How do vibrations affect measurements on bearings of rotating machinery?
Vibration measurements are carried out on machines with rotating parts. These include steam turbines, gas turbines, centrifugal compressors, fans, pumps, mixers, rollers, gearboxes, fans, motors, etc. Vibration measurements can also be performed on reciprocating compressors.
When measuring vibrations on bearings, measurements are taken in 3 directions: axial, horizontal, and vertical. This allows for precise measurement of the direction in which the vibration occurs and at what frequencies it happens. With the right knowledge, a cause or hypothesis can be provided in the form of a report through spectrum analysis and orbit analysis.
P-F interval
The graph above is a visual representation of the point at which it is determined that a failure is developing (potential failure), to the point that a failure actually occurs (functional failure). The points in between show what happens in the path to the occurrence of a failure and how long it generally takes until the actual failure occurs.
Proactive Methods
As seen in the graph, changes in vibrations are first measurable and it takes the longest from this point until an actual failure occurs. There is still enough time to intervene and ensure that the developing failure never actually occurs. Other proactive methods to prevent an actual failure include analyzing wear particles in oil and using infrared thermography to detect heat differences. However, these methods can only be used at a later stage of failure development.
Reactive Methods
After this, only reactive methods can be used to limit damage resulting from a failure. The development of a failure is then sequentially noticeable by a change in sound from the machine, heat upon touching the machine, and the emergence of smoke. Intervention is required before these characteristics occur.
Vibration Specialist
By performing vibration measurements on bearings, the potential development of a failure can be quickly discovered and addressed. For this, competent vibration specialists are needed who are capable of performing these measurements and drawing the right conclusions from them, to then provide maintenance advice where needed. Measuring vibrations and assessing/analyzing the data is indeed a specialty. Istec’s vibration specialists are certified with ISO 18436-2 level 3 and level 4 certification, with level 4 being the highest achievable in relation to vibration measurements on rotating machinery.
Causes of Vibrations
To give an idea of how vibrations arise, we describe below some possible causes. These are just a few examples that can emerge in condition monitoring:
- Poor maintenance of bearings
Poor lubrication, overload, corrosion, contamination, and overheating are possible causes of bearing damage to the outer ring, inner ring, cage, or rolling elements. The latter applies, of course, only to rolling bearings. For more information, download this whitepaper: Damage to Rolling Bearings (PDF) - Incorrect alignment
When two machines are coupled, it may be that the centerlines of both shafts do not align. This can result in radial, angular, or axial misalignment. - Imbalance
This occurs when the center of gravity of the rotating parts is not exactly in the center. This can cause the pressure on a bearing to be too high with each rotation, leading to wear on the bearings.
Also read our knowledge article: The 4 most common causes of vibrations in rotating machines »
