SHAFT, DRIVING, is a bar of steel revolving in bearings and part of a mechanical power transmission system. It drives a machine or a set of machines, and is distinguished from a spindle which is incorporated in the machine itself (e.g., a drill spindle) to carry some tool or appliance, and from an axle, which carries road wheels. The shaft is necessary to transmit power from a hand drive, or a power unit, and may be located overhead in a factory, or on the ground or below it. Some machines have several shafts incorporated in the construction, conveying power by means of belts, couplings, clutches or gears. Overhead shafting is usually chosen as a matter of convenience for driving several machines by means of belts. Two essentials apply to all shafts: (I) sufficient rigidity to withstand twisting and bending under load, and (2) running with the minimum of friction. The first requirement is a matter of sufficient diameter and suitable mate rial, generally a good class of steel, and bearings placed close enough together, especially where there is a heavy lateral pull. The frictional difficulty is not troublesome with only two bearings located close together, but when a shaft is long, and the bearings are numerous, alinement is not always easy to ensure or maintain. Settlements in buildings and foundations cause bearings to alter their position. Wedges or other adjusting devices are therefore necessary, while a swivel fitting with ball-and-socket action allows the bearings to float and automatically adjust themselves to the shaft.
The three classes of shaft bearings (q.v.) are the journal, or peripheral, the thrust or end pressure type, and the footstep, which sustains the bottom end of a vertical shaft, such as that of a water turbine. When it is necessary to drive a spindle at different heights, as with the rolling-mills, the shaft connection must be made with a universal coupling, permitting the varying angular positions. Much greater flexibility occurs in the small flexible shafts, which are so constructed that they will bend freely in any direction and transmit the power. These are utilized for driving portable tools for drilling, reaming, grinding, polishing, boiler-tube expanding and so on.
Special machinery is laid down for the manufacturing of shaf t ing, which may be rolled, drawn or finished accurately, by turn ing or grinding. To make up lengths as required from stock sizes couplings are used, or if frequent disconnection should be desired a clutch is fitted instead. To a considerable extent shafting has been eliminated in many factories, machines being driven by self contained electric motors, so that overhead lighting is improved, and costs for installations and the frictional losses are reduced.
(F. H.)