Hydraulic System Fundamentals and Fluid Mechanics

Calculating Oil Column Weight and Pressure

1. Calculate the weight of a column of oil that is 15 meters high with a one square centimeter cross-section (using an oil specific gravity of 0.88 Kp/dm³).

2. What is the pressure at the bottom of a column of oil 15 feet high? (Using an oil specific gravity of 0.88 Kp/cm³).

How Pressure is Created in Hydraulic Systems

1. How do I create pressure in a hydraulic system?

Pressure occurs when the flow encounters resistance to its progress. This resistance may be due to the load of the actuator or restrictions (such as orifices) in the piping.

Regulating Flow in Hydraulic Systems

2. Describe how you can regulate the flow in a hydraulic system.

To regulate flow, we need a flow regulator and a pressure-limiting valve. The flow control causes a pressure drop; the valve will open partially to limit and divert a portion of the flow. The resulting effect is a reduction in the flow rate through the flow regulator.

Primary Functions of Industrial Hydraulic Fluids

3. Enumerate and describe the objectives of the fluid in an industrial hydraulic system.

• Power Transmission: As a means of transmitting power, the fluid should be able to move easily through the lines and orifices in the elements. Excessive flow resistance causes significant power losses. The fluid should also be as incompressible as possible so that when a pump or valve is actuated, the action is instantaneous.
• Lubrication: In most hydraulic elements, internal lubrication is provided by the fluid. Pump components and other wear parts slide over each other on a fluid film. To ensure the longevity of the ingredients, the oil must contain the necessary additives to ensure good wear characteristics. Note that not all hydraulic oils contain these additives.
• Sealing: In many cases, the fluid is the only seal against the pressure inside a hydraulic component. The mechanical adjustment and oil viscosity determine the rate of leakage.
• Cooling: The movement of oil through lines and around the vessel walls allows the heat in the system to be dissipated.

Understanding and Preventing Cavitation

4. Describe the phenomenon of cavitation and what can be done to prevent it.

If it were possible to create a full vacuum at the pump inlet, 1.013 bar would be available to boost the oil. However, the available pressure difference is virtually much smaller. One reason is that liquids evaporate in a vacuum, which introduces gas bubbles into the oil. These bubbles are dragged through the pump and collapse with considerable force when exposed to pressure at the outlet (cavitation), causing damage that may impair the operation of the pump and reduce its service life.

To avoid this, we must limit the vacuum at the pump inlet. Most pump manufacturers recommend a vacuum not exceeding 0.84 bar at the entrance to the pump. With an atmospheric pressure of 1.013 bar available in the tank, this leaves only a difference of 0.173 bar of pressure to push the oil into the pump.