Other factors such as machine builder specifications, additive requirements and compatibility must also be taken into consideration. The viscosity selection guidelines are designed to assist in this analysis. However, it involves a careful analysis, particularly if failure to use a fluid from the manufacturer's qualified product list voids the machine warranty. Temperature Operating Window for 13 to 860 mm2/s (cSt), 100 VI Hydraulic FluidĬonsolidating hydraulic fluids can reduce storage space requirements and decrease inventory costs. 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.įigure 2. Consequently, inventory may be consolidated by using either an ISO VG 46 or ISO VG 68 antiwear hydraulic oil throughout the plant. ISO VG 46 and ISO VG 68 fluids cover the upper and lower bounds of the 45✯ to 150✯ range. Compare this temperature range to the TOW chart in Figure 3. For instance, consider a machine shop with low temperature startups of 45✯ and a top system temperature of 150✯. To select a hydraulic fluid using the TOW criteria, determine the lowest ambient temperature at startup and the highest fluid temperature in use. Methods for selecting hydraulic fluids for mobile applications and maximum efficiency will appear in upcoming issues of Machinery Lubrication. The National Fluid Power Association document T2.13.13 provides guidelines for selecting multigrade hydraulic fluids for mobile hydraulic fluid applications. Hydraulic systems operating in an outdoor environment generally require multigrade hydraulic fluids for efficient performance at high and low temperatures. These systems typically incorporate coolers and thermostats to stabilize the oil temperature. The TOW chart can be used to identify the viscosity requirements for hydraulic equipment found in most climate-controlled manufacturing environments. A fluid that has a TOW corresponding to the hydraulic oil reservoir temperature will provide satisfactory performance, at least from a viscosity standpoint. Based on this viscosity range, the temperature operating window (TOW) chart, shown in Figure 2, was developed for straight-grade hydraulic fluids. The majority of equipment was found to provide satisfactory performance with an operating viscosity range of 13 to 860 cSt. Hydraulic pump and motor manufacturers were surveyed regarding the fluid viscosity requirements for their pumps and motors. Effect of Viscosity upon the Volumetric and Mechanical Efficiency of Hydraulic Pumps To accomplish this, the requirements of the hydraulic components, as well as the operating temperature range, must be taken into consideration.įigure 1. Therefore, selecting the proper fluid involves optimizing oil viscosity in terms of volumetric and mechanical pump efficiency, as shown in Figure 1. A fluid that is too high in viscosity, on the other hand, will cause poor mechanical efficiency, startup issues and wear due to cavitation. In addition, low viscosity fluids can lead to increased friction and pump wear. This is because a hydraulic fluid that is too low in viscosity will reduce the volumetric efficiency of pumps and cause fluid overheating. 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. One of the most important criteria in the selection of a hydraulic fluid is viscosity. Other factors such as machine builder specifications, additive requirements and compatibility must also be considered. The guidelines shown in Figure 2 are designed to assist in this analysis. Consolidating hydraulic fluids can enhance system performance and reduce costs however it requires a careful analysis. An abridged version of ISO viscosity grades is listed in Table 1.Īccommodating a variety of equipment builder preferences can lead to an abundance of fluids and high inventory costs. They may also identify qualified lubricants by brand name or the ISO viscosity grade. Machine builders recommend hydraulic fluids for their equipment by specifying characteristics such as viscosity, antiwear performance and oxidation stability.
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