WHEELS. In machinery, wheels are generally used for transmitting motion, regu lating velocity, converting one species of mo tion into another, reducing friction, or equaliz ing the effect of forces applied in an intermit tent or irregular manner.
The simplest mode of transmitting motion from one wheel to another is by causing their peripheries to revolve in contact and pressing them together, in order to prevent slipping. Another mode of transmitting rotatory motion more effectually than by simple contact is by the use of endless bands or straps passing over the peripheries of the wheels which are intended to revolve together. Straps or belts are much used in cotton-factories, and other works in which moving-power has to be com municated to a great number of machines in different parts of a building, and they are pre ferred to cog-wheels in cases where sudden strains are liable to occur, because of the yielding character of the connection effected by them. Such straps were formerly made of leather ; but caoutchouc and gutta percha are now largely used. Sometimes a chain of ' links is used instead of a band.
To convert rotatory motion into rectilinear, or rectilinear into rotatory, various forms of the rack and pinion are used. In its simplest form this contrivance is applied in raising sluice-gates, in lifting-jacks, and various other machines in which a fixed pinion or small toothed wheel is made to give motion to a straight toothed bar capable of moving in the direction of its length. A crank is also ap plied in various ways for converting rotatory motion into rectilinear, or rectilinear motion into rotatory. Of the Ereentric and other wheels for producing rectilinear or irregular motion from a revolving or for producing uniform rotation from an intermittent force, the varieties are numerous.
The escapement -wheels of clocks and wheels furnish types of another important class of wheels for modifying motion ; and snail-wheels, pin-wheels, ratchet-wheels, and fusee wheels, are among the ingenious contri vances by which the rotatory motion of a wheel and axle may be made to motion a train of complicated machinery, or to regulate and vary motion at pleasure. Another class of
wheels, called Frielion-Wheels are intended to lessen the evils of friction in machinery.
Wheels introduced into machinery for the purpose of overcoming inertia, or of rendering uniform and eteady a motion derived from an intermittent or, variable source, are called Fly Wheels. Since they owo their effect to their weight, fly-wheels are usually heavy, and as much as possible of their weight is disposed in the rim, where, owing to the effect of cen trifugal force, it is of far more value than when near the centre. The steam-engine, the stamping-press, the common lathe, the coffee mill, and a variety of other machines exhibit the useful application of 'a fly-wheel. In roasting-jacks, musical -hoses, the striking apparatus of clocks, and various other contri vances in which a retardingforceis required to prevent the moving power of a spring or weight from running down too rapidly, wheels with projecting vanes, which encounter suffi cient resistance from the air to moderate their velocity, are used under the name of Flys or. Flyers'.
A small wheel having cylindrical staves or spindles fixed between two circular boards or plates of metal, in positions parallel to the axis of rotation, is called a lantern wheel: and when a wheel acts with one which is smaller in diameter, whatever be the form of the teeth, the latter is usually called a pinion. Wheels having the teeth formed on their circumfe rences so as to project from thence in the direction of the radii are called spar-wheels : but when the teeth are perpendicular to the plane of the wheel, the latter is called a crown or eontrate wheel. If the teeth are cut on the circumference of a wheel in a direction ob lique to its plane, the wheel is said to be bevilled ; and two wheels may have their teeth so bevilled as to revolve in planes making any angles with one another.