Up to now we've dealt with circuit symbols, simple piston cylinders, non-electronic regulation and certain hydraulic applications. At this point, we'll include a further practical application. It's all about construction and building site machinery and how they generate hydraulic pressure and how they feed feed it to their various working cylinders.

Let's start with the working cylinder. We'll assume it has to bring a loader-bucket into tipping position. If, to do this, it has to be extended, it needs pressure on the piston-surface and the pressure on the ring-surface. There may of course, be counter-pressure on the other side of the piston from the previous movement of the loader-bucket. This pressure must, at the same time, be decteased. Thus, one can alternate between the tipping- and the transport position.

Of course, when more force is needed, two parallel cylinders are normally used, at best, not directly next to each other but one on either side of the bucket. First of all, we'll assume there is a simple control valve in the cabin of the bucket-loader. What however, must be there, in all cases, is the pressure generation, whose torque can be created either by an electric motor or a combustion engine.

We are here assuming an axial-piston motor, which gets it's name from the direction in which the piston moves, when idling, parallel to it's output axle. In this case the hydraulic pressure is not converted into mechanical energie. Only when the axially arranged pistons tip away from being parallel, depending on which way they tip, is the rotation direction decided, either clockwise or anti-clockwise.

The transferable torque increases proportionally with the tilting angle. Every hydraulic motor has a hydraulic pump. If this drives e.g., a rear axle, this is done by a Diesel engine with the pump. Here the power of the Diesel engine is turned into respective pressure. The position of the Diesel engine is not dependent on the axles to be driven, because there is no mechanical connection.

The hydraulic pump is also called the primary-part because it is mounted at the beginning of the power transfer. The hydraulic motor is described as being the secondary part. It makes up the end of the hydraulic part and leads once again to the mechanics. There are motors with a fixed angle (see picture) and those which are adjustable. The degree of adjustment is also decided by pressure lines and control valves.

The lines can be laid out both as rigid metal piping and also as flexible hoses. They not only connect the hydraulic motors and the pumps, they also connect these with the valve-block, thus, with the operation unit. This is where the RPM of the combustion engine and the respective adjustment angle is decided, very often of course, by electronics. Torque build up- and output must stand in a reasonable relationship to each other.

Of there are any number of different hydraulic pumps, e.g., one is similar to the pump part of a hydraulic clutch. If this is installed in the vehicles drive, the mechanical clutch is not necessary. In addition, a Diesel engine can drive several hydraulic pumps. They can even be flanged on in various places, e.g., directly onto the clutch or on the gearbox output.

The efficiency of the hydraulic power transfer is inferior compared with that of the mechanical transfer, except perhaps in the case of very high transmission ratios. This is why a special optimising in view of the CO2- emission is particularly useful and important. None of this can be seen on the operating unit. The press of a button or a small movement of the joystick is sufficient to activate thousands of kW. 06/12

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