Glossary · Pneumatics and hydraulics
Hydraulic fluid
Also known as: Hydraulic oil, Pressure fluid
German: Hydraulikflüssigkeit
In hydraulics, hydraulic fluid is the liquid that transmits power in a hydraulic system and also lubricates, cools and protects components; it is classified by type (for example mineral oil, fire-resistant or biodegradable fluids) and viscosity grade.
- Fluid power
In one sentence
Hydraulic fluid transmits power in a hydraulic system and also lubricates, cools and protects components.
Example
The hydraulic power unit is filled with an HLP 46 mineral oil, and the cleanliness class is checked by oil analysis twice a year.
How it applies
- Engineering: The fluid type is chosen by temperature range, fire risk and environmental requirements; seals and components must be compatible. Cleanliness is expressed in codes according to ISO 4406.
- Operation: Temperature affects viscosity; too hot or too cold fluid causes wear, cavitation or sluggish response.
- Maintenance: Fluid and filters are changed by condition or interval; mixing fluid types can cause damage.
- Documentation: State the approved fluid type, viscosity grade, fill quantity and cleanliness requirement in the technical data. Refer to the safety data sheet for handling, and cover disposal, skin contact and fire hazards of leaking fluid.
Hydraulic fluid vs. lubricant
Hydraulic fluid lubricates too, but its primary job is transmitting power. Lubricants specified for gearboxes or bearings are not interchangeable with hydraulic fluid; list both in the Lubrication specification.