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Chapter 6. HYDROSTATIC TRANSMISSION

6.1 Hydrodynamics Vs. Hydrostatics

Hydraulic transmission is a power transmission using fluid as medium. Today, there are many thousands of pressure-operated machines and they are so distinct from earlier devices . They are used very broadly in industries and in our life . The name hydraulics was applied to systems using liquids. In modern terminology, hydraulics implies a circuit using mineral oil. Hydraulics are divided into two sciences - hydrodynamics and hydrostatics.

Hydrodynamics deals with the characteristics of a liquid in motion, especially when the liquid impacts on an object and releases a part of its energy to do some useful work.

Hydrostatics deals with the potential energy available when a liquid is confined and pressurized. This potential energy also known as hydrostatic energy is applied in most of the hydraulic systems. This field of hydraulics is governed by Pascal's law.

It can thus be concluded that pressure energy is converted into mechanical motion in a hydrostatic device whereas kinetic energy is converted into mechanical energy in a hydrodynamic device.

Hydrodynamics can be called the science of moving liquids under pressure (We have studied already). A water wheel or turbine (Shown in the Figure) represents a hydrodynamic device. Energy is transmitted by the impact of a moving fluid against vanes. We are using the kinetic energy that the liquid contains.

A hydrodynamic transmission consists of non-positive displacement pumps and motors, such as turbine and centrifugal pumps the flow is continuous from inlet to outlet and results from energy being directly imparted from the fluid stream. These machines are basically low pressure with high volume output.

Transmission of energy using kinetic energy

The underlying principle of how fluids transmit power is revealed by Pascal's law. Pascal's law states "Pressure in a confined fluid is transmitted undiminished in every direction, and acts with equal force on equal areas, and at right angles to the container walls".

The transmitted pressure acts with equal force on every unit area of the containing vessel and in a direction at right angle to the surface of the vessel exposed to the liquid.

Pascal's law can be illustrated by the following example.

In a hydrostatic device, power is transmitted by pushing on a confined liquid (the below figure). The liquid must move or flow to cause motion, but the movement is incidental to the force output. A transfer of energy takes place because a quantity of liquid is subject to pressure. Most of the hydraulic machines in use today operate hydrostatically - that is through pressure.

6.2 Hydrostatic transmission - General conception

This type of transmission bases on uncompressible property of fluids to transfer energy, other words, fluid power is the use of a confined fluid flowing under pressure to transmit power from one location to another. The medium of transmission is a flowing fluid. Other words, HST is transmission of power by fluid using positive displacement

pumps and motors.

A hydrostatic transmission consists of a fixed or variable-displacement pump and a fixed or variable displacement motor, operating together in a circuit.

The primary function of any hydrostatic transmission (HST) is to accept rotary power from a prime mover (usually an electric motor or an internal combustion engine) having specific operating characteristics and transmit that energy to a load having its own operating characteristics. In the process, the HST generally must regulate speed, torque, power, or, in some cases, direction of rotation. Depending on its configuration, the HST can drive a load from full speed in one direction to full speed in the opposite direction, with infinite variation of speed between the two maximums - all with the prime mover operating at constant speed.

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