Vibration monitoring of rolling bearings to maximise asset reliability
04 July 2016
The results from a series of in-depth studies on the vibration monitoring of rolling bearings in wind turbine gearboxes and generators, have been published in a report by Dr. Steve Lacey, Engineering Manager at Schaeffler UK.
As greater demands are being placed on existing plant assets, either in terms of higher output or increased efficiency, the need to understand when things are starting to go wrong is becoming more important than ever.
As plant and equipment becomes more complex and automated, the need to have a properly structured and funded maintenance strategy is critical. There is a need to properly understand the operation of equipment such that improvements in plant output and efficiency can be realised.
Rolling element bearings
Almost every type of rotating equipment uses rolling contact bearings to locate and allow accurate rotation of the shaft. During operation, equipment reliability very much depends on the type of bearing selected as well as the precision of all associated components i.e. shaft, housing, spacers, nuts etc. Bearing engineers generally use fatigue as the normal failure mode based on the assumption that the bearings are properly installed, operated and maintained.
Today, due to improvements in manufacturing technology and materials, generally bearing fatigue life, which is related to sub-surface stresses, is not the limiting factor and probably accounts for less than 3 percent of failures in service.
Unfortunately, many bearings fail prematurely in service because of contamination, poor lubrication, misalignment, temperature extremes, poor fitting/fits, unbalance and misalignment – factors that can all lead to an increase in bearing vibration. Condition monitoring has been used for many years to detect degrading bearings before they catastrophically fail with the associated costs of downtime or significant damage to other parts of the machine.
Vibration monitoring is probably the most widely used predictive maintenance technique and can be applied to a wide variety of rotating equipment. Since the mass of the rolling elements is generally small compared to that of the machine, the velocities generated are typically small and result in even smaller movements of the bearing housing, making it difficult for the vibration sensor to detect.
Machine vibration comes from many sources such as bearings, gears, imbalance, etc. and even small amplitudes can have a severe effect on the overall machine vibration, depending on the transfer function, damping and resonances. Each source of vibration will have its own characteristic frequencies, which can manifest itself as a discrete frequency or as a sum and/or difference frequency.
At low speeds it is still possible to use vibration but a greater degree of care and experience is required and other techniques such as measuring shaft displacement or Acoustic Emission (AE) may yield more meaningful results although the former is not always easy to apply. Also, while AE may detect a change in condition it has limited diagnostic capability.
Vibration monitoring has now become a well accepted part of many Predictive Maintenance regimes and relies on the well-known characteristic vibration signatures which rolling bearings exhibit as the rolling surfaces degrade. However, in most situations, bearing vibration cannot be measured directly and so the bearing vibration signature is modified by the machine structure and this situation is further complicated by vibration from other equipment on the machine i.e. electric motors, gears, belts, hydraulics, structural resonances, etc.
This often makes the interpretation of vibration data difficult and can lead to a misdiagnosis resulting in unnecessary machine downtime and costs.
Wind turbine drive train
Vibration-based condition monitoring systems have become well established for monitoring the mechanical condition of a wind turbine drive train (rotor, gearbox and generator) during operation. However, vibration signals from this type of equipment can be very complex as they often contain a number of different bearing types and gears, which can include multi-stage planetary systems. At times, this can make the detection and diagnosis of a problem very difficult and often several different techniques may have to be used to diagnose a problem.
Vibration monitoring can also be used to assess the condition of the drive train components prior to installation. Consequently, over a number of years, Schaeffler UK conducted a series of in-depth vibration monitoring studies on wind turbine gearboxes and generators, prior to these systems being installed on the wind turbine.
Wind turbine gearbox study
Rolling element bearings are manufactured to high accuracy and great care is taken over the geometrical accuracy, form and surface finish of the rolling surfaces. It is important therefore, that all associated bearing components i.e. shafts, housings, spacers, are all made to these high standards. In addition, assembling the bearings and associated components in a clean and controlled environment with the correct tools is also critical, as failure to do so can seriously compromise the performance and reliability of the bearing in service.
Assembling large gearboxes is a skilled task and it is not uncommon to find that some damage has been introduced to the bearing rolling surfaces during the assembly process.
While it is easy to introduce damage, detecting it is almost impossible without conducting some form of operational test. This often takes the form of running the gearbox on a purpose built test stand under a range of operating conditions. In some cases, only operating temperatures may be measured to quality assure the gearbox, but often this is not sufficient and any damage to the bearing rolling surfaces may go undetected.
Vibration measurements obtained from various positions on the gearbox e.g. input shaft, intermediate shaft and output shaft are often the best approach, enabling any damage to the bearings or gears to be detected.
As part of Schaeffler UK’s studies, a 1.2MW gearbox was run at 1500rpm on a purpose built test stand and vibration measurements were obtained at various positions on the gearbox housing.
The calculated BPFI (Ball Pass Frequency of the Inner race) for the type NU228 cylindrical roller bearing on the high speed shaft (HSS) was 271.26Hz and present in the spectrum is a large amplitude vibration at 270.64Hz, which matches very closely with the calculated frequency. Either side of the vibration at 270.64Hz are a few sidebands at shaft rotational speed (fs = 25Hz). In the envelope spectrum, the BPFI is also evident at 272.50Hz, along with the third harmonic (817.52Hz).
This indicates that some damage may be present on the inner ring raceway and the absence of any significant harmonics of BPFI suggests that the damage is fairly localised. This is further supported by the impulsive nature of the time signal showing impulses at the output rotational speed (40ms, 25Hz).
As a result, the gearbox was dismantled and examined and a localised fault was found on the inner ring raceway of the type NU228 cylindrical roller bearing.
This damage occurred during the assembly process, the most likely cause being misalignment between the inner ring and outer ring/rollers as the inner ring-shaft and outer ring-housing were aligned and assembled together.
During running of the gearbox on the test stand, all the operating temperatures were normal and this damage would otherwise have gone undetected without the vibration measurements. Such damage would result in a shortened service life and premature failure of the gearbox. In this case, the value of a detailed vibration analysis is obvious; in the long run, this saves the customer both time, money and reputation.
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