New Holland CX720 Repair Manual page 2466

POWER TRAIN - TRANSMISSION Hydrostatic
If the pressure in the high pressure line exceeds 450 bar (6525 psi) (shock) then the relief valve (8c), will open
allowing oil to flow directly from the high pressure line (11), into the return line (12), through the charge check valve
(9a) (the pump is bypassed in this instance).
When carrying out a hydrostatic stop (rapid movement of the hydrostatic control lever from drive to neutral) the drive
motor (16), will operate as the pump, this will make the pump (4) operate as the motor, possibly causing the engine
to over rev which may damage the engine and the hydrostatic pump (4).
To prevent this if the drive motor return oil pressure exceeds 280 bar (4060 psi) (CX720, 740, 760 and 880) or 180
bar (CX780, 820, 840 and 860) brake flow valves,(15a), and (15b), in the brake manifold will begin to close. The oil
flowing through brake flow valves (15a), and (15b), will convert much of the energy into heat thereby absorbing part
of the brake energy.
The orifice (15f), will allow the brake valves (15a), and (15b), to close fully from a certain point (flow) onwards. Pres-
sure towards the pump will again increase and as displacement decreases, the torque towards the engine will be kept
at maximum.
At 400 bar (5800 psi) (delta p) the pressure limiter valve (15c), will open allowing oil to flow to the hydrostatic pump
forward servo cylinder piston (4a), which will attempt to reduce the braking pressure by keeping the pump at a dis-
placement which results in a pressure of 400 bar (5800 psi) (delta p).
To make sure the pressure acting on the forward servo cylinder does not exceed charge pressure, the 400 bar (5800
psi) pressure flowing through the pressure limiter valve (15c), will open the check valve (14), connecting forward
servo line directly to the charge pressure circuit. In this circuit the pressure is limited by the charge pressure relief
valve (7).
If the pressure exceeds 450 bar (6525 psi) (shock) then the relief valve (15d), will open allowing oil to flow directly
from the return line (12), (high pressure) into line (11).
Reverse
When the hydrostatic lever is moved to reverse the system operates the same as in forward other than high pressure
line (11), becomes the return line and high pressure line (12), becomes the drive line. Also the drive pressure flows
through the check valve (15e), in the brake manifold block (15).
In reverse braking is achieved by the motor acting as a pump and its pressure is controlled by the multi function valve
(8) only and not by the brake manifold.
continued.....
Schematic Circuit for CX780, 820, 840, 860 and 880 machines
TRANSMISSION Hydrostatic - Hydrostatic schema (C.20.F - C.20.B.25)
Gear Changing
When changing gear under certain conditions (i.e. changing gear while stood on a hill side) the pressure in the system
may prevent the selection of a different drive gear.
To assist in the gear selection, when a gear change operation is required the drive motor (16), electric solenoid is
energized moving the pressure release solenoid valve (16c), to connect the charge pressure line through the shuttle
spool valve (16a), to the pressure release spool (16d).
The pressure on the pressure release valve moves the spool (16d), connecting the high pressure line to the return
line relieving the pressure in the system. This will allow the gear selection to take place.
Temperature Control
The hydrostatic system also includes a high temperature control. The motor case drain line temperature sender
monitors the temperature in the return line sending the information to the Control Module.
If the temperature exceeds 95 °C (203 °F), the Control Module CCM1 will automatically reduce the current to the elec-
tric displacement proportional solenoid, reducing the swash plate angle independent of the hydrostatic lever position.
Also an alarm message 0009 will be displayed on the instrument panel display. The temperature in the system will
be reduced by the reduced power in the system.
Schematic Circuit for CX780, 820, 840, 860 and 880 machines
TRANSMISSION Hydrostatic - Hydrostatic schema (C.20.F - C.20.B.25)
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C.20.F / 14