Torque loss of type C40 FZG gears lubricated with wind turbine gear oils

Five fully formulated wind turbine gear oils were characterised. The gear oils have 320 ISO VG grade and different formulations: ester, mineral, PAO, PAG and mineral+PAMA.A back-to-back FZG test machine, with re-circulating power, was used and a torque-cell was included on the test rig in order to measure the torque loss. Eight thermocouples were included to monitor the temperatures in different locations of the test rig.Tests at 1.13, 2.26 and 6.79 m/s were performed for different FZG load stages: K1, K5, K7 and K9. Both gearboxes were jet-lubricated with an oil flow of 3 l/min. The input flow temperature was kept almost constant (80 ±1 °C).Friction generated between the meshing teeth, shaft seals and rolling bearing losses was predicted.     

Micropitting performance of nitrided steel gears lubricated with mineral and ester oils

This study compares the gear micropitting performance of high pressure nitriding (HPN) steel gears, lubricated with three different gear oils: a standard mineral lubricant, containing a special micropitting additive package, and two biodegradable esters with low toxicity additivation. The physical, chemical and wear properties of the three lubricants were determined, as well as their biodegradability and toxicity characteristics. The gear material and the corresponding heat treatment are presented.Gear simulation tests were performed in a Falex machine, using a roller-disc geometry, in order to evaluate the lubricant temperature and friction coefficient corresponding to each gear oil.Gear micropitting tests were performed on the FZG machine, using type C gears, and lubricant samples were collected during the tests for wear particle analysis. Post-test analysis included the mass loss measurement of the gear (pinion and wheel), the ferrometric analysis of the lubricant samples and the teeth flank roughness measurement below and above the pitch line. The teeth flanks were inspected using scanning electron microscopy (SEM) and surface topography measurements to assess the number and depth of micropits. Metallurgical cuts were done to observe the size and depth of micropits as well as contact fatigue crack initiation and propagation.The ester lubricants show better micropitting performance than the mineral oil, confirming the potential of environmental friendly fluids as high-performance gear oils.     

Friction torque of thrust ball bearings lubricated with wind turbine gear oils

Planetary gearboxes used in wind turbines very often have premature bearing and gear failures, some of them related to the lubricants used. Five fully formulated wind turbine gear oils with the same viscosity grade and different formulations were selected and their physical characterization was performed. The lubricant tribological behaviour in a thrust ball bearing was analyzed. A modified Four-Ball Machine was used to assemble the bearings. They were submitted to an axial load and the tests were performed at velocities ranging between 150 and 1500 rpm. Experimental results for the operating temperatures and for the internal friction torque are presented.     

Torque loss in a gearbox lubricated with wind turbine gear oils

In this study, four different fully formulated ISO VG 320 wind turbine gear oils were select: a mineral oil‐based, a polyalphaolefin‐based, an ester‐based and a polyalkyleneglycol‐based fluids. Their physical properties (viscosity, thermoviscosity, piezoviscosity etc.) were characterised for a wide range of operating temperatures. A two‐stage multiplying gearbox, with helical gears, was selected to evaluate the influence of the wind turbine gear oil formulation on torque loss with the gearbox operating at low speed (130–230 rpm) and high torque (500–1000 Nm). The results obtained showed that each wind turbine gear oil formulation generated very different torque losses, evacuated heat flows and operating temperatures, with differences above 20 °C under the most severe operating conditions. A numerical model was developed, simulating all power loss mechanisms inside the gearbox, in particular the churning and friction losses. The coefficients of friction, between gear teeth and between rolling elements and bearing raceways, were calculated for all the tested oils. Copyright © 2013 John Wiley & Sons, Ltd.     

Power losses at low speed in a gearbox lubricated with wind turbine gear oils with special focus on churning losses

In this study gear oils were tested for power loss behaviour in a two stage multiplying gearbox, on a back-to-back test rig with recirculating power. The tests were performed at low input speeds and high input torques with oil sump temperature set free.A power loss model simulating the power loss mechanisms was implemented to evaluate gear power losses, but failed to correctly describe the gear churning.Two lubricant flow regimes were identified, which are related to the nature of the fluid circulation, as well as with the gearbox case. A calibration method for the gear churning loss is proposed based on these results and a method to identify the transition between the fluid flow regimes inside the gearbox.     

