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What is Viscosity?
Viscosity is a measure of a fluid’s resistance to flow. The ISO Viscosity Grade defines an oil’s resistance to flow at a temperature of 40c. The viscosity of any oil grade will vary with temperature, it will be more resistant (thicker) at low temperatures and less resistant (thinner) at higher temperatures.
Viscosity in a hydraulic system
The equipment manufacturer will have designed your machine’s hydraulic system to operate reliably within a temperature range, from start-up temperature on a cold morning in winter, to maximum operating temperature on a hot day in summer. The selected oil must be thin enough to move through the pipes and valves on the cold morning, but not become too thin when operating on a hot day.
If the oil is too thick, the pump will have trouble drawing it from the reservoir. If the oil becomes too thin it loses its ability to provide adequate lubrication, and more oil will flow through internal leakage paths making the system less efficient.
If the equipment manufacturer has specified a particular grade of oil then you should use it!
Below are some examples of fluid velocities at different temperatures (measured in centistokes at temperatures in degrees celcuis).
Water – 1 at 20C, 0.66 at 40C and 0.48 at 60C
Honey – 9,000 at 20C, 800 at 40C and 200 at 60C
Hydraulic Oils at common Viscosity Grades:
ISO VG 32 - 85 at 20C, 32 at 40C and 15 at 60C
ISO VG 46 - 140 at 20C, 46 at 40C and 20 at 60C
ISO VG 68 - 210 at 20C, 68 at 40C and 30 at 60C
Article written by:
David Todd BE (Mech)
Fluid Power Engineer
Hydraulic Distributors Pty Ltd
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