214 & 224 WHEEL-TYPE EXCAVATORS Variable Double Pump Caterpillar

Variable Double Pump
1.1. Function
2.1. Total Capacity Controller
3.1. Function

With total capacity controller and integrated control-oil gear pump.

The variable double pump with total capacity controller consists of two axial piston pumps, the actual pumping units and the pump regulating unit. The outer casing contains the pivoting double pump casing which, in turn, contains the two pumps. These pumps draw up the oil from the surrounding outer casing and discharge it into the pressure line through the pivot-mounted flanges. Each of the pumps has its own pressure connection, allowing two separate circuits with different pressures. Also, in the outer casing is the control-oil gear pump and the control-oil pressure limiting valve.


The drive motor powers the two drive shafts. These drive the seven articulated piston rods located in the flanges of the shafts, which in turn cause the cylinder to rotate. The rotating cylinder slides on the fixed control plate, which is provided with two ports.

If a piston moves in the cylinder, due to the rotation of the drive shaft, a stroke is carried out whose length depends on the angle of pivot. This causes a volume of oil corresponding to the piston surface and the stroke to be drawn in from the pump casing through one of the ports. As the drive shaft continues to rotate, the drawn-in oil is discharged again through the other port. If in control procedure the pumps are displaced towards the smaller angle, the piston stroke and thus the pump delivery are reduced.

The driving gear bearings are in the pump casing. Both control plates and the cylinders with journals and the pistons are located in a pivot casing. The casing itself is held via the two pivot bearings by flanges, in which are located the elements for the high pressure seals between the moving pivot casing and the stationary flanges. The two pumps are pivoted by means of two control pistons located in the pivot casing.

The outer casing can only be filled through the tank above. When refilling or changing the oil, the outer casing must be bled at its highest position.

The hydraulic oil is drawn in through the end plates of the pivot casing from the pump casing and passes to the actuators by way of the pressure sides of the end plates of the pressure channels in the pivot casing and the two pressure flanges.

Total Capacity Controller

The total capacity controller is an adjustment dependent on the sum of both operating pressures. The pressures present in the two hydraulic circuits are added and converted into force. The total force acts through the control pistons and the pivot casing on the measuring spring system. The range of spring which is established in a state of balance between hydraulic force and elastic force is the range of displacement about which the pumps are pivoted. If the total pressure exceeds a certain value, the pivot casing and thus the pumps are pivoted back simultaneously and uniformly.

1. Thus, in spite of increasing pressure, the drive motor is always subjected to the same output torque. Thus, in conjunction with constant output speed, constant output power is produced.
2. The delivery volume of the pumps is reduced in such a way that, in conjunction with a specific constant output speed, the resulting reduction of flow provides a constant hydraulic delivery.

If one pump takes up less power, the power released is available to the other pump. In a borderline case, either pump can transmit full engine power.


The pressure prevailing in each hydraulic circuit as a result of operating resistance is transmitted to the measuring surfaces of the control pistons through the control-pressure channels inside the pivot casing. Both measuring surfaces are equal in size. The hydraulic force resulting from the pressure in both systems and the measuring surfaces displaces the pivot casing against the spring and thus provides infinitely variable adjustment of the angle of pivot of the two pumps within the set range between maximum and minimum delivery volume. The range of displacement is determined by regulating screws.

Increasing pressure: pumps pivot back - flow reduced

Decreasing pressure: pumps pivot out - flow increased