Terex TR60 Maintenance Manual page 434

Braking System - Treadle Valve
Section 250-0070
TO FRONT
BRAKES
P x
T
SUPPLY
P 1
FROM
BRAKE
MANIFOLD
B 1
13
Fig. 2 - Normal Service Brake Application
DESCRIPTION
Numbers in parentheses refer to Fig. 1.
Note: The treadle valve can be identified as item 4 in
Section 250-0000, BRAKING SYSTEM SCHEMATIC.
The treadle valve is a closed centre controller which
controls the level of hydraulic oil pressure applied to
the front and rear brakes and the maximum pressure
available to these circuits. It is operated by a foot
pedal in the operators cab and with the engine
running, is automatically applied by the brake manifold
valve. Refer to Section 250-0050, BRAKE MANIFOLD
VALVE.
The differing maximum operating pressures required
for the front and rear brakes are achieved within the
treadle valve. These pressures can be adjusted as
described under 'Testing and Adjustments'.
In the normal position, brake pedal (50) and regulator
spools (8) are in the up position. In this condition the
notched area near the top of regulator spools (8) is
exposed to the tank cavity in valve body (20), the
lower portion of the notched area is exposed to the
regulated pressure outlet ports of the valve. This
effects a direct link between the tank port 'T' and the
regulated outlet ports 'B1' and 'B2'. While regulator
spools (8) are in this position the brakes are released.
Stop light pressure switch (55) sends a signal to
illuminate the brake lights at the rear of the vehicle on
2
SM - 1656
4
TO REAR
BRAKES
SUPPLY
P 2
FROM
BRAKE
B 2
MANIFOLD
17
SUPPLY
FROM
BRAKE
MANIFOLD
actuation of the treadle valve.
Lockup drop out pressure switch (54) sends a signal
to actuate the lockup drop out solenoid, which in turn
drops the transmission out of lockup on actuation of
the treadle valve.
OPERATION
Numbers in parentheses refer to Fig. 1.
Normal Service Brake Application
Refer to Fig. 2. When the operator depresses pedal
(50), actuator plungers (2) are moved down and push
on regulator springs (5 & 6) which in turn, push
regulator spools (8) down. As regulator spools (8)
move down, the metering notches move out of the
tank cavity and close outlet ports 'B1' and 'B2' off to
tank port 'T'. Regulator spools (8) continue to move
downward until the lower edge of the metering notches
become exposed to the inlet pressure ports. This
movement opens inlet ports 'P1' and 'P2' to outlet
ports 'B1' and 'B2' respectively.
As hydraulic pressure builds in the brake, oil flows
through the small orifice in the side of regulator spools
(8) and into the cavities below the spools. As pressure
rises in the brake it also rises in the cavities below the
spools forcing regulator spools (8) upwards closing
outlet ports 'B1' and 'B2'. Regulator spools (8) are now
balanced between the brake pressure and the
pressure of regulator springs (5 & 6) generated by the
FROM
FRONT
BRAKES
P x
T
P 1
B 1
13
Fig. 3 - Normal Service Brake Release
SM - 1657
4
FROM
REAR
BRAKES
P 2
B 2
17
SUPPLY
FROM
BRAKE
MANIFOLD
SM 1225 Rev 1 9-99