474 To Article 263, page 298 (Clasp-Nut), To Table V., page 482 (Strength and Elasticity of Silk and Flax),. To Article 465, page 547, and Article 466, page 549 (Braced Shaft), 682 382 A MANUAL OF MACHINERY AND MILLWORK. INTRODUCTION ART. 1. Nature and Use of Machinery in General.-The use of machinery is to transmit and modify motion and force. The parts of which it consists may be distinguished into two principal divisions,the Mechanism, or moving parts; and the Frame, being the structure which supports the pieces of the mechanism, and to a certain extent determines the nature of their motions. In the action of a machine the following three things take place:-First, Some natural source of energy communicates motion and force to a part of the mechanism called the Prime Mover; Secondly, The motion and force are transmitted from the prime mover through the train of mechanism to the working piece; and during that transmission the motion and force are modified in amount and in direction, so as to be rendered suitable for the purpose to which they are to be applied; and, Thirdly, The working piece, by means of its motion, or of its motion and force combined, accomplishes some useful purpose. 2. Distinction between the Geometry and the Dynamics of Machinery.—The modification of motion in machinery depends on the figures and arrangement of the moving pieces, and the way in which they are connected with the frame and with each other; and almost all questions respecting it can be solved by the application of geometrical principles alone. The modification of force depends on the modification of motion; and those two phenomena always take place together; but in solving questions relating to the modification of force, the principles of dynamics have to be applied in addition to those of geometry. Hence, in treating of the art of designing machinery, arises a division into two departments, the "Geometry of Machinery," or "Science of Pure Mechanism" (to use a term introduced by Professor Willis), which shows how the figure, arrangement, and mode of connection of the pieces of a machine are to be adapted to the modification of motion which they are to B produce; and the "Dynamics of Machinery," which shows what modifications of force accompany given modifications of motion, and what modifications of motion are required in order to produce given modifications of force. 3. Strength of Machinery.—In order that a machine may be fit for use, every part, both of the machinery and of the framework, must be capable of bearing the utmost straining action which can be exerted upon it during the working of the machine, without any risk of being broken or overstrained; and the dimensions required for that purpose are to be determined by the proper application of the principles of the strength of materials. 4. The Art of the Construction of Machinery consists of three departments, the selecting and obtaining of suitable materials for the parts of the mechanism and framework; the shaping of those parts to the proper figures and dimensions by means of suitable tools; and the fitting-up of the machine, by putting its parts together. 5. Division of the Subject. For the reasons explained in the preceding Articles, the subjects of this work are treated of under four principal heads,-Geometry of Machinery, or Pure Mechanism; Dynamics of Machinery; Materials, Construction, and Strength of Machinery. |