WHEEL MANUFACTURE. The simplest form of wheel that can be used for any purpose is that of a plain circular disc, such as might be procured by a transverse section of the trunk of a tree of tolerably regular form. Solid wooden wheels are still occasionally used in machinery, but if large they are usually formed of two or three thick nesses of planking fastened together, with the grain crossing in various directions. Wooden wheels for millwork, when not required to bo solid, usually have the periphery formed of segments, the inner sides or edges of which are left straight. The periphery may consist of three thicknesses of planking, each composed of six or eight such seg ments; and if the three thicknesses are properly break-jointed, a wheel of considerable strength may be thus produced. The arms, or radii, of the wheel are fitted to the loner or straight sides of the segments by bolting or other modes of fastening. Wooden wheels are occasionally morticed into their shafts or axes ; but a preferable plan is to use four arms, arranged in two pairs crossing at right angles, and halved into each other in the centre, where their intersection leaves a square open ing for the shaft. This opening should be somewhat larger than the shaft itself, and the difference of size should be made up by the inser tion of wedges, which afford the means of adjusting the wheel per fectly true upon the axis. In very large wheels, such as water-wheels, two complete sets of clasp arms, one on each side of the wheel, are used. In mounting face-wheels it is not unusual to add stays or braces from the back of the wheel to a point at some distance along the shaft, to resist the tendency of the trundle or pinion to force the wheel out of its true position at right angles with the shaft. Hornbeam is considered to be the best kind of wood for the cogs or teeth of wheels.
In modern machinery cast-iron has almost entirely superseded the use of wood for cog-wheels of every description. If they do not ex ceed eight or ten feet in diameter, they may be cast in one piece; but if above that size it is desirable to form them into two or more parts, because of the difficulty of cooling a very large casting without unequal contraction. Where the diameter does not exceed twelve or fourteen feet, the rim may still be formed in one piece, and the centre and arms m another, the two to be united by bolts; but when those dimensions are exceeded, a further subdivision is necessary. The rim may then
be cast in three segments, the box or centre in one piece, and the arms in several pieces, each terminating in a rib forming ball the thickness of an arm, for convenience of bolting together. Large iron wheels are adjusted accurately on their axes by wedges or keys ; but small ones may, in many cases, be adjusted by turning the periphery in a lathe after mounting.
Carriage-wheels are those in which the greatest ingenuity of con struction is called for, as they are exposed to strains far greater for their size and weight than ahnost any others. The peculiar nature of these strains requires not only that the wheels be made exceedingly strong, but also that they possess a degree of elasticity sufficient to enable them to bear the violent concussions to which they are con tinually exposed, without risk of fracture or without the starting of any of their numerous joints. An ordinary carriage-wheel consists of the nitre, a cylindrical block of wood, usually elm, which forms the centre of the wheel, and is pierced longitudinally with a hole to receive the axle ; the spokes, which are radiating arms framed into the nave at equal distances ; and the felloes, which are circular segments framed on to the outer extremities of the spokes, and forming collec tively the periphery or rim of the wheel. The external surface of the felloes is usually protected by a covering of iron, called the tire, which may either be put on in several pieces, or strokes, the joints of which arc made to alternate with those of the felloes, or in a single piece, forming a hoop-tire. The simplest form in which such a wheel as here alluded to could be formed, would be that in which the spokes would stand at right angles with the axis, and would form a that or plane figure when the wheel is viewed edgewise; hut such a wheel would be ill adapted to meet the lateral shocks to which a carriage-wheel is exposed. The more common form is that called the dished-wheel, in which the centre or nave is made to fall back a little from the plane of the felloes, so that the face of the wheel is not flat, but slightly con cave. The elasticity of this form is a very great recommendation.