Using Time Domain Plots To Find: Rubs

What does a rub cause on the spectrum ? Well, it is quite unpredictable but can, because of the wave shape, cause extensive harmonics, half-harmonics (0.5, 1.5, 2.5, etc. x rpm) or even sub-harmonics. But what does the time domain look like ?  Figure 1

Once the rotor contacts the side (begins the rub), it is prevented from moving any further in that direction. It will rub until the forces present pull it away from the contacted surface in the other direction. This is another example of a problem where the diagnostic capabilities of the time plot far exceed those of the spectrum. For instance, the length of the flat portion (the rub) relative to the length of the entire cycle will tell you how much of the rotation is rubbing. You should note that this condition would not be apparent if the was mounted horizontally because the rub is strictly in the vertical direction.

This is also known as a TRUNCATED wave shape and a rub is one of the  problems that would generate such a wave shape.