'Hydraulics 1.0' training

This is an extract from ‘Hydraulics 1.0’ training by Custom Fluidpower registered training organisation (RTO).

Pump fundamentals

A volume of hydraulic oil has a varying weight due to viscosity variances in the range of 8639.81 to 9111.07 newtons per cubic metre in normal operating conditions. In the imperial scale, this would be stated as 55 to 58 pounds per cubic foot.

Liquids are regarded as non-compressible, allowing power to be transmitted instantaneously in a hydraulic system. However, the most common form of fluid used in a hydraulic system is petroleum oil (mineral oil). The most important aspect of mineral oil is its lubricating properties. Mineral oil transmits power readily because it is only very slightly compressible. Mineral oil is known to compress approximately one half of one per cent of its volume at 68.98 bar or 6894 kPa of pressure. This would be 1000 pound per square inch (psi) in imperial terminology.

Note: The compressibility of mineral can pose a safety hazard. Do not attempt to work on any system until all energy has been released in a safe manner back to the reservoir.

A very important factor that needs to be understood relates to the weight of the oil and the effect it has on a positive displacement pump’s inlet condition. The weight of the oil will cause a pressure of about 0.03 bar (2.76 kPa) at the bottom of a 300 mm (0.3 m) column of oil. For each additional 300 mm of height, the pressure will increase by 0.03 bar (2.76 kPa). To estimate the pressure at the base of a fluid column, simply measure the height of the column, divide this by 300 mm (0.3 m) and then multiply by 0.03 bar.

When considering this principle, there are two possibilities as to where the reservoir can be located in relationship to the pump. These are:

• Reservoir mounted above the pump
• Reservoir mounted below the pump

The terminology surrounding the weight of the oil is called a head of oil. The other important fundamental aspect of pump operation relates to atmospheric pressure or head of air.

Atmospheric pressure is caused by the total weight of air based on a height starting at sea level up as high as the atmosphere. To explain this, a column of air 6.45 cm² (1 in²) in cross section extending as high as the atmosphere weighs 65.39 newtons (14.7 lb) at sea level. Simply, this means atmospheric pressure is 101.3 kPa or 1.01 bar (14.7 psi).

The atmospheric pressure of 101.3 kPa was derived from the force pressure area ratio F = p x A. Substituting the values: Force = 65.39 newtons, Area = 6.45 cm² and pressure 1.01 bar, hence the final atmospheric pressure of 101.3 kPa (14.7 psi) that we are now familiar with. Remember this is based at sea level.

At higher altitudes, the weight of the air column is less. As a result, the atmospheric pressure at higher altitudes is less. Similarly, the atmospheric pressure below sea level is higher than 101.3 kPa. Any condition where pressure is less than the atmospheric pressure is called a vacuum or partial vacuum. A complete vacuum exists where the pressure is zero kPa.

The mercury barometer was invented by an Italian scientist named Toricelli. The principle of a mercury thermometer and an understanding of how a column of mercury can be held at a height of 759.97 mm (29.92 inches) in a vacuum are relevant to how the oil from a reservoir fills the pump inlet cavity. For an example, in a mercury barometer, a tube full of mercury is inverted in a container filled with mercury. The column of mercury inside the tube falls to a certain height before it stops. The height of the column of fluid is a constant, and equal to 759.97 mm Hg. This is because the atmospheric pressure on the surface of mercury in the container balances the weight of the column of mercury in the tube.

The space above the mercury column in the tube is absolute vacuum. Thus, 759.97 mm Hg (rounded to 760 mm) is also a standard representation of the pressure of one atmosphere.

Note: Most vacuum gauges are calibrated in inches of mercury.

For more information about fluid power training from Custom Fluidpower, Registered Training Organisation, please visit www.custom.com.au/training or email training@custom.com.au.