Final answer:
Various factors like uneven heating, improper lubrication, or mechanical damage can cause the formation of hard spots in the rotor.
Step-by-step explanation:
The formation of hard spots in the rotor can be caused by various factors, such as uneven heating or cooling, improper lubrication, or mechanical damage. When the rotor undergoes uneven heating or cooling, it can lead to thermal stress and the formation of hard spots. Similarly, if the rotor is not properly lubricated, excessive friction can occur, resulting in the formation of hard spots. Mechanical damage, such as impact or wear, can also cause hard spots to form in the rotor.