The correct answer is: C. large
The critical speed of a shaft is the speed at which the shaft will start to vibrate and eventually break. The critical speed depends on the size and shape of the shaft, as well as the material it is made of. The larger the shaft, the higher the critical speed.
In large machines, the shaft is often the most critical component. The shaft must be strong enough to support the weight of the machine and the loads it is subjected to. It must also be able to withstand the vibrations caused by the machine’s operation.
The size of the shaft is determined by the critical speed. The critical speed must be higher than the maximum speed at which the machine will operate. This ensures that the shaft will not vibrate and break.
The other options are incorrect because they do not take into account the critical speed of the shaft. Small machines do not have to worry about the critical speed of the shaft because the shaft is not subjected to the same loads and vibrations as a large machine. Medium-sized machines may have to worry about the critical speed of the shaft, but it is not as critical as it is for large machines.