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Hydraulic Oil Guide: Performance, Viscosity, and Selection Criteria

Hydraulic Oil Guide: Performance, Viscosity, and Selection Criteria


Hydraulic Oil Guide: Performance, Viscosity, and Selection Criteria

The efficiency of hydraulic systems is directly related to the quality of the oil used and its compatibility with the system. Incorrect oil selection can lead to high energy consumption and critical part failures. In this article, we detail the technical specifications of hydraulic oils and how to make the right choice for your business.

What is Hydraulic Oil and What Are Its Functions?

Hydraulic oil is an incompressible fluid partner that is used to transmit power from one point to another. However, its role is not limited to power transmission:

  • Power Transmission: Carries energy from the pump to the cylinders or motors.
  • Lubrication: Reduces friction between moving parts such as pumps, valves, and cylinders.
  • Heat Transfer: It provides cooling by moving the heat generated in the system to the tank.
  • Sealing: Prevents internal leaks by filling tolerance ranges.
  • Protection: Protects metal surfaces from corrosion and oxidation.

Basic Technical Parameters: Viscosity and VI

The most important technical property of a hydraulic oil is viscosity. Viscosity is the resistance of the fluid to flow.

  • ISO VG (Viscosity Grade): Generally, oils numbered 32, 46 and 68 are preferred. The lower number represents thin (fluid) oil, the high number represents thick oil.
  • Viscosity Index (VI): The ability of the oil to maintain its viscosity against temperature changes. The higher the VI, the oil does not become so thin in extreme heat and not so thick in extreme cold.

Types of Hydraulic Oils and Additives

Hydraulic oils are classified based on their base oil type and the additive packages they contain:

  1. Mineral Based Oils: These are the most widely used economical solutions.
  2. Synthetic Oils: Excels under extreme temperatures and pressures, with a long change life.
  3. HLP / HM Class: Oils containing anti-wear (AW - Anti-Wear) additives, ideal for general industrial use.
  4. HVLP / HV Grade: Suitable for machines operating outdoors and at variable temperatures, with a high viscosity index.

How to Choose the Right Hydraulic Oil?

When choosing the right oil, you should definitely consider the following three criteria:

  • Operating Temperature: The temperature range within which the system operates continuously determines the choice of viscosity.
  • Pump Type: Piston, vane, or gear pumps each require different levels of wear protection.
  • Environmental Factors: Oils with high water separation (demulsibility) properties should be selected for humid environments.

Pro Tip: To extend the life of your oil, have regular oil analysis and ensure your filtration systems are functioning properly (at the micron level).

How is Hydraulic Oil Analysis Performed?

Step 1: Accurate Sampling (Most Critical Stage)

The accuracy of the analysis result depends on how representative the sample is. An incorrectly taken sample can make a clean system look "dirty" or malfunctioning.

  • Timing: Samples must be taken when  the system is  at normal operating temperature (approximately 45-60°C) and running. This prevents the particles from settling and ensures a homogeneous mixture.
  • Equipment: A clean, dry, and previously unused vacuum pump or sample bottle should be used.
  • Method:
    • Vacuum Pump: It is taken from the dipstick hole or tank so that the end of the hose does not touch the bottom (at the middle level).
    • Drain Valve: The  valve should first be flushed to remove some stagnant contamination in the line, and then a sample should be taken.
    • Bottle Filling: The bottle should be filled to approximately 80%, leaving room for agitation.

Step 2: Laboratory Analysis Parameters

In the laboratory environment, the physical and chemical condition of the oil is examined with the following tests:

  • Viscosity Test (ASTM D445): The fluidity of the oil is measured. Viscosity loss indicates that the oil has deteriorated or mixed with another fluid.
  • Particulate Count (ISO 4406): The level of contamination in the oil is determined. It is usually  expressed in codes between 14/12/10 (very clean) and 22/18/13 (very dirty).
  • Water Content (Karl Fischer): The moisture content in the oil is measured in ppm (parts per million). High water causes corrosion and oil degradation.
  • Spectrometric Analysis: Wear metals (Iron, Copper, Aluminum) in the oil are detected. This tells you which part (pump, valve, etc.) is starting to wear out.
  • TAN (Total Acid Count): Indicates the oxidation level and chemical life of the oil.

Step 3: Interpreting the Analysis Report

When the report comes in, you should focus on these three key pillars:

Parameter

Normal Values

Critical Warning

ISO 4406

NAS 7 or lower

NAS 9 and later (Filtering required)

Water Ratio

< 300 ppm

> 500-1000 ppm (Sealing problem)

Viscosity

±10% change

More than 20% change (Oil life ended)

 

Remember: Oil analysis predicts not only the condition of the oil, but also the "storm" inside your machine.