Hydraulic Fluid Properties
Comprehensive reference for hydraulic fluid characteristics including ISO VG grades, viscosity-temperature relationships, and fluid type comparisons.
ISO VG Viscosity Grades
| ISO VG | Viscosity @ 40C (cSt) | Min (cSt) | Max (cSt) | Typical Applications |
|---|---|---|---|---|
| VG 10 | 10 | 9.0 | 11.0 | Spindle oils, low-temp applications |
| VG 15 | 15 | 13.5 | 16.5 | Light hydraulics, cold climates |
| VG 22 | 22 | 19.8 | 24.2 | Machine tools, moderate loads |
| VG 32 | 32 | 28.8 | 35.2 | General purpose, most common indoor |
| VG 46 | 46 | 41.4 | 50.6 | Industrial hydraulics, standard |
| VG 68 | 68 | 61.2 | 74.8 | High-pressure systems, warm climate |
| VG 100 | 100 | 90.0 | 110.0 | Heavy equipment, high temps |
| VG 150 | 150 | 135.0 | 165.0 | Very high pressure, hot environments |
Viscosity vs Temperature
Viscosity decreases significantly as temperature increases. This table shows approximate kinematic viscosity (cSt) for mineral-based hydraulic oils at various temperatures.
| ISO VG | -20C (-4F) |
0C (32F) |
20C (68F) |
40C (104F) |
60C (140F) |
80C (176F) |
100C (212F) |
|---|---|---|---|---|---|---|---|
| VG 15 | 450 | 110 | 38 | 15 | 7.5 | 4.5 | 3.0 |
| VG 22 | 800 | 175 | 55 | 22 | 10 | 5.8 | 3.8 |
| VG 32 | 1400 | 280 | 80 | 32 | 14 | 7.8 | 5.0 |
| VG 46 | 2400 | 430 | 115 | 46 | 19 | 10 | 6.2 |
| VG 68 | 4500 | 720 | 175 | 68 | 27 | 13 | 7.8 |
| VG 100 | 8500 | 1200 | 270 | 100 | 38 | 18 | 10 |
| VG 150 | 15000 | 1900 | 400 | 150 | 55 | 25 | 13 |
Physical Properties by Grade
| ISO VG | Density @ 15C (kg/m3) |
Specific Gravity | Bulk Modulus (psi x 10^5) |
Bulk Modulus (bar x 10^4) |
VI Typical |
|---|---|---|---|---|---|
| VG 15 | 855 | 0.855 | 2.1 | 1.45 | 95-105 |
| VG 22 | 860 | 0.860 | 2.15 | 1.48 | 95-105 |
| VG 32 | 865 | 0.865 | 2.2 | 1.52 | 95-105 |
| VG 46 | 870 | 0.870 | 2.25 | 1.55 | 95-105 |
| VG 68 | 875 | 0.875 | 2.3 | 1.59 | 95-105 |
| VG 100 | 880 | 0.880 | 2.35 | 1.62 | 95-105 |
| VG 150 | 885 | 0.885 | 2.4 | 1.65 | 95-105 |
Viscosity Index (VI)
Measures viscosity change with temperature. Higher VI = more stable viscosity. Mineral oils: 90-105. Premium oils with VI improvers: 120-160. Synthetics: 130-200+.
Bulk Modulus
Resistance to compression. Higher values = stiffer system response. Decreases ~0.5% per 1% air entrainment. Critical for servo systems.
