One can never become a machine designer by studying a book. The true designer is one whose judgment is ripened by experience in constructing and operating machines. Mr. William B. Bement, a designer in the true sense, once said that he thought it useful to figure the strength of machine parts, because the results were suggestive to the designer. One may know thoroughly the laws which govern the transmission of energy; may understand much concerning the nature of constructive materials; may know how to obtain results by mathematical processes; and yet be unable to design a good machine. One needs also to know the thousand and one things connected with practice, which constantly modify design, so that one can take the results of computation and accept, reject, and modify, until the machine will, when constructed, do its required work satisfactorily and enduringly. The writer once heard Professor John E. Sweet say : "It is comparatively easy to design a good new machine, but it is very hard to design a machine which will be good when it is old." This quality of foresight only comes with long experience.

There is, however, a certain part of the designer's mental equipment which may be furnished in the class-room, or by books. This is the writer's excuse for the following pages.

Even Elementary Machine Design cannot be treated exhaustively. The kinds of machines are too numerous, and their differences are too great. An effort is made here to suggest methods of reasoning, rather than to give rules. A knowledge of the usual university course in pure and applied mathematics is presupposed.

The part upon "Motion in Machines" could not have been written without the use of the excellent book "The Mechanics of Machinery," by Prof. A. B. W. Kennedy. To him and to Prof. L. M. Hoskins, to whom the writer has so often gone for the help which never failed, grateful acknowledgment is here made. In several places acknowledgment is made to others ; yet the writer feels that he has failed, though unintentionally, to give credit for much of the best he has received.

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

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 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 yielded under the stresses sustained; it was replace by a new part made stronger; it yielded again, and again was enlarged, or perhaps made of some more suitable material; it the sustained the applied stresses satisfactorily. Some other 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 no changed; only the parts that yield are perfected.

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 increas-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 reduced weight which prevents undue vibrations, (b) He may design special tools to reduce labor without reduction of the standard o 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 and which usually find their level in the scrap heap; 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 ; may reduce the expense incident upon repairs and delays, thereby bettering the reputation of the machine, and increasing sales.

Suppose, to illustrate further, that a machine part is to be designed, and either of two forms, A or B, will serve equally well. The part is to be of cast iron. The pattern for A will cost twice as much as for B. In the foundry and machine shop, however, A can be produced a very little cheaper than B, Clearly then, if but one machine is to be built, B should be decided on ; whereas, if the machine is to be manufactured in large numbers, A is preferable. Expense for patterns is a first cost. Expense for work in. the foundry and machine shop is repeated with each machine.

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 friction al 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 the machine designer as it is a part of the duty of an architect.


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