WRITING THE MECHANISM DESCRIPTION
The outline below presents the usual fonn for writing a mechanism description. This basic approach, with slight variations, will work in most instances:
A. Definition and purpose
B. Overall description (size, weight, shape, material)
C. Main parts
II. Body: Description of Mechanism
A. Main part A (definition followed by detailed description of size, shape, material, location, and method of attachment) B. Main part B (definition followed by overall description, then identification of subparts)
1. Subpart X (definition followed by detailed description of size, shape, material, location, and method of attachment)
2. Sub part Y (same as for sub part X)
3. Optional conclusion
The introduction gives the reader a framework for understanding the mechanism. In the introduction’ you should define the mechanism, tell its purpose, present an overall description, and preview the main parts. In the following introduction, the writer does all four:
To: Emily BroJn Date: July 18, 19XX
From: Steve V,andeWalle
Subject: Tablejep Paper Micrometer
This memo provides the information you requested at our July 17 meeting dealing with my department’s paper micrometer. A paper micrometer is a small measuring instrument used to measure the thickness of a piece of paper. The micrometer, roughly twice as large as a regular stapler , has four main parts: the frame, the dial, the hand lever, and the piston.
Body: Description of Parts
The body of the paper contains the detailed description. Usually you identify each main part with a heading, then describe it in a single paragraph. Each paragraph will follow the outline explained earlier in the section on The Common Elements of Description, p. 175. In the example below, notice that each section describes only one item. If necessary, you can divide a secnonmro subsections.
The frame of the paper micrometer is a cast piece of steel that provides a surface to which all of the other parts are attached. The frame, painted gray, looks like the letter C with a large flcttdisk on the bottom and a round calibrated dial on top. The disk is 4112 inches in diameter and resembles a flat hockey puck. The frame is 5% inches high and 7112 inches long. Excluding the bottom disk, the frame is approximately 1V4 inches wide. The micrometer weighs 8 pounds.
The dial shows the thickness 01 the paper. The dial looks like a watch djal except that it has only one moving hand. The frame around the dial is made of chrome-plated metal. A piece of glass protects the face of the dial in the same way that the glass crystal on a watch protects the face and hands. The dial, 6 inches in diameter and 7/8 inches thick, is calibrated in .001 inch marks, and the face of the dial is numbered every .010 inches. The hand is made from a thin, stiff metal rod, pointed on the end.
The Hand Lever
The hand lever, shaped like a handle on a pair of pliers, raises and lowers the piston. It is made of chrome-plated steel and attaches to the frame near the base of the dial. The hand lever is 4 inches lorg, V2 inch wide, and V4 inch thick. When the hand lever is depressed, the piston moves up, and the hand on the dial rotates. When the hand lever is released and a piece of paper is positioned under the piston, the dial shows the thickness of the paper.
The piston moves up and down when the operator depresses and releases the hand lever. This action causes the paper’s thickness to register on the dial. The piston is % inches in diameter, flat on the bottom, and made of metal without a finish. The piston slides in a hole in the frame. The piston can measure the thickness of paper up to .300 inches
Other Patterns for Mechanism Descriptions
Two other patterns fire useful for describing mechanisms: the function method and the generalized method. The Function Method One common way to describe a machine is to name its main parts and then give only a brief discussion of the function of each part. This function method is used extensively in manuals. The following paragraph is an example of a function paragraph.
The four function buttons, located under the liquid crystal display, work in conjunction with the function switches. The four switches are hertz (Hz), decibels (dB), continuity (c), and relative (REL). The hertz function can be selected to measure the frequency of the Input signal by pressing button 1. Press the button again to disable. The decibel function allows you to measure the intensity of the input signal, which is valuable for measuring audio signals. It functions the same as the hertz button. The continuity function allows you to turn on a visible bar o~the display, turn on an audible continuity signal, or disable both of them. The relative function enables you to store a value ‘as a reference value. For example, say you have a value of 1.09 volts stored: every signal that you measure with this value will have 1.00 volt subtracted from it.
The Generalized Method
The generalized method does not focus on a part-by-part description; instead the writer conveys many facts about the machine. This method of describing is commonly found in technical journals and in technical reports. With the generalized method, writers use the following outline Gordan 19-22, 35):
1. General detail
2. Physical description
3. Details of function
4. Other details
General detail consists of a definition and a basic statement of the operaticIn principle. Physical description explains such items as shape, size, arrea’ ance, and characteristics (weight, hardness, chemical properties, methods of assembly or construction). Details of function explain these features the mechanism:
how it works, or its operational principle its applications how well and how efficiently it works special constraints, such as conditions in the environment how it is controlled how long it performs before it needs service Other details include information about background marketing general information, such as who makes it The article on anti-static foam in Chapter 6 (p. 110) and the EVA hot melt model at the end of this chapter are examples of this kind of description.