Operating Temperature Limits
| Fluid Type | Min Startup | Min Operating | Optimal Range | Max Operating | Flash Point |
|---|---|---|---|---|---|
| Mineral Oil (Standard) | -20C (-4F) | 10C (50F) | 40-55C (104-131F) | 70C (158F) | 200C+ (392F+) |
| Mineral Oil (Premium) | -30C (-22F) | 5C (41F) | 40-60C (104-140F) | 80C (176F) | 220C+ (428F+) |
| Synthetic (PAO) | -45C (-49F) | -20C (-4F) | 40-70C (104-158F) | 120C (248F) | 250C+ (482F+) |
| Water Glycol (HFC) | -20C (-4F) | 5C (41F) | 35-50C (95-122F) | 55C (131F) | N/A (FR) |
| Phosphate Ester (HFD-R) | -20C (-4F) | 10C (50F) | 40-65C (104-149F) | 90C (194F) | 245C+ (473F+) |
| HEES (Vegetable) | -25C (-13F) | 5C (41F) | 40-55C (104-131F) | 70C (158F) | 300C+ (572F+) |
Optimal 40-55C
Best efficiency, minimal wear, stable viscosity. Target this range for continuous operation.
Acceptable 55-70C
Reduced oil life, increased oxidation. Monitor closely, consider cooling.
Caution >70C
Accelerated degradation, seal damage risk. Cooling required, check for root cause.
Fluid Type Comparison
| Property | Mineral Oil (HM/HLP) |
Synthetic (PAO/Ester) |
Water Glycol (HFC) |
Phosphate Ester (HFD-R) |
HEES (Biodegradable) |
|---|---|---|---|---|---|
| Specific Gravity | 0.86-0.90 | 0.82-0.87 | 1.04-1.09 | 1.13-1.17 | 0.91-0.93 |
| Viscosity Index | 95-110 | 130-200 | 140-200 | 0-30 | 180-220 |
| Bulk Modulus (psi) | 220,000 | 200,000 | 350,000 | 320,000 | 200,000 |
| Fire Resistance | Poor | Poor-Moderate | Excellent | Excellent | Poor |
| Lubricity | Good | Excellent | Fair | Good | Good-Excellent |
| Oxidation Stability | Good | Excellent | Good | Good | Fair |
| Temperature Range | Wide | Very Wide | Limited | Wide | Moderate |
| Seal Compatibility | Standard | Requires FKM | Special seals | Special seals | Standard |
| Paint Compatibility | Good | Good | Fair | Poor | Good |
| Relative Cost | 1x (baseline) | 3-5x | 2-3x | 8-15x | 2-4x |
| Typical Applications | General industrial | Extreme temps, aerospace | Steel mills, foundries | Aviation, military | Forestry, marine |
Viscosity Grade Selection Guide
| Application / Environment | Ambient Temp Range | Recommended ISO VG |
|---|---|---|
| Cold climate, outdoor mobile | -30 to +15C (-22 to 59F) | VG 15 or VG 22 |
| Temperate climate, indoor | +10 to +30C (50 to 86F) | VG 32 or VG 46 |
| Warm climate, general industrial | +15 to +40C (59 to 104F) | VG 46 or VG 68 |
| Hot environment, heavy duty | +25 to +50C (77 to 122F) | VG 68 or VG 100 |
| Wide temperature variation | -20 to +40C (-4 to 104F) | VG 32 multigrade (HV) |
| High-pressure piston pumps | - | VG 32 to VG 68 |
| Vane pumps | - | VG 32 to VG 68 (AW type) |
| Gear pumps | - | VG 46 to VG 100 |
| Servo/proportional systems | - | VG 32 to VG 46 (high VI) |
Additional Fluid Properties
Pour Point
Lowest temperature at which oil will flow. Typically -20C to -40C for standard mineral oils. Must be 10-15C below minimum startup temperature.
Thermal Conductivity
Mineral oil: ~0.13 W/m-K. Water glycol: ~0.40 W/m-K. Affects heat dissipation in reservoir and coolers.
Specific Heat
Mineral oil: ~1.9 kJ/kg-K. Water glycol: ~3.3 kJ/kg-K. Higher values = more heat storage capacity.
Thermal Expansion
~0.07% per C for mineral oil. Consider for closed systems and accumulator sizing. 10C rise = 0.7% volume increase.
Air Release
Time for entrained air to separate. Premium fluids: 2-5 minutes. Important for pump cavitation prevention.
Foaming Tendency
Measured per ASTM D892. Anti-foam additives reduce surface foam but may slow air release. Balance required.