American School of Correspondence.

Machine design; a manual of practical instruction in the art of creating machinery for specific purposes,including many working hints essential to efficiency in the operation and care of machines, and increase of output online

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LIBRARY

OF THE

UNIVERSITY OF CALIFORNIA.

Class



Machine Design



A Manual of

PRACTICAL INSTRUCTION IN THE ART OF CREATING MACHINERY FOR
SPECIFIC PURPOSES, INCLUDING MANY WORKING HINTS ESSEN-
TIAL TO EFFICIENCY IN THE OPERATION AND CARE
OF MACHINES, AND INCREASE OF OUTPUT



By CHARLES L. GRIFFIN, S.B.

American Society of Mechanical Engineers. Mechanical Engineer with

the Semet-Solvay Company. Formerly Professor cf Machine

Design, Pennsylvania State College.



ILLUSTRATED




CHICAGO

AMERICAN SCHOOL OF CORRESPONDENCE
1908



COPYRIGHT 1907 BY
AMERICAN SCHOOL OF CORRESPONDENCE;



Entered at Stationers' Hall, London
All Rights Reserved



Foreword




recent years, such marvelous advances have been
made in the engineering and scientific fields, and

O o

so rapid has been the evolution of mechanical and
constructive processes and methods, that a distinct
need has been created for a series of practical
, of convenient size and low cost, embodying the
accumulated results of experience and the most approved modern
practice along a great variety of lines. To fill this acknowledged
need, is the special purpose of the series of handbooks to which
this volume belongs.

C, I n the preparation of this series, it has been the aim of the pub-
lishers to lay special stress on the practical side of each subject,
as distinguished from mere theoretical or academic discussion.
Each volume is written by a well-known expert of acknowledged
authority in his special line, and is based on a most careful study
of practical needs and up-to-date methods as developed under the
conditions of actual practice in the field, the shop, the mill, the
power house, the drafting room, the engine room, etc.

C, These volumes are especially adapted for purposes of self-
instruction and home study. The utmost care has been used to
bring the treatment of each subject within the range of the com-



179729



mon understanding, so that the work will appeal not only to the
technically trained expert, but also to the beginner and the self-
taught practical man who wishes to keep abreast of modern
progress. The language is simple and clear; heavy technical terms
and the formulae of the higher mathematics have been avoided,
yet without sacrificing any of the requirements of practical
instruction; the arrangement of matter is such as to carry the
reader along by easy steps to complete mastery of each subject ;
frequent examples for practice are given, to enable the reader to
test his knowledge and make it a permanent possession; and the
illustrations are selected with the greatest care to supplement and
make clear the references in the text.

C, The method adopted in the preparation of these volumes is that
w r hich the American School of Correspondence has developed and
employed so successfully for many years. It is not an experiment,
but has stood the severest of all tests that of practical use which
has demonstrated it to be the best method yet devised for the
education of the busy working man.

C. For purposes of ready reference and timely information when
needed, it is believed that this series of handbooks will be found to
meet every requirement.




Table of Contents



PRINCIPLES AND METHOD Page 3

Object of Machine Design Mechanical Thought, Develop-mesrt, and
Specification Importance of Details Relation of Design to Problems
it Seeks to Solve Theory and Production Invention Use of Hand-
books and Empirical Data Calculations, Notes, and Records
Sketches Method of Design Analysis of Conditions and Forces
Theoretical Design Practical Modification Delineation and Specifi-
cation Constructive Mechanics Forces, Moments, and Beams Ten-
sion, Compression, and Torsion Friction and Lubrication Working
Stresses and Strains.



APPLICATION TO A PRACTICAL CASE Page 23

Design of an Elevator Wire-Rope Drive Preliminary Sketch Rope
and Drum Driving Gears Pulleys Torsional Moment around Shaft
Axes Calculation of Width of Belt Length of Bearings Height of
Centers Data on Sketch Sizes of Shafts Preliminary Layoui
Pulleys Gears Brackets and Caps Drum and Brake Base Brake-
Strap Bracket Foot-Lever Gear Guard Brake-Relief Spring The
General or Assembled Drawing- "Reversed" Machine Design.



