A common material used in machine shafts is __________.

Cold rolled steel is a widely used material for machine shafts due to its properties that provide strength and durability. This type of steel undergoes a cold rolling process, which enhances its mechanical properties by refining its grain structure. The resulting material has increased tensile strength and improved surface finish, making it suitable for precision applications where reliable performance is essential.

In contrast, the other materials listed have limitations in this context. Copper, while excellent for electrical conductivity, does not have the necessary strength for most shaft applications. Pewter is a softer alloy and would not withstand the stresses that a machine shaft typically endures. Iron, particularly in its cast form, may not possess the same combination of hardness and ductility as cold rolled steel, making it less ideal for shafts where resistance to wear and fatigue is crucial. Thus, cold rolled steel emerges as the optimal choice for machine shafts in various applications.