Hydraulic pressure and its variants

Generally speaking, hydraulic pressure is the pressure that exists within a hydraulic system. It is the force per square millimeter of area pressing against the inner wall of the piping system. Depending on the application, different types of hydraulic pressure can occur. These variants have defined designations.

Static hydraulic pressure

Static pressure is the internal pressure in the hydraulic system that prevails when the hydraulic fluid is stationary. It is the available working pressure when the volume flow is at rest. When pressure is drawn, the volume flow begins to move and the static pressure changes—for example, into working pressure or dynamic pressure.

Static pressure is important for calculating the behavior of the hydraulic system when the pump is switched off. It must never be assumed that a hydraulic system becomes depressurized immediately after shutdown. Opening such a hydraulic line poses a risk of serious injury and extensive damage to the machine. Mechanical pressure gauges therefore remain very important as backups today. In the event of a power failure, they reliably display the static pressure within a deactivated hydraulic system.

Hydrostatic pressure or gravitational pressure

Hydrostatic or gravitational pressure takes the height of the stationary liquid column into account. This is important, for example, when designing large hydraulic tanks. These are almost pressureless at the top. At the bottom, however, the weight of the stationary liquid column can already have built up significant pressure. Hydraulic specialists can work with this “natural” pressure by, for example, selecting a smaller, energy-saving pump.

Dynamic pressure or back pressure

Dynamic pressure is the pressure that exists within a flowing volume stream. It is also considered “working pressure”—the pressure that can be tapped in a hydraulic system to operate the connected actuators.

Distinguishing this from static pressure is very important. If static pressure is tapped—for example, by an extending hydraulic cylinder—its value drops rapidly. This can mean that the initially high pressure is insufficient to fully extend the cylinder. With dynamic pressure, however, the same contact force is applied in every state. This makes the movements of linear or radial motors fluid, controllable, and uniform.

Hydraulic pressure ranges

Finally, hydraulic pressure is divided into four pressure ranges:

1. Low pressure (LP) refers to all pressures from 1–100 bar
2. Medium pressure (MP) covers the pressure range from 100–200 bar
3. High pressure (HP) stands for pressures up to 600 bar
4. Ultra-high pressures are the pressure ranges beyond 600 bar

Hydraulic pressure in conventional hydraulic systems lies between the medium and high-pressure ranges. Construction machinery, for example, operates within this pressure spectrum.