Answer: 1. Correct
It is correct that the shafts are designed on the basis of rigidity and stiffness.
The shafts are designed on the basis of strength or rigidity or both. The goal of a strength-based design is to ensure that stress at any point on the shaft does not exceed the yield stress of the material. The goal of rigidity-based design is to keep the shaft's maximum deflection (due to bending) and maximum twist (due to torsion) within acceptable bounds. In some circumstances, rigidity is also significant. For example, if the shaft is deflected, the position of a gear mounted on the shaft may change, and if this value exceeds some permissible limit, it may cause excessive dynamic loads and noise in the gears.
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