A A shaft is a cylindrical mechanical component, though some are square, that passes through bearings, wheels, or gears. It supports rotating parts and rotates with them to transmit motion, torque, or bending moment. Typically made of metal in round bar form, a shaft may have different diameters along its length. Rotating machine elements are mounted on it. Shaft materials are primarily carbon steel or alloy steel, and sometimes ductile iron or alloy cast iron. A shaft's working capacity depends on strength, stiffness, and at high speeds, vibration stability.

A Common shaft types include crankshafts, straight shafts, and flexible shafts. Straight shafts are further classified into three types. Rotating shafts carry both bending moment and torque, like those in gear reducers. Spindle shafts support rotating parts under bending moment only without transmitting torque; some rotate, like railway axles, others stay fixed. Transmission shafts transmit torque without bending moment, such as long drive shafts in crane mechanisms or automobile drive shafts. Each type is designed for its specific load and function.

A A spindle shaft supports rotating components and bears only bending moment, not torque. A railway axle is a classic example—it carries the weight but does not transmit driving torque. Some spindles rotate, others remain stationary. A transmission shaft, by contrast, is designed primarily to transmit torque without significant bending load, like the long propeller shaft connecting a vehicle's transmission to the differential. The key distinction lies in the type of load each is engineered to carry.

A A hollow shaft features a central through-hole running along its axis. Inside, it may have an internal keyway. The outer surface is machined with stepped cylindrical sections and external keyways. The center hole slides over a mating spindle, and power is transmitted through external gears mounted on the stepped diameters. Although it occupies more space, a hollow shaft significantly reduces weight compared to a solid shaft of equivalent outer diameter, an advantage in rotating machinery where inertia must be minimized.

A According to material mechanics, when a shaft transmits torque, the outer radial region contributes most to effective moment transmission. The core material near the center carries very little torque. Therefore, when a large diameter is needed to handle high torque, removing the central material to create a hollow shaft saves substantial weight without significantly compromising torsional strength. This weight reduction lowers rotational inertia, improves dynamic response, and saves material cost.

A A hollow shaft is mounted over a spindle or drive shaft via its central bore. External gears or pulleys are mounted on the shaft's stepped outer diameters and keyed to prevent rotation. Input power is delivered to these gears, which then turn the hollow shaft. The shaft, in turn, drives the internal spindle it is sleeved over, often through internal keyways. This configuration allows compact power transmission and is common in machines like screw presses where the main shaft passes through the hollow bore.