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Hydraulic Oil Selection | Hydraulic System Performance with the Right Oil

Hydraulic Oil Selection | Hydraulic System Performance with the Right Oil


Hydraulic Oil Selection: Critical Factors Determining Hydraulic System Performance

Hydraulic Oil Selection: The Unseen Performance Element of the System

When designing a hydraulic system, mechanical components such as pumps, valves, cylinders and power units are usually considered in the foreground. However, one of the most critical elements for the reliability and long-lasting operation of the system is often overlooked: hydraulic oil.

A hydraulic oil that is not selected correctly; It can cause energy losses, wear, high temperature, component failures and unplanned downtime. Therefore, hydraulic oil selection is not just an "oil adding" process, but an important engineering phase of system design.

What is the Function of Hydraulic Oil in the System?

The primary job of hydraulic oil is not just to transmit pressure. At the same time:

  • Provides lubrication between moving parts
  • Reduces wear
  • Carries the heat generated in the system
  • Protects against corrosion
  • Helps precise operation of pumps and valves
  • Allows contaminants to be transported to the filtration system

Therefore, the properties of the oil used are directly related to the performance of the system components.

The Most Important Criterion in Hydraulic Oil Selection: Viscosity

One of the most important parameters in hydraulic oil selection is viscosity.

Viscosity is the resistance of oil to flow.

If very high viscosity oil is used:

  • The first work becomes difficult
  • Pump consumes more power
  • Energy loss increases
  • Flow problems may occur at low temperatures

If very low viscosity oil is used:

  • Lubricating film weakens
  • Wear increases
  • Internal leaks rise
  • System efficiency may decrease

Commonly used oil grades in hydraulic systems:

  • ISO VG 32 Hydraulic Oil
  • ISO VG 46 Hydraulic Oil
  • ISO VG 68 Hydraulic Oil

is as follows.

For example, ISO VG 46 or ISO VG 68 is preferred in industrial systems operating at high temperatures, while different viscosity values may be required in mobile equipment operating at low temperatures.

How Does Operating Temperature Affect Oil Selection?

Oil temperature in hydraulic systems can change constantly.

Specifically:

  • Mobile machines
  • Press systems
  • Busy production lines

can create high temperature.

High Temperature:

  • Accelerates the oxidation of fat
  • Reduces the performance of additives
  • Shortens the life of the oil

Therefore, the system operating temperature must be taken into account.

The following table  is given for standard mineral-based hydraulic oils (approximately VI ≈ 100). Actual values  may vary depending on the viscosity index (VI) of the oil manufacturer, additive package and base oil. The ISO VG value  refers to the kinematic viscosity (cSt / mm²/s) at 40°C.

ISO VG 32/46/68 Temperature Dependent Viscosity Change in Hydraulic Oils

Oil Grade

-10°C

0°C

20°C

40°C

60°C

80°C

ISO VG 32

~650 cSt

~220 cSt

~70 cSt

32 cSt

~15 cSt

~8 cSt

ISO VG 46

~950 cSt

~320 cSt

~100 cSt

46 cSt

~21 cSt

~11 cSt

ISO VG 68

~1400 cSt

~500 cSt

~145 cSt

68 cSt

~30 cSt

~15 cSt

Comment from an engineering point of view:

ISO VG 32

  • It provides better flow at low temperature.
  • It is preferred in fast working systems.
  • Advantageous in precision servo systems or low-temperature applications.

ISO VG 46

  • It is the most widely used grade in the industry.
  • Balanced choice for general hydraulic power packs, presses, machinery systems.
  • It is suitable for most applications in pump, valve and cylinder systems.

ISO VG 68

  • It provides an advantage in systems operating at higher temperatures.
  • Large hydraulic units are preferred in systems where heavy operating conditions and high leakage tolerance are required.
  • Cold flows more difficult in the first works.

Example of practical selection:

Working Condition

Recommended Oil

Sub-0°C environment, mobile machines

ISO VG 32 or low temperature oils

20–60°C standard factory environment

ISO VG 46

Above 60°C, heavy duty

ISO VG 68

Precision servo/proportional systems

Usually ISO VG 32–46

In hydraulic system design, it's not just temperature; pump type, operating pressure, flow rate, component tolerances and manufacturer recommendation should be evaluated together.

Hydraulic Oil Selection Based on Pump Type

Not every hydraulic pump works with the same oil characteristics.

For example:

Gear pumps

It is generally more tolerant, but proper viscosity selection is still important.

Vane pumps

It is more sensitive to clean oil and appropriate viscosity values.

Piston pumps

It requires more attention on oil quality due to high pressure and precise tolerances.

Incorrect oil selection can lead to premature wear, especially in piston pumps.

Why is Hydraulic Oil Cleaning Important?

The root cause of many hydraulic failures is not mechanical, but oil contamination.

The system contains:

  • Metal particles
  • Powder
  • Water
  • Chemical contaminants

may cause damage to components.

For this reason, criteria such as ISO 4406 cleanliness class should be followed in hydraulic systems.

Oil cleanliness is at a critical level, especially in servo valves and precision control systems.

Mineral Oil or Synthetic Oil?

Mineral-based hydraulic oils are commonly used in standard industrial applications.

But:

  • Very high temperature
  • Very low temperature
  • Long maintenance interval
  • Harsh working conditions

In such cases, synthetic-based oils can provide an advantage.

When choosing, not only the oil price but also the total operating cost should be evaluated.

Consequences of Using Incorrect Hydraulic Oil

Incorrect oil selection can lead to the following problems:

  • Pump noise
  • Pressure fluctuations
  • Irregularity in roller movements
  • Snagging in valves
  • High oil temperature
  • Reduced component life

For the efficient operation of a hydraulic system, oil selection is as important as component selection.

Choosing the Right Oil Means a Longer Lasting Hydraulic System

In order to achieve successful results in hydraulic system design, pump, valve, cylinder and oil should be evaluated as a whole that complements each other.

Hydraulic oil selected in accordance with the right viscosity, appropriate additive properties and system operating conditions;

  • Lower maintenance cost,
  • Higher system efficiency,
  • Longer component life

provides.

As Hidroman, we develop solutions with an engineering approach that takes into account not only component selection but also all operating conditions of the system in hydraulic system design.