INTRODUCTION

Machine design is a subject which is often neglected by men who have the ambition to graduate from the drafting and machine work of our factories into the more important departments dealing with the design of tools and machines, and the numerous devices which are made by means of them. The draftsman is liable to forget that, in order to be a really good draftsman, he must add to his ability to draw objects the power of visualizing accurately the device or machine, the different views of which he is called upon to draw, and be a good enough mechanic to know "how things work" and how the machine is to be manufactured. Similarly, the machinist is liable to lose sight of the fact that he is all the better workman if he can couple with his skill in turning out the finished product machined to the proper size, the ability to read the draftsman's drawings down to the smallest detail and to know the why and wherefore of every element of the design.

It is plain, therefore, that machine designing in its broadest sense demands a familiarity with the point of view of both draftsman and machinist; it demands a knowledge of materials their strength, their characteristics, and their behavior under different machine operations. The designer must know which parts should be cast and which should be machined from steel or brass; he must also know the standard designs and specifications for bolts, screws, nuts, pins, keys, etc.; he must know the different types of transmission, the proportions of pulleys and gears, the strength of shafts, the design of bearings, and the methods of lubrication.

It is with the idea of offering a simple treatment of these phases of machine design that this text was prepared. It is the hope of the publishers that the presentation will appeal to young men who are anxious to get ahead in their profession as well as to those of greater experience in the machine-design field.

The fundamental lines of thought and action which every designer follows in the solution of a problem in design are four in number. The expert may carry all these in mind at the same time, without definite separation into a step-by-step process; but the student must master them in their proper sequence, and thoroughly understand their application. In these four fundamentals is concentrated the entire art of Machine Design, and when they have become so familiar as to be instinctively applied on any and all occasions, good design is the result. Experience is the only other factor which will facilitate still further the design of good machinery; and it cannot be taught, it must be acquired by actual work.


ANALYSIS OF CONDITIONS AND FORCES

First, take a good square look at the problem to be solved. Study it from all sides, view it in all lights, note the worst conditions which can possibly exist, the average conditions of service, and any special or irregular service likely to be called for.

With these conditions in mind, make a careful analysis of all the forces, maximum as well as average, which may be brought into play. Although it is hard and sometimes impossible to determine exactly the forces acting on a given piece, their nature whether sudden or slowly applied, rapid in action or only occurring at intervals and their approximate magnitude, are always capable of analysis. Make a rough sketch of the piece under consideration, put in these forces, and go over the analysis carefully. A hasty and poor analysis will in the end be time lost, and, if the machine actually fails from this reason, heavy financial loss in material and labor will occur.

Machines are nothing but a collection of parts upon which forces are acting directly, or parts acting as loaded beams. Where the force has no leverage it acts directly on the sustaining part. Forces acting with leverage produce a moment; the sustaining member is a beam, and the effect produced therein depends on the theory of beams.

An example of the first is the load on a rope, the force acting without leverage, and the rope, therefore, having a direct pull put upon it.

An example of the second is a push of the hand on the crank of a grindstone. A moment is produced about the hub of the crank; the arm of the crank is a beam.

After it is determined by careful analysis what stress the machine part has to sustain, the next step is so to design it that it will theoretically resist the applied forces with the least expenditure of material.

Machinery is often constructed with the metal of which it is made distributed in the worst possible manner. In places where the stress is heavy and a rigid member is needed, a weak, springy part is sometimes found; while in other parts, where there are no forces to be resisted, or vibration to be absorbed, there seems to be a waste of good material. Whether in such case the "analysis of the forces was poor, or perhaps not made at all, or whether a knowledge of how to design so as to resist the given forces was wholly absent, cannot be told. At any rate, lack of either or both is clearly shown in the result.

Any member of a machine may vary in form from a solid block or chunk of material to an open ribbed structure. The solid chunk fills the requirement as far as strength is concerned, unless it is so heavy as to fail from its own weight. But such construction is poor design, except in cases where the concentration of heavy mass is necessary to absorb repeated blows like those of a hammer. The possibility of these blows should, however, have been determined in the analysis; and the solid, anvil construction then becomes theoretical design for that analysis.

