The ISO classification of hydraulic oils
ISO standard 11158 defines the types of hydraulic fluids according to their properties:
- HH: mineral oil without additives. No longer used in industrial applications.
- HL: mineral oil with oxidation and corrosion inhibitors. For low-pressure systems.
- HM (= HLP in DIN): adds anti-wear additives. The standard for most industrial hydraulic systems.
- HV (= HVLP in DIN): high viscosity index. For outdoor machinery with wide thermal cycles.
- HF: fire-resistant fluids (HFA, HFB, HFC, HFD). For environments with fire risk.
- HBD: biodegradable. For applications where environmental contamination is unacceptable.
How to choose the correct ISO VG viscosity grade
The ISO VG viscosity of the hydraulic oil must be selected so that the viscosity in operation (at the actual working temperature) is within the pump's optimal range. This range is typically 25-54 cSt for vane pumps, 25-100 cSt for gear pumps, and 10-100 cSt for piston pumps.
Procedure: 1) Determine the maximum oil operating temperature in the reservoir. 2) Consult the viscosity-temperature curve of the candidate oil. 3) Verify that the viscosity at that temperature is within the pump's optimal range. 4) Also verify the minimum viscosity on a cold start.
Quick rule: for standard indoor industrial hydraulic systems with operating temperature 40-60°C, ISO VG 46 is the most common choice. For outdoor use or wide thermal cycles, ISO VG 46 HVLP.
Contamination: the greatest enemy of the hydraulic system
75-85% of hydraulic system failures are related to fluid contamination. Particles in the oil act as an abrasive in the tight tolerances of pumps and proportional valves. The cleanliness level according to ISO 4406 (for example 16/14/11) determines what level of contamination is acceptable for each component.
Maintaining the cleanliness level is as important as oil selection. High-efficiency filters, purging condensation water, and always filling through filtration are the three measures that most reduce hydraulic failures.
When to consider switching to a synthetic hydraulic oil
- Continuous operating temperature above 80°C in the reservoir.
- Frequent cold starts in extreme cold (<-15°C).
- Systems with proportional valves or servovalves with very tight tolerances.
- Working pressures continuously above 400 bar.
- When oil analysis shows accelerated degradation of the mineral oil.
Hydraulic oil selection starts with the pump type and its optimal viscosity range, continues with the actual operating temperature, and ends with environmental conditions. ISO VG 46 HLP covers 70% of standard industrial hydraulic systems. The remaining 30% requires more specific selection that always merits a technical consultation.
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