Any mechanical looseness in a rotating machine will typically generate a peak in the spectral data at the 1X speed (synchronous speed). There are a few types of looseness that may occur:
1. Rotating looseness
2. Structural looseness
This form of mechanical looseness is caused by excessive clearances between rotating and non-rotating or stationary components, such as bearings. Looseness may occur due to improper machine design, however, in many cases, it is caused by wear and tear of components.
With rotating looseness in the case of bearings, you may observe harmonics of the 1X operating speed that are visible in the spectral data over several orders.
Figure 1, below, shows an example of a spectral signal caused by rotating looseness.
Figure 1: Spectral data for a structure with rotating looseness
As rotating looseness becomes worse, it is expected that the number of harmonics will increase as well. Extremely severe rotating looseness will also lead to a harmonic at 0.5X of the running speed. If left unchecked, this condition can worsen and cause other frequencies to become excited, leading to severe vibration and mechanical failure.
This form of mechanical looseness usually due to looseness in the foundation of a machine. It can be caused due to improperly tightened bolts, cracking, corrosion, or even a flexible base.
Structural looseness can be identified due to a higher 1X vibration in the axis with the lesser stiffness. I many cases, the horizontal axis is usually less stiff than the vertical axis, but this depends on the specific mounting and installation for any particular equipment.
As an example, Figure 2 shows spectral data in the horizontal and vertical axis for a pump with structural looseness. Note that that 1X vibration is higher in the horizontal axis as this axis is less stiff than the vertical axis.