One thing that most field vibration analysts will become aware of is how the directional stiffness of a typical machine will manifest itself in vibration data. The typical horizontal machine train (driver and driven) is fastened down to a very rigid foundation, having little to no freedom of movement in the vertical plane. Although the machine components are bolted and welded to one another in the horizontal plane, there is nothing like the rigidity of the foundation preventing the movement of the machine frame in the horizontal plane. This being the case for most horizontal machine trains the vibration amplitude in the horizontal direction is typically higher than in the vertical direction. How much higher depends on the structural make-up of the machine. This is a tipoff to the savvy analyst who may notice a rise in the ratio of the vertical amplitude to the horizontal. When this happens, it usually means the machine has lost some stiffness in the vertical direction. This can be due to loose fasteners, cracked welds, or even compromised foundations.
Maintenance Tips, Vibration Analysis by Mike Fitch CRL