
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:
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:
If very low viscosity oil is used:
Commonly used oil grades in hydraulic systems:
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:
can create high temperature.
High Temperature:
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
ISO VG 46
ISO VG 68
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:
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:
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:
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;
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.




