Hydraulic Oil

The terms hydraulic oil and hydraulic fluid are at times used interchangeably, although they do similar jobs they are quite different products and cannot safely be interchanged in most applications. They are both fluids, but hydraulic fluid is made up of a number of different fluids. This can include pure, fresh water, water-oil emulsions and salt solutions. An important difference is that hydraulic fluid, unlike hydraulic oil is not flammable, a major disadvantage, which is why it’s used in situations where flammability is an issue such as in vehicles, aircraft electrical and marine applications.

Multi grade oils with viscosity index improvers were starting to be used eighty years ago in high performance vehicles and for other high speed applications. Now this common and very well tested technology is being used to make most of the standard high viscosity index oils used in today’s automotive transmission fluids as well as gear oils. Unfortunately, these viscosity improvers do not usually have the right share stable characteristic of hydraulic oil and so are mostly not suitable for heavy, modern, high pressure, hydraulic systems.

Hydraulic oils with its inherent bulk modulus properties can have its viscosity index improved by using high base stocked synthetics and polymers to meet the high demands of industrial hydraulics.

Properties of Hydraulic Oil

For hydraulic oil to be effective it needs a range of properties that other lubricants share as well as special qualities that make it unique, such as:

Foaming resitance as well as good air release characteristics
To be stable at high temperatures
Have a low oxidation rate
To be hydrolytically stable
High anti-wear performance
Be easily filtered
Demulsibility
Have a good anti-rusting and corrosion inhibiting properties
Able to maintain viscosity regarding film thickness

Hydraulic oils most important qualities are its ability resist reducing in volume when under high pressure (having a high bulk modulus) and a low rate of change in its viscosity with high or low temperature (High viscosity index).

A good analogy is to consider the way a fan belt works and the importance of the right tension on it. If it’s too loose it will slip with the result of a loss of power as it becomes inefficient. With hydraulic oil, if it loses its viscosity or and its bulk modulus, it can be affected in a similar way as the torque or power is transferred from it in the hydraulic system.

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