of the servo valve spool
(10c), moving the spool and allowing oil to flow to the forward servo cylinder (4a).
The charge pressure oil acting on the forward servo cylinder (4a), will move the hydrostatic pump swash plate creating
axial movement of the pistons increasing the pressure in the forward high pressure line (11), towards the drive motor
(16).
The increased pressure in the forward high pressure line (11), will create rotational movement of the drive motor
pistons, causing the drive motor output shaft to turn in the forward direction. The oil will return by the return line (12),
through the brake manifold and to the hydrostatic pump (4).
Also the increase in pressure will close the forward multi function valve charge check valve (8a), preventing the charge
pressure circuit receiving drive pressure from the hydrostatic pump (4).
The charge pump (3) will continue to charge the reverse high pressure line (12), through the reverse multi function
valve charge check valve (9a), to replace the oil used for lubrication and flushing and any oil lost through internal
leakage.
The return oil from the drive motor (16), is passed through the flow valves of the brake manifold and back to the pump.
If there is more than a 13 bar (188.5 psi) pressure difference across the drive motor (16), the drive pressure from
the hydrostatic pump (4) moves shuttle spool valve (16a), in the drive motor assembly (16), connecting the return line
(12), to the flushing relief valve (16b).
The flushing relief valve (16b), will open at a lower pressure than the charge pressure relief valve (7) causing some of
the charge pressure oil to flow through the flushing relief valve (16b), to the drive motor case drain. The oil will then
flow to the oil cooler through the pump case drain, cooling the motor and pump bodies.
continued.....
Schematic Circuit for CX780, 820, 840, 860 and 880 machines
TRANSMISSION Hydrostatic - Hydrostatic schema (C.20.F - C.20.B.25)
If the resistance on the drive motor (16), increases (machine travelling up hill) then the pressure in the forward high
pressure line (11), will increase. As the pressure reaches 420 bar (6090 psi) (delta p) the pressure limiter valve (8b),
will open allowing oil to flow to the hydrostatic pump reverse servo cylinder piston (4a), reducing the swash plate angle
and pump flow, protecting the hydrostatic system from overpressure.
To make sure the pressure acting on the reverse servo cylinder piston (4a), does not exceed charge pump pressure,
as the oil flows through the pressure limiter valve (8b), the pressure (420 bar [6090 psi]) will open the check valve (13),
and the charge check valve (9a), allowing the oil to flow into the charge pressure circuit. In this circuit the pressure
is limited by the charge pressure relief valve (7).
2 4/7/2006
C.20.F / 13