Experimental Investigations on Wear in Oscillating Grease-Lubricated Rolling Element Bearings of Different Size and Type

Grease-lubricated rolling element bearings can suffer from wear due to lubricant starvation under certain oscillating operating conditions. Especially for large-scale slewing bearings, such as blade bearings in wind turbines, experimental investigations are complex compared to small-scale reference testing. For an easier manner of testing, it is desirable to know whether the results of small-scale testing are applicable to larger-sized bearings. In this work, three different bearing types were tested and compared to already published results from a small-scale ACBB with a pitch diameter of 60 mm. The newly tested bearing types comprise a downscaled blade bearing (4-point contact double row ball bearing) with a pitch diameter of 673 mm, a small-scale CRTB with a pitch diameter of 77.5 mm and another ACBB with a pitch diameter of 95 mm. Qualitatively, all tested bearings show similar wear behaviour in terms of friction energy when operation parameters are varied. With higher oscillation frequency, damage becomes more severe. The oscillation amplitude shows three distinctive regimes. Within the range of small amplitudes, an increase in amplitude leads to more pronounced damage. We observe a threshold amplitude where this is no longer the case; a further increase in amplitude counteracts wear initiation until a final threshold is reached, beyond which no more wear is observed. These findings are in accordance with the reference results of the small-scale ACBB. Direct comparison between point and line contact shows that the latter is more prone to wear initiation under grease-lubricated, oscillating operating conditions. Furthermore, a previously introduced empirical number shows good performance in assessing critical operating parameters of the different bearing types. Specifically, harmful operating conditions can be classified for all studied bearing types with an accuracy of 78%. This method can be useful to assess operating conditions of greased, oscillating, rolling element bearings, e.g., to assess different pitch controllers or designs of slewing bearings

Wear Development in Oscillating Rolling Element Bearings

Rotor blade bearings enable rotor blades to pivot about their longitudinal axis and thus control the power output and reduce the loads acting on the wind turbine. Over a design period of 20 years, rolling bearings are exposed to frequent oscillation movements with amplitude ratios of x/2b > 1, especially due to new control concepts such as Individual Pitch Control, which can lead to wear and a reduction in service life. The objective of this paper was to identify the dominant wear mechanisms and their consequences for the operation of oscillating bearings. Oscillating experiments with an increasing number of cycles on the angular contact ball bearings of two different sizes (types 7208 and 7220) show that the damage initiation starts with adhesive and corrosive wear mechanisms, which result in a sharp increase in the torque as well as the wear volume on the bearing raceway. As the number of cycles increases, an abrasive mechanism occurs, resulting in a lower slope of the wear curve and a smoothing of the resulting wear depressions. The wear and torque curves were evaluated and classified using an energy-wear approach according to Fouvry

Ambient conditions prevailing during hail events in central Europe

Around 26 000 severe convective storm tracks between 2005 and 2014 have been estimated from 2D radar reflectivity for parts of Europe, including Germany, France, Belgium, and Luxembourg. This event set was further combined with eyewitness reports, environmental conditions, and synoptic-scale fronts based on the ERA-Interim (ECMWF Reanalysis) reanalysis. Our analyses reveal that on average about a quarter of all severe thunderstorms in the investigation area were associated with a front. Over complex terrains, such as in southern Germany, the proportion of frontal convective storms is around 10 %–15 %, while over flat terrain half of the events require a front to trigger convection. Frontal storm tracks associated with hail on average produce larger hailstones and have a longer track. These events usually develop in a high-shear environment. Using composites of environmental conditions centered around the hailstorm tracks, we found that dynamical proxies such as deep-layer shear or storm-relative helicity become important when separating hail diameters and, in particular, their lengths; 0–3 km helicity as a dynamical proxy performs better compared to wind shear for the separation. In contrast, thermodynamical proxies such as the lifted index or lapse rate show only small differences between the different intensity classes.ISSN:1561-8633ISSN:1684-998

Oscillating rolling element bearings: A review of tribotesting and analysis approaches