CLASSIFICATION OF MACHINERY . . . . . . Page 62

Machine Tools (Lathe, Planer, Milling Machine, etc.) Motive-Power
Machinery (Steam Engine, Air-Compressor, Steam Pump, etc.)
Structural Machinery (Cranes, Elevators, Locomotives, Cars, Cable-
W^ays, etc.) Mill and Plant Machinery (Rolling Mills, Mining Ma-
chinery, Crushers, Stamps, Drills, etc.) Use of Cast and Wrought
Iron and Steel Suggestions on Original Design.



DESIGN OF COMPONENT PARTS OF MACHINERY . . . . Page 75

Belts Strength of Leather Belting Horse-Power Transmitted by
Belts Speed of Belting Material of Belting Initial Tension in Belt
Pulleys (Rim, Arms, Hub) Split Pulleys Special Forms of Pulleys
Shafts Simple and Combined Stresses of Torsion, Bending, Com-
pression Deflection Centrifugal Whirling Horse-Power of Shaft-
ing Bessemer, Open-Hearth, and Nickel Steel Spur Gears Involute
and Cycloidal Curves Pitch Circles Mortise Teeth Shrouding
Hook-Tooth Gear Stub Tooth Web Gear Bevel Gears Worm and
Worm Gear Friction Clutches Couplings (Flange, Clamp, Claw)
Bolts, Studs, Nuts, and Screws Keys, Pins, and Cotters Spline
Bearings, Brackets, and Stands.



INDEX Page 183




MACHINE DESIGN,



PART I.

Definition. Machine Design is the art of mechanical thought
development, and specification.

It is an art, in that its routine processes can be analyzed and
systematically applied. Proficiency in the art positively cannot
be attained by any " short cut " method. There is nothing of a
spectacular nature in the methods of Machine Design. Large
results cannot be accomplished at a single bound, and success is
possible only by a patient, step-by-step advance in accordance
with well-established principles.

" Mechanical thought " means the thinking of things strictly
from their mechanical side; a study of their mechanical theory,
structure, production, and use; a consideration of their mechanical
fitness as parts of a machine.

"Mechanical development" signifies the taking of an id^a in
the rough, in the crude form, for example, in which it comes from
the inventor, working it out in detail, and refining and fixing it in
shape by the designing process. Ideas in this way may become
commercially practicable designs.

" Mechanical specification " implies the detailed description
of designs, in such exact form that the shop workmen are enabled
to construct completely and put in operation the machines repre-
sented in the designs.

The object of Machine Design is the creation of machinery
for specific purposes. Every department of a manufacturing
plant is a controlling factor in the design and production of the
machines built there. A successful design cannot be out of

o

harmony with the organized methods of production. Hence in
the high development of the art of Machine Design is involved a
knowledge of the operations in all the departments of a manu-
facturing plant. The student is therefore urged not only to
familiarize himself with the direct production of machinery, but to
study the relation thereto of the allied commercial departments-



MACHINE DESIGN



He should get into the spirit of business at the start, get into the
shop atmosphere, execute his work just as though the resulting
design were to be built and sold in competition. He should visit
shops, work in them if possible, and observe details of design and
methods of finishing machine parts. In this way he will begin
to store up bits of information, practical and commercial, which
will have valuable bearing on his engineering study.

The labor involved in the design of a complicated automatic
machine is evidenced by the designer's wonderful familiarity with
its every detail as he stands before the completed machine in
operation and explains its movements to an observer. The intri-
cate mass of levers, shafts, pulleys, gears, cams, clutches, etc., etc.,
packed into a small space, and confusing even to a mechanical
mind, seems like a printed book to the designer of them.

This is so because it is a familiar journey for the designer's
mind to run over a path which it has already traversed so many
times that he can see every inch of it with his eyes shut. Every
detail of that machine has been picked from a score or more of
possible ideas. One by one, ideas have been worked out, laid
aside, and others taken up. Little by little, the special fitness of
certain devices has become established, but only by patient, care-
ful consideration of others, which at first seemed equally good.

