AP-800 ASPHALT PAVER Hydraulic Systems Caterpillar

Hydraulic Systems
1.1. General
2.1. Ground Drive And Conveyor Drive Hydrostatic Transmissions
3.2. Charge Pump Circuit
4.2. Main Pump and Motor Circuit
5.2. Cooling Circuit
6.2. Displacement Control
7.1. Vibrator Drive Hydraulic System
8.1. Auxiliary Hydraulic System


Hydraulic oil for all the hydraulic systems is stored in the 30-gallon (113.5-liter) tank on the left side of the machine. A sight gauge on the tank allows quick and accurate checking of the hydraulic oil level.

The hydraulic oil flowing from the hydraulic oil tank to the ground drive pump flows through a 10-micron (nominal) filter before reaching the pump. Suction oil for the vibrator drive pump passes through a strainer. The hydraulic oil for the conveyor drive pumps is filtered after it passes through the charge pump by a 10-micron (nominal) filter. The auxiliary system hydraulic oil is filtered when it leaves the auxiliary pump by a 5-micron (nominal) filter. All hydraulic oil returning to the tank flows through a 10-micron (nominal) filter.

The hydraulic oil cooler mounted on the engine radiator cools part of the hydraulic oil returning to the oil tank. Cooling air drawn through the radiator by the engine fan passes through the oil cooler first. Any obstructions to the air flow will cause overheating of both the hydraulic oil and the engine.

Ground Drive And Conveyor Drive Hydrostatic Transmissions

The machine ground drive and conveyor drives are hydrostatic transmissions which are closed loop hydraulic systems. The main components of each system are a variable, positive displacement pump and a positive displacement motor.



The output of the variable displacement pump is determined by engine speed and by the tilt angle of the pump swashplate. Engine speed is controlled by the engine speed switch while the pump swashplate tilt angle is determined by the speed selection control (ground drive) or the automatic feeder control system.

The speed and torque output of the motors depend in turn on the output of the pump and the load placed on the system. The ground drive motor has two different displacements set by the motor speed selection control. Combined with the two speed transmission, this yields four travel speed ranges.

Charge Pump Circuit

Hydraulic oil flows from the hydraulic oil tank to the charge pumps on the main pumps. The charge pumps provide the flow of oil needed for cooling purposes, to maintain a positive pressure on the low pressure side of the main pump/motor circuit, for pump control purposes and for internal leakage makeup.

Main Pump and Motor Circuit

The oil in the main circuit of the hydrostatic transmission flows in a continuous closed loop from the pump to the motor and back to the pump. The direction of oil flow in the circuit, and thus which side of the circuit is the high pressure side, is determined by the direction in which the pump swashplate is tilted from the neutral position. Oil from the charge pump is directed to the low pressure side by one of two check valves. The second check valve is held closed by the oil in the high pressure side of the circuit.

A manifold valve assembly connected across the main circuit of the ground drive system contains two pilot operated pressure relief valves. The pressure relief valves serve to prevent high pressure surges in either side of the circuit by dumping oil from the high to the low pressure side. High pressure surges may occur during rapid acceleration or braking, or with sudden application of a load.

The manifold valve assembly also contains a shuttle valve and a charge pressure relief valve. The shuttle valve establishes a circuit between the low pressure side of the main circuit and the charge pressure relief valve so that the relief valve can dump excess cooling oil added by the charge pump, and thereby control the charge oil pressure. A spring centers the shuttle valve in the closed position so that no high pressure oil is lost from the main circuit during the transition period when the pressures are being reversed.

Cooling Circuit

Excess charge oil (cooling fluid) from the ground drive manifold charge pressure relief valve enters the motor case, then flows through case drain lines to the pump case, through the pump case to the hydraulic oil tank.

Excess charge oil (cooling fluid) from the conveyor drive pump charge pressure relief valve flows through the pump case, through the left motor, through the right motor, and then through the 10-micron return filter and the hydraulic oil cooler to the hydraulic oil tank.

When the main pump in any of the hydrostatic transmissions is in neutral, the shuttle valve is centered and the excess flow from the charge pump is directed to the cooling circuit by the charge relief valve in the charge pump. Cooling flow is not admitted to the motor case, but flows through the pump case, the 10-micron return filter and the oil cooler to the oil tank.

Displacement Control

The displacement of the hydrostatic transmission pump is controlled by the displacement control valve which is positioned by the speed selection control (ground drive) or the automatic feeder control system (conveyor drive).

Vibrator Drive Hydraulic System

The vibrator drive hydraulic system is an open-center system that incorporates a variable positive displacement pump and two or more motors. The hydraulic oil flows from the tank to the pump, to the motors, to the 10-micron return filter, to the oil cooler and then back to the tank.

The speed (rate of vibration) of the vibrators is controlled by the vibrator speed control located on the screed. The vibrator speed control sends an electrical signal to the pump to set the angle of the swashplate and, thereby, set the speed of the vibrators.

If the pressure in the supply line to the screed gets too high, as when the quick disconnects are unplugged, a pressure relief valve will open to relieve the pressure. The hydraulic oil from the pressure relief valve flows back through the return line to the return filter, to the oil cooler and then to the hydraulic oil tank.

Auxiliary Hydraulic System

The auxiliary hydraulic system incorporates the auxiliary pump, a 5-micron filter and the valves, sensors and cylinders necessary for auxiliary machine functions such as raising the hopper wings, raising the screed, steering, and if the machine is so equipped, grade control and slope control.

The auxiliary pump is a variable displacement pump with a pressure compensator that maintains a constant system pressure.

After the output of the auxiliary pump is filtered through the 5-micron filter, it is directed to the required end of a hydraulic cylinder by a solenoid valve, a steering valve or an optional electric sensor. The solenoid valves are controlled by the operator through switches on the control console and the steering valves are controlled through the steering wheels. The optional electric sensors (grade sensor or slope sensor) operate automatically, when the grade control system or slope control system is activated, to direct hydraulic oil to the appropriate end of one of the optional tow point cylinders.