Rolling element bearings, when subjected to small oscillating movements or vibrations, run the risk of being damaged by mechanisms such as Standstill Marks and False Brinelling. Damages resulting from these phenomena can decrease bearing fatigue life and increase wear-induced friction torque. These failures do not correlate well with standard life estimation approaches. Experimental studies play a crucial role in gaining knowledge in this area. The review integrates knowledge from experiments ranging from single contacts to laboratory and full-scale bearings in wind power and aerospace applications. The generalization is achieved using a non-dimensional amplitude parameter that relates rolling element travel during an oscillation to the Hertzian contact size. The review encompasses testing methods, procedures, reporting practices, result scaling, and application-specific considerations

Starvation and Re-lubrication in Oscillating Bearings: Influence of Grease Parameters

A common application for grease-lubricated oscillating rolling element bearings is, e.g., rotor blade bearings in wind turbines. These bearings mainly operate under conditions that are prone to starvation. If the grease is unable to provide enough inlet lubricant supply for the contact between rolling element and bearing raceway, wear in the form of False Brinelling and thus premature bearing failure is possible. Bearing experiments with different lithium complex model greases, which differ mainly in their base oil viscosity and oil separation rate, were carried out to show the influence of the grease parameters on wear initiation. The results show that the ability of the grease to release a high amount of base oil with high mobility into the track of the rolling element is a crucial mechanism to prevent wear, especially at small oscillation angles. For oscillation angles larger than a critical angle, a secondary replenishment mechanism may prevent early wear initiation. The experimental results are used to validate a starvation model proposed in earlier work (Wandel et al. in Tribol Int 165:107276, 2022)

On the critical amplitude in oscillating rolling element bearings

Small oscillating amplitudes (~0. 1°) and larger (> 1°) can both lead to wear, w hich complicates the evaluation of the operating conditions. In this work, a simulation model is used to discuss critical amplitudes. The parametric study, which includes 125 simulations, shows the effect of load and coefficient of friction on the frictional work density for amplitudes ranging from 0.1 to 2.5°. The study concludes that the frictional work density increases with the oscillating amplitude up to a certain point, where it becomes almost constant in relation to the amplitude. This point is reached when the amplitude is so large that a continuous overlapping of the contact zone no longer occurs. It is precisely this point that marks the critical oscillation amplitude

The Lick AGN Monitoring Project 2011: Dynamical Modeling of the Broad Line Region in Mrk 50

We present dynamical modeling of the broad line region (BLR) in the Seyfert 1 galaxy Mrk 50 using reverberation mapping data taken as part of the Lick AGN Monitoring Project (LAMP) 2011. We model the reverberation mapping data directly, constraining the geometry and kinematics of the BLR, as well as deriving a black hole mass estimate that does not depend on a normalizing factor or virial coefficient. We find that the geometry of the BLR in Mrk 50 is a nearly face-on thick disk, with a mean radius of 9.6(+1.2,-0.9) light days, a width of the BLR of 6.9(+1.2,-1.1) light days, and a disk opening angle of 25\pm10 degrees above the plane. We also constrain the inclination angle to be 9(+7,-5) degrees, close to face-on. Finally, the black hole mass of Mrk 50 is inferred to be log10(M(BH)/Msun) = 7.57(+0.44,-0.27). By comparison to the virial black hole mass estimate from traditional reverberation mapping analysis, we find the normalizing constant (virial coefficient) to be log10(f) = 0.78(+0.44,-0.27), consistent with the commonly adopted mean value of 0.74 based on aligning the M(BH)-{\sigma}* relation for AGN and quiescent galaxies. While our dynamical model includes the possibility of a net inflow or outflow in the BLR, we cannot distinguish between these two scenarios.Comment: Accepted for publication in ApJ. 8 pages, 6 figure

