Área de influencia

In many vehicles it is preferred to have hydrostatic transmission rather than mechanical transmission despite its lower efficiency. This is because the benefits of hydrostatic transmission outweigh the

disadvantages: Hydrostatic transmission provides superior maneuvering abilities and continuously variable speed control between minimum and maximum speeds. The direction of rotation of the wheels is easily reversed by using the features of the variable mobile pump.

In mechanical transmission systems, the wheels of a vehicle are driven by shafts and gears from an internal combustion engine. But in hydrostatic transmission, the combustion engine drives a pump and the pump supplies oil for the hydraulic motor(s) which make the wheels turn. The pump’s flow rate is proportional to input speed.

But due to limitations of mobile hydraulics such as reservoir size, weight of components etc, the pump and the motor(s) do not operate as they do in a conventional hydraulic system, which is known as an open circuit. Instead, they operate in a closed circuit where both inlet and outlet ports of the pump and motor are connected. In other words, the oil delivered by the pump makes the hydraulic motor rotate and the oil out of the motor goes to the suction port of the pump again. The oil circulates between the pump and the motor in a continuous loop.

The above diagram is the basic representation of a closed hydraulic circuit. The oil circulates in the circuit making the motor(s) rotate. The flow direction is reversed and the flow is adjusted by tilting the swash plate of the pump.

For various reasons, the circuit in the diagram cannot operate.

1. There are leakages in both pump and the motor. So the oil in the circuit gets less as the system runs, which make the system impossible to operate. (These leakages are necessary for cooling and cleaning functions.)

2. The suction side of the pumps in closed circuits must always have positive pressure, otherwise the system may not run. But due to load on the motor, this pressure may be too low to make the system run.

3. The oil in the system gets hotter due to extreme working conditions and pressures and has to be replaced with fresh oil.

© Festo Didactic GmbH & Co. KG 574166 I-11 In closed circuits, a charge pump is necessary to overcome internal and auxiliary leakages, provide flow for

cooling and maintain pressure in the line which acts as a suction side (where the flow is into the pump). As the flow direction should be reversible in closed circuits, two non return valves are used to ensure the supply pump oil flows only in the suction side of the main pump as the non return valve connected to the pressure line will close due to higher pressure and the one on the suction side will open due to lower pressure.

In most mobile pumps (variable axial piston pump), the charge pump is in the main pump block and is driven by the same shaft connected to internal combustion engine.

No oil can circulate in the main pump when it is idle. For this reason, the charge pump also lubricates the main pump. The charge pump is usually placed in the main pump block, but is separate in some

applications.

As the diagrams show, the leakage lines from both pump and motors are connected to tank. These leakages are intentionally built in for cooling and lubricating purposes, but as the system wears out, the amount of leakage increases to undesirable levels. As the clearances in mobile applications are smaller, the cleanliness of the oil affects the life of the components as well as the amount of leakage.

Because of higher pressures, smaller clearances and limited cooling sources, the oil very soon gets hot in mobile systems. And because of the smaller clearances, the oil should always maintain a certain level of cleanliness.

To ensure efficient cooling and removal of contamination, instead of running all the available oil in the circuit, oil which is not required by the pump is flushed to tank, ensuring that the charge pump always receives fresh oil from the tank.

The flushing is better done as soon as the oil gets hot. This is done by flushing valve as seen in the circuit. The flushing valve receives relatively hot and contaminated oil from the outlet of the motor and immediately diverts it to the tank. The flushing valve is pressure-operated by the pressure in the pressure line. In most applications, the flushing valve is in the motor block as it has to be placed very near to where the oil gets hot. There are cases where the flushing valve is in the pump block.

The setting of the pressure relief valve connected to the flushing valve determines the pressure of the suction line.

The last important detail which was missing in the previous diagrams was the shock valve. As the wheels connected to hydraulic motors can receive sudden and severe iMPacts as the result of surface conditions, the pressure in the lines may go over the circuit’s limitations. For this reason, a relief valve is placed in each line to protect the system pressure against spikes resulting from external effects. And these relief valves are called shock valves and set at a higher value than the pump’s pressure setting.

The major components in a closed transmission system are:

1 Main pump

2 Charge pump

3 Charge pump pressure relief valve 4 Non return valve

5 Flushing valve

6 Flush valve pressure relief valve

7 Shock valve

8 Hydraulic motor

9 Heat exchanger

10 Reservoir 11 Filter

© Festo Didactic GmbH & Co. KG 574166 I-13 Some of these components can also be seen on the following diagram.