AP-1200 ASPHALT PAVER Hydraulic Systems Caterpillar

Hydraulic Systems
1.1. General
2.1. Ground Drive, Conveyor Drive And Vibrator 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. Auxiliary Hydraulic System


Hydraulic oil for all the hydraulic systems is stored in the 33-gallon (124.9-liter) tank on the right side of the machine. A sight gauge on the front of the tank allows quick and accurate checking of the hydraulic oil level.

Hydraulic oil drawn from the tank passes through a 100-mesh strainer that is mounted inside the tank. The hydraulic oil leaving the charge pumps on the ground drive pump, vibrator drive pump and the conveyor drive pumps flows through 10-micron filters before entering the main pump/motor hydraulic circuit, entering the pump control hydraulic lines or joining the case drain flow back to the tank. A dual-element 10-micron filter cleans the return oil before it enters the hydraulic oil cooler.

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, Conveyor Drive And Vibrator Drive Hydrostatic Transmissions

The machine ground drive, conveyor drives, and vibrator drive 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.

Forward Typical Heavy Duty Variable Pump - Variable Motor Transmission Schematic


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 travel speed control, the automatic feeder control system or the vibrator speed control.

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 speed range control. Combined with the two speed transmission, this yields four travel speed ranges.

Charge Pump Circuit

Hydraulic oil flows through the 100-mesh strainer in 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 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.

Each 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 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, through the 10-micron filter, through the hydraulic oil cooler to the hydraulic oil tank.

Excess charge oil (cooling fluid) from the conveyor drive and vibrator drive charge pressure relief valves flows directly to the respective pump case, through the pump case, through the 10-micron filter, through 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, 10-micron filter, and 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 travel speed control lever (ground drive), automatic feeder control system (conveyor drive) or the vibrator speed control (vibrator drive).

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, 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 five-micron filter, it is directed to the required end of a hydraulic cylinder by a solenoid valve, a steering valve or an 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 electric sensors (grade sensors 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 tow point cylinders.