Every line, and corner, and surface of each piece, however
small that piece may be, has been through the refining process of
theoretical, practical, and commercial design. Every piece has
been followed in the mind's eye of its designer from the crude
material of which it is made, through the various processes of fin-
ashing, to its final location in the completed machine; thus its
bodily existence there is but the realization of an old and familiar
picture.

What wonder that the machine seems simple to the designer
of it! As he looks back to the multitude of ideas invented,
worked out, considered and discarded, the machine in its final
form is but a trifle. It merely represents a survival of the fittest.

No successful machine, however simple, was ever designed
that did not go through this slow process of evolution. No
machine ever just simply happened by accident to do the work
for which it is valued. No other principle upon which the sue-



MACHINE DESIGN



cessful design of machinery depends is so important as this careful,
patient consideration of detail. A machine is seldom unsuccessful
because some main point of construction is wrong. Thejprincipal
features of a machine are usually the easiest to determine. It is
a failure because some little detail was overlooked, or hastily con-
sidered, or allowed to be neglected, because of the irksome labor
necessary to work it out properly.

There is no task so tedious, for example, as the devising of
the method of lubricating the parts of a complicated machine.
Yet there is no point of design so vital to its life and operation as
an absolute assurance of an adequate supply of oil for the moving
parts at all times and under all circumstances. Suitable means
often cannot be found, after the parts are together, hence the
machine goes into service on a risky basis, with the result, per-
haps, of early failure, due to "running dry." Good designers
will not permit a design to leave their hands which does not pro-
vide practically automatic oiling, or at least such means of lubri-
cation that the operator can offer no excuse for neglecting to oil
his machine. This is but a single illustration of many which
might be presented to impress the definite and detail character
necessary in work in Machine Design.

Relation. The relation which Machine Design should cor-
rectly bear to the problems that it seeks to solve, is twofold; and
there are, likewise, two points of view corresponding to this two-
fold relation, from which a study of the subject should be traced.
Neither of these can be discarded and an efficient mastery of the
art attained. These points are
I. Theory.

II. Production.

I. Theory. From this point of view, Machine Design is
merely a skeleton or framework process, resulting in a repre-
sentation of ideas of pure motion, fundamental shape, and ideal
proportion. It implies a working knowledge of physical and
mathematical laws. It is a strictly scientific solution of the
problem at hand, and may be based purely on theory which has
been reasoned out by calculation or deduced from experiment.
This is the only sure foundation for intelligent design of any sort.

But it is not enough to view the subject from the standpoint



8 MACHINE DESIGK

of theory alone. If we stopped here we should have nothing but
mechanisms, mere laboratory machines, simply structures of
ingenuity and examples of fine mechanical skill. A machine may
be correct in the theory of its motions ; it may be correct in the
theoretical proportions of its parts; it may even be correct in its
operation for the time being; and yet its complication, its mis-
directed and wasteful effort, its lack of adjustment, its expensive
and irregular construction, its lack of compactness, its difficulty
of ready repair, its inability to hold its own in competition any
of these may throw the balance to the side of failure. Such a
machine, commercially considered, is of little value. No shop
will build it, no machinery house will sell it, nobody will buy it
if it is put on the market.

Thus we see that, aside from the theoretical correctness oi
principle, the design of a machine must satisfy certain other
exacting requirements of a distinctly business nature.

II. Production. From this point of view, Machine Design
is the practical, marketable development of mechanical ideas,
Viewed thus, the theoretical, skeleton design must be so clothed
and shaped that its production may be cheap, involving simple
and efficient processes of manufacture. It must be judged by the
latest shop methods for exact and maximum output. It must
possess all the good points of its competitor, and, withal, some
novel and valuable ones of its own. In these days of keen com-
petition it is only by carefully studied, well-directed effort toward
rapid, efficient, and, therefore, cheap production that any machine
can be brought to a commercial basis, no matter what its other
merits may be. All this must be thought of and planned for in
the design, and the final shapes arrived at are quite as much a
result of this second point of view as of the first.

