Learning without thought is labor lost;
thought without learning is perilous.
CONFUCIUS, 6TH CENTURY B.C.
1.1
DESIGN
What is design? Wallpaper is designed. You may be wearing “designer” clothes. Automobiles are “designed” in terms of their external appearance. The term design clearly encompasses a wide range of meaning. In the above examples, design refers primarily to the object’s aesthetic appearance. In the case of the automobile, all of its other aspects also involve design. Its mechanical internals (engine, brakes, suspension, etc.) must be designed, more likely by engineers than by artists, though even the engineer gets to exhibit some artistry when designing machinery.
The word design is from the Latin word designare meaning “to designate, or mark out.” Webster’s dictionary gives several definitions of the word design, the most applicable of which is “to outline, plot, or plan as action or work . . . to conceive, invent, contrive.” We are more concerned here with engineering design than with artistic design. Engineering design can be defined as “The process of applying the various techniques and scientific principles for the purpose of defining a device, a process, or a system in sufficient detail to permit its realization. ”
Machine Design
This text is concerned with one aspect of engineering design—machine design. Machine design deals with the creation of machinery that works safely, reliably, and well.
A machine can be defined in many ways. The Random House dictionary[1] lists twelve definitions, among which are these two:
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MACHINE DESIGN -
An Integrated Approach
1
Machine
1. An apparatus consisting of interrelated units, or
2. A device that modifies force or motion.
The interrelated parts referred to in the definition are also sometimes called ma-
chine elements in this context. The notion of useful work is basic to a machine’s func-
tion, as there is almost always some energy transfer involved. The mention of forces
and motion is also critical to our concerns, as, in converting energy from one form to
another, machines create motion and develop forces. It is the engineer’s task to de-
fine and calculate those motions, forces, and changes in energy in order to determine
the sizes, shapes, and materials needed for each of the interrelated parts in the machine.
This is the essence of machine design.
While one must, of necessity, design a machine one part at a time, it is crucial to
recognize that each part’s function and performance (and thus its design) are dependent
on many other interrelated parts within the same machine. Thus, we are going to at-
tempt to “design the whole machine” here, rather than simply designing individual el-
ements in isolation from one another. To do this we must draw upon a common body
of engineering knowledge encountered in previous courses, e.g., statics, dynamics, me-
chanics of materials (stress analysis), and material properties. Brief reviews and ex-
amples of these topics are included in the early chapters of this book.
The ultimate goal in machine design is to size and shape the parts (machine ele-
ments) and choose appropriate materials and manufacturing processes so that the result-
ing machine can be expected to perform its intended function without failure. This
requires that the engineer be able to calculate and predict the mode and conditions of
failure for each element and then design it to prevent that failure. This in turn requires
that a stress and deflection analysis be done for each part. Since stresses are a func-
tion of the applied and inertial loads, and of the part’s geometry, an analysis of the forces,
moments, torques, and the dynamics of the system must be done before the stresses and
deflections can be completely calculated.
If the “machine” in question has no moving parts, then the design task becomes
much simpler, because only a static force analysis is required. But if the machine has
no moving parts, it is not much of a machine (and doesn’t meet the definition above);
it is then a structure. Structures also need to be designed against failure, and, in fact,
large external structures (bridges, buildings, etc.) are also subjected to dynamic loads
from wind, earthquakes, traffic, etc., and thus must also be designed for these condi-
tions. Structural dynamics is an interesting subject but one which we will not address
in this text. We will concern ourselves with the problems associated with machines that
move. If the machine’s motions are very slow and the accelerations negligible, then a
static force analysis will suffice. But if the machine has significant accelerations within
it, then a dynamic force analysis is needed and the accelerating parts become “victims
of their own mass.”
In a static structure, such as a building’s floor, designed to support a particular
weight, the safety factor of the structure can be increased by adding appropriately dis-
tributed material to its structural parts. Though it will be heavier (more “dead” weight),
if properly designed it may nevertheless carry more “live” weight (payload) than it did Title-page photograph courtesy of
Boeing Airplane Co. Inc., Seattle,
before, still without failure. In a dynamic machine, adding weight (mass) to moving Wash.
parts may have the opposite effect, reducing the machine’s safety factor, its allowable
Chapter 1