The strong development of electronics and computers, and its application to dynamic signal analysis, has prompted the appearance of reliable predictive maintenance systems. This type of maintenance is also known as condition-based maintenance, on- condition maintenance or condition-directed maintenance and it is conceived to detect the onset of a failure, avoiding critical damages of high cost components before they might happen, thus reducing overall maintenance costs. Possible faults are detected by monitoring representative parameters by signal analysis techniques and comparing signals during normal and abnormal conditions.