MACHINERY, COMPOSITION OF. The composition of a ma chine is usually understood, by writers on this branch of the arts, to involve all the detailed arrangements by which the power exerted, however that power be obtained, is made to produce the required effects; or in other words, those contrivances by which the initial force is transmitted with the smallest loss, or with the least waste of material. Front the endless modifications of machinery it must evidently be impossible hero to notice all their component parts; but a few general observations upon wheels, shafts and spindles, cranks and excentrics, drums and band wheels, clutches and coupling-gearing, may suffice to call attention to some of the leading principles connected with the componition of machinery.
IS heelwork, whereby the power is transmitted by means of teeth ,• working into ono another, is usually known under the name of gearin , and this part of the subject has already been noticed [Gitaitrao ; but it may perhaps bo worth while to add that wheels are technical y subdivided into teethed, trundle, pin, crown, annular, and bevilled wheels, according to the shape, or tho position, of the parts gearing, or working, Into one another. Sometinies also a pinion may be mado to revolve in, or upon, a straight piece of gearing, for the purpose of changing a continuous rotary to a horizontal motion, or vice verad ; or, again, the wheels may be susceptible of motion in alternate directions, or they may be intended only to revolve in one, in which case they either are made of such a form as themselves to act as ratchet wheels, or they are prevented from turning in the wrong direction by such a contrivance. In some cases also motion is given by the mere friction of the peripheries of the wheels upon one another, but evidently this can only be accomplished in very small machinery, or where the force to be transmitted is small. The principles upon which wheels of the various descriptions above-mentioned must be formed have been elaborately discussed in Camas, the Teeth of Wheels,' by J. T. Hawkins, 1842 ; Moseley's Mechanical Principles of Engineering,' 1843 ; Warr's Dynamics,' &c., 1851; they will also be found in a condensed form under GEARINO ; MACHINE; 'WHEELS, TEETH OF.
Spindles, or shafts, are the axes upon which the various wheels are fixed, and by which the initial motion is distributed as may be desired. They may be of wood, or of metal ; and if of the latter class of materials, either solid or hollow, square, hexagonal, or circular, as may be desired ; the object to be aimed at being, in all cases, to make the shaft as light as possible consistently with the requisite degree of strength, and to reduce the dimension of the bearings to the minimum in order to diminish the friction. When shafts are made to revolve on a vertical axis, they turn on a pivot, or gudgeon ; when horizontal shafts of great length are used, they turn in plummer blocks, or carriages. As it has been found practically that two loaded surfaces work upon one another with the greatest amount of friction when they are of the same material, it is usual to make the bushes or steps, in which the shafts or gudgeons work, of a different material to those parts of the machinery themselves ; or in fact to make iron work on brass or gun metal, wood upon iron, &c.
Cranks and eccentrics are the contrivances by means of which, in addition to wheels, the direction, or the nature, of the initial motion of a machine is transmitted to the work ; or in other words, by means of which circular, and rectilineal, motion aro reciprocally converted, or uniform velocity is changed into a variable one. The bell crank lever is the best' illustration of the parts of machinery for transmitting a rectilineal motion in one direction to a shaft working iu another direc tion; and according as the other end of that shaft may be attached to a crank working on a fixed axle around which it may revolve freely, or be attached to a part of the machine susceptible only of rectilineal motion, it will be able to convert the original rectilineal motion into another one of the same description, or into a curvilinear motion. The rack and pinion have been already cited as an illustration of the means of converting circular into rectilineal motion ; and the common bow drill may he cited as an ordinarily adopted method of converting alternate rectilineal motion into alternate circular motion. Excentric wheels are, however, the most generally adopted means of converting curvilinear into rectilinear motion, and they may be either formed of circular wheels keyed upon shafts, whose axes do not pass through the centre of the former; or they may be what are called heart-shaped eccentrics; or they may communicate movement by pins working in slots. The cam is a contrivance frequently used for the purpose of converting continuous circular motion into an alternate vertical one, and this is effected in the case of tilt hammers by means of a series of eccentrics able to lift a lever, fixed at one end, in a gradual manner through a path corresponding to the highest point of the cam itself, alter attaining which the lever is suddenly released, and of course falls. Varying rectilinear motion may be produced by modification in the shape of the cam in any way which may be required; and there are countless modifications of the traverses of machinery, whereby con tinucd alternate motion is obtained from wheels or axles revolving in a circle. A reversing motion may be obtained by the use of a sliding bush working in a groove ; and in the numerous tool-making machines of Mr. Whitworth, every description of change of motion from circular to horizontal may be observed. The crank is, it may he added, the most important element of machinery for the purpose of converting an alternate vertical, or horizontal, motion round a fixed centre, into a circular motion ; and the parallel rods, so universally applied to connect the piston heads to the balance beams of steam engines, may be cited as amongst the most important combinations for the con version of alternate vertical motion into a partial alternate circular motion round a fixed axis. CRANK; PARALLEL MOTION; STEAM ENOINE.