Engineering materials - Ferrous metals

Engineering materials - Ferrous metals

Engineering materials - selection

In the design of machinery in general, a vast variety of materials of both organic and inorganic origin is utilized. We generally think of metals as the usual materials of design, but, although used to a lesser degree, such materials as wood, leather, rubber, and other plastics have widespread use, and others, such as fabrics, cork, special minerals, etc., have limited use.

In making a selection of a material we must first decide what constitutes a "proper materials". A proper material may be defined as one which best performs the functions required with the least total cost. This does not mean that the material having the lowest unit cost is best, because a more expensive material may permit reduction of weight, eaier heat tretment or fabrication, or it may process other advantages that make the final result less costly. And at times, of course, luxury, aooearance, or extreme safety is desired even at great expense.

Designers are interested principally in the physical properities and the cost of the finished part, and only incedentally in the chemical constituents and methods of preparation from the raw material. The physical properities of most importance are strenght, regidity, resistance to corrosion and to fatigue failure, and in some cases, weight. Other properities that may be of importance are hardness, impact resistance, heat and electrical conductivity, wear resistance, low friction, machinability, and weldability, When several of these characteristics are desired simultaneosly, selection of the most suitable and the most economical material is sometimes difficult.

Ferrous metals - Iron.

In general, we may say that certain groups of materials are used mainly because they are abundantly available and cheap. This condition is particularly true of the ferrous group of metals.

Ferrous metals are the most commonly used, and, with proper alloying and treatment, they may be adapted to almost all simple needs. The advantages of iron as a base metal, in addition to its abundance and low cost, are its strength and its adaptability to fabrication. It may be readily cast, forged, machined, and welded. Principal limitations are its weight and its susceptibility to corrosion.

Plain carbon steels

Steel differs from cast iron in that it has no carbon in free state. The percentage of carbon varies from 0.08 to 1.5 with consequent differences in properities. Steel is classified according to carbon content approximately as follows: "Very mild", "mild", or "low carbon"; "medium carbon", and "high carbon" or "hard". Both low and medium-carbon steels are generally used for machine parts, whereas high-carbon steels are used for springs or tools. Low-carbon steels are readily welded and foged since they are plastic over an extensive temperature range. They are very ductile and hence are resistant to shock and impect, but are not responsive to heat treatment by quenching. Medium-carbon steels are more difficult to forge and weld, but tensile strenght and elastic limit can be increased considerably by quenching at the expense of lessened ductility. High-carbon steels are difficult to forge and weld but may be hardened to a good cutting edge by quenching.

Alloy steels.

When metals are dissolved in each other and then solidified, an alloy results. Alloy steel is obtained when the other elements added to the iron and carbon are in sufficient quantities to influence the physical properities. Practically all alloy steels must undergo special heat treatment to obain the properities desired. The alloying elements used in steel are as follows: Nickel, silicon, chromium, vanadium, tungsten, molybdenum, manganese, and copper.

Several of these may be used simultaneously to obtain special physical properities when given a double heat treatment, High elastic limit with ample ductility, hard wearresisting surfaces combined with high core strength and toughness, high impact and fatigue resistance, are some of the properities that are readily attainable.   

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Engineering mechanichs - Part two

Engineering mechanichs - Part two

Statics

Static deals with the conditions of equilibrium of bodies acte upon by forces and is one of the oldest branches of science. When several forces of various magnitudes and directions act upon a body, they are said to constitute a system of forces. The general problem of statics consist of finding the conditions that such a system must satisfy in order to have equilibrium of the body. The various methods of solution of this problem are based on several axioms, called the principles of statics, which are given here in brief.

 

Parallelogram law

If two forces, represented by vectors AB and AC, acting under an angle α, are applied to a body at point A, their action is equivalent to the actionof one force represented by the vector AD, obtained as the diagonal of the parallelogram constructed on the vectors AB and AC.

