The goal of a mechanism description is to give readers all the information they need to know about the mechanism. Obviously you can’t describe every part in minute detail, so you select various key parts and their functions. Most important you must plan the description before writing it. When you plan a description of a mechanism, you must consider the audience, select an organizational principle, choose visual aids, and follow the usual form for writing descriptions.

Consider the Audience

Consider the audience’s knowledge level and why they need the information. Both will determine how much detail you include. In the following mechanism description, the writer uses very specific detail to show an expert reader that the part exactly fulfills the specifications and then explains the significance of these details.

The IRU contains an Inertial Sensor Assembly (ISA), power supply, eight elecjfonic boards, and a chassis containing a motherboard. Its form and dimensions (7.60 in. high x 12.69 in. wide x 12.76 in. long) meet the requirements of ARINC 60010 MCU. With the implementation of second-generation electronics (14 boards reduced down to 8 boards), six empty electronic card slots are left for future growth. (Honeywell 3-10)

But in the following description, from a different document, the writer does not include any specific details, aiming instead at a reader who needs only a general understanding of the part:

The pump creates fluid flow within the system. The system  has a gear-type pump made of two components: a drive gear and a driven gear, both in a closely fitted housing. The drive  gear, which is powered by an electric motor, turns the driven gear ‘in the opposite direction. As the gears turn, they mesh at  a point in the housing between the inlet and the outlet ports. The fluid trapped between the teeth and the housing is pushed through the outlet port by atmospheric pressure due to the low pressurefreated by the rotation of the gears. This creates
fluid flow.

Select an Organizational Principle

You can choose from a number of organizational principles. For instance, you can describe an object from

• top to bottom (or bottom to. top)

• outside to inside (or inside to outside)

• most important to least important (or least important to most important)

For example, if you were going to describe a secretarial chair from top to bottom, you might start with the backrest, then go to the seat, and then move down to the casters. Or you could do the reverse. If you wished to describe it from most important to least important, you might start with the seat, then describe the backrest, and then the casters.

To make your decision, consider the audience’s potential use of your’ document. If the audience needs a general introduction, then an easy sequence, from top to bottom, is best. If your audience needs to know special details for secretaries’ safety and comfort, you might start with the caster system, which prevents tipping, then go to the adjustment system, which eases back strain.

An easy way to check if your organization is working is to look for “backtracking.” Your description should move steadily forward, starting
with basic definitions or concepts that the audience needs to understand later statements. If your description is full of sections in which you have to stop and backtrack to define terms or concepts, then your sequence is probably inappropriate.

Choose Visual Aids

Use visual aids to assist your description of a complex mechanism. The type of visual you select depends on the mechanism and the reader. H your readers need an overview of a secretarial chair so that they can see how each part is constructed, then you should use a drawing or photograph of the entire unit. If, however, they need to know how the backrest can be adjusted to a comfortable height, the overall drawing is useless. Instead, provide a drawing or photograph of the adjusting assembly behind the backrest.

Follow the Usual Form for Writing Descriptions

Generally, descriptions follow a similar form: an introduction or an overview followed by a body in which each part is described in turn (see pp. 180-181). Physical descriptions generally require no conclusion because a good description leaves nothing to conclude. Sometimes, however, you can place in a conclusion material that needs clarification but that does not fit elsewhere. If the operating principle of the mechanism, for instance, is complicated, you might explain it at the end of the description.