WHEEL-GEARING, a general term ap plicable to all forms of mechanical devices by which motion is transmitted by means of toothed or cog wheels. The different parts of the part to which that motion is transmitted by the driver is termed the °follower.' A train of mechanism consists of a series of such ele mentary combinations, the motion being trans mitted to each piece from the one immediately a machine may transmit motion to each other by 'direct contact' or by means of an 'inter mediate connector': in the latter case the mo tion of the connecting part being usually of no preceding it, and so on throughout the entire series; therefore, in order to ascertain the ac tion of a complete machine, the •velocity ratios and the °directional relation' of each combina tion of the series has to be determined. These factors depend upon the mechanical connection of the twoparts, forming the combination, and are absolutely independent of the true velocities or directions of the parts themselves, which may or may not be liable to continual variation. For example, in two circular wheels, in contact importance since the desired object is simply the proper relative motion of the two parts thus connected. In elementary combinations of either kind, the part receiving the motion from the source of power is termed the 'driver' and with each other, and rotating upon fixed axles, the velocity ratio is constant If the diameter of one is twice that of the other, the angular velocity of the larger will be one-half that of the smaller, but during any changes of velocity whatsoever since the length of the respective radii of the wheels remains unchanged, the velocity ratio at any instant is the same. As to the directional relation, if the wheels are in internal contact, they will rotate in the same direction, and if in external contact, in opposite directions; but in both cases the directional re btions will remain unchanged regardless of any On the absolute direction of the driver. e other hand, if the wheels are elliptical, while the directional relation will remain con stmt, the velocity ratio will be subject to varia tion according to varying lengths of the contact radii All elementary combinations may be di vided into four classes — rolling contact, sliding contact, link work and wrapping connections.
The first two are direct methods while the two last named communicate the motion through in termediate connectors.
The theoretical forms required to transmit motion by rolling contact are cylinders, cones and hy-perboloids of revolution. In the various forms of "friction gearing,' employed to drive tight machinery, the rolling pieces are made of different materials, one of woodand the other of iron, or one of them is covered with leather or rubber, and the necessary amount of adhesive contact is secured by adjusting the bearings of the. i pieces or by the application of weights or springs. It is impossible to transmit motion against any considerable amount of resistance by means of smooth surfaces, and toothed gears are, therefore, generally employed and may be conveniently divided into five classes, according to the form of the pitch or contact surfaces of the equivalent toothed wheels, and designated as 'spur' gears, 'bevel' gears, 'skew' gears, 'screw' or 'worm' gears and 'face' gears.
In spur gearing, the wheels act upon each other in the same plane; the pitch surfaces are cylinders and the teeth engage along straight fines parallel to the sides of the cylinders. If one is considerably smaller than the other it is usually termed the 'pinion' while the larger is commonly known as the 'wheel.' When the teeth are formed on the inner side of the wheel, or convergent to its centre, it is called an in ternal wheel. Wheels in external contact rotate in opposite directions, but wheels in internal contact rotate in the same direction. As the diameter of the pitch circle of a 'wheel' is in creased its curvature decreases and finally be comes a straight line and results in a 'rack and pinion' combination.