Summary:

- Dynamic and kinematic viscosity in SI and British unit
- Understanding the optimum operating viscosity range
- Selecting the ISO VG (Viscosity Grade) for your system
- 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/m^{2}.

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 = 6.89476×10^{6} 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 m^{2}/s but the more common unit is the centistoke (cSt):

1 cSt = 1 mm^{2}/s = 10^{-6} m^{2}/s

1 St = 1 cm^{2}/s = 10^{-4} m^{2}/s

1 m^{2}/s = 10^{6} cSt = 10^{4} 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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Gerardo E. Suellohydraulic oil pressure not included in the chart?

Dzyanis(Post author)The way viscosity measured at the test procedure requires atmospheric pressure and static condition of the fluid (see ASTM D445 – ‘Test Method for Kinematic Viscosity of Transparent and Opaque Liquids’). The way viscosity calculated to make this chart doesn’t involve pressure (see ASTM D341 – ‘Standard Practice for Viscosity-Temperature Charts for Liquid Petroleum Products’).

André MartineauAt what Centipoise number hydraulic oil will be too viscous to be pumped by the hydraulic system of an heavy machinery like an excavator in cold weather, like minus 40 Celcius?

ChrisIt is not just the viscosity you need to consider.

Look at the pump manufacturer’s specification for the operating viscosity range. Danfoss states for the H1P closed circuit pump min. 7mm²/s max 1600. mm²/s . However they recomend a continuous working viscosity of 12-80 mm²/s. The same manufacterer states the the open circuit axial piston pump from the series 45 has a working range from 9-1000 mm²/s. This will be different for other style pumps. The pump inlet pressure is important. Is the fluid to thick (high viscosity), your pump will suffer and be damaged by cavitation due to a high vacuum. That means that flow will be a part of your concern. In cases with such cold weather you will generally find that it it would make sense to run the equipment at a low engine speed until the fluid is warm and the viscosity within working range. When chosing a hydraulic fluid look for the viscosity grade and index they work together and are equally important. A high viscosity index may help.