Friction torque in grease lubricated thrust ball bearings

Thrust ball bearings lubricated with several different greases were tested on a modified Four-Ball Machine, where the Four-Ball arrangement was replaced by a bearing assembly. The friction torque and operating temperatures in a thrust ball bearing were measured during the tests. At the end of each test a grease sample was analyzed through ferrographic techniques in order to quantify and evaluate bearing wear.A rolling bearing friction torque model was used and the coefficient of friction in full film lubrication was determined for each grease, depending on the operating conditions.The experimental results obtained showed that grease formulation had a very significant influence on friction torque and operating temperature. The friction torque depends on the viscosity of the grease base oil, on its nature (mineral, ester, PAO, etc.), on the coefficient of friction in full film conditions, but also on the interaction between grease thickener and base oil, which affected contact replenishment and contact starvation, and thus influenced the friction torque.     

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.     

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.     

Influence of grease rheology on thrust ball bearings friction torque

The friction torque and the operating temperatures in a thrust ball bearing were measured for seven different types of greases, including three biodegradable greases having low toxicity. These friction torque tests were performed using a modified Four-Ball machine.Rheological evaluations of the lubricating greases were made using a rheometer. Bleed oils were extracted from the greases and the dynamic viscosities were measured.In order to compare the performance of the lubricant greases in terms of friction, the grease characteristics were related to experimental results, showing that the interaction between thickener and base oil have strong influences in the bearing friction torque.     

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.     

Analysis of hydrodynamic slider thrust bearings lubricated with non-newtonian fluids

A procedure for solving the Navier-Stokes equations for the steady three-dimensional flow of a non-newtonian fluid within a finite width slider thrust bearing is described. The method uses a finite difference approach together with a technique known as SIMPLE (semi-implicit method for pressure-linked equations) which has now become well established in the field of computational fluid dynamics. The concept of “effective viscosity”, used to describe the non-linear dependence of shear stress on shear rate, is also employed.To illustrate the capabilities of the procedure, results are presented for a wide range of non-linearity factors and these are compared with results from another source.     

Micropolar fluid lubricated squeeze films and thrust bearings

A theoretical study of squeeze films between two infinitely long rectangular plates, between two circular plates and a thrust bearing assuming the lubricant a micropolar fluid is reported. The effects of the material constants of the fluid on the bearing characteristics are discussed. It is shown that the squeeze with a micropolar fluid is slower than that with a Newtonian fluid. With a given flow rate a thrust bearing can sustain more load, while a given load can be supported with less pump work when the lubricant is micropolar than when it is Newtonian.     

Prediction of the friction torque in grease lubricated angular contact ball bearings using grey system theory

The development of special grease makes it possible for angular contact ball bearings to operate at high speed and temperature; however, as an important performance parameter, friction torque of bearings lubricated with grease is much greater than that of bearings lubricated with oil-air, and heat generation due to frictional loss is also greater, so it is necessary to predict the friction torque occurring in grease lubricated angular contact ball bearings. Based on grey system theory, a new prediction methodology for bearing friction torque is proposed which capitalizes on the notion that the information about friction torque of angular contact ball bearing is generally poor, incomplete and uncertain. A grey prediction model, GM (1, N) model, is presented to predict the friction torque in grease lubricated angular contact ball bearings. Several experiments on the friction torque of grease lubricated angular contact ball bearings were conducted to model and validate the effectiveness of the GM (1, N) model through on-line and off-line approaches. Experimental results show that about 90% of bearing friction torque under varying speed can be predicted in the on-line prediction; above 85% of bearing friction torque under varying speed and different loads can be predicted in the off-line prediction. Comprehensive analysis shows that, the GM (1, N) model performs very well for both modeling data and model validation data under different loads, varying bearing speed and work cycles, the proposed methodology can be used to predict bearing friction torque with good accuracy and robustness.

     

Ball motion in thrust loaded ball bearings

This paper reports on a study of ball motion, including the measurement of the ball rolling axis, in thrust loaded angular contact bearings. Experimental results of cage speed, ball speed and orientation of the rolling axis are compared with theoretical values based on realistic elastohydrodynamic traction and viscoelastic force expressions. The excellent agreement between theory and practice enables a skidding threshold for the test bearing to be presented, and explains actual ball motion more fully.     

Friction coefficient in FZG gears lubricated with industrial gear oils: Biodegradable ester vs. mineral oil

Two industrial gear oils, a reference paraffinic mineral oil with a special additive package for extra protection against micropitting and a biodegradable non-toxic ester, were characterized in terms of their physical properties, wear properties and chemical contents and compared in terms of their power dissipation in gear applications [Höhn BR, Michaelis K, Döbereiner R. Load carrying capacity properties of fast biodegradable gear lubricants. J STLE Lubr Eng 1999; Höhn BR, Michaelis K, Doleschel A. Frictional behavior of synthetic gear lubricants. Tribology research: from model experiment to industrial problem. Elsevier 2001; Martins R, Seabra J, Seyfert Ch, Luther R, Igartua A, Brito A. Power Loss in FZG gears lubricated with industrial gear oils: biodegradable ester vs. mineral oil. Proceedings of the 31th Leeds-Lyon symposium on tribology. Elsevier; to be published; Weck M, Hurasky-Schonwerth O, Bugiel Ch. Service behaviour of PVD-coated gearing lubricated with biodegradable synthetic ester oils. VDI-Berichte Nr.1665 2002.]. The viscosity–temperature behaviors are compared to describe the feasible operating temperature range.Standard tests with the Four-Ball machine and the FZG test rig [Winter H, Michaelis K. FZG gear test rig—desciption and possibilities. In: Coordinate European Council second international symposium on the performance evaluation of automotive fuels and lubricants; 1985.] characterize the wear protection properties. Biodegradability and toxicity tests are performed in order to assess the biodegradability and toxicity of the two lubricants.Power loss gear tests are performed on the FZG test rig using type C gears, for wide ranges of the applied torque and input speed, in order to compare the energetic performance of the two industrial gear oils. Lubricant samples are collected during and