Which hydraulic oil should I use?

In a previous article, I discussed the use of multigrade, detergent engine oil as a hydraulic fluid in mobile equipment.

Following its publication, I received a lot of feedback. Some folk thought my recommendations were not definitive enough. But when it comes to hydraulic fluid, it is not possible to make one, definitive recommendation that covers all types of hydraulic equipment in all applications.

But in an effort to clarify this matter, I decided to pose (and answer) the following question:

What type of oil would I use in my hydraulic excavator and why?

Assuming that my hydraulic excavator is a late model machine fitted with variable displacement piston pump(s) which operate at pressures up to 5,000 PSI or higher, my selection would be based on the following factors.

Multigrade versus monograde

The operating temperature range of the excavator is the factor that will determine whether I use a multigrade or monograde oil. If my excavator is required to operate in freezing temperatures in winter and tropical conditions in summer, then multigrade oil will be required to maintain oil viscosity within optimum limits across this wide operating temperature range.

If my excavator has a narrow operating temperature range and it is possible to maintain optimum fluid viscosity using monograde oil, I wouldn't chose a multigrade. This is because the viscosity index (VI) improvers used to make multigrade oils can have a negative effect on the air separation properties of the oil. This is not ideal in a mobile hydraulic machine like an excavator, which typically has a relatively small reservoir with correspondingly poor de-aeration characteristics. In addition, by not using a multigrade oil when it's not required, I don't need to concern myself with loss of viscosity as a result of VI improver sheardown.

Detergent versus non-detergent

Detergent oils have the ability to emulsify water, and disperse and suspend other contaminants such as varnish and sludge. This keeps components free from deposits but means that contaminants are not precipitated out - they must be filtered out. These are desirable properties in a mobile hydraulic machine like an excavator which, unlike an industrial hydraulic system, has little opportunity for the settling and precipitation of contaminants at the reservoir, due to its relatively small volume.

I'll be using an oil with detersive/dispersive additives in my excavator and closely monitoring water content on the oil condition reports. Water accelerates aging of the oil, reduces lubricity and filterability, reduces seal life and leads to corrosion and cavitation. And emulsified water can be turned into steam at highly loaded parts of the system. I'll be avoiding these problems by maintaining water content below 100 ppm.

Anti-wear versus no anti-wear

The purpose of anti-wear additives is to maintain lubrication under boundary conditions. The most common anti-wear additive used in engine and hydraulic oil is Zinc dialkyl dithiophosphate (ZDDP).

The presence of ZDDP is not always seen as a positive, due to the fact that it can chemically break down and attack some metals. However, it is an essential additive in oil used in any high-pressure, high-performance hydraulic system, such as a modern excavator fitted with piston pumps and motors. I'll be selecting an oil containing ZDDP at a concentration of at least 900 ppm for use in my excavator hydraulics.

Fill 'er up

In summary, if optimum fluid viscosity can be maintained using monograde oil, then I would select and use one that contains detersive/dispersive additives and at least 900 ppm of ZDDP. If multigrade oil were required, I would use a multigrade hydraulic fluid with a minimum ZDDP content of 900 ppm.

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