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Unofficial Home of Old Simplicity & Allis-Chalmers Garden Tractors

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    The desire to have more useful work performed by the machinery used to care for our outdoor spaces has led to the application of hydraulic principles to accomplish these tasks. The most basic description of hydraulics is that it is the science of transmitting force or motion through a fluid. A fluid can be described as any liquid or gas.

    A child blowing a pinwheel, or a stream of water flowing over a mill wheel are simple examples of the hydraulic science called hydrodynamics. The energy is transmitted by the impact of moving fluid against an object that is able to respond. This science is   however, too inefficient for our use.

    The hydraulic science we are concerned with is called hydrostatics, and deals with the ability to transmit force or motion through a fluid that is confined. Because liquids are practically incompressable, they are the choice fluid. Gasses can be compressed in volume, so are therefore unacceptable for our application.

    Flexibility is the most distinct advantage in the use of liquid to transmit power. It can change shape, divide into parts, reunite as a whole, and fit exactly the container it is in. The liquid usually used in hydrostatics is oil, which provides the high lubricating qualities required for the components in the system.


    In the mid sixteenth century a French mathematician named Pascal described the basic law of hydrostatics. It says ˜Pressure in a confined fluid is transmitted undiminished in every direction, acts with equal force on equal areas, and acts at right angles to container   walls. In a cylinder containing oil, a close fitting piston can be used to apply pressure to the oil to illustrate Pascal's law. (Fig. 1)

    As we study the basic principle, we can see that a second piston exposed to the oil will react to the forces applied by the first piston (Fig. 2). A transfer of energy takes place because a quantity of liquid is subject to pressure. The flexibility of fluid can be noted also as it fills the confines of the cylinders regardless of their shape.


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    The piston applying force to the oil is, in effect, a pump. The piston that can actuate as a result of the pressure upon its surface can be described as a motor. The quantity of oil positively displaced by the pump must be accepted by the motor (assuming no other path of less resistance exists). This piston to   piston energy transfer is the basis for the operation of the hydrostatic transmission.

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    [Source: Simplicity publication, Hydraulic Systems Training Information, #840172, Principles and Operation of Tractor Hydraulic Systems]

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