Classification of Hydraulic Oils. Types of Hydraulic Oil

This article’s topic will be the classification of hydraulic oils and types of hydraulic oil. We’ll look at what basis hydraulic oils are classified on. We’ll cover where the different types of hydraulic oil are used.

Classification of Hydraulic Oils

If we classify hydraulic oils by the additives they contain and the properties those additives give the hydraulic oil, the types of hydraulic oil are as follows:

HH Category Hydraulic Oils

These are plain mineral oils. They contain no additives. They’re low-cost products. Suitable for use in non-critical systems.

HL Category Hydraulic Oils

These are R&O-type mineral oils. The distinguishing feature of these hydraulic oils is that they contain antioxidants and rust inhibitors.

In short, this is hydraulic oil with anti-rust additives.

They have a longer service life than HH oils. Suitable for systems where anti-wear performance isn’t required.

HM Category Hydraulic Oils

These are similar to HL-type oils. The distinguishing feature of these hydraulic oils is that they also contain anti-wear additives. They’re used where a longer service life and protection against wear are needed. They’re mostly used in mobile and industrial systems.

HV Category Hydraulic Oils

The distinguishing feature of HV category hydraulic oils is their high viscosity index. They’re used in places that encounter very extreme temperatures, or where only small changes in viscosity are allowed as temperature changes. For example, iron and steel industry plants.

Viscosity-temperature relationship diagram by type of hydraulic oil

ISO VG Classification

Hydraulic oils are classified under the ISO VG (Viscosity Grade) standard with numbers such as 32, 46, and 68.

Viscosity-Temperature Relationship

As the oil’s number gets higher, the oil’s viscosity (resistance to flow) at 40°C increases.

Importance of Choosing the Right Oil

Using oil with the wrong viscosity can cause pump wear or a drop in system efficiency.

What Does a Hydraulic Oil’s Number Mean?

The number 46 printed on the hydraulic oil in the image above indicates the hydraulic oil’s viscosity.

In other words, when we say “grade 46 hydraulic oil,” we’re actually saying the oil’s viscosity is 46 cSt. For optimum lubrication, numbers between 10 and 50 are used.

NOTE: Viscosity: The resistance shown to flow. As viscosity increases, flowability decreases.

Related Questions

HL oils contain antioxidant and anti-rust additives, while HH oils contain no additives at all. These additives extend service life by slowing down the oil’s degradation over time and the corrosion it causes in the system.

These oils contain anti-wear additives in addition to the properties found in the HL category. Since friction is greater between frequently moving parts, this extra protection is decisive for extending part life in such systems.