Description of Mechanism

A mechanism is generally defined as any object or system that has a working part or parts. Most often the term suggests tools, instruments, and machines. But other examples of mechanisms could be the human body and systems like the universe or a city, which is composed of parts that work together like parts of a machine. A technical man constantly works with mechanisms and always needs to understand them; what they do, what they look like, what parts they have, and how these parts work together.
There are three fundamental divisions of the description and these are the introduction, the part-by-part description, and the conclusion.
Because the description of a mechanism seldom constitutes an article or report by itself, the introduction required is usually rather simple. The two elements that need most careful attention are:
1. the initial presentation of the mechanism
2. the organization of the description

The Initial Presentation

At the beginning of a discussion of an unfamiliar mechanism, a reader immediately needs three kinds of information: what it is, what its purpose is, and what it looks like.

Organization of the description. It is possible to divide almost every mechanism into parts. In the initial presentation of the description, the purpose indicates the organization of the discussion that is to follow. Since it is logical to describe the principal parts one at a time, a list of the principal parts in the order in which one wishes to discuss them is a clear indication of the organization of the remainder of the description. The order in which the parts are taken up will normally be determined by either their physical arrangement or their function.

The list of the principal parts should be in parallel form.

The parts are usually named in normal sentence form, like,
“The principal parts of the lathe are (1) the bed, (2) the headstock, (3) the tailstock, and (4) the carriage.” But if the parts are numerous, it may be preferable to present them in the form of a list.

Part-by-Part Description

After the introduction and the mechanism logically divided into parts, the description of the first part follows. But the fact is that now, as far as method goes, it is almost as if one had not written a line. The part that is to be discussed is a brand new mechanism. The reader wants to know what it is. So again, it has to be introduced.
The lathe machine has been divided into the bed, the headstock, the tailstock and the carriage and now we are about to describe the bed. The first problem is to tell the reader what the bed is. The general procedure will be – as before – to define the part, to state its purpose, to indicate its general appearance, and finally, if necessary, to divide it into subparts.
What is done to the principal parts is also done to the subparts. In other words, the mechanism as a whole is progressively broken down into smaller and smaller units until common sense says it is time to stop. Then, each of these units is described in detail. The value of this system is for general policy, it is simply not true that all description should be handled in this way. Sometimes, for example, instead of giving a preliminary statement of all the subparts that will be described in a given section of the description, it is desirable not to mention a certain minor subpart at all except when it is actually described.
Aspects of the mechanism that needs careful attention when describing in detail a subpart of the mechanism:

Shape
Size
Relationship to other parts
Methods of attachment
Material
Finish

Each of these matters need not be labored over mechanically, in the order stated, in every description. Which one needs attention, and what kind of attention, depends upon the reader and the subject. The same line of reasoning can be applied throughout the description. There is no formula that will fit every situation. The important thing is to decide what information the reader needs, and to provide it as clearly as possible.

Conclusion of the Description

The last principal function of the description of a mechanism is to let the reader know how it works, or how it is used, if this has not been done in the general introduction. Emphasis should naturally fall upon the action of the parts in relation to one another. This part of the writing constitutes in effect a description of a process usually highly condensed, and this will be discussed elaborately in the next technique of development.

Summary of the Principles of Organization
Description of a Mechanism
I. Introduction
A. What the mechanism is
B. Purpose
C. General appearance
D. Division into principal parts II. Part-by-part description
A. Part number 1
1. What the part is
2. Purpose
3. Appearance
4. Division into subparts
a. Subpart number 1
(1) What the subpart is
(2) Purpose
(3) Appearance
(4) Detailed description
(a) Shape
(b) Size
(c) Relationship to other parts
(d) Methods of attachment
(e) Material
(f) Finish
b, c, etc.- same as “a”
B, C, etc. – same as “A”

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