Hydrostatic laws

The technical processes in hydraulics are essentially based on the following basic laws of physics:

Pascal's law:

(Blaise Pascal, 1623—1662). Every force acting on a stationary fluid generates a pressure that propagates in all directions with the same magnitude.

This the basic law of hydrostatics. All force and pressure processes in fluids are based on this law. As the hydraulic fluid must be transported from the place it is generated (pump) to the place the energy is given up (motor, cylinder), other laws of hydrokinetics need to be taken into account:

Continuity law (conservation of mass):

This law states that a flow rate always remains the same even if it flows through cross-sections of different size. This means that as the cross-section become smaller, the velocity must be higher:

Bernoulli's law:

(Daniel Bernoulli, 1700—1782). The total energy content of a flowing fluid remains the same. From this law it can be derived that the sum of the static pressure and dynamic pressure is constant:

This means that in places of high flow velocity the static pressure drops such that air separation or even cavitation can occur.

Orifice law:

This law defines the outlet velocity of a flow from a chamber or a pipe that is defined by a pressure difference generated:

In this way the flow rate through a cross-section A of an orifice is obtained:

Law of the conservation of momentum:

This law states that a fluid flow that changes its direction and/or its velocity, causes reaction forces. On the actuation of directional valves with a spool this situation can result in undesirable incorrect switching.

In addition, there is of course also the behaviour of technical fluids that behave in accordance with laws:

• The viscosity of a fluid,
• The compressibility of a fluid,
• The flow behaviour of a fluid,
• The hydraulic surge (compression surge) in a fluid.