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Hydraulic fluids properties

Hydraulic fluid is the power transfer medium in machines and oil-hydraulic systems. In addition to transmitting force and motion, the fluid lubricates components, cools the circuit, protects against corrosion, carries contaminants to the filters, and helps seal internal clearances, including O-rings. The most common base is mineral oil with additives, although synthetic formulations, phosphate esters, polyglycols, and water-in-oil emulsions exist for specific applications.

Functions and properties

Section titled “Functions and properties”
Function Required property
Power transmission and control High bulk modulus (high dielectric rigidity), rapid air release, low foaming tendency, low volatility
Heat transfer Good thermal capacity and conductivity
Sealing of clearances Adequate viscosity and high viscosity index, shear stability
Lubrication Sufficient viscosity to maintain film, low-temperature fluidity, thermal and oxidative stability, hydrolytic stability, cleanliness and filterability, demulsibility, anti-wear characteristics, corrosion control
Pump protection Appropriate viscosity to minimize internal leakage, high viscosity index to maintain it across the thermal range
Special Fire resistance, friction modifiers, radiation tolerance
Environmental impact Low toxicity (of both new fluid and its degradation products), biodegradability
Service life Compatibility with system materials (seals, paints, metals)

ISO VG viscosity classification

Section titled “ISO VG viscosity classification”

ISO standard 3448 establishes the ISO VG (Viscosity Grade) grades for industrial oils. The reference kinematic viscosity is measured at 40 °C (104 °F) and expressed in centistokes (cSt), equivalent to mm²/s. Each grade is designated by a number that matches the mean kinematic viscosity at that temperature; the minimum and maximum limits are ±10 % of the nominal value. The grades range from ISO VG 2 to ISO VG 1500, and the step between consecutive grades represents an approximate 50 % increase in viscosity.

ISO 3448 viscosity grade table

Section titled “ISO 3448 viscosity grade table”
ISO VG Grade Kinematic viscosity at 40 °C – Min. (cSt / SUS) Mean viscosity (cSt / SUS) Kinematic viscosity at 40 °C – Max. (cSt / SUS)
ISO VG 2 1,98 cSt / 33 SUS 2,2 cSt / 33 SUS 2,42 cSt / 34 SUS
ISO VG 3 2,88 cSt / 35 SUS 3,2 cSt / 37 SUS 3,52 cSt / 38 SUS
ISO VG 5 4,14 cSt / 39 SUS 4,6 cSt / 41 SUS 5,06 cSt / 43 SUS
ISO VG 10 9,0 cSt / 55 SUS 10 cSt / 59 SUS 11,0 cSt / 63 SUS
ISO VG 15 13,5 cSt / 72 SUS 15 cSt / 77 SUS 16,5 cSt / 83 SUS
ISO VG 22 19,8 cSt / 96 SUS 22 cSt / 106 SUS 24,2 cSt / 116 SUS
ISO VG 32 28,8 cSt / 135 SUS 32 cSt / 152 SUS 35,2 cSt / 165 SUS
ISO VG 46 41,4 cSt / 198 SUS 46 cSt / 213 SUS 50,6 cSt / 234 SUS
ISO VG 68 61,2 cSt / 284 SUS 68 cSt / 315 SUS 74,8 cSt / 347 SUS
ISO VG 100 90 cSt / 417 SUS 100 cSt / 463 SUS 110 cSt / 509 SUS
ISO VG 150 135 cSt / 626 SUS 150 cSt / 695 SUS 165 cSt / 764 SUS
ISO VG 220 198 cSt / 918 SUS 220 cSt / 1019 SUS 242 cSt / 1121 SUS
ISO VG 320 288 cSt / 1334 SUS 320 cSt / 1482 SUS 352 cSt / 1631 SUS
ISO VG 460 414 cSt / 1918 SUS 460 cSt / 2130 SUS 506 cSt / 2344 SUS
ISO VG 680 612 cSt / 2835 SUS 680 cSt / 3149 SUS 748 cSt / 3465 SUS
ISO VG 1000 900 cSt / 4169 SUS 1000 cSt / 4632 SUS 1100 cSt / 5095 SUS
ISO VG 1500 1350 cSt / 6253 SUS 1500 cSt / 6948 SUS 1650 cSt / 7643 SUS

Note: The conversion to Saybolt Universal Seconds (SUS) at 37.8 °C (100 °F) has been calculated using the formula SUS ≈ ν × 4,632 for ν ≥ 50 cSt and by tabulated interpolation for lower values. 1 cSt = 1 mm²/s.

