| Intermediate Technology Education | Control Technology | Pre-Design | Topic 3 | Activity 6 |

Hydraulic Systems

This activity looks at the basics of hydraulics, with examples that illustrate how hydraulics may be used
  • Review basic principles of hydraulics
  • Review applications for hydraulics

Hydraulics

Hydraulic cylinders employ pressurized liquids, called hydraulic fluid—typically some form of oil (even vegetable oil)—to do work.  The output is usually in the form of linear or rotary motion.

Hydraulics is similar in many ways to Pneumatics.  They both use pressure to do work, employ pressurized cylinders with moving pistons, employ pressurized rotary devices, and employ a variety of similar control mechanisms. 

However, they differ in these important ways. 

  • Liquids cannot be compressed.  As a result there are two major behaviour differences from pneumatics
    • Hydraulic systems do not exhibit the 'bounce' that is present in pneumatic systems which is the result of the air compressing.  Control can be more accurate with respect to positioning.
    • Hydraulic systems can operate at much higher pressures, and can therefore handle much heavier loads.
  • Pneumatic systems allow air to escape to the atmosphere when the work is done.  Hydraulic systems cannot do that.  It would be very expensive to continuously replace the oil, and it would be very polluting to let it escape.  As a result, hydraulic systems collect the oil after it has done its work and return it to the system to be re-pressurized.

The following video clips illustrate simple hydraulic mechanisms using syringes as cylinders, plastic tubing as hydraulic lines, and coloured water as the hydraulic fluid.  Compare the action in these to the similar mechanisms used for pneumatics in the previous activity.

                  
Video Hydraulics        Video Hydraulic Arm

Hydraulic Systems

A simplified version of a typical hydraulic system is illustrated in the next two drawings.  In the first drawing, pressure is causing the piston to move out of the cylinder. In the second drawing, pressure is causing the piston to move into the cylinder.  Note the direction of the hydraulic fluid through the control valve.  The valve has passageways (not shown) in it that control the fluid flow.  Usually a lever or plunger mechanism is used to change the valve position.  A plunger is shown


Figure Hydraulic System Piston being pushed out—Note the Plunger position

Figure Hydraulic System Piston being pushed in—Note the Plunger position

Hydraulics systems are used as a power source in heavy equipment, machinery, and even as oil pumps in car engines.  A car's brake system is also hydraulic.

For More Information

You may want to check these sites for more information on hydraulics

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