Determining Hydraulic Fluid Viscosity Requirements

Steven Herzog, RohMax; Thelma Marougy, Eaton Corporation

Machine builders recommend hydraulic fluids for their equipment by specifying characteristics such as viscosity, antiwear performance and oxidation stability. They may also identify qualified lubricants by brand name or the ISO viscosity grade. An abridged version of ISO viscosity grades is listed in Table 1.

Accommodating a variety of equipment builder preferences can lead to an abundance of fluids and high inventory costs. Consolidating hydraulic fluids can enhance system performance and reduce costs; however it requires a careful analysis. The guidelines shown in Figure 2 are designed to assist in this analysis. Other factors such as machine builder specifications, additive requirements and compatibility must also be considered.

Viscosity and Efficiency

One of the most important criteria in the selection of a hydraulic fluid is viscosity. There is a common misconception that reducing hydraulic fluid viscosity will decrease operating temperatures, when in fact higher viscosities may lead to a reduction in operating temperatures in hydraulic applications. This is because a hydraulic fluid that is too low in viscosity will reduce the volumetric efficiency of pumps and cause fluid overheating.


Table 1. Abridged Table of ISO Viscosity Grades

In addition, low viscosity fluids can lead to increased friction and pump wear. A fluid that is too high in viscosity, on the other hand, will cause poor mechanical efficiency, startup issues and wear due to cavitation.

Therefore, selecting the proper fluid involves optimizing oil viscosity in terms of volumetric and mechanical pump efficiency, as shown in Figure 1. To accomplish this, the requirements of the hydraulic components, as well as the operating temperature range, must be taken into consideration.


Figure 1. Effect of Viscosity upon the Volumetric and Mechanical Efficiency of Hydraulic Pumps

Viscosity Selection Criteria

Hydraulic pump and motor manufacturers were surveyed regarding the fluid viscosity requirements for their pumps and motors. The majority of equipment was found to provide satisfactory performance with an operating viscosity range of 13 to 860 cSt. Based on this viscosity range, the temperature operating window (TOW) chart, shown in Figure 2, was developed for straight-grade hydraulic fluids. A fluid that has a TOW corresponding to the hydraulic oil reservoir temperature will provide satisfactory performance, at least from a viscosity standpoint.

The TOW chart can be used to identify the viscosity requirements for hydraulic equipment found in most climate-controlled manufacturing environments. These systems typically incorporate coolers and thermostats to stabilize the oil temperature. Hydraulic systems operating in an outdoor environment generally require multigrade hydraulic fluids for efficient performance at high and low temperatures. The National Fluid Power Association document T2.13.13 provides guidelines for selecting multigrade hydraulic fluids for mobile hydraulic fluid applications. Methods for selecting hydraulic fluids for mobile applications and maximum efficiency will appear in upcoming issues of Machinery Lubrication.

To select a hydraulic fluid using the TOW criteria, determine the lowest ambient temperature at startup and the highest fluid temperature in use. For instance, consider a machine shop with low temperature startups of 45ºF and a top system temperature of 150ºF. Compare this temperature range to the TOW chart in Figure 3. ISO VG 46 and ISO VG 68 fluids cover the upper and lower bounds of the 45ºF to 150ºF range. Consequently, inventory may be consolidated by using either an ISO VG 46 or ISO VG 68 antiwear hydraulic oil throughout the plant. Because fluids tend to operate on the left side of the optimum range in Figure 1, an ISO 68 fluid would be expected to provide better volumetric and overall pump efficiency.


Figure 2. Temperature Operating Window for 13 to 860 mm2/s (cSt), 100 VI Hydraulic Fluid

Fluid Consolidation

Consolidating hydraulic fluids can reduce storage space requirements and decrease inventory costs. However, it involves a careful analysis, particularly if failure to use a fluid from the manufacturer's qualified product list voids the machine warranty. The viscosity selection guidelines are designed to assist in this analysis. Other factors such as machine builder specifications, additive requirements and compatibility must also be taken into consideration. If carried out properly, consolidation of fluids can enhance the performance of hydraulic systems and reduce operating costs.


Figure 3. Temperature Operating Window for Hydraulic System Operating at 45ºF to 150ºF

Read more on hydraulic system best practices:

10 Hydraulic Reliability Checks You Probably Aren't Making

The Seven Most Common Hydraulic Equipment Mistakes

How Do You Know if You're Using the Right Hydraulic Oil?

Top 5 Hydraulic Mistakes and Best Solutions


1. ISO 3448 - Industrial Liquid Lubricants - ISO Viscosity Classifications.

2. C. Neveu, S. Herzog, D. Placek, M. Alibert, K. Hedrich. "Influence of Viscosity Increase on the Rate of Temperature Increase of Hydraulic Fluids." NCFP paper I05-13.4 presented at the International Fluid Power Exposition, March 17, 2005, Las Vegas, Nev.

3. P. Michael, S. Herzog, and T. Marougy. "Fluid Viscosity Selection Criteria for Hydraulic Pumps and Motors." Proceedings of the 48th National Conference on Fluid Power, 2000, Chicago, Ill.

4. NFPA/T2.13.13-2002 Recommended Practice - Hydraulic Fluid Power - Fluids - Viscosity Selection Criteria for Hydraulic Pumps and Motors, Milwaukee, Wis.

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