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Pascal's law is the basis of hydraulic drive systems. As the pressure in the system is the same, the force that the fluid gives to the surroundings is therefore equal to pressure x area. In such a way, a small piston feels a small force and a large piston feels a large force.
The same counts for a hydraulic pump with a small swept volume, that asks for a small torque, combined with a hydraulic motor with a large sweptvolume, that gives a large torque.
In such a way a transmission with a certain ratio can be built.
Most hydraulic drive systems make use of hydraulic cylinders. Here the same principle is used- a small torque can be transmitted in to a large force.
By throttling the fluid between generator part and motor part, or by using hydraulic pumps and/or motors with adjustable swept volume, the ratio of the transmission can be changed easily. In case throttling is used, the efficiency of the transmission is limited; in case adjustable pumps and motors are used, the efficiency however is very large. In fact, up to around 1980, a hydraulic drive system had hardly any competition from other adjustable (electric) drive systems.
Nowadays electric drive systems using electric servo-motors can be controlled in an excellent way and can easily compete with rotating hydraulic drive systems. Hydraulic cylinders are in fact without competition for linear (high) forces. For these cylinders anyway hydraulic systems will remain of interest and if such a system is available, it is easy and logical to use this system also for the rotating drives of the cooling systems.