The lubricated wear of ceramics : Part II: The wear and friction of silicon nitride and steel in the presence of mineral oil or an ester based lubricant

The friction and wear characteristics of combinations of silicon nitride, alumina and AISI 52100 steel in the presence of mineral oil containing anti-wear, dispersant and detergent additives have been investigated in a tri-pin-on-disc machine. The tests were carried out at a nominal temperature of 100°C for a range of sliding speeds, loads and total sliding distances. In Part II of this two-part paper a comparison will be made between the tribological performance of these sliding pairs of materials in mineral oil and ester based lubricant environments. The results of the investigation showed that the alumina performed relatively poorly under these test conditions, whereas silicon nitride showed good potential as an improved wear resisting material compared with 52100 steel. Wear factors of the order of 10⁻¹⁰ mm³/Nm were deduced for the alumina, while values as low as 10⁻¹¹ mm³/Nm were typical of the silicon nitride sliding against 52100 steel discs. The alumina pins wore by a process of brittle fracture at the surface, whereas the silicon nitride pins wore primarily by a tribochemical polishing mechanism. The rate of tribo-chemical wear was found to be proportional to the nominal contact area.     

Differential diagnosis of spall vs. cracks in the gear tooth fillet region: Experimental validation

This paper presents a technique to differentially diagnose two types of localized gear tooth faults: a spall and a crack in the gear tooth fillet region. These faults could have very different prognoses, but existing diagnostic techniques only detect the presence of localized tooth faults without being able to differentiate between a spall and a crack. The effects of spalls and cracks on the behaviour of gear assemblies were studied using static and dynamic simulation models. Changes in the kinematics of a pair of meshing gears due to a gear tooth fillet crack (TFC) and a tooth flank spall were compared using a static analysis model. The difference in the variation of the transmission error (TE) caused by the two faults reveals their characteristics. The effect of a tooth crack depends on the change in stiffness of the tooth while the effect of a spall is dominantly determined by the geometry of the fault. A technique has previously been proposed to detect spalls [M. EL Badaoui, J. Antoni, F. Guillet, J. Daniere, Use of the moving cepstrum integral to detect and localize tooth spalls in gears, Mechanical System and Signal Processing, 15 (5) (2001) 873–885; M. EL Badaoui, V. Cahouet, F. Guillet, J. Daniere P. Velex, Modelling and detection of localized tooth defects in geared systems, Transaction of ASME, 123 (2001) 422–430], using the cepstrum to detect a negative echo in the signal (from entry into and exit from the spall) and successfully performed differential diagnosis on the simulated vibration signals. While the result of the experimental study showed some differences from the result of the simulation study, the differential diagnosis was successfully performed based on the technique presented in this paper. Further investigation revealed non-linear gearmesh behaviour which was causing differences in the experimental and simulation model results.     

Lubricant oils additivated with polymers in EHD contacts: Part 1. Rheological behaviour

This paper reports on the influence of a polymer additive on the traction behaviour of a mineral oil investigated using a two‐disc machine at different temperatures and contact pressures. A semi‐empirical approach was used for determining the effective lubricant rheological parameters ‐ the elastic shear modulus, the viscosity of the lubricant, and the limiting and Eyring stresses ‐ in elastohydrodynamic contacts. Using this approach, the effect of polymer concentration on the rheological parameters that appear in both the Johnson‐Tevaarwerk and Bair‐Winer models was quantified. The influence of operating conditions, such as pressure, oil temperature, and polymer concentration, on the traction coefficient, limiting shear stress (from the Bair‐Winer model), Eyring stress (from the Johnson‐Tevaarwerk model), shear modulus, and apparent viscosity was also investigated.     

Lubricant oils additivated with polymers in EHD contacts: Part 2. Tests using a four‐ball machine

A mineral oil of low viscosity was additivated with different concentrations of low‐density polyethylene. The wear behaviour of the additivated samples and the base oil was evaluated using a four‐ball wear tester at constant speed and variable load. Steel and ceramic (silicon nitride) were chosen for the balls. The scuffing resistance of the ceramic balls was higher than that of the steel balls. No scuffing appeared in the case of an upper steel ball in contact with lower ceramic balls. As far as the minimum wear‐scar diameter on the lower balls was concerned, an optimum concentration of polymer added to the base oil was found from the experimental data, for both types of ball. For the systems investigated, the optimum concentration was about 1.0% polyethylene.