Starvation in oscillating rolling element bearings

Oszillierende Wälzlager sind Teil eines breiten Spektrums industrieller Anwendungen. Neben Anwendungen in denen Oszillationen als unerwünschte Vibrationen auftreten, sind sie in anderen Anwendungen funktionsrelevant. Besonders der Einsatz von Rotorblattlagern in Windenergieanlagen hat in letzter Zeit an Bedeutung gewonnen. Durch neue Regelungskonzepte wie 'Individual Pitch Control' (IPC) sind die Lager einer hohen Anzahl von Oszillationen bei variierenden Betriebsparametern ausgesetzt. In kontinuierlich rotierender Anwendung ist der Einsatz von Wälzlagern tiefgehend erforscht, sodass diese in der Regel bei Schmierungsbedingungen betrieben werden können, unter denen ein frühzeitiger Ausfall aufgrund von Verschleiß unwahrscheinlich ist. In oszillierenden Lagern, insbesondere unter Fettschmierung, sind die Schmierungsmechanismen nur unzureichend verstanden, sodass frühzeitige Lagerschäden durch Mangelschmierung und daraus resultierende Verschleißerscheinungen wie 'False Brinelling' nicht vorhergesagt werden können. Die vorliegende Arbeit setzt an diesem Punkt an. Die Schmierungsmechanismen in oszillierenden Wälzkontakten werden anhand von Bauteilversuchen an Schrägkugellagern des Typs 7208 und Modellversuchen auf einem optischen EHL-Tribometer analysiert. Der Fokus der Untersuchungen liegt auf dem Einfluss des Grundölrückflusses sowie der Wälzkörper-Käfig-Interaktion auf die Schmierstoffversorgung des Kontaktes. Auf Grundlage der identifizierten Schmierungsmechanismen wird ein Mangelschmierungsmodell entwickelt, das es erlaubt, anhand der Betriebs- und Schmierstoffparameter zu beurteilen, ob das Lager Gefahr läuft, frühzeitigt Schaden aufgrund von Mangelschmierung zu nehmen. Das Modell wird anhand von Bauteilversuchen validiert und diskutiert. Die Versuche werden im Frequenzbereich von f=0.2-5 Hz und kleinen Oszillationsswinkeln (2-60°) unter maximalen Kontaktpressungen von p_max=1.5-2.5 GPa durchgeführt. In der Regel sind die Oszillationswinkel klein genug, sodass die Wälzkörperteilung nicht überschritten wird, jedoch so groß, dass das Amplitudenverhältnis kleiner eins ist (x/2b>1). Kleinste Schwingungen, die zu Stillstandsmarkierungen führen könnten, werden nicht untersucht.Oscillating rolling element bearings are part of a wide range of industrial applications. In addition to applications in which the oscillations occur as unwanted vibrations, in other applications they are used intentionally. One application that has gained particular importance in recent times is the use of rotor blade bearings in wind turbines. Newcontrol concepts such as individual pitch control (IPC) expose them to a high number of oscillations under varying operating parameters. Continuously rotating bearings are well understood in most cases and can usually be operated under lubrication conditions that avoid early bearing failure due to wear. In oscillating bearings, especially under grease lubrication, the lubrication mechanisms are insufficiently understood, so early bearing damage due to starvation and resulting wear phenomena such as false brinelling cannot be predicted. The present work addresses this point and investigates the lubrication mechanisms in oscillating rolling contacts by means of component tests on angular contact ball bearings of type 7208 and model tests on an optical EHL tribometer. The focus of these investigations is primarily on the influence of the base oil re-flow as well as on the influence of the rolling bearing cage on the lubricant supply to the oscillating contact. Based on the identified lubrication mechanisms, a starvation model is proposed, which allows to assess, based on the operating and lubricant parameters, whether the bearing is at risk of early damage due to starvation. This model is validated and discussed on the basis of component tests. These tests are executed in the frequency range of = 0.2 − 5 Hz and small oscillation angles (2−60◦) under contact pressures of max = 1.5−2.5 GPa. In general, the oscillation angles are small enough that the rolling element pitch is not exceeded, but large enough that 2 > 1 applies to the amplitude ratio. Smallest vibrations, which could lead to standstill marks, are therefore not examined