Question

After full analysis of a solid steel shaft considering deflection, strength and vibrations, we come to know that the shaft design is limited by deflection, which option as a designer would you pursue, provide reasoningfor your choice.

(a)Increase the shaft diameter but make the shaft hollow while keeping the weight constant

(b)Keep the shaft diameter constant and select a shaft with higher strength

(c)Increase the shaft diameter to increase the critical speed and select a steel with higher strength

(d)Increase the shaft diameter and use a cheaper steel

(e)Increase the shaft diameter to increase the critical speed and reduce deflection as well as selecting a steel

with higher strength to improve the strength.

Answer 1

The designer must choose option (e) because ,on increasing the diameter, the critical speed at which the solid shaft runs increases and the strength of the shaft can be increased only by choosing a good quality of steel. Option (a) cannot be choosed because , though the diameter of shaft is increased it is made hollow.The stiffness criteria comes to picture i.e on making the shaft hollow, the stiffness of the structure reduces thus there is a failure.Option (b) cannot be chosen because, keeping the diameter constant means there in no increase in critical speed hence is not safe to use.Option (d) cannot be chosen because,using cheap steel means reducing the strength which may lead to failure. Option (c) and (e) are same.