A shaft is defined as a rotating machine element, usually circular in cross-section, which is used to transmit power from one part to another, or from a machine that produces power to a machine that absorbs power.
Shaft form the important element of machines. They support rotating parts like gears and pulleys and are themselves supported by bearings resting in the rigid machine housing.
The shafts perform the function of transmitting power from one rotating member to another supported by it or connected to it. Thus, they are subjected to torque due to power transmission and bending moment due to the reactions of the members that are supported by them.
Shafts are to be distinguished from axles which also support rotating members but do not transmit power.
Shafts are always made to circular cross-sections and could be either solid or hollow. The shafts are classifieds as straight, cranked, flexible, or articulated. Straight shafts are the commonest to be used for power transmission.
Such shafts are commonly designed as stepped cylindrical bars, that is, they have various diameters along their length, although constant diameter shafts would be easy to produce. The stepped shafts correspond to the magnitude of stress which varies along the length.
Moreover, the uniform diameter shafts are not compatible with assembly, disassembly, and maintenance such shafts would complicate the fastening of the parts fitted to them, particularly the bearings, which have restricted against sliding in an axial direction.
While determining the form of the stepped shaft it is borne in mind that the diameter of each cross-section should be such that each part fitted onto the shaft has convenient access to its seat.
