Can you use engine oil like hydraulic oil?

Two Reasons to Use Multigrade Hydraulic Fluid

Multigrade hydraulic fluids can be used to improve system efficiency. However, their potential to increase machine productivity could have a greater impact on your bottom line.

A case study was published in December 2009 issue Hydraulics & Pneumatics. It showed that a hydraulic excavator had been shipped to the wrong site with the wrong hydraulic oil. This mistake led to four major pump failures that cost $20,000 each, three swing motor failures and two-track motor failures. All of these occurred within 27 months of the excavator’s operation. The total cost of these failures, plus downtime, was $193,872 in the first 27 months of the machine’s existence.

Although your hydraulic equipment may not be the same, the principle is the same: Hydraulic systems that aren’t properly lubricated will not perform well and won’t last long.

What makes Hydraulic Fluid different from other lubricants?

Hydraulic fluid differs from other oils. Hydraulic fluid is a lubricant that also controls how hydraulic power is transmitted throughout the system. It’s both a lubricant as well as a power transmission device. Its dual function makes it unique.

Temperature-viscosity diagram for the “perfect” hydraulic fluid. You will notice that viscosity flatlines at 25 centiStokes, regardless of temperature.

Hydraulic oil must have properties that are similar to other lubricants to be effective and reliable. These properties include anti-wear performance, foaming resistance, hydrolytic stability, thermal, oxidative, and hydrolytic stability, filterability, deductibility, rust, and corrosion inhibition, and viscosity concerning its effect on lubricating films thickness. This is crucial for hydraulic components’ maximum service life.

Hydraulic oil must have high bulk modulus (high resistance against volume reduction under pressure) as well as high viscosity index (low-temperature rate of change in viscosity).

Consider a V-belt drive as an example. The belt will slip if it is not adjusted properly. This results in a lower percentage of heat-related input power. This results in less power being available to do any useful work. This means that the drive is less efficient. Similar results can be seen with hydraulic oil. Hydraulic oil can change in bulk modulus and viscosity to affect how efficiently it is used in hydraulic systems.

The ideal hydraulic fluid for power transmission would be incompressible (infinite stiff) and have a constant viscosity around 25 CST regardless of temperature. Unfortunately, such fluid is not available.

Multigrade Fluids: The Benefits

Bulk modulus, which is an inherent property in a base oil, cannot be altered by additives. However, viscosity index (VI), which is a property of the base oil, can be increased by adding high VI base stocks such as synthetics or polymers called viscosity improvers to your formulation.

In the 1940s, multigrade engine oils were made using viscosity index enhancers. This well-tested technology can be used to make multigrade VI oils. These VI enhancers are typically not shear-stable in modern hydraulic systems. Modern VI enhancer technology has made it possible to sell mineral hydraulic oils that have a shear-stable viscosity index of 150 to 200. (Monograde, mineral hydro oils typically have a VI between 90–100.

Multigrade hydraulic oils are used for their ability to keep viscosity within acceptable limits in all temperatures, including winter and summer. This eliminates the need for seasonal oil changes. Multigrade hydraulic oil can also be used in other situations. There are limits to the viscosity that can be used to maintain sufficient film thickness for hydraulic components. This allows power loss to be minimized, which maximizes power transfer. This allows for faster machine cycles and increases productivity.

Transmission “sweet spot” is a greater operating temperature range. This could be compared to the installation of an automatic tensioner on a V-belt drive, which is used to maintain optimal power transfer conditions.

Based on a simple cost/benefit analysis we can conclude that $200 is a reasonable price to spend. However, this amount should be recouped if there’s a good return.

Multigrade hydraulic oil that is shear-stable and more expensive than monograde is better. The same analysis should be used when weighing the benefits and costs of high VI hydraulic oils. However, savings from increased hydraulic machine performance are more complex than the V-belt drive.