Equilibrium law

Two concurrent forces can be in equilibrium only of their resultant is zero. This will be the case if we have two forces of equal magnitude acting in opposite directions along the same line. We shall now generalize this conclusion as the second principle of statics, usually called the equilibrium law: Two forces can be in direction, and collinear in action.

Law of superposition

When two forces are in equilibrium (equal, opposite, and collinear), their resultant is zero and their combined action on a rigid body is equivalent to that of no force at all. A generalization of this observation gives us the third principle of statics, sometimes called the law of superposition: The action of a given system of forces on a rigid body will in no way be changed if we add to or subtract from them another system of forces in equilibrium.

Law of action and reaction

Very often we have to investigate the conditions of equilibrium of bodies that are not entirely free to move. Restriction to the free motion of a body in any direction is called constraint. A body that is not entirely free to move and is acted upon by some applied force (of forces) will, in general, exert pressures against its supports. These actions of a constrained body against its supports induce reactions from the supports on the body, and as the fourth principle of statics we take the following statement: Any pressure on a support causes an equal and opposite pressure from the support so that action and reaction are two equal opposite forces.

 Kinematics and kinetics

In statics we have considered rigid bodies that are rest. In dynamic we shall consider bodies that are in motion. For convenience, dynamics is comonly divided into two branches called kinematics adn kinetics. In kinematics we are concerned only with the space-time relationship of a given motion of a body and not at all with the forces that cause the motion. If we see that a wheel rolls along a straight level track with uniform speed, the determination of the shape of the path described by a point on its rim and of the position along this path that the chosen point will occupy at any given instant are problems of kinematics.

In kinetics we are concerned with finfing the kind of motion that a given body or system of bodies will have under the action of given forces, or with what forces must be applied to produce a prescribed motion. If a constant horizontal force is to be applied to a given body that reset on a smooth horizontal plane, the prediction of the way in which the body will move is a problem of kinetics.

The whole science of dynamics is based on the natural laws governing the motion of a particle under the action of a given force. Whenever a particle moves through space, it describes a curve that is called the path. The path of a particle may be either a pace curve, called a tortuous path, or a plane curve, called a plane path. In the simplest case the path will be a straight line, and the particle is said to have rectilinear motion. This can be uniform of nonuniform. If the rectilinear motion of a particle is nonuniform, its velocity is changing with time and we have acceleration. In the case of uniform motion, however, the velocity remains constant describes a curved path, it is said to have curvilinear motion.

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Engineering mechanics - Part one

Engineering mechanics - Part one

The importance of mechanics in the preparation of young engineers for work in specialized fields cannot be overmphasized. The demand from industry is more and more for young men who are soundly grounded in their fundamental subjects rather than for those with specialized raining. There is good reason for this trend: The industrial engineer is continually being confronted by new problems, which do not always yield to routine methods of solution. The man who can successfully cope with such problems must have a sond understanding of the fundamental principles that apply and be familiar with various general methods of attack rather than proficient in the use of any one. It seems evident, then, that university training in such a fundamental subject as mechanics mus seek to build a strong foundation, to acquaint the student with as many general methods of attack as possibile, to illustrate the application of these methods to practical engineering problems, but to avoid routine drill in the manipulation of standardized methods of solution.

The solution of a problem in mechanics usually consists of three steps:

1. The reduction of a complex physical problem to such a state of idealization that it can be expressed algebraically or geometrically;
2. The solution of this purely mathematical problem;
3. The interpretation of the results of the solution in terms of the given physical problem. It is too often the case that the student's attention is called only to the second step so that he does not see clearly the connection between this and the true physical problem. 

Rigid body 

We shall be mostly concerned in engineering mechanics with problems involving the equilibrium of rigid bodies. Physical bodies, such as we have to deal with in the design of engineering structures and machine parts, are never absolutely rigid but deform slightly under the action of loads which they have to carry.

