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INTRODUCTIONHydraulic drives are used for transmission mechanical energy by the working fluid from place it is produced to place it is use. Because of the way of energy transmission it could be distinguished two groups of hydraulic drives:- hydrokinetic drives using kinetic energy of the working fluid; - hydrostatic drives mostly using pressure energy of the working fluid. Usualy energy supplies in hydraulic systems are various pumps. The task of that kind of pump is to supply hydraulic system big enough amounts of working fluid on right pressure. Basic characteristic parameters of the pumps are efficiency (flowrate) and pressure, both on fixed rotational speed of the propellant element. Working pump generates pressure p in space where fluid is pumping. Power gives by the pump is equal to product its flowrate and pressure. Power consumed by the pump from external source is greater then power produced because of inevitable losses of the power. Ratio of given power to consumed power is defined as a mechanical efficiency factor. Pump efficiency n is equal to product of volume efficiency and mechanical efficiency. Nominal power consumptionPower consumption is a power that has to be provided to pump in order to receive flowrate Q with pump load p. It can be defined: N -- power consumption, the SI unit [W] however it is usefull to represent it in greater unit [kW]; p -- pump load (work pressure), the SI unit is [Pa] but it is more common to use greater unit [MPa]; n -- overall efficiency of the pump. Hydraulic devices
Hydraulic cylinders are used to move or hold working elements e.g. Industrial device.
Force as pressure function is defined:
Cylinder uses thrust of the working fluid on specific pressure that is recived from energy source and directed by the valve device. It can be distinguished between single- and double-acting cylinders. 
In single-acting cylinder piston move is result of the effect of pressure differential caused by direction of the working fluid on specific pressure.
Retraction of the piston is due to external pressure caused by e.g. appropriately tense spring (with possibility of outflow of provided fluid).
In double-acting cylinder moves of the piston in two oposite directions are forced due to right pressure differential which means it depences on appropriate inflow of the working fluid on the specific pressure to the cylinder only. It has to be noted that free outflow from the oposite side of the cylinder is possible at the same time.
The working fluid creates specific axial thrust on cylinder's piston rod. It depences on piston and piston pod gemetrical dimensions and fluid pressure. 
 Where: p- working pressure [Pa]; D, d- dimensions (accorging to the figure above) [m]; F- axial thrust [N]. Hydraulics -- symbols
Symbols used on this site were designed basing on actual Polish norm PN/ISO-1219-1.
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