Articles about hydraulic, pneumatic, lube, etc. systems design.
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Hydraulic oil viscosity


  • 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/m2.

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

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Cylinder’s Area Ratio


  • Understanding the Cylinder Area Ratio

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By the definition, the “Cylinder Area Ratio” is the relation between the complete area of the bore and the same area minus the area of the rod:

\[ CR = \frac{A_b}{A_b – A_r} \]


For the ratio there is important a diameter of the rod (but not rod side annular area!) because when you choose the cylinders for your project in hydraulic catalogs you pick out them using Piston and Rod diameters.

This is why the area ratio CR is always >1.

Sometimes, you can meet records like:

\[ \frac{A_b}{A_b – A_r} : 1 \]

For example, for cylinder 4″ bore diameter and 2.5″ rod diameter, the ratio is:

\[ CR=\frac{A_b}{A_b-A_r}=\frac{\pi\frac{D^2_b}{4}}{\pi\frac{D^2_b}{4}-\pi\frac{D^2_r}{4}}=\frac{D^2_b}{D^2_b-D^2_r}=\frac{4^2}{4^2-2.5^2}=1.64 \]

So, \(CR=1.64\) or \(1.64:1\)

Hydraulic manifolds material


  • Selecting the type of material for manifold
  • Manifold’s Blackening

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Choosing between ductile steel and aluminum can mean the difference between a failed product or machine and a reliable, productive machine. Consider the following factors:

System Pressures
When system pressure will be consistently above 3500 psi, ductile steel is required. Also, when normal operating pressures are at or near or at 3500 psi, and when pressure “spikes” might exceed 3500 psi, ductile steel should be considered.

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Pilot ratio & pilot pressure of counterbalance valves


  • Understanding the pilot ratio of the counterbalance valves
  • Calculations the pilot pressure of the counterbalance valves for different applications

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Lower pilot ratios will increase system stability and provide better motion control. Therefore, the vast majority of counterbalance applications are satisfied with a 3:1 pilot ratio.

Higher ratios will be more efficient (reduce heat generation) but at the cost of stability and smooth motion control. This is why 10:1 pilot ratio valves, generally, should be avoided. But, sometimes, on motors high pilot ratios will provide adequate dynamic control.

Two areas work to open a counterbalance valve, the relief area (port 3) and the pilot area (port 1); the pilot area divided by the relief area equals the pilot ratio:

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