Mechanical looseness

There are two types of mechanical looseness, rotating and non-rotating. Rotating looseness means that there is too much room between rotating and stationary elements in the machine. Non rotating looseness means that the distance between two stationary parts is too big. Both types generate extensive vibrations at 1X harmonics in all three directions.

Rotating looseness
Bearing clearance in journal bearings or rolling element bearings generates 1X harmonics, which sometimes streches to 10X. If the high harmomics are dominating collisions can be suspected.

Non rotating looseness
Non rotating looseness causes the highest vibrations in the direction where the stiffness is the smallest. The stiffness is usually least in the horizontal direction, but it depends on the physical layout of the machine. Loose foundation can be causes by loose bolts, rust or cracks.

Source of vibration Exciting frequency Dominating direction Amplitude Spectral characteristic Comments
Loose foundation 1 X multiples normally tangential constant narrow band Indicated by loose foundation
Bearing clearance in babbitt bearing 1 X multiples radial constant narrow band Multiples may stretch to 10X
Extreme bearing clearance in babbitt bearing 0,5 X subsynchronous multiples radial constant   Sometimes also 0,25 X multiples

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