For steadily applied loads an open, ribbed, or hollow box structure can be made which will distribute the metal where it is theoretically needed, and each fiber will then sustain its proper share of the load. In this way weight, cost, and appearance are heeded; and the service of the piece is as good as, and probably better than, it would be with the clumsy, solid form.

There is no such thing as putting too much theory into the design of machinery. The strongest trait which an engineer can have is absolute faith in his analysis and calculations, and their reproduction in his theoretical design. Theoretical design is an indication of scientific advance in the art, and some of the greatest steps of progress which have been made in recent years have been accomplished through a purely theoretical study of machine structure.

It will never do to be satisfied with theoretical design when it is not in accord with modern commercial and manufacturing considerations. Hence the next step after the determination of the theoretical design is the study of it from the producing standpoint.

All theoretical design viewed from the business standpoint is worthless, unless it has been subjected to the test of cheap and efficient production. Each machine detail, though correct in theory, may yet be improperly shaped and unfit for the part it is to play in the general scheme of manufacture.

The conditions here involved are changeable. What is good design in this decade may be bad in the next. In this light the designer must be a close student of the signs of the times; he must follow the march of progress, closely applying existing resources, conditions, and facilities, otherwise he cannot produce up-to-date designs. The introduction of new raw materials, the cheapening of production of others, the changing of shop methods, the use of special machinery, the opening of new markets, the development of new motive agents all these and many others are constantly demanding some modification in design to meet competition.

Illustrative of this, note the change which has been wrought by the development of electric power, the rise and decline of the bicycle business, the present manufacture of automobiles, the last named especially with reference to the development of the small motive unit, the gasoline engine, the steam engine, etc. The design of much machinery has been materially changed to meet the exacting demands of these new enterprises.

Practical modifications of design necessary to meet the limitations of construction in the pattern shop, foundry, and machine shop are of daily application in the designer's work. He must keep in his mind's eye at all times the workmen and the processes they use to create his designs in metal in the shop.

"How can this be made?" "Can it be made at all?" "Can it be made cheaply?" "Will it be simple in operation after it is made?" "Can it be readily removed for repair?" "Can it be lubricated?" "How can it be put in place?" "How can it be gotten out?" "Will it be made in small quantities or large?" "Will it sell as a special or standard machine?" etc., etc.

The consideration of such questions as these is a practical necessity from a business standpoint, as no other factor affects the design of machinery more. Designs which cannot be built as business propositions are no designs at all.

The student, it is true, may not have the extended shop knowledge which is essential to this; But he can do much for himself by visiting shops whenever possible, getting hold of shop ways of doing things, and invariably treating his work as a business matter. Though a man may not be a pattern maker, moulder, blacksmith, or machinist, yet he can soon gain ideas of the processes of each of these branches which will be of immense advantage to him in his designing work.


DELINEATION AND SPECIFICATION

Delineation and specification mean the clear and concise representation of the design by mechanical drawings, and are as much a part of the routine method of Machine Design as the three preceding fundamentals. The mere act of putting the results of mechanical thinking ort paper is one of the greatest helps for bringing the thinking machinery into systematic and definite action. A designer never thinks very long without drawing something, and the student must bring himself to feel that a drawing in its first sense is a means of helping his own thought, and must freely use it as such.

In its second and final sense, the drawing is an order and specification sheet from the designer to the workman. Design which stops short of exact, finished delineation in the form of working shop drawings, is only half done. In fact, the possibility of a piece being thus exactly drawn is often the crucial test of its feasibility as a part of a machine. It is easy to make general outlines, but it is not so easy to get down to finished detail. It is safe to say that there is no one thing productive of more trouble, delay, and embarrassment, and waste of time and money in the shop, when there need be none from this cause, than a poor detail drawing. The efficiency of the process of design is not fully realized, and failures are often recorded where there should be success, merely because the indefiniteness permitted by the designer in the drawings naturally transmitted itself to the workman, and he in turn produced a part indefinite in form and operation.

The actual process of drawing in the development of a design may be outlined as follows: