Hydraulic fluids for durable power transmission

Hydraulics is the study of energy transmission through fluids. The property of fluids that is of interest for hydraulic systems is their incompressibility. This would make practically any liquid suitable for hydraulics. In practice, however, there are several other requirements for hydraulic fluids. This significantly limits the selection of usable liquids.

Requirements for a hydraulic fluid

When speaking of the properties and requirements of a hydraulic fluid, this refers to its suitability for closed technical-hydraulic systems. Although open systems, such as deep-well pumps, also function according to hydraulic principles, they do not play a role in this context.

The primary task of a hydraulic fluid is to transmit energy with low losses. In a hydraulic context, energy refers to operating pressure and volume flow. However, the hydraulic engineer must also consider the waste heat generated by friction or the drive when selecting the working liquid. Furthermore, hydraulic fluids must meet numerous other requirements. In summary, these are the prerequisites that make a liquid suitable for use in hydraulic systems:

• Incompressibility
• Good lubricating properties
• Resistant to aging processes
• Good wetting and adhesion
• High flash point
• Acid-free
• Non-resinating
• Consistent viscosity across the entire temperature range
• No foaming
• Protective properties against corrosion
• Non-hygroscopic

These requirements are met by specially mixed mineral oils. They are referred to as “hydraulic oils.” Hydraulic applications in the feed and food industries present a particular challenge. They may only be operated with hydraulic oils that have the appropriate certification.

Selection of suitable hydraulic fluids as hydraulic oil

Hydraulic oils are identified by the letter H. The following classification has emerged in this technical application:

H: The “ancestor” of all hydraulic oils according to DIN 51524.
It contains no additives and is hardly used today.

HL: Hydraulic oil with additives for improved corrosion protection and against premature aging. This oil is approved for pressures up to 200 bar.

HLP: High-pressure oil, largely identical to HL but approved for pressures above 200 bar.

HM: Oil with improved tribological properties, specifically against “scuffing.”
This makes it particularly suitable for use in gear pumps.

HV: Same as HM, but with an increased temperature range.

HLPD: Hydraulic oil with high self-cleaning properties.
It efficiently transports dirt particles to the filters.

Hydraulic fluids and environmental protection

The problematic aspect of hydraulic oils is their contaminating properties in the environment. In the event of leaks or improper disposal, they can be just as toxic as engine oil. For critical environments such as nature or water protection areas, special oils have therefore been developed that are biodegradable. They can be recognized by the HE designation.

Fire-resistant hydraulic fluids

If hydraulic oils must not have any fire-promoting effects, flame-retardant oils are the medium of choice. They can be identified by the letter combination HFD.