WHEEL A mechanical contrivance, by means of which the inter mittent and limited action of the hiver is extended to any distance, and made to act continuously and uniformly. The direction and the velocity of movement of a machine are commonly regulated by the disposition and character of the whcelwork which enters into its composition; but the principle upon which all wheels really act, con sists iu the application, in a continuous manner, and in a circular direction, of the power obtained by leverage.
Wheels are either of the kind known as carriage wheels, or friction wheels, or teethed including under the second division band wheels, and under the third the various kinds of cog, trundle, spur, crown, and befitted wheels. In a system of wheelwork the wheels may be either multiplying or diminishing, according to their relative sizes ; they may either perform complete revolutions, or, as in the case of balance wheels, only revolve over small arcs ; they may revolve either horizontally or vertically ; they may be used to produce motion or to communicate it ; or, finally, they may be used for the purpose of regulating the velocity of the machinery to which they are annexed, as in the case of fly wheels. Water-wheels constitute a sepa rate class of machinery, known under the same generic name of wheels, in which the power is produced by the direct action upon the wheel itself. [Werza-WnEsts.) In ordinary wheels the power is applied originally to the shaft bearing them, by the intervention of cranks, handles, winches, or levers.
In carriage wheels the object sought to be attained is to convert a eliding friction into a rolling one, in order to facilitate the horizontal movement of heavy loads. The efficiency of a carriage wheel, there fore, consists in the length of the lever it offers (or, in other words, upon its diameter); upon the direction in which the power is applied to it ; and upon the small extent of surface producing friction; always provided that the surface should be sufficient to prevent the load from forcing the wheels into the materials over which they run. It is for the avowed object of insuring the application of the power exercised by a horse, in drawing a cart or carriage, above the horizontal line passing through the centre of the fore wheels, that they are made smaller than the bind wheels ; but, as an abstract proposition, it may be stated that the larger a wheel is, the greater is time useful effect it produces, provided the lino of draught pass horizontally through its centre. Another abstract proposition with respect to carriage wheels
is, that the narrower their surfaces, the less friction they must de velop° ; but evidently the character of the roadway over which the load travels must re-act upon this condition, for upon a soft surface a narrow wheel, heavily laden, would compress the road materials, and thus create a continually occurring series of obstacles to its own pro gress. Upon soft roadways, under heavy loads, carriage wheels must therefore be made wide upon the face of the fences ; for quick traffic, on hard roads, when the carriages do not convey heavy loads, narrow wheels are generally resorted to.
In the earliest wheeled carriages, the wheels were made of solid planks, and the axles were fastened to them, so that wheel and axle turned together; the load in such cases being borne upon collars worked upon the axles. Rude carts of this description have been used in England within a very short period, and they may still be seen upon the provincial roads of such countries as Spain or southern Italy. When the roads are habitually of a superior character, how ever, carriage wheels are made as lightly as the work they are required to perform will adroit of ; and they consist usually of a nave, cr centre boss, into which the spokes or radiating arms are fastened at one cad, whilst the spokes at their other clads bear the Echoes; the whole assemblage being bound together by an iron tire, nailed to the felloes whilst it is hot, in order that the shrinkage of the iron in cooling may " force the various joints home," as workmen ay. The nave is usually bored out to receive a box of iron (or of gun metal in some crmes), in which the collar of the axle is inserted, so that the wheels on the respective sides of the carriage can revolve independently of one another in their horizontal positions, but are maintained at fixed distances apart horizontally. In wheeled carriages the bodies are attached to the axles, either with, or without, the intervention of springs; in four-wheeled carriages the front pair of wheels is usually made to revolve on a pivot fixed under the fore part of the body. Of course the dimensions of all the details of carriage wheels unmet depend upon the weights they are intended to transport, and upon the velocity at which they are intended to travel; in railway carriages the naves, spokes, felloes, and tires of the wheels are executed of iron, or of steel ; in common carriages, wood is the material principally used.