Force

For the investigation of problems of statics we most introduce the concept of force, which may be defined as any action that tends to change the state of rest of a body to which it is applied. There are many kinds of force, such as gravity force, with which we are all familiar, and the simple push or pull that we can exert upon a body with our hands. Other examples of force are gravitational atraction between the sun and planets, the tractive effort of a locomotive, the force of magnetic attraction, steam or gas pressure in a cylinder, win pressure, atmospheric pressure and frictional resistance between contiguous surfaces.

Characteristics of a force

For the complete definitions of a force we must know 1 its magnitude, 2 its point point of application, and 3 its direction. These three quantities, which completely define the force, are called its characteristics or specifications.

The magnitude of a force is obtained by comparing it with a certain standard, arbitarily taken as the poundm which represents the weight of a certain platinum cylinder kept in the Tower of London. The magnitudes of forces are commonly measured by using various kind of dynamometers.

The point of application of a force acting upon a body is thet point in the body at which the force can be assumed to be concentrated. Physically, it will be impossible to concentrate a force at a single points; that is, every force must have some finite area or volume over which its action is disturbed. However, we often find it convenient to think of such disturbed force as being concentraded at a single point of application wherever this can be done without sensibily changing the effect of the force on the contitions of equilibrium. In the case of gravity force disturbed throught the volume of a body, the ooint of application at which the total weight can be assumed to be concentrated is called the center of gravity of the example is always directed vartically downward. 

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Descriptive geometry and mechanical drawing

Descriptive geometry and mechanical drawing

Descriptive geometry

Is a mathematical-graphical procedure that has for its purpose the visualization of structures and their exact representation in drawings. After analysic of any structure, each element is shown in the drawing in its exact geometrical relation to the other elements.

The basic methods of descriptive geometry are the projection method and the direct method. There are two general types of views, perspective and orthographic. A perspective view is observed from a fixed station point, or point of view, by means of converging rays of light that meet at the eye of the observer. An orthogographic view of an object is observed in a chosen direction by means of paralel rays of light.

Mechanical drawing

Language is a defined as the expression of thought. But if we attempt to describe in words the appearance and details of a machine, or bridge, or building, we find it not only difficult but in most cases impossible. Here we must use another language, the universal graphic language of drawing.

A writen description of a new machine part would have to be very long to tell about it and even then might be misunderstood. A picture of it would serve the purpose much better, but the picture would not show the exact method of construction. It would gove only the external appearence without telling what was inside. It would be impossible to construct a locomotive or an airplane from either a word description or a picture.

Fortunately, another form of description has been developed by which the exact shape of every detail of any structure may be defined accurately and quickly. This method consist of the making of a series of views arranged according to a definite system, with figures added to tell the sizes. This is know as "mechanical drawing" and it forms so important a part of all undustrial an mechanical work that it is called the "language of industry".

Shape description

There are two things that a deigner, invertor or builder must be able to do: 

• first, he must be able to visualize what an object looks like without actually having the objects; 

• second, he must be able to describe it so that it could be built. 

His problem then is how to represent solid objects on a sheet of paper in such a manner as to tell the exact shape. This is done drawing a system of views of object as seen from different positions..

A picture of an hydraulic jack for an automobile (fig. 1) shows this tool as it ordimarily appears to us, but it does not show the true shapes of the parts. The top of the cylinder appears as an ellipse, although we know it really is circular. If we look down at the jack from above, we obtain a view showing the exact shape of the cylinder, and the outline of the other parts as seen above. This is called a top view or plan. This view deos not tell us the height of the jack, so it is necessary to take another view from a postition directly in front view or side viewm to show the height, is added. Often, as in this case, both the front and side views, in addition to the top view, are needed to describe the object (fig. 2). The three views taken together completely define the shapes of all visible parts of the jack and their exact relations to each other. 

