Hydraulic actually means, as indicated by the Greek word, 'hydro', the causing of movement through 'water'. Starting in the 19th century, very soon instead of water, an oil-hydraulic was developed, in modern times, it should actually be described as an 'emulsion'. This is due to the mixing of the two substances, whereby the oil provides the corrosion protection and the water, the two-fold ability to transfer heat.
In the motor car spectrum however, we only need to consider the oil-hydraulics, even though recently there have been interesting advances made as far as transformations under high water-pressure are concenrned. Hydraulics can be found all over the place in vehicles, although in the field of power-steering (see pictures 5 & 6), particularly in the motor car sector, it has to take a back seat compared with the electric system. Indeed, there are still enough applications, e.g., in the case of the fuel- (see pictures 7 & 8) and oil filled units of the drive-train, the shock absorbers, possibly with a compound system and the transfer of the braking forces, where a fully electrical system has been thought about, but seems nowhere to be ready for serial production.
The minor areas should also be mentioned here, like the operation of the convertible roof or the tipping of a trailer floor (see picture 4), we won't even touch on the possibilities for utility vehicles. Consider the possibilities for e.g., the motor car workshop, where cars have to be raised components have to be installed or de-installed by pressing them in or out and where things have to be straightened or aligned. Whereby here, we've only touched on the production of, and the important regulations concerning vehicles.
What makes the hydraulic power transfer more attractive compared with the purely mechanical, electrical or pneumatic transfer? If one has the opportunity to watch the hydraulic system in action, e.g., when pressing out a solidly jammed wheel-bearing, one is aware of the enormous power at work, which can be achieved through manual application, we won't even mention the pumping power (see picture 1). To do the same job, a mechanical system would need any number of gear-wheels - thus energy - or long leverage, thus, plenty of space to work. The power is transfered directly, compared with pneumatics, where the pressure has to first be built up and there is the risk of too much pressure.
Electric systems, e.g., still have difficulties as far as the gentle dosage of the hydraulic systems is concerned. Consider the procedure of automatic clutching, where the torque converter still dominates the electrically operated clutch. Here however, a disadvantage of the hydraulics also becomes obvious. Should the manual pressure build-up, e.g., in the workshop-press, not be possible, then a complex process of building up and feeding the pressure into lines must be undertaken, and this means the use of energy. This pressure has to be constantly available, even if, at the time, it's not being used.
What I could further mention, is the more simple to realise, overload-protection, the rapid reversal of the movement, the power transfer over long distances and the lower wear and tear using oil as a medium. Of course, there are also disadvantages. The hydraulic sysem is often complicated, the oil can overheat and if one considers e.g., an excavator, the purely hydraulic driving of the wheels is much easier for the engine, indeed, more energy is also lost, which, because of the short working range of the excavator, can be tolerated.
Oh yes, the permanent threat of leakage and that even when carrying out the smallest repairs one may have to drain, and refill large amounts of oil and then bleed off any air in the system as well. On top of that, the manufacturers low tolerances make a certain industrial standard necessary. 04/12