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All Tests

  Transmission - Hydraulics

Agriculture is already well advanced in the field of autonomous driving. It is favoured by a limited territory that can be precisely measured and does not usually have to reckon with surprises such as pedestrians or oncoming traffic. So it is already possible today for machines to work fields meticulously without a driver on board.

Such syszems need suitable automatic transmissions. What is special about an automatic transmission in agricultural machines? The fact that they often have to do their work in a very small, finely adjustable speed range, where a kind of idling is not enough because large loads have to be moved, e.g. as a plough on heavy soil.

The transmission to be described here is assisted by hydrostats, which have similarities in construction to the compressor for the air conditioning system shown in the picture. Smaller cylinders with pistons are arranged in a cylinder parallel to its axis of rotation, which then move from BDC to TDC and back during a rotation of the cylinder when they are connected via rods and ball joints to an oblique swash plate that also rotates.

Here is a version with a fixed angle for the swash plate. Although this is a pump, you could still run it as a motor. This even works with compressed air and allows a rough function test without all the hassle with the hydraulic oil connections.

Such a unit can therefore work both as a pump or as a motor, as in the two examples above. At the top you can see two identical units, but the upper one works in conjunction with the Diesel engine as a pump and the lower one with the axle drive as a motor. Its swash plate is therefore firmly connected to the output shaft of the gearbox.

By the way, you won't find a clutch in the whole construction. The somewhat unusual part at the end of the crankshaft is only an absorber for torsional vibrations. If torque is transmitted is determined by the hydraulic pump at the top, which is also driven when the engine is idling. As above, the cylinder and swash plate can rotate, but there is no delivery stroke and thus no pressure in the flexible lines to the engine below.

As you can see in the picture, the pump must first be swivelled so that an effective pressure is created in one of the two hydraulic lines. The angle of the pump determines the flow rate and thus the pressure on the motor. Below you can see that the pump is swivelled in the other direction. The pressure in the hydraulic lines changes. This is then used to engage the reverse gear steplessly.

It is important to note that the hydrostatic system does not transmit the torque alone. Rather, a branching takes place via the planetary gear at the top right. This is to combine the good efficiency of a gear transmission with the shifting and starting comfort of hydraulics. The greater the transmitted speed and thus the flow velocity and resistances would be, the more the gears would take over the power transmission.

English undertitles possible . . .

Here is another version of the power split. The hydraulic motor is no longer directly connected to the output shaft of the gearbox, but to the ring gear of the planetary. The pump now swivels for start-up from one side to the centre. In this way, the speed of the hydraulic motor goes further and further towards zero. The vehicle is finally driven only mechanically.

If the pump swings further in the other direction, the forces of the two branches add up. In this way, the highest driving speed can be achieved with maximum swinging out. The reverse gear must be additionally realised with this design.

English undertitles possible!
Here are two further developments . . .

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