INTRODUCTION TO DESIGN

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example is the ordinary bicycle. Though inventors continue to attempt to improve this machine, the basic design has become fairly static after more than a century of development.

In machine design, the early design-process steps usually involve the Type Synthesis of suitable kinematic configurations which can provide the necessary motions.

Type synthesis involves the choice of the type of mechanism best suited to the problem.

This is a difficult task for the student, as it requires some experience and knowledge of the various types of mechanisms that exist and that might be feasible from a performance and manufacturing standpoint. As an example, assume that the task is to design a device to track the constant-speed, straight-line motion of a part on a conveyor belt and attach a second part to it as it passes by. This has to be done with good accuracy and repeatability and must be reliable and inexpensive. You might not be aware that this task could be accomplished by any of the following devices:

• a straight-line linkage

• a cam and follower

• an air cylinder

• a hydraulic cylinder

• a robot

• a solenoid

Each of these solutions, while possible, may not be optimal or even practical. Each has good and bad points. The straight-line linkage is large and may have undesirable accelerations, the cam and follower is expensive but is accurate and repeatable. The air cylinder is inexpensive but noisy and unreliable. The hydraulic cylinder and the robot are more expensive. The inexpensive solenoid has high impact loads and velocities. So, the choice of device type can have a big effect on design quality. A bad choice at the type-synthesis stage can create major problems later on. The design might have to be changed after completion, at great expense. Design is essentially an exercise in trade-offs. There is usually no clear-cut solution to a real engineering design problem.

Once the type of required mechanism is defined, its detailed kinematics must be synthesized and analyzed. The motions of all moving parts and their time derivatives through acceleration must be calculated in order to be able to determine the dynamic forces on the system. (See reference 2 for more information on this aspect of machine design.)

In the context of machine design addressed in this text, we will not exercise the entire design process as described in Table 1-1. Rather, we will propose examples, problems, and case studies that already have had steps 1–4 defined. The type synthesis and kinematic analysis will already be done, or at least set up, and the problems will be structured to that degree. The tasks remaining will largely involve steps 5 through 8, with a concentration on synthesis (step 5) and analysis (step 6).

Synthesis and analysis are the “two faces” of machine design, like two sides of the same coin. Synthesis means to put together and analysis means to decompose, to take apart, to resolve into its constituent parts. Thus they are opposites, but they are sym-biotic. We cannot take apart “nothing,” thus we must first synthesize something in order to analyze it. When we analyze it, we will probably find it lacking, requiring further

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MACHINE DESIGN -

An Integrated Approach

 

 

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synthesis, and then further analysis ad nauseam, finally iterating to a better solution. You will need to draw heavily upon your understanding of statics, dynamics, and mechanics of materials to accomplish this.

 

 

1.3

PROBLEM FORMULATION AND CALCULATION

It is extremely important for every engineer to develop good and careful computational habits. Solving complicated problems requires an organized approach. Design problems also require good record-keeping and documentation habits in order to record the many assumptions and design decisions made along the way so that the designer’s thought process can be later reconstructed if redesign is necessary.

A suggested procedure for the designer is shown in Table 1-2, which lists a set of subtasks appropriate to most machine-design problems of this type. These steps should be documented for each problem in a neat fashion, preferably in a bound notebook in order to maintain their chronological order.*

 

Definition Stage

In your design notebook, first Define the Problem clearly in a concise statement. The

givens” for the particular task should be clearly listed, followed by a record of the assumptions made by the designer about the problem. Assumptions expand upon the given (known) information to further constrain the problem. For example, one might assume the effects of friction to be negligible in a particular case, or assume that the weight of the part can be ignored because it will be small compared to the applied or dynamic loads expected.

 

 

 

Preliminary Design Stage

 

 

Once the general constraints are defined, some Preliminary Design Decisions must be made in order to proceed. The reasons and justifications for these decisions should be documented. For example, we might decide to try a solid, rectangular cross section for a connecting link and choose aluminum as a trial material. On the other hand, if we recognized from our understanding of the problem that this link would be subjected to significant accelerations of a time-varying nature that would repeat for millions of cycles, a better design decision might be to use a hollow or I-beam section in order to reduce its mass and also to choose steel for its infinite fatigue life. Thus, these design decisions can have significant effect on the results and will often have to be changed or abandoned as we iterate through the design process. It has often been noted that 90%

* If there is a possibility of a

of a design’s characteristics may be determined in the first 10% of the total project time, patentable invention resulting

from the design, then the

during which these preliminary design decisions are made. If they are bad decisions, notebook should be permanently

it may not be possible to save the bad design through later modifications without es-bound (not loose-leaf), and its

pages should be consecutively

sentially starting over. The preliminary design concept should be documented at this numbered, dated, and witnessed

stage with clearly drawn and labeled Design Sketches that will be understandable to by someone who understands the

technical content.

another engineer or even to oneself after some time has passed.

 

Chapter 1