Nuggets Of Gold
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Nuggets Of Gold - Tricks Of The Vibration Analysis Trade
by Nelson Baxter
Abstract
This paper can be described as a list of tips to remember, tricks to use, and pitfalls to avoid when performing vibration analysis & general inspection of machinery. Organization by machine type, problem type, type of corrective action, etc is provided to aid the reader. It represents essentially more than two decades of “lessons learned” by a recognized leader in the field of vibration analysis, Mr. Nelson Baxter. This paper is one of the few that I would recommend not only for one-time reading, but also for annual review as it is of that high value and applicable to nearly every situation a vibration analyst might find him or herself in.
PREVIEW
“General Thoughts:
When excessive vibration is present, the analyst must consider three possibilities. The first is that there might be a large force present. The second is that the supporting structure is statically weak and the third is that there might be resonance amplification, which is equivalent to the system being dynamically weak at the forcing frequency. These three possibilities should always be present in the mind of the analyst during an investigation of a vibration related problem.
"Balancing:
1
When balancing, plot the original and trial vectors on all bearings and orientations. When all vectors point towards the same solution; you are in the right plane. When the vectors do not point towards the same solution, try another plane. If this still doesn’t work, try a multi-plane solution.
Case History- A drag line had a motor generator set that had two generators, a synchronous motor and then three more generators all coupled together. The vibration on the first generator was the highest so balance weight was installed on that rotor. When the solution vectors for the horizontal and vertical directions on that generator were compared, they showed conflicting results. The horizontal vectors indicated that the trial weight needed to be doubled and rotated in one direction, the vertical vectors showed that the weight needed to be reduced and rotated in the other direction. In other words, there was not going to be a solution the balance problem by adding weight in the first generator rotor. The solution turned out to be to add weight to the synchronous motor. When the weight was added to the synchronous motor, all the vectors came up with the same solution
2
When a rotor runs above its 1st critical and there is an indication of a bow, translate a portion of the static balance component from the ends to the center of the rotor. If this is not done, the rotor will run good on a balance machine but bad in its own bearings at high speed. This is due to internal bending moments produced by unbalance forces acting on the axial distance between the mass unbalance and the correction weights.
Case History: A large high pressure turbine rotor after several years of operation developed a bow. The rotor was removed and sent to a shop for balancing. The shop added weights in each end of the rotor at equal angles to offset the unbalance introduced by the bow.”
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