Sometimes a left-side view describes the object or construction more clearly than the tight-side view and in such cases it should be used. It is sometimes desirable or necessary to show the rear view or the bottom view of an object. Views can then be projected to all six faces or planes of an object.

Sections 

We know that the parts of an object that cannot be seen are repesented by hidden lines composed of short dashes.

This method is satisfactory where the object is solid or the interior simple. There are many cases, especially where there is considerable interior detail or where several pieces are shown together, in which the hidden lines become confusing or hard to read. This diffictly is avoided by using a sectional view. A sectional view is obtained by supposing the piece to be cut apart by an imaginary cutting plane, and the front part removed, thus exposing the interior.

 

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Power and Efficiency

Power and Efficiency

Rate of doing work: power

 In practice, where work is done upon a body, both the amount of work and also the time during which that work is done are important. For example, if a motordriven soist has to raise its load quikly, a more powerful hoist and a larger driving motor are needer than if more time were allowed. Usually the size of machinery is determined, not by the total amount of work to be done, but by the rate ar which it to be done; that is, the amount of work required per unit of time, The time rate of doing work is caled power.

Since power is the time rate of doing work, the unit for power in any system of units is found by dividing the work unit in that system by the time unit. Thus, in the Sl system, power is expressed in watts, and in the British gravitational system it is expressed in foot-pounds (pound-force feet) per second.

In addition to the units of the standard systems, other practical units are in general use. The horsepower (hp) is the power prowided by an agent while doing work at the rate of 33 000 (ft x lb)/min. , or 550 (ft x lb)sec. The watt is a rate of doing work equal to l joule/sec. The kilowatt (kw) is a power unit used in rating electric machines.

Simple machines

It is a matter of common expirience that a stone firmly embedded in the ground can be dislodged with a crowbar, and that a heavy automobile can be raised by means of a jack. The crowbar or jack serves as an intermediate device upon which work can be done and which in turn does work upon some other object. A device that accomplishes this results is technicall called a machine. The complex machines used in industry are found upon analysis to be made up largely of certain elements that maybe considered simple machines in themselves. These simple machines comprise the lever, the wheel and axle, the pulley, the inclined plane, the screw, and the wedge.

Ushually a machine is epmployed order to lessen the force required in doing a certain piece of work. Thus, if a 500-lb weight is to be lifted, a machine can be used to exert this amount of upward force upon it while the person operating the machine exerts perhaps only 50 pounds. It is thus possible, and indeed usual, to obtain a larger force from a machine than that which is exerted upon it. Of course, this statement applies to force and not to energy; according to the law of conservation of energy, more work cannot be obtained from a machine than the energy supplied to it. Since work = force x distance, when the operator exerts a smaller force thandoes the machine, he exerts the smaller force through a correspondingly greater distance. 

The ratio of the force extred by a machine on a load to the force extred by an operator on the machine is called the mechanical advant of the machine.

Efficiency of a machine. Friction is present in all moving machinery, however well designed; consequently, the energy delivered by a machine is less than that supplied to it. More definitely, the principle of conservation of energy, of energy shows that 

energy input = energy output + energy wasted

if no energy is stored up in the machine, Since energy used per unit time is power, it can also be said that

power input = power output + power wasted.

The efficiency of machine is defined as the ratio of its output to its input, both input and output being expressed in the same units of energy or power. The ratio of output to input is always less than unity; in practice, it is usually multiplied by 100 and expressed in per cent. High efficiency in a machine implies that in a given time a largde part of the energy supplied to it is delivered by the machine to its loadand a small part wasted. The efficiency of a large electric generator may be as high as 98 per cent. In some of the simple machines - a screw jack, for exapmle - considerable friction is necessary to prevent the load from running down after it has been raised; because of the energy wasted in friction the efficiency of a terms of their output; thus, a 5-hp motor is one that can deliver 5 hp without exceeding its design limitaions; if its efficiency is 80 per cent, the power input to machine is 5 hp/0.80 - 6.25 hp.

 

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