Another key aspect is to monitor overall vibration level over time. The general trend is depicted in the figure below. Normally, for most of the machines after an initial period of settling or “break-in”, the vibration level is kept acceptable level. Later, due to wear, the vibrations levle begin to increase to levels of attention and, later, alarm. The periodicity of the control and the type of maintenance intervention will depend on the type of machinery. It ‘also important to pay attention not only to the absolute level of vibration, but also to a given percentage of changes compared with the previous. A too abrupt variation of the vibrational level, although below acceptable levels is however an index of abnormal behavior of the machine.