As a good illustration of this, may be cited the effect of the
present somewhat remarkable development of the so-called "high
speed " steels. The speeds and feeds possible with tools made of
these steels are such that the driving power, gearing, and feed
mechanism of the ordinary lathe are wholly inadequate to the
demands made upon tnem when working the tool to its limit
This means that the basis of design as used for the ordinary tool
steel will, not do, if 'the machine is expected to stand up to the



MACHINE DESIGN"



cuts possible with the new steels. Hence, while the old designs
were right for the old standard, a new one has been set, and a
thorough revision on a high-speed basis is. imminenV-else the
market for them as machines of maximum output will be lost.

From these definitions it is evident that the designer must
not only use all the theory at his command, but must continually
inform himself on all processes and conditions' of manufacture,
and keep an eye on the tenderly of the sales markets, both
of raw material and the finished machinery product. This is
what in the broadest sense is nieant by the term " Mechanical
Thought," thought which is directed and controlled, not only by
theoretical principle but by closely observed practice. From the
feeblest pretenders of design to .those engineers who consummate
the boldest feats and control the largest enterprises, the process
which produces results is always the same. Although experience
is necessary for the best mechanical judgment, yet the student-
must at least begin to cultivate good mechanical sense very early
in his study of design.

Invention. Invention is closely related to Machine Design,
but is not design itself. Whatever is invented has yet to be
designed. An invention is of little value until it has been refined
by the process of design.

Original design is of an inventive nature, but is not strictly
invention. Invention is usually considered as the result of genius,
and is announced in a flash of brilliancy. We see only the flash,
but behind the flash is a long course of the most concentrated
brain effort. Inventions are not spontaneous, are not thrown off
like sparks from the blacksmith's anvil, but are the result of hard
and applied thinking. This is worth noting carefully, for the
same effort which produces original design may develop a valuable
invention. But there is little possibility of inventing anything
except through exhaustive analysis and a clear interpretation of
such analysis.

Handbooks and Empirical Data. The subject matter in
these is often contradictory in its nature, but valuable nevertheless,
Empirical data are data for certain fixed conditions and are nor
general. Hence, when handbook data are applied to some specific
case of design, while the information should be used in the freest



MACHINE DESIGN



manner, yet it must not be forgotten that the case at hand is prob-
ably different, in some degree, from that upon which the data were
based, and unlike any other case which ever existed or will ever
again exist. Therefore the" data should be applied with the greatest
discretion, and when so applied will contribute to the success of
the design at least as a check, if not as a positive factor.

The student should at the outset purchase one good handbook,
and acquire the habit of consulting it on all occasions, checking
and comparing his own calculations and designs therefrom. Care
must be taken not to become tied to a handbook to such an extent
that one's own results are wholly subordinated to it. Independence
in design must be cultivated, and the student should not sacrifice
his calculated results until chey can be shown to be false or based
on false assumption. Originality and confidence in design will be
the result if this course be honestly pursued.

Calculations, Notes, and Records. Accurate calculations are
the basis of correct proportions of machine parts. There is a right
way to make calculations and a wrong way, and the student will
usually take the wrong way unless he is cautioned at the start.

The wrong way of making calculations is the loose and shift-
less fashion of scratching upon a scrap of detached paper marks
and figures, arranged in haphazard form, and disconnected and
incomplete. These calculations are in a few moments' time totally
meaningless, even to the author of them himself, and are so easily
lost or mislaid that when wanted they usually cannot be found.

Engineering calculations should always be made systemati-
cally, neatly, and in perfectly legible form, in some permanently
bound blank book, so that reference may always be had to them at
any future time for the purpose of checking or reviewing. Put
all the data down. Do not leave in doubt the exact conditions
under which the calculations were made. . Note the date of calcu-
lation.

