Lesson 4: Fluid Systems Even as you sleep at night, your body works to operate different hydraulic and pneumatic systems. For example, your circulatory system is pumping blood from your heart through big and small vessels, to the tiny capillaries, and back again through veins. Your strong muscular heart provides the force to push the fluid around and many valves direct the blood flow. Your blood pressure' transmits force through the fluid and is a measurable indicator of the health of this vital hydraulic system. Meanwhile, your respiratory system is also working hard. Air is being drawn in by your contracting diaphragm and expanding rib cage and is directed through your nostrils, bronchi, and lungs. Because this system involves the movement of air, it is a pneumatic system. There are of course many hydraulic and pneumatic systems that we encounter and use in our daily lives. All of them consist of a variety of components linked together, arranged such that they can transmit and control forces in a fluid to perform a useful task. While the order of the components may vary, each system begins with a source of energy. This may be an electric motor that drives a pump to exert a force on a liquid or a compressor to exert a force on a gas, transforming the mechanical force into a fluid force. At the end of the system is a device that transforms the fluid forces back into mechanical forces. This device is commonly known as the actuator. The force exerted in the beginning by the pump or compressor often gets multiplied in the fluid system, resulting in achievement of mechanical advantage. Mechanical Advantage In an earlier activity, we studied different types of simple machines and calculated the mechanical advantage of each. That's why we use machines to gain an advantage; we multiply what force we put into a machine and get a lot more force out of the machine. If one simple machine gives us a ten fold mechanical advantage and another gives us a five fold mechanical advantage, then we could link them together to give us a fifty fold mechanical advantage! Note: In the example below, notice that output force divided by input force gives mechanical advantage for a system. Similarly, the output area of the actuator cylinder of fluid divided by input area of the initial cylinder of fluid gives the amount of mechanical advantage provided by a system. Example Let's look at the handy little car jack that you might have in your trunk to change a flat tire. The diagram below shows its two main components. It's basically a combination of two machines: a simple lever and a hydraulic press. This is how it works: You step on the lever with a force of 100 N (let's say).
The lever gives you an advantage of 10 times because the effort arm is 10 times longer than the load arm. The lever's magnified force pushes on the hydraulic press. This 'liquid lever' gives an advantage of 5 times because the area of the outlet piston is 5 times bigger than the area of the input piston; So your initial input force of 100 N has been multiplied by 10 times and then by 5 times. You get a total of a 50 times advantage for this car jack. Your output force is 5,000 N not bad for a machine stored in your car trunk. Check Your Understanding Answer in the space provided
Circuit Diagrams Just as electricians use drawings of electric circuits that incorporate specific symbols for specific components, mechanics use circuit diagrams to communicate the pathway of a fluid system. Common symbols used in these diagrams are shown below. Gauge Pressure versus Absolute Pressure One of the above symbols is the symbol for a pressure gauge. Pressure gauges allow measurement of the amount of pressure in a system. The measurement given by a pressure gauge, called gauge pressure, is the measurement relative to the atmospheric pressure. That is, the gauge pressure is the pressure in the system minus the atmospheric pressure. The absolute pressure in the system would be the gauge pressure and the atmospheric pressure added together.
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System Circuit Diagrams The following circuit shows a car hoist system similar to ones that may be used in a car repair shop. In this system, both gas and liquid are used making it a combination of pneumatic and hydraulic components. A small electric motor pumps air into a pressurized storage tank. The air follows a path through a valve, a filter, and another valve to arrive at a cylinder of oil. The pressure transmitted by the air pushes the oil to the hydraulic cylinder which then raises the car. Study the diagram and take time to recognize and understand the symbols used. Follow the arrows in the transmission lines, remembering that white arrows represent gas flow and black arrows represent liquid flow. Can you see how a mechanic might find a diagram like this helpful in diagnosing and repairing such a machine? Mechanical or Speed Advantage? Keep in mind that you can amplify your force with a high mechanical advantage, but the output movement is slower since it requires a large distance by the small input force to operate the machine. Some machines have a mechanical advantage of less than 1 so they actually employ an input force larger than the output force, but they achieve a speed advantage. Primitive catapults such as trebuchets utilized both advantages; they employed a high mechanical advantage to load and ready the catapult which allowed human effort to lift large weights high into place. Then the release of the catapult utilized the massive force of a large falling weight to gain a speed advantage where the output speed of the projectile was much larger than the speed of the falling weight.
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