Kinematic viscosity (absolute viscosity divided by density) [m2/s] is used to determine the value of viscosity of industrial lubricants, such as working fluid. Practically, mm square per second [mm2/s] is used, and its value is the same as the value of customary employed centistokes [cSt]. The process of viscosity calculation is specified in “Determination of Kinematic Viscosity and Calculation of Viscosity Index from Kinematic Viscosity” in JIS K 2283, which recommends the use of a thin tube and scale of [mm2/s] for the calculation. Saybolt Seconds Universal[SSU] measured by Saybolt viscosity gauge is also practically used.

The viscosity of working fluid is very important for hydraulic equipment. Working fluid with inappropriate viscosity causes poor pump suction, lubrication, and valve operations, and enhances internal leakage and heat in circuits, which eventually lead to shortened lives and can eventually lead to shorter machine life and an increased possibility of equipment failure or accidents. On the other hand, working fluid with low viscosity is suitable for saving energy by reducing piping resistance. Also, high viscosity index fluid helps to shorten warm-up time in winter. JIS K 2001 “Industrial Liquid Lubricants-ISO Viscosity Classification,” classifies the viscosity spectrum of ISO VG 2 to 3 000 into 20 grades. Figure 14.2 shows the spectrum. It is only relevant to hydraulics. Figure 14.3 clarifies viscosity-and- temperature performance of working fluid with viscosity grades of VG22- VG150.

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