Hydraulic oil is not just a fluid for power transmission; it is a vital component that
lubricates,
cools, seals, and cleans every part of the system. Choosing the right hydraulic
oil directly affects the life and performance of your pumps and valves.
Did You Know?
70% to 80% of hydraulic system
failures
are
caused by incorrect oil selection or contamination. Regular analysis and correct
selection
are the cheapest insurance for your system.
1. Understanding Viscosity (ISO VG)
Viscosity is the most important property of hydraulic fluid. If the viscosity is too
low,
the oil
film thins, causing metal-to-metal friction and leakage. If it is too high, pump
cavitation
occurs and energy consumption increases.
| ISO Class |
Kinematic Viscosity (40°C)
|
Typical Application Areas
|
| ISO
VG
32 |
28.8 - 35.2
cSt
|
Cold
climates,
mobile equipment, precision machine tools. |
| ISO
VG
46 |
41.4 - 50.6
cSt
|
Standard
industrial
usage (Injection molding, Presses). |
| ISO
VG
68 |
61.2 - 74.8
cSt
|
High
temperature
environments, heavy duty iron-steel industry. |
2. Mineral vs. Synthetic Oils
While mineral oils perform adequately in standard conditions, synthetic oils are
indispensable
for fire resistance or extreme temperature ranges.
- Mineral Oils (HLP): Economical, high lubrication capability,
suitable
for
general use.
- Synthetic Oils (HEES/HFD): High fire resistance, long life,
environmentally
friendly.
"The cleanest oil is the cheapest spare part. In hydraulic systems, 1 gram of dirt
causes 1
ton of damage."
— BRS PROSES Engineering Team
3. Monitoring and Maintenance (NAS 1638)
Don't Mix!
Mixing different brands or types of oil can cause
chemical
reactions that disrupt the additive structure and clog filters. Always clean the
storage
tank when changing oil.
We recommend periodic analysis to monitor the health of your oil. Parameters such as
particle
count (NAS 1638 / ISO 4406) and water content determine the life of your equipment.
4. BRS PROSES Oil Maintenance Checklist
- Check oil level and temperature
daily
- Check filter pollution
indicators
weekly
- Comprehensive oil analysis
(Viscosity,
Water, Particle) every 6 months
- Clean tank breathers Annually
- Check for leakage at connections
6. DIN 51524 Classifications & Viscosity Index ($VI$) Optimization
Selecting hydraulic fluid solely by its nominal ISO VG viscosity rating (e.g., ISO VG 46) overlooks its behavior under broad operating temperature swings. The German industrial standard DIN 51524 categorizes mineral hydraulic fluids into three primary engineering tiers:
- DIN 51524-1 (HL): Refined mineral oils containing active corrosion and oxidation inhibitors, suited for low-pressure machine tools and air compressors ($P < 100 ext{bar}$).
- DIN 51524-2 (HLP): Enhanced with anti-wear (AW) zinc dialkyldithiophosphate (ZDDP) additives. The global standard for industrial stationary presses, injection molding machines, and machine tools ($VI approx 95–105$).
- DIN 51524-3 (HVLP): Formulated with shear-stable polymeric Viscosity Index Improvers (VII). High Viscosity Index ($VI ge 150$) ensures fluid remains pumpable at -30°C while maintaining sufficient load-carrying film thickness at +90°C. Essential for mobile cranes, forestry harvesters, and offshore winches.
7. Biodegradable (ISO 15380) & Fire-Resistant (ISO 12922) Fluids
In sensitive environmental zones (hydropower dams, waterways, forestry) or high fire-risk plants (steel mills, die-casting foundries), mineral oils pose unacceptable liability. Alternative fluid chemistries are governed by ISO standards:
| Fluid Category |
Chemical Base |
Operating Limits |
Primary Application Field |
| ISO 15380 HEES |
Synthetic Esters (Saturated / Unsaturated) |
-35°C to +85°C / >90% biodegradable |
Hydropower dam gates, offshore marine systems |
| ISO 15380 HEPG |
Polyalkylene Glycol (PAG water-soluble) |
-30°C to +80°C / High lubricity |
Tunnel boring machines, marine dredging |
| ISO 12922 HFC |
Water-Glycol Solutions (≥35% water) |
+5°C to +60°C / Fire-resistant |
Hot metal rolling mills, die casting machines |
| ISO 12922 HFD-R |
Phosphate Esters (Synthetic anhydrous) |
-10°C to +150°C / Auto-ignition >500°C |
Steam turbine electro-hydraulic governor controls |
Hydraulic Fluid Chemistry: Viscosity Index, Oxidation Stability & ISO Cleanliness
Hydraulic oil is the most critical structural component of a fluid power system. It functions simultaneously as an incompressible energy transmission medium, a boundary lubricant preventing metal-on-metal contact, a thermal coolant, and a corrosion inhibitor. Selecting fluid solely by nominal ISO VG rating without verifying Viscosity Index (VI) and chemical base chemistry leads to rapid pump failure.
1. Viscosity Index (VI) & Temperature Operating Windows
The Viscosity Index measures how fluid viscosity changes across temperature swings. Standard HLP mineral oils feature a VI of ~100. For mobile machinery or outdoor industrial plants exposed to seasonal swings from -20°C to +40°C, HVLP fluids with VI (ge 150) must be specified to maintain adequate kinematic viscosity (typically 16 cSt to 36 cSt) and prevent lubricating film collapse at peak operating temperatures.
2. Monitoring Total Acid Number (TAN) & Water Saturation
- Total Acid Number (ASTM D664): Fresh mineral oil has a baseline TAN of 0.2 to 0.5 mg KOH/g. When thermal oxidation drives TAN above 1.5 mg KOH/g, aggressive organic acids corrode bronze bearings and generate insoluble sludge, demanding immediate oil replacement.
- Moisture Limits: Dissolved water must not exceed 200 ppm in mineral oil. Free emulsified water (milky appearance) destroys anti-wear additives (ZDDP) and accelerates hydrogen embrittlement in rolling-element bearings.
