A hydraulic system is defined as a, ‘machinery unit which uses fluid as a pressure medium’. It has a number of advantages over an electrical system of a similar working capacity.
The power to weight ratio is at least 14 times better than an equivalent electrical system. (2)
Self-resetting overload protection.
Infinitely variable speeds.
No static electricity.
Low speed, high torque with motors.
However, there is a downside!
Large pipework is often employed (in contrast to the relatively thin wiring used in electrical systems). (2)
Units are often physically larger.
Large diversity of units available which can sometimes mean that spares are difficult to obtain. (4)
Hydraulic units, particularly those designed to work under high pressures, can be very expensive. (5)
Hydraulic fluids under pressure are dangerous and under certain conditions, flammable.
PRINCIPLES OF HYDRAULIC SYSTEM OPERATION
Hydraulic systems employ fluids to transmit energy from the point of application, ie power source to the output actuator. A fluid is defined as, ‘a substance that will readily take up the shape of its container’ and it may be either a liquid or a gas. However, one of the main characteristics of gases is that they are compressible and this means that they are not suitable for use in hydraulic systems. Liquids, on the other hand are incompressible and this means that any movement at one extreme of a column of fluid in a pipe, or other component, is immediately reproduced at the other. This characteristic of, ‘positive displacement hydraulics’, together with ‘Pascal’s Law’ (see below) are the fundamental principles on which hydraulic systems operate.
Pascal’s Law: States that, a force applied to a liquid in an enclosed container, will be transmitted, undiminished throughout the liquid and act on, acts with equal force on equal areas and at right angles to, the container’s surfaces.
It is the pressure that is developed in the fluid that ‘carries’ this force and gives rise to another important property of a hydraulic system,
ie that of, Force Magnification (see below).
Pressure does not occur simply because a force is applied - pressure only occurs if there is some opposition to that force, ie pressure results when there is a resistance to fluid flow or when a force acts to make a fluid flow.
Force Magnification: In the diagram below consider a force of 1 lb applied to the small piston which has an area of 1 square inch. From the equations above we know that a pressure of 1 lb will be developed at the piston’s face and, because of Pascal’s Law, this pressure will be transmitted throughout the fluid. This means that this pressure will act on the output piston (Area 10 sq in.) and that since, Force = Pressure x Area, the force developed at the output will be 10 lb, ie the system has a force magnification of 10, a ‘mechanical advantage’ of 10.
The HYDRAULIC JACK - a basic application of force magnification in a hydraulic system.
Operation: Force applied to the input lever means that pressure is developed at the small input piston face. Due to Pascal’s Law this pressure is transmitted via Non-Return Valve (NRV) V1 to the much larger output piston and the force developed there is sufficient to lift a much larger load than would otherwise be possible (force magnification). Note also that for the load to move:
Not only has the energy required to move it to be transmitted through the fluid in the form of pressure;
but also, the fluid must move....
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