Basic for Hydraulic System
Before getting into the specifics of Hydraulics system it would be helpful for us to understand the objectives we are trying to accomplish through its use. We are not trying to rewrite physics ofcource, but only wish to summarize those elements that a hydraulic system must contend with.
One of the most important topics of today is energy and conservation thereof. However, most people discuss and debate the use of energy without understanding what energy really is. By simple definition , energy is ability to do work. Although energy shows itself in the form of mechanical, electrical, sound, light, heat, or chemical, its origin is usually form our natural resources, namely oil or coal. In physics the law of conservation states that energy can neither be created nor destroyed, although it can be converted from one to another with some forms more useful to us than others.
In a hydraulic system the energy input is called a prime mover. Example of prime movers are electric motors and internal combustion engines. Prime movers and hydraulic pumps are not create energy , they simply put it in a form that can be utilized by a system.
In defining energy we had to use the term work. Simply stated, work is done by when something is moved. If we lift a book from the table we have done a work , however , there are two components which must exist to do work on the book, first of all, if we are to move the book we must exert a force on it in the form of push and pull. This force must be equal in magnitude to the weight of the book and opposed in direction. Consequently, a force has the units of pounds (lbs) and we must know the pounds of force required to do any type of work.
Secondly, if we move the book it must move through some distance which can measured in inches, feet or miles. Thus, we have define the other unit of work; namely distance represented in inches or feet.
It enough force in pounds is exerted to move the book through a distance (ft) then work is done.
Work=Force(lbs.) x Distance (Ft)= ( ft – lbs)
In Hydraulics. The load is the object we want to do work upon. By definition load is resistance to work. Any force which tends to hinder the movement of an object is resistance. Resistive force can be frictional, inertial, or force due to acceleration.
Although frictional forces are well understood, inertial forces are somewhat more complicated. Inertia is the tendency of a body in motion to stay in motion, or if at rest to stay at rest, unless acted on by an external force. Inertial forces are directly related to the mass or weight of an object. The heavier an object is, the harder t start that object moving or to stop it once it’s in motion.
On high speed equipments is it to often necessary to move relatively heavy objects fromest to high speed in short amount of time. This Requires fast acceleration. Isaac Newton’s (1642-1727) second law of motion states that the force required is simply the products of the object’s mass times the required acceleration rate. it must be remembered in the English systems of units, mass is the weight of an object in pounds divided by the acceleration due to gravity. Consequently :
Where F = force In (lbs.)
M = Mass in ( slugs)= w/g = weight/ 32.2 ft/sec2
G= acceleration due to gravity = 32.2 ft/sec2
A = acceleration (ft/sec)
Power is defined as the rates of doing work. To better describe this term will use the example we cited earlier Assuming the book weight 1 pound and we lift it 3 feet the table we have done 3 ft-lbs. of work. It does not matter we lift it fast ( 1 second ) or slow ( 1 hour), we always do the same amount of work . it does, however, take more power to lift the book in a lesser amount of time. Consequently , the unit of power are defined as the amount of work (ft-lbs ) per unit time ( seconds) or:
Power = Ft- lbs./sec.
The common method of measuring power is known as horsepower. Horsepower is defined as the amount of weight (lbs.) a horse could lift one foot n one second by experiments it was found that the average horse could lift 550 lbs. one foot is one second, consequently :
Horsepower = 550 ft. –lbs./sec.
Heat and wasted Energy
Energy which is not use in useful work, can be considered wasted. Although energy cannot be destroyed it can be converted into heat or noise, which is useful for performing work through hydraulics. As a hydraulic engineer it is your responsibility to calculate the power to do work in both directions. Once you know the power you needed you can than design a system which delivers that the power only when needed. A truly efficient system never generates heat by dumping high pressure oil over a relief valve for long periods of time. We will discuss the effect of heat in hydraulic later