Hydraulic fluid plays many different roles in the operation of a hydraulic equipment, whether mobile or stationary. Heat transfer, power transfer, and lubrication are all critical aspects of hydraulic fluid performance. The chemical makeup of a hydraulic fluid is the basis for many other decisions when selecting hydraulic fluid for your equipment. Hydraulic oil can range from full synthetic (to handle drastic temperature and pressure swings) to water-based fluids (used in applications where there is a risk of fire). But often it is the many additive, detergent and other finer points of choosing a hydraulic oil that leads to confusion or misunderstandings.
While it is not possible to make one definitive recommendation that covers all types of hydraulic equipment in all applications, there are a few key questions to ask yourself when considering different additive packages and other hydraulic fluid options. Once you understand the basics yourself, you can ask better questions of a potential supplier or oil as a service partner to identify your lubricant needs and choose the right grade, additive, or detergent properties for your equipment.
Is multigrade hydraulic oil always better?
Environment and equipment type are two main considerations for choosing a multigrade type of hydraulic oil. If the system is required to operate in freezing temperatures in winter and tropical conditions in summer, multigrade will do a better job of maintaining viscosity within permissible limits across a wide range of temperatures. But it is important to remember that multigrade does not automatically mean “one size fits all.”
If fluid viscosity can be maintained in the optimum range, typically 25 to 36 centistokes, the overall efficiency of the hydraulic system is maximized (less input power is given up to heat). This means that under certain conditions, the use of a multigrade can reduce the power consumption of the hydraulic system.
For mobile hydraulic equipment users, this translates to reduced fuel consumption.
But, as we said, multigrade fluids are not always ideal. 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, particularly in mobile hydraulic systems which have a small reservoir with poor deaeration characteristics. The high shear rates and turbulent flow conditions often present in hydraulic systems destroy the molecular bonds of the VI improvers over time, resulting in loss of viscosity.
When selecting a high VI or multigrade fluid, a good guideline is that the hydraulic component manufacturers’ minimum permissible viscosity values should be increased by 30 percent to compensate for VI improver sheardown.
This adjustment reduces the maximum permissible operating temperature that would otherwise be allowable with the selected oil, thereby providing a margin of safety for viscosity loss through VI improver shearing.
Monograde hydraulic oil’s best use
A narrow operating temperature range can make it possible to maintain optimum fluid viscosity using a monograde oil. This is preferable when possible for the reasons stated above. These considerations, along with other, broader hydraulic oil selection criteria, should all be factored into your final decision.
When to use antiwear (AW) hydraulic fluid
The purpose of antiwear additives is to maintain lubrication under boundary conditions. The most common antiwear additive used in engine and hydraulic oil is zinc dialkyl dithiophosphate (ZDDP).
While ZDDP is effective, it may not be optimal when used with certain metals or oil filtration systems. Because it can chemically break down and attack these metals while also reducing filterability. Today’s ZDDP chemistry has minimized these problems, making AW hydraulic oils one of the most popular types, especially in high-pressure, high-performance systems, such as those with piston pumps and motors.
A ZDDP concentration of at least 900 parts per million can be beneficial in heavy mobile applications as well.
Detergents in hydraulic fluid
Antiwear hydraulic fluids that contain detergents and dispersants are approved for use by most major hydraulic component manufacturers. Detergent oils emulsify water and disperse and suspend other contaminants such as varnish and sludge.
This keeps components free from deposits, but it also means that contaminants do not settle out — they must be filtered out. These can be desirable properties in mobile hydraulic systems, which, unlike industrial systems, have little opportunity for the settling and precipitation of contaminants at the reservoir, due to its small volume.
The main concern with these fluids is that they have excellent water emulsifying ability, which means that if present, water is not separated out of the fluid. Water accelerates the aging of the oil, reduces lubricity and filterability, reduces seal life and leads to corrosion and cavitation.
Emulsified water can be turned into steam at highly loaded parts of the system. Avoid these problems by maintaining water content below the oil’s saturation point at operating temperature.
Following OEM recommendations
The original equipment manufacturer (OEM) is always the foundation of any good lubricant decision. If you have considered all of the above questions, review the OEM's specifications again to consider how warranties and other manufacturer recommendations might impact your choice.
While there may be a warranty-related reason to follow the equipment manufacturer’s recommendations exactly, some applications in extreme temperatures or other unusual operating conditions may perform better and more reliably by stepping outside strict OEM recommendations. Using something different than what the manufacturer recommends may actually serve you better and keep costs lower even than following warranty restrictions.
However, if you do not have the proper training to make such a decision, it's a good idea to talk with a service provider or technical specialist who can discuss your options and advise you on solutions that fit your needs.