Most hydraulic systems will operate satisfactorily using a variety of fluids. These include multigrade engine oil, automatic transmission fluid and more conventional antiwear hydraulic oil. But which type of fluid is best for a particular application? While it is not possible to make one definitive recommendation that covers all types of hydraulic equipment in all applications, the following are some of the factors to consider when selecting a hydraulic fluid.

Multigrade or Monograde
Viscosity is the single most important factor when selecting a hydraulic fluid. It doesn’t matter how good the other properties of the oil are if the viscosity grade is not correctly matched to the operating temperature range of the hydraulic system. In this situation, maximum component life will not be achieved. Defining the correct fluid viscosity grade for a particular hydraulic system involves consideration of several interdependent variables. These include:

  • starting viscosity at minimum ambient temperature
  • maximum expected operating temperature, which is influenced by maximum ambient temperature
  • permissible and optimum viscosity range for the system’s components

Typical minimum permissible and optimum viscosity values for different types of hydraulic components are shown in Figure 1.

If the hydraulic system is required to operate in freezing temperatures in winter and tropical conditions in summer, then it’s likely that multigrade oil will be required to maintain viscosity within permissible limits across a wide operating temperature range. 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.

There are some concerns when using multigrade fluids in hydraulic systems. The viscosity index (VI) improvers used to make multigrade oils can have a negative effect on the air separation properties of the oil.1 This is not ideal, particularly in mobile hydraulic systems which typically have a relatively small reservoir with correspondingly 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, it is recommended that the hydraulic component manufacturers’ minimum permissible viscosity values (Table 1) 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 and thereby provides a margin of safety for viscosity loss through VI improver shearing.

If the hydraulic system has a narrow operating temperature range and it is possible to maintain optimum fluid viscosity using a monograde oil, it is recommended not to use a multigrade for the reasons stated above.


Table 1. Typical Minimum Viscosity Values for

Hydraulic Components

Detergent or No Detergent
DIN 51524; HLP-D fluids are a class of antiwear hydraulic fluids that contain detergents and dispersants. The use of these fluids is approved by most major hydraulic component manufacturers. 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, however, 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 relatively 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. These problems can be avoided by maintaining water content below the oil’s saturation point at operating temperature.

Antiwear or No Antiwear
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). 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, and reduce filterability. Stabilized ZDDP chemistry has largely overcome these shortcomings, making it an essential additive to the fluid used in any high-pressure, high-performance hydraulic system, such as those fitted with piston pumps and motors. A ZDDP concentration of at least 900 parts per million can be beneficial in mobile applications.

As far as hydraulic oil recommendations go, for commercial reasons relating to warranty, it is wise to follow the equipment manufacturer’s recommendations. However in some applications, the use of a different type of fluid to that originally specified by the equipment manufacturer may increase hydraulic system performance and reliability. Always discuss the application with a technical specialist from your oil supplier and the equipment manufacturer before switching to a different type of fluid.

Reference

1. Mannesmann Rexroth. “Mineral Oil-based Pressure Fluids for Vane Pumps, Radial Piston Pumps and Gear Pumps as Well as GM, GMRP, MCS, MCR, MR and MKM/MRM Motors.” (RE 07 075/07.98), p.2. 1998.