Dr Steve Lacey describes how the operational testing of wind turbine gearboxes can help to identify potential gear- or bearing-related problems prior to installation, saving the customer time, money and their reputation.
Vibration-based condition monitoring (CM) is a well-established technique for monitoring the mechanical condition of the wind turbine drive train (i.e. rotor, gearbox and generator). CM tools are commonly used for the early detection of faults and failures in order to minimise downtime and maximise productivity.
In the case of wind turbines, vibration monitoring can be used to assess the condition of drive train components prior to installing these on the wind turbine.
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 therefore important that all associated bearing components such as shafts, housings and spacers are all manufactured to high standards. In addition, assembling the bearings and associated components in a clean, controlled environment using the correct tools is critical. Failure to do so can seriously compromise the performance and reliability of the bearing in service.
Assembly of large gearboxes is a skilled task and so 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.
Operational tests typically involve running the gearbox on a purpose-built test stand under a range of operating conditions. In some cases, only operating temperatures may be measured in order 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 (eg, input shaft, intermediate shaft and output shaft) are often the best approach in enabling any damage to the bearings or gears to be detected.
In a recent operational test of a wind turbine gearbox, a 1.2MW gearbox was run at 1,500rpm on a purpose-built test stand under a range of operating conditions. Vibration measurements were taken at various positions on the gearbox housing.
The vibration spectrum obtained from the housing close to the high speed shaft (HSS). The calculated BPFI (Ball Pass Frequency of the Inner ring) for the type NU228 cylindrical roller bearing on the 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 that shows impulses at the output rotational speed (40ms, 25Hz).
As a result of these tests, the gearbox was stripped down and examined. A localised fault was discovered 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 typically 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 term, the customer saves time, money and reputation.
New bearing cage designs
As well as running operational tests on gearboxes in order to detect any potential damage to gears and bearings, the design of the bearing itself can be improved, while simplifying bearing assembly and mounting.
Bearings for wind turbine gearboxes face the challenge of dealing with high dynamic forces, extreme peak and minimum loads, sudden alternating loads, as well as fluctuating operating temperatures. Bearings therefore need to be supplied with high static safety ratings and reliable dynamic designs.
Due to new developments in cage designs, the latest high capacity cylindrical roller bearings for wind turbine gearboxes offer higher load ratings and reduced friction. The basic static and dynamic load ratings of these types of bearings are now higher than those of conventional cylindrical roller bearings with solid brass cages, resulting in up to 20 per cent increases in rating life.
In addition, high capacity cylindrical roller bearings can be supplied with sheet steel cages, which enable a much more compact design compared to conventional solid brass cage designs. A more slimline cage frees up space for at least one additional rolling element in the bearing, which again, increases the load rating significantly.
Due to new cage designs, these high capacity bearings have a significantly lower friction than full complement cylindrical roller bearings with maximum basic load rating. A self-retaining cage also simplifies bearing mounting because the rolling elements are retained even when the ring is removed.
Today, most planetary stages of wind turbine gearboxes use full complement cylindrical roller bearings. These bearings have no cage, the function of which is to guide the rolling elements and to maintain a set distance between them. These full complement bearings have the highest load ratings, since more rolling elements are fitted to bearings that have no cage.
However, full complement roller bearings have relatively high friction because the rolling elements are in direct contact with each other. These kinematic conditions cause higher friction losses. In contrast, cylindrical roller bearings with conventional solid brass cages have significantly lower friction, but often do not achieve the required load ratings due to the reduced number of rolling elements.
Schaeffler has developed a new high capacity cylindrical roller bearing that incorporates a new sheet steel cage, which ensures low friction and provides sufficient space for an additional rolling element in the bearing (due to very narrow crosspieces).
The cage comprises two cage rings lying one inside the other and joined together. The crosspieces of the inner cage guide the rolling elements. At the same time, these retain the rolling elements in the bearing, even when the bearing ring is removed. This means that the bearing is much easier to mount, as no separate device is required to help prevent the rolling elements from falling out.
In addition, the special geometrical shape of the cage pockets improves lubricant flow, which reduces friction and prevents damage to the bearing.
The basic static (C) and dynamic (Co) load ratings of the high capacity cylindrical roller bearing are 6 per cent higher than those of a conventional cylindrical roller bearing with a solid brass cage. This means that the rating life is increased by more than 20%.
Dr Steve Lacey is engineering manager at Schaeffler (UK) Ltd, Sutton Coldfield, West Midlands, UK.