# Hydraulic Cylinders

Explain how power is created using hydraulics and how a little cylinder on the tractor can raise or lower extreme amounts of implement weight. Explain the system including parts and functions.

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The basic concept of hydraulics is using the force of a liquid to take up a volume and to push on a movable cylinder. Hydraulics use the physics described by "Pascal's Law" which states that pressure applied at any point upon a confined liquid is transmitted undiminished in all directions. Thus the cylinder will supply the least amount of opposing force the the pressurized liquid and thus is moved by the pressurized liquid (usually oil). This movable cylinder is usually connected to the part of the machine that you want to move. The benefit of hydraulics is that you are able to supply large amounts of force through movable pipes and hoses using an incompressible liquid.

By definition, power is equal to the rate at which energy is transferred in units of Joule/second. In the case of hydraulics, power is most easily understood by looking at the equation:

P = F*v where P is power, F is force and v is velocity

Force is found by multiplying the pressure that is placed on the cylinder by the effective area of the cylinder.

In the simplest terms power is generated by pressurizing a liquid by use of a motor or an opposing cylinder which supplies a force to an effective area on a cylinder and causes the cylinder to move.

Now to get to the crucial information:

Above I talked about Pascal's Law, "basically, the principle states that the pressure in a closed container is the same at all points. Pressure is described mathematically by a Force divided by Area. Therefore if you have two cylinders connected together, a small one and a large one, and apply a small Force to the small cylinder, this would result in a given pressure. By Pascal's Principle, this pressure would be the same in the larger cylinder, but since the larger cylinder has more area, the force emitted by the second cylinder would be greater. This is represented by rearranging the pressure formula P = F/A, to F = PA. The pressure stayed the same in the second cylinder, but Area was increased, resulting in a larger Force. The greater the differences in the areas of the cylinders, the greater the potential force output of the big cylinder. A hydraulic jack is simply two cylinders connected as described above."(1).

Here is an example to clarify the principles: Please view the attach file. This has a diagram for the problem below.

"Assume a small piston (one square inch area) applies a weight of 1 lbs. to a confined hydraulic fluid. That provides a pressure of 1 lbs. per square inch throughout the fluid. If another larger piston with an area of 10 square inches is in contact with the fluid, that piston will feel a force of 1 lbs/square inch x 10 square inches = 10 lbs.

So we can apply 1 lbs. to the small piston and get 10 lbs. of force to lift a heavy object with the large piston. Is this 'getting something for nothing'? Unfortunately, no. Just as a lever provides more force near the fulcrum in exchange for more distance further away, the hydraulic lift merely converts work (force x distance) at the smaller piston for the SAME work at the larger one. In the example, when the smaller piston moves a distance of 10 inches it displaces 10 cubic inch of fluid. That 10 cubic inch displaced at the 10 square inch piston moves it only 1 inch, so a small force and larger distance has been exchanged for a large force through a smaller distance." (2)

The example problem above displays all the necessary physics to clearly explain why the tractor can raise or lower extreme amounts of weight, including force, power, work, and pressure.

Now for the parts of a hydraulic system on a tractor:

1. Oil Reservoir- holds the oil

2. Hosing - provides a pathway for the oil to move through. You can have narrow high pressure hoses, or wider low pressure hoses.

3. Pump - provides the pressure to the oil.

4. Spool Valve - directs the oil whether to flow against the cylinder or go back to the reservoir.

5. Cylinder - object which moves due to pressure of oil and provides necessary force.

References:

(1) physlink.com. edited by R C Rosignol. 2012. Oct, 30, 2012. http://www.physlink.com/education/askexperts/ae526.cfm

(2) physlink.com. edited by Paul Walorski. 2012. Oct. 30, 2012. http://www.physlink.com/education/askexperts/ae526.cfm

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