Centaur 2001 950, Service Manual

Page: 426 / 450

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Axial Piston Pumps, Motors, and Transmissions Series 70 / 15 Series
Description of Operation for Series 70 and 15 Series Products (Continued)
Bypass Valve Circuit
BYPASS
VALVE
MF
OUTPUT
Charge Pump
Axial piston pumps and motors use a small amount of
fluid for internal lubrication. This results in fluid being
lost from the closed circuit that must be replenished.
A fixed displacement gerotor pump is used to replen-
ish the lost oil. This gerotor pump, called a charge
pump, is driven by the prime mover through the piston
pump drive shaft.
Since the piston pump and piston motor are con-
nected in a closed circuit, either side of the hydro-
static loop may be pressurized. To allow charge oil to
enter the closed circuit, two check valves are used to
direct charge flow to the side of the loop with the
lowest pressure.
The pressure in the charge pump circuit is limited by
a direct operating relief valve. Any fluid not used as
replenishing oil is discharged over this valve, either
into the transmission case or recirculated back to the
charge pump inlet. Flow across a small fixed orifice
connecting the charge circuit with the transmission
housing, supplements the cooling flow in the Series
70 transmissions.
Inlet Filter
It is imperative that only clean fluid enters the hydro-
static transmission circuit, therefore a 20 micron
(nominal rating) inlet filter is required in the charge
pump inlet line. This filter should not have a bypass
and should be changed regularly to ensure system
reliability.
Bypass Valve
In some applications, it is desirable to move the
machine for short distances at low speeds without
starting the engine. A bypass valve allows oil to be
routed from one side of the pump/motor circuit to the
other, thus allowing the motor to turn. The bypass
valve must be fully closed during normal vehicle
operation.
Series 70 BDU-10L and 21L transmissions utilize a
spool type bypass valve. A spring closes this valve on
the 10L transmission, while charge pressure closes
the valve on the 21L transmission. The BDP-10L
pump utilizes a screw type bypass valve.
15 Series PV, PT, and in-line units utilize a screw type
bypass valve which is fully open at 1/2 revolution of
the valve stem. 15 Series U transmissions utilize the
charge check valves for the bypass function. External
plungers are depressed to hold the charge check
balls off of their seats, allowing oil to bypass from one
side of the pump/motor circuit to the other.
Charge Circuit – Series 70-BD
TO
CASE
DRAIN
INPUT
CHARGE
PUMP
CHARGE
RELIEF
PV PF
INLET
FILTER
RESERVOIR
COOLING ORIFICE
CHARGE
CHECK
VALVES
Charge Circuit – 15 Series
TO
CASE
DRAIN
INPUT
CHARGE
RELIEF
CHARGE
PUMP
PV PF
INLET
FILTER
RESERVOIR
CHARGE
CHECK
VALVES