INTRODUCTION
DESCRIBING THE REAL PURPOSE AND SCOPE OF THE BOOK, AND INDICATING
THE BEST METHOD OF MAKING USE OF IT

In view of criticism of this work which has occasionally found
expression by certain readers, who appear to have expected to find it
designs of Machines, Structures, etc., and complaints, based on the same
misunderstanding, that the book is, consequently, not kept up to date, the
Author has thought it desirable to append a few explanatory observations.

These impressions it may be stated arise from a complete misconception
of the intention of the Author and the purpose of the book.

This work <i>is not a treatise on Design, nor a textbook on Mechanical
Construction</i>. Its object is to supply the designer with the <i>basic
elements of mechanical and structural composition</i>, classified and
displayed in such a way as to facilitate the selection of those details
which may be most efficiently combined into an effective and harmonious
whole.

A little reflection will show that a Sketch Book of <i>Designs</i>
is almost an impossible ideal, since it would be out of date as soon
as published.

To deal with designs in a diagrammatic matter a large book would be needed
to treat of each subject-such as Boilers, Cranes, Steam Engines, Pumps,
etc. On the other hand, the details on which these constantly changing
designs are built up remain fundamentally the same. It is surprising
to realise how few really new <i>basic elements</i> are invented. The
Author's experience seems to show that only one or two such innovations
are produced annually, and, as these novelties are usually subject to
Patent protection, it is only after a lapse of several years that they
can be included in a book of this kind.

Further, the inclusion of anything more than the simplest elements would
lead to this book becoming unwieldy in the extreme. Endless duplication
would be unavoidable, and efficient classification would become an
impossibility.

An example of the manner in which the book should be used will now be
given. It will serve to indicate in a practical way the method to be
pursued by a reader in order to select from its Sections the various
details required to build up a given piece of mechanism.

Suppose it is required to design a reciprocating carriage or frame.

In the first place the designer should reduce his problem to its
elements-nature of the material to be treated (assuming that the frame is
for sieving or segregation of a material), output required, speed, size,
method of drive, suitability to surrounding conditions, etc. Having done
this, the following points will come up for consideration.

Is the drive to be direct, from motor or shaft, or indirect, through
chain, gear, rope or belt? If the former, a reference to Clutches, Section
15, pages 46 to 49, and to Couplings, Section 16, pages 48 to 51, will be
helpful. If the latter, Friction Gear, Section 38, pages 118 to 121, Rope
Gear, Section 66, pages 212 to 214, Belt Gear, Section 3, pages 8 to 11,
and Toothed Gear, Section 84, pages 276 to 283, may be consulted. Section
40, pages 126 to 133, showing Various Devices in Gearing, may be looked
up, as a solution of some special problem in transmission may possibly
be found applicable to the case under consideration.

The next question is the design of a satisfactory reciprocating movement,
having regard to the conditions-material to be treated (<i>i.e.</i>
heavy or light, fine or gross, adhesive or not, etc.) or other duty,
rate of output, etc.

Section 21, pages 68 to 75, gives diagrams of practically every device
utilisable for the conversion of Circular into Reciprocating Motion. If
an intermittent movement is required, Section 62, pages 202 to 209,
will provide the solution. At this point Section 72, pages 226 to 229,
dealing with Screening, may offer some useful information.

The carriage or frame will require support according to the circumstances
(speed, kind of vibratory movement, accessibility etc.), and an inspection
of Section 46, pages 142 to 149, showing Bearings, or Section 48, pages
152 to 159 showing Link Suspension devices, may be made. On the other
hand, if a sliding support or guide is preferred, Section 39, pages 120
to 127, should be consulted.

A mechanism equipped with a Variable Speed device, either for driving
the carriage or for feeding it, may possibly be of service. In this case,
Section 88, pages 286 to 295, will be found to disclose the elements of
every <i>practical</i> means of attaining this end.

Reciprocating bodies, especially if they are heavy, call for means of
checking their motion at the end of their travel. This may be effected
by a braking device or by a cam or similar gear. Information on Brakes
and Retarding appliances will be found in Section 5, pages 12 to 17, and
on Cams, Tappets and Wipers in Section 9, pages 26 to 31. Section 29, on
Cushioning, may provide some ideas on this subject, see pages 96 and 97.

Lubrication is a subject that is often neglected, nevertheless it
is often the most vital and consequently the most vulnerable part of
an appliance called on to perform severe duty. Forced or mechanically
operated lubrication is now generally recognised as an essential of the
design of continuously operating mechanisms. In Section 51, pages 174
to 177, will be found indications of the various methods of attacking
this problem.

This example, in which the possible references are by no means exhausted,
will demonstrate-it is hoped-that an intelligent and diligent use
of this book will provide the searcher with very wide and thoroughly
<i>practical</i> assistance, bearing on every aspect of the design he
has to produce.

Information on Materials is given under Section 99, pages 330 to 341,
but it will be obvious that such information in a book of this character
must necessarily be suggestive only. Books on structural design and the
characteristics of materials must be consulted if information as to the
method of calculating strains and designing cross-sections is desired.

The sketches given in this work are <i>practical drawings</i> intended for
Engineers, by an Engineer, showing just what is needed and no more. They
will give him the inspiration he requires, the guidance he wants-in fact
a "lead" in the right direction-and he will not waste his time wading
through a lot of "pictures" that have no analytical value. Descriptions
have intentionally been made as brief and to the point as possible for
the same reason.

The Author submits that nothing more is called for by the practical
man, and that he probably would resent any attempt to impose on him
more complete or elaborate solutions of his problems. The many years
of service that this book has already rendered to the Engineering
Profession would seem to justify the claim that it has met a real need,
and encourages the Author to hope that the work has not yet exhausted
its period of usefulness.

If the searcher does not find precisely what he wants, or finds the
proffered solution difficult to interpret, the Author will always be
glad to endeavour to help if a communication is sent to him through
the Publishers.

It may be pointed out that in Schools of Design more use of this book
might be made. It is not only informative but intensely practical, and
will convey ideas to the Student that are mechanically sound and which
may at some future time be of the very greatest utility to him.

The solution by him of a given problem in design may be effected by
following the synthetic method indicated in the example given above,
and interesting alternatives may be developed by adopting variants in
the details and elements utilised to build up a complete mechanism.

T.W.B.
<i>July</i> 1934