Designing machines must have reference to adaptation, endurance, and the expense of construction. Adaptation includes the performance of machinery, its commercial value, or what the machinery may earn in operating; endurance, the time that machines may operate without being repaired, and the constancy of their performance; expense, the investment represented in machinery.
The adaptation, endurance, and cost of machines in designing become resolved into problems of movements, the arrangement of parts, and proportions.
Movements and strains may be called two of the leading conditions upon which designs for machines are based: movements determine general dimensions, and strains determine the proportions and sizes of particular parts. Movement and strain together determine the nature and area of bearings or bearing surfaces.
The range and speed of movement of the parts of machines are elements in designing that admit of a definite determination from the work to be accomplished, but arrangement cannot be so determined, and is the most difficult to find data for. To sum up these propositions we have:—
1. A conception of certain functions in a machine, and some definite object which it is to accomplish.
2. Plans of adaptation and arrangement of the component parts of the machinery, or organisation as it may be called.
3. A knowledge of specific conditions, such as strains, the range and rate of movements, and so on.
4. Proportions of the various parts, including the framing, bearing surfaces, shafts, belts, gearing, and other details.
5. Symmetry of appearance, which is often more the result of obvious adaptation than ornamentation.
To illustrate the practical application of what has preceded, let it be supposed, for example, that a machine is to be made for cutting teeth in iron racks ¾ in. pitch and 3 in. face, and that a design is to be prepared without reference to such machines as may already be in use for the purpose.