BİLGE AKUSTİK
Noise and Vibration Control Engineering
“I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”
Machinery Vibration
The most important benefit that a ship's crew can derive from their own efforts in the field of vibration relates to machinery.
Relatively simple measurements can be used to monitor the mechanical condition of pumps, generators, motors. Also, with a little training and simple equipment, a limited amount of diagnostics is possible.
However, any questions concerning the main propulsion machinery should be left to the vibration engineers.
1. Rotating Machines
Frequency, amplitude and vibration direction can be used as indicators to identify the cause of high vibration in rotating machinery. The table presents approaches to vibrations occurring in machines.
2. Tracking of Machines
Vibration measurements on machines are increasingly used periodically to detect deterioration before the machine reaches the point where it must be shut down. This allows the crew to plan maintenance operations instead of performing maintenance in an emergency. Often vibration measurements can replace more time-consuming disassembly and Inspection.
Machine monitoring measurements are usually taken every month, so they usually need to be performed by the test equipment and crew on board. Experience with particular machines will suggest whether measurements should be made more or less frequently.
Measurements can be taken with very simple instruments or very complex instruments. Most often, they will include one of the following types of systems:
• Counter Converter
• Converter with Filter and Counter
• Transducer with Spectral Analyzer
Each of these will be discussed in this section as needed.
Whatever instruments are used, the first step is to obtain the "baseline" measurements. Measuring locations must be selected for each machine in the maintenance program.
The vibration of each location on each machine is measured when the machine is in good working order. These are "baseline" vibration levels and will be compared with periodic measurements at the same locations. Measurements must also be made after a machine has been overhauled.
General guidelines can be given for interpreting vibration data. Both the absolute vibration level and the variation from the main baseline are important. If the absolute level exceeds the given safe operating levels, the machine must be shut down. If the vibration doubles from the baseline measurement, it may not be a problem, but measurements should be made more often, perhaps weekly, to see if the vibration continues to increase or is stable. If the vibration is increasing 10 times above the baseline level, it should be repaired even if it does not exceed the corresponding levels. If there are two identical machines it is often helpful to compare their vibration levels.
The discussion so far was about whether there was a problem and there was no idea of the nature of the problem. How the machine is diagnosed depends on the type of measuring equipment used.
If only a transducer and measuring device is used, there is no indication of the frequency of the increased vibration and the measurement is of little help in diagnosis. An exception is a vibration meter that will read displacement and velocity, velocity and acceleration, or all three. If the displacement increases more than the velocity or acceleration, the frequency will be lower; such as in an imbalance problem. If the acceleration increases more than the velocity or displacement, the frequency is high, for example, it can cause bearing deterioration. The list of common problems and the corresponding frequency range are as follows:
Low Frequency
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Unbalanced rotor - defeated, worn, broken parts
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Misalignment - Causes significant axial vibration
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camshafts
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sliding clutches
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mechanical slack
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loose foundation bolts
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Oil cycling or slip - half or less times the spindle speed
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Worn belts
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unadjusted belts and pulleys
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Modified reciprocating elements causing additional torsional vibration
High frequency
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Vibration Due to Impacts
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Defective bearings - random or coarse vibration
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weak gears
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sliding clutches
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Friction or fasteners
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air leaks
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Hydraulic leaks
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Colliding parts
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Broken or loose parts
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Electromagnet driven loose parts
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water hammer
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Swelling
If amplitudes of individual frequency components are desired for periodic measurements, the use of a filter will probably not be beneficial. The same data can be obtained much faster via a spectral analyzer. Some of these are cost-effective and create a hard copy of the frequency spectrum for each location. They are easy to compare with base spectra and are convenient to file in machine history.
3. External Driven Machines
Sometimes a machine will cause its supporting structure to be caused by another machine, propeller forces, etc. It will vibrate excessively as a result of being stimulated by another source, such as The source may perhaps be defined by its frequency. Vibration can also be observed when nearby machinery is operating in sequence, or while cruising and in port. In any case, such externally stimulated machines can often be treated in the same way as a local structure or equipment issue.