Equivalences between ISO VG and SAE (crankcase oil)

Section titled “Equivalences between ISO VG and SAE (crankcase oil)”
ISO VG Grade SAE Crankcase Oil Grade
22 5W
32 10W
46 15W
68 20W
100 30
150 40
220 50
320 60

Equivalences between ISO VG and SAE (aircraft oils)

Section titled “Equivalences between ISO VG and SAE (aircraft oils)”
ISO VG Grade SAE Aircraft Oil Grade
100 65
150 80
220 100
320 120

Equivalences between ISO VG and SAE (gear lubricants)

Section titled “Equivalences between ISO VG and SAE (gear lubricants)”
ISO VG Grade SAE Gear Lube Grade
46 75W
100 80W-90
220 90
460 85W-140
1500 250

Equivalences between ISO VG and AGMA

Section titled “Equivalences between ISO VG and AGMA”
ISO VG Grade AGMA Regular AGMA EP
46 1 –
68 2 2 EP
100 3 3 EP
150 4 4 EP
220 5 5 EP
320 6 6 EP
460 7 7 EP
680 8 8 EP

Calculation formulas

Section titled “Calculation formulas”

  • Kinematic viscosity (ν) from dynamic viscosity (μ) and density (ρ):

    ν (cSt) = μ (cP) / ρ (g/cm³)

    (1 cP = 1 mPa·s; 1 cSt = 1 mm²/s)

  • Approximate conversion between cSt and SUS (valid for ν ≥ 50 cSt):

    SUS ≈ ν (cSt) × 4,632

  • Temperature conversion:

    °F = (9/5) °C + 32

    °C = (5/9) (°F − 32)

  • Relationship between consecutive ISO VG grades:

    νₙ₊₁ ≈ 1,5 × νₙ (~50 % increase in kinematic viscosity)

Frequently Asked Questions (FAQ)

Section titled “Frequently Asked Questions (FAQ)”

What is ISO VG grade and how is it determined?

Section titled “What is ISO VG grade and how is it determined?”

ISO VG grade 46, for example, has a mean kinematic viscosity of 46 cSt at 40 °C (104 °F). It is defined per ISO 3448, with a range of ±10 % about the nominal value, so an ISO VG 46 oil can exhibit between 41,4 cSt and 50,6 cSt at the reference temperature.

Section titled “What is the recommended viscosity for a typical hydraulic system?”

In industrial machinery, viscosities between 15 cSt and 100 cSt at 40 °C are recommended, corresponding to ISO VG grades 15 to 100. The most frequent values are ISO VG 32, ISO VG 46, and ISO VG 68, which provide a balance between lubrication, friction losses, and dynamic response.

How is kinematic viscosity converted from cSt to SUS?

Section titled “How is kinematic viscosity converted from cSt to SUS?”

For fluids with viscosity above 50 cSt, the linear relationship SUS ≈ cSt × 4,632 can be used; for example, an oil of 68 cSt is approximately equivalent to 315 SUS at 100 °F (37,8 °C). For lower viscosities, more precise conversion tables exist.

What viscosity index should a good hydraulic fluid have?

Section titled “What viscosity index should a good hydraulic fluid have?”

A viscosity index (VI) above 95 is considered desirable. High-performance fluids achieve values of 150 or more, which allows viscosity to vary less with temperature, maintaining an effective lubricating film from cold starts (e.g., −20 °C / −4 °F) up to continuous operation at 80 °C (176 °F).

Section titled “How often is it recommended to change the hydraulic fluid?”

The replacement interval ranges between 2000 and 4000 service hours under normal conditions, although it may be shortened to 1000 hours if the working temperature continuously exceeds 70 °C (158 °F). Oil analyses determine parameters such as oxidation, water content (typical limit < 500 ppm), and residual viscosity to decide the actual change.

What effects does using a fluid of incorrect viscosity have?

Section titled “What effects does using a fluid of incorrect viscosity have?”

A fluid that is too viscous (e.g., ISO VG 100 in a system designed for ISO VG 46) causes overheating, power losses greater than 10 %, and slow response. A fluid that is too thin (ISO VG 22 instead of ISO VG 46) leads to increased internal leakage, premature wear of pumps and valves, and a drop in working pressure below design values.

References

Section titled “References”