If a mistake in figures is made, or a change is found neces-
sary, never rub out the figures or tear out the leaf, or in any way
obliterate the figures. Simply draw a bold cross through the wrong
part and begin again. Often a calculation which is supposed to
be wrong is later shown to be right, or the facts which caused the
error may be needed for investigation and comparison. Time which



MACHINE DESIGN 9

is spent in making figures is always valuable time, time too pre-
cious to be thrown away by destroying the record.

The recording of calculations in a permanent .formr-as- just
described, is the general practice in all modern engineering offices.
This plan has been established purely as a business policy. In
case of error it locates responsibility and settles dispute. Con-
sistent designing is made possible through the records of past
designs. Proposals, estimates, and bids may often be made
instantly, on the basis of what these record books show of sizes
and weights. This bookkeeping of calculations is as important a
factor of systematic engineering as bookkeeping of business
accounts is of financial success.

The student should procure for this purpose a good blank book
with a firm binding, size of page not smaller than 6 by 8 inches
(perhaps 8 by 11 inches may be better), and every calculation, how-
ever small and apparently unimportant, should be made in it.

Sample pages of. engineering calculations are reproduced in
Figs. 3 to 9. Note the sketch showing the forces. Note the clear

c5 . O

statement of data. Note the systematic writing of the equations,
and the definite substitutions therein. Note the heavy double
underscoring of the result, when obtained. There is nothing in
the whole process of the calculation that cannot be reviewed at
any moment by anybody, and in the briefest time.

The development of a personal note-book is of great value to
the designer of machinery. The facts of observation and experi-
ence recorded in proper form, bearing the imprint of intimate
personal contact with the points recorded, cannot be equalled
in value by those of any hand or reference book made by another.
There is always a flavor about a personal note-book, a sort of
guarantee, which makes the use of it by its author definite and
sure.

The habit of taking and recording notes, or even knowing
what notes to take, is an art in itself, and the student should
begin early to make his note-book. Aside from the value of the
notes themselves as a part of his personal equipment, the facility
with which his eye will be trained to see and record mechanical
things will be of great value in all of his study and work. How
many men go through a shop and really see nothing of the opera-



10 MACHINE DESIGN

tions going on therein, or, seeing them, remember nothing ! An
engineer, trained in this respect, will to a surprising degree be
able to retain and sketch little details which* fall under his eye for
a brief moment only, while he is passing through a crowded shop.

Some draftsmen have the habit of copying all the standard
tables of the various offices in which they work. While these are
of some value in a few cases, yet this is not what is meant by a
good note-book in the best sense. Ideas make a good note-book,
not a mere tabulation of figures. If the basis upon which stan-
dards are founded can be transferred to permanent personal record,
or novel methods of calculation, or simple features of construc-
tion, or data of mechanical tests, or efficient arrangement of
machinery if these can be preserved for reference, the note-book
will be of greatest value.

Whatever is noted down, make clear and intelligible, illus-
trating by a sketch if possible. Make the note so clear that
reference to it after a long space of years would bring the whole
subject before the mind in an instant. If this is not done the
author of the note himself will not have patience to dig out the
meaning when it is needed ; and the note will be of no value.

METHOD OF DESIGN.

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

i. 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, note the average conditions of service, note any special or
irregular service likely to be called for.



MACHINE DESIGN 11

With these conditions well in mind, make a careful analysis
of all the forces, maximum as well as average, which may be
brought into play. Make a rough sketch of the piece under con-
sideration, and put in these forces. Be sure that these forces are
at least approximately right. Go over the analysis carefully
again and again. Remember that time saved at the beginning
by hasty and poor analysis will actually be time lost at the end ;
and if the machine actually fails from this reason, heavy financial
loss in material and labor will occur. Any haste toward com-
pletion of the structure beyond the roughest outline, without this
careful study of forces, is a blind leap in the dark, entirely un-
scientific, and almost certain to result in ultimate failure.

On the other hand this principle may be carried too far. In
trying to make the analysis thorough and the forces accurate, it is


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Online LibraryAmerican School of CorrespondenceMachine design; a manual of practical instruction in the art of creating machinery for specific purposes,including many working hints essential to efficiency in the operation and care of machines, and increase of output → online text (page 1 of 14)