Machine design - Smith

One can never become a machine designer by studying books. Much help may come from books, but the true designer must have judgment, ripened by experience, in constructing and operating machines. One may know the laws that govern the development, transmission and application of energy; may have knowledge of constructive materials; may know how to obtain results by mathematical processes, and yet be unable to design a good machine. There is also needed a knowledge of many things connected with manufacture, transportation, erection and operation. With this knowledge it is possible to take results of computation and accept, reject and modify until a machine is produced that will do the required work satisfactorily.

Professor John E. Sweet once said, "It is comparatively easy to design a good new machine, but it is very hard to design a machine that will be good when it is old." A machine must not only do its work at first, but must continue to do it with a minimum of repairs as long as the work needs to be done. The designer must be able to foresee the results of machine operation; he must have imagination. This is an inborn power, but it may be developed by use and by engineering experience.

But there is a certain part of the designer's mental equipment that may be furnished in the class-room, or by books. This is the excuse for the following pages. Machine design cannot be treated exhaustively. There are too many kinds of machines for this and their differences are too great. In this book an effort is made simply to give principles that underlie all machine design and to suggest methods of reasoning which may be helpful in the designing of any machine. A knowledge of the usual university course in pure and applied mathematics is presupposed.

In general there are five considerations of prime importance in designing machines: I. Adaptation, II. Strength and Stiffness, III. Economy, IV. Appearance, V. Safety.

I. This requires all complexity to be reduced to its lowest terms in order that the machine shall accomplish the desired result in the most direct way possible, and with greatest convenience to the operator.

II. This requires the machine parts subjected to the action of forces to sustain these forces, not only without rupture, but also without such yielding as would interfere with the accurate action of the machine. In many cases the forces to be resisted may be calculated, and the laws of mechanics and the known qualities of constructive materials become factors in determining proportions. In other cases the force, by the use of a "breaking-piece," may be limited to a maximum value, which therefore dictates the design. But in many other cases the forces acting are necessarily unknown; and appeal must be made to the precedent of successful practice, or to the judgment of some experienced man, until one's own judgment becomes trustworthy by experience.

In proportioning machine parts, the designer must always be sure that the stress which is the basis of the calculation or the estimate, is the maximum possible stress; otherwise the part will be incorrectly proportioned. For instance, if the arms of a pulley were to be designed solely on the assumption that they endure only the transverse stress due to the belt tension, they would be found to be absurdly small, because the stresses resulting from the shrinkage of the casting in cooling are often far greater than those due to the belt pull.

The design of many machines is a result of what may be called "machine evolution." The first machine was built according to the best judgment of its designer; but that judgment was fallible, and some part ruptured under the stresses sustained; it was replaced by a new part made stronger; it ruptured again, and again was enlarged, or perhaps made of some more suitable material; it then sustained the applied stresses satisfactorily. Some other part yielded too much under stress, although it was entirely safe from actual rupture ; this part was then stiffened and the process continued till the whole machine became properly proportioned for the resisting of stress. Many valuable lessons have been learned from this process; many excellent machines have resulted from it. There are, however, two objections to it: it is slow and very expensive, and if any part had originally an excess of material, it is not changed; only the parts that yield are perfected. Modern analytical methods are rightly displacing it in all progressive establishments.

III. The attainment of economy does not necessarily mean the saving of metal or labor, although it may mean that. To illustrate: Suppose that it is required to design an engine -lathe for the market. The competition is sharp; the profits are small. How shall the designer change the design of the lathes on the market to increase profits? (a) He may, if possible, reduce the weight of metal used, maintaining strength and stiffness by better distribution. But this must not increase labor in the foundry or machine-shop, nor reduce weight which prevents undue vibrations.

(b) He may design special tools to reduce labor without reduction of the standard of workmanship. The interest on the first cost of these special tools, however, must not exceed the possible gain from increased profits. (c) He may make the lathe more convenient for the workmen. True economy permits some increase in cost to gain this end. It is not meant that elaborate and expensive devices are to be used, such as often come from men of more inventiveness than judgment; but that if the parts can be rearranged, or in any way changed, so that the lathes-man shall select this lathe to use because it is handier when other lathes are available, then economy has been served, even though the cost has been somewhat increased, because the favorable opinion of intelligent workmen means increased sales.

In (a) economy is served by a reduction of metal; in (b) by a reduction of labor; in (c) it may be served by an increase of both labor and material.

The addition of material largely in excess of that necessary for strength and rigidity, to reduce vibrations, may also be in the interest of economy, because it may increase the durability of the machine and its foundation, or may reduce the expense incident upon repairs and delays, thereby bettering the reputation of the machine and increasing sales.

Economy of operation also needs attention. This depends upon the efficiency of the machine ; i.e., upon the proportion of the energy supplied to the machine which really does useful work. This efficiency is increased by the reduction of useless frictional resistances, by careful attention to the design and means of lubrication of rubbing surfaces.

In order that economy may be best attained, the machine designer needs to be familiar with all the processes used in the construction of machines pattern -making, foundry work, forging, and the processes of the machine-shop and must have them constantly in mind, so that while each part designed is made strong enough and stiff enough, and properly and conveniently arranged, and of such form as to be satisfactory in appearance, it also is so designed that the cost of construction is a minimum.

IV. The fourth important consideration is Appearance. There is a beauty possible of attainment in the design of machines which is always the outgrowth of a purpose. Otherwise expressed, a machine to be beautiful must be purposeful. Ornament for ornament's sake is seldom admissible in machine design. And yet the striving for a pleasing effect is as much a part of the duty of a machine designer as it is a part of the duty of an architect.

As a guiding principle, the general rule may be laid down that simplicity and directness are always best. Each member should be studied with strict reference to the function which it is to perform and the stresses to which it is subjected and then given the form and size best suited to meet the conditions with the greatest economy of material and workmanship. When combined, the parts must be modified in such manner as may be found necessary to the harmonious effect of the whole.

V. Safety of the operator and others who come into the vicinity of the machine is the fifth important point in design. It is really a sub-division of Adaptation but, for emphasis, may be given the prominence of a separate head. Beyond the provisions for Strength and Stiffness, it requires that all moving parts shall be so formed and guarded as to eliminate, so far as may be foreseen, all danger of bodily accident.


CHAPTER I - PRELIMINARY.

1. Definitions. The study of machine design is based upon the science of mechanics, which treats questions involving the consideration of motion, force, work, and energy. Since it will be necessary to use these terms almost continually, it is well to make an exact statement of what is to be understood by them.

Motion may be defined as change of position in space.

A Force is one of a pair of equal, opposite, and simultaneous actions between two bodies by which the state of their motion is altered, or a change in the form or condition of the bodies themselves is effected.

Work is the name given to the result of a force in motion.

Energy is the capacity possessed by matter to do work.

A Machine is a combination of resistant bodies whose relative motions are completely constrained, and whose function it is to transform available energy into useful work.

The law of Conservation of Energy underlies every machine problem. This law may be expressed as follows: The sum of energy in the universe is constant. Energy may be transferred in space; it may be stored for varying lengths of time; it may be changed from one of its several forms to another; but it cannot be created or destroyed.

The application of this law to machines is as follows: A machine receives energy from a source, and uses it to do useful and useless work.

A single cycle of action of a machine is that sequence of operations during which each member of the machine has gone once through all the relative motions possible to it. A complete cycle of action is such a period that all conditions (velocities, etc., as well as relative positions) in the machine are the same at its beginning and end.

During a single cycle of action of the machine, the energy received equals the total work done. The work done may appear as (a) useful work delivered by the machine, or as (b) heat due to energy transformed through frictional resistance, or as (c) stored mechanical energy in some moving part of the machine whose velocity is increased. The sign of the stored energy may be plus or minus, so that energy received in one cycle may be delivered during another cycle; but for any considerable time interval of machine action the algebraic sum of the stored energy must equal zero.

2. Efficiency of Machines. In general, efficiency may be denned as the ratio of a result to the effort made to produce that result. In a machine the result corresponds to the useful work, while the effort corresponds to the energy received. The designer must strive for high efficiency, i.e., for the greatest possible result for a given effort.

3. Function of Machines. Nature furnishes sources of energy, and the supplying of human needs requires work to be done. The function of machines is to cause matter possessing energy to do useful work.


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