UNDERCARRIAGE RT880E SERVICE MANUAL
8-14 Published 11-10-2014, Control # 524-00
BRAKE SYSTEM
Description
The brake system includes all the components necessary for
the application of the service brakes and the parking brake.
Service Brakes
The service brakes are full power hydraulic disc brakes
which are hydraulically controlled and are used to apply the
brake assemblies on all four wheels. The system consists of
the tandem brake valve with treadle pedal, the dual
accumulator charge valve, two hydraulic accumulators, the
brake assemblies, and all the associated hoses and tubing.
The operator depresses the pedal on the tandem brake
valve, located on the cab floor, and the valve modulates the
brake line pressure to the brake assemblies at each wheel.
The full powered brake system supplies a high brake system
pressure with relatively low reactive pedal forces, while
controlling the maximum brake line pressure. The service
brake dual accumulator charge valve regulates flow to the
hydraulic accumulators to provide fully powered
independently separate, primary (front) and secondary
(rear), service brake circuits. Hydraulic pressure is
constantly maintained in the brake circuits by the
accumulators and the charging valve.
Parking Brake
The parking brake is a hydraulic release, spring apply, disc-
type brake, located on the front axle. The system consists of
a two-position switch, a three-way solenoid valve, actuator, a
brake assembly, and all the associated hardware and tubing.
The selector switch, located on the front console in the cab,
is used to activate the solenoid valve which controls the park
brake actuator, which applies and releases the park brake.
There is one brake actuator on the front axle.
NOTE: For description and maintenance of the tandem
brake valve with treadle pedal, the accumulators,
and the dual accumulator charge valve, refer to
Hydraulic System, page 2-1.
Theory of Operation
Service Brakes
Braking begins when the operator depresses the brake
pedal in the cab. Mechanical linkage transfers the force
created by the lever action of the brake pedal to the hydraulic
brake valve which modulates the brake line pressure to the
brake assemblies at each wheel.
Hydraulic oil from Pump No. 1 flows to swivel port 7 to the
dual accumulator charge valve. The dual accumulator
charge valve charges the accumulators from the open center
circuit upon demand and within its present operating charge
rate and the high limit pressure setting. However, when the
open center circuit pressure reaches the brake relief setting,
which is higher than the high accumulator charge limit, then
the accumulators will be charged to the regulated maximum
pressure setting. The dual accumulator charge valve
regulates flow to the hydraulic accumulators to provide fully
powered independently separate, primary (front) and
secondary (rear), service brake circuits. Hydraulic pressure
is constantly maintained in the brake circuits by the
accumulators and the charging valve. The charged
accumulators supply pressurized fluid to the closed tandem
brake valve.
Once the operator depresses the brake pedal, the tandem
brake valve modulates fluid out to the brakes to provide the
means of braking. The tandem brake valve will modulate the
pressure in the brake system by increasing or decreasing
pressure as required in proportion to the input force from the
operator via the brake pedal. The hydraulic force acts within
the brake assemblies to force the brake pads against the
brake discs, acting to slow wheel rotation. Fully powered
separate primary (front) and secondary (rear) braking circuits
are provided with independent accumulators. A low pressure
warning switch is used to sense the accumulator pressures
and warn the operator through visual brake warning indicator
light on the cab console in the event the pressure in the
accumulators drops to an unsafe operating level. In the
event of engine failure, the accumulators are pre-charged
with dry nitrogen gas and properly sized to provide power-off
stopping capacity for secondary braking.
Parking Brake
Hydraulic flow from the transmission charge pump is routed
to the parking brake control valve. When the Park Brake
Switch is in the ON position, the parking brake solenoid valve
shifts to route flow from the hydraulic parking brake actuator
back to the transmission sump. The actuator spring pulls on
the lever on the brake assembly, applying the parking brake.
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