Hydraulic Oils and Their Properties

In this article, we’ll cover topics like hydraulic oils, the properties of hydraulic oils, the functions of hydraulic oils and hydraulic filters, choosing hydraulic oil, what viscosity is, and what a hydraulic oil’s viscosity should be.

Hydraulic Oils

Hydraulic oils have wide-ranging applications in many areas of industry, in motion and power-transmission systems. For this reason, to be able to choose the right oil for the system being used, it’s necessary to know the main functions of hydraulic oils and take properties like compressibility, power transmission, foaming, and viscosity into account.

For choosing a hydraulic oil suited to working conditions, the properties of the anti-wear, anti-corrosion, and water-separating additives found in hydraulic oils also need to be known.

When choosing hydraulic oil, ISO viscosity grades, choosing viscosity according to the equipment, using the viscosity selection table, and the properties a safe hydraulic oil should have are also of great importance.

Advances in hydraulic lubricant technology have made it possible, through the use of additives, to reduce foaming in hydraulic oils, increase load-carrying capacity, improve anti-wear properties, lower the pour point, and improve the viscosity index. Fire-resistant hydraulic oils were developed for locations with fire risk.

The Importance of Hydraulic Oils

Hydraulic systems are found in many areas, from everyday life to industrial machinery. For this reason, hydraulic oils are naturally the most heavily consumed type of oil.

Faults like pump failure, hose bursts, and oil leaks are common in hydraulic systems. On machines with an oil leak, new oil is continuously added to replace the amount lost. Even so, if you ask people running these systems, you’ll often be told they don’t have any problems. This is because these faults have become so normalized that they’re accepted as routine.

However, today, under manufacturing conditions where mass production, production cost calculations, and production time — down to the second — matter, and where production goals like high machine performance, achieving maximum efficiency with minimum energy, minimal breakdowns, and minimal waste are expected, choosing the right hydraulic oil for the job also matters a great deal for superior efficiency in hydraulic systems.

For a hydraulic system’s efficiency, it’s very important for the system’s components to be high quality. But the single biggest factor causing faults in hydraulic systems is the hydraulic oil. Even in a hydraulic system built from the world’s best materials, if a suitable, quality hydraulic oil isn’t used, faults are inevitable in a very short time.

Hydraulic oils are a combination of a suitable base oil with suitable additives. There are many factors that can prevent this combination from maintaining its stability. During operation, the oil heats up due to friction and compression. This heating changes the oil’s chemical structure. Oxidation also changes the oil’s structure.

In particular, some of these altered molecules formed in the oil’s additives build up on precision surfaces, especially on valves. For precision parts operating under high pressure, this buildup can cause a system failure.

Properties of Hydraulic Oils

Viscosity of Hydraulic Oils

A hydraulic oil’s most important property is its viscosity. The oil’s viscosity should be high enough to meet the following requirements:

1- It should be efficient enough to lubricate the system’s components, especially the pump.
2- It should provide effective sealing to prevent leakage through the working tolerances of pumps, motors, and valves.
3- The oil’s viscosity should allow it to flow freely within the hydraulic circuit, while also being low enough to provide effective cooling.

Viscosity and Operating Temperature in Hydraulic Oils

What is viscosity?: It’s the name given to the resistance a liquid shows to flowing. If viscosity is low, flowability is high; if viscosity is high, flowability is low.

What is the viscosity index?: In an environment where the oil operates at high temperature for long periods, an oil with a high viscosity index will wear out less than an oil with a low viscosity index.

What should a hydraulic oil’s viscosity index be?

A fluid’s viscosity index should be chosen high enough, taking into account the operating temperature range of the hydraulic system it’s used in.

The fluid should maintain its lubricating properties even at the highest operating temperatures, while also flowing easily at the lowest operating temperatures, without causing difficulty, especially during cold starts.

Many fluids have a viscosity index around 100, but in fields with very large operating-temperature swings, such as aerospace, viscosity indexes of 150 or higher are needed.

Foaming in Hydraulic Oils

When hydraulic fluid containing trapped air returns to the system’s reservoir, a tendency toward foaming is seen as the air bubbles rise to the surface. If this foam enters the hydraulic circuit, the hydraulic circuit’s efficiency drops. System components can also be damaged, because foam is a far less effective lubricant than a film of oil. For this reason, a hydraulic fluid is expected to have good anti-foaming properties.

If needed, anti-foam additives can be added to prevent foam formation. Anti-foam additives can slow down the rate at which air is released, so choosing the right type and amount of additive is very important.

The Roles of Hydraulic Oils

Hydraulic fluids perform three important functions:

Transmitting power, protecting the hydraulic system, and coping with operating conditions.

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