# Hydraulic oil viscosity

Summary:

• Dynamic and kinematic viscosity in SI and British unit
• Understanding the optimum operating viscosity range
• Understanding the Viscosity Index

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The viscosity is a measure of the fluid’s resistance to flow.

There are dynamic and kinematic viscosity are usually common for calculations.

The symbol for dynamic viscosity is the Greek letter mu (µ). The SI unit for dynamic viscosity is the pascal-second (Pa·s), but the more common unit is the centipoise (cP):

1 P = 0.1 Pa·s
1 cP = 0.001 Pa·s = 0.001 N·s/m2.

For example, the dynamic viscosity of water at 20°C is 1.00 cP

British unit of dynamic viscosity, is the reyn, named in honour of Osbourne Reynolds:

1 reyn = 1 lbf·sec·inches−2
1 reyn = 6.89476×106 centipoise
1 reyn = 6890 Pa·s

It is easier to measure and more common to report the kinematic viscosity of a ﬂuid. The kinematic viscosity is the ratio of the dynamic viscosity μ to the density of the fluid ρ:

ν = µ / ρ

The symbol for kinematic viscosity is the Greek letter nu (ν).

The SI unit for kinematic viscosity is m2/s but the more common unit is the centistoke (cSt):

1 cSt = 1 mm2/s = 10-6 m2/s
1 St = 1 cm2/s = 10-4 m2/s
1 m2/s = 106 cSt = 104 stokes

In North America are more popular kinematic viscosity units Saybolt Universal Seconds (SUS) or Seconds Saybolt Universal (SSU). The conversion from Centistokes to Saybolt Universal Seconds in terms of calculations is specified by the ASTM D2161 and is not simple. For a quick and an approximate conversion you can use next formulas depends on viscosity range:

SUS to cSt
SUS 32..100 $cSt=0.2253\times SUS-\frac{194.4}{SUS}$
SUS 100..240 $cSt=0.2193\times SUS-\frac{134.6}{SUS}$
SUS >240 $cSt=\frac{SUS}{4.635}$
cSt to SUS
1 > cSt > 20.6 $SUS=\frac{cSt+\sqrt{cSt^2+175.2}}{0.4506}$
20.6 > cSt > 52 $SUS=\frac{cSt+\sqrt{cSt^2+118.07}}{0.4386}$
cSt > 52 $SUS=cSt\times4.635$

NOTE: Equations above are for fluid with specific gravity 0.876 (like petroleum oil) and at fluids temperature 37.8°C (100°F).

Kinematic viscosity for some common liquids you can see at The Engineering ToolBox.

Viscosity is a function of temperature. As the temperature increases, liquid viscosity decreases and leakage becomes more significant, reducing the volumetric efficiency. As the viscosity decreases (when temperature increasing), the mechanical efficiency will increase due to low forces:

Hydraulic components will operate efficiently only within a specific viscosity range, optimum operating range for each of them. A fluid which is too viscous may prompt cavitation. Conversely, a fluid which is too thin may allow an accelerated rate of wear and additional slip losses.

Generally, optimum operating viscosity of the hydraulic oil should be  between 16 cSt (80 SUS) and 40 cSt (180 SUS).

As a rule, the manufacturers of hydraulic components give hydraulic fluid viscosity recommendations in accordance with type of their pump you use in the system. In general, an oil which matches the viscosity requirements of the pump, will also be satisfactory for valves. For example, see recommendations from EATON.

This is a generic chart for viscosity grade selection in depend of ambient temperature:

The International Standards Organization created the ISO VG (Viscosity Grade) in response to the need for a globally recognized viscosity designation. The actual VG value signifies a lubricant’s average viscosity at 40 degrees C. For example, a lubricant with a VG value of 22 will have an average viscosity of 22 cSt (centistokes) at 40 degrees C:

Viscosity-Temperature chart for most popular hydraulic oil:

You can download full chart in PDF: Viscosity-Temperature-Chart.pdf or use online tool: TEMPERATURE-VISCOSITY CHART.

The way viscosity changes with temperature is reflected by the Viscosity Index: the smaller the viscosity change is the higher the viscosity index. The viscosity index of hydraulic system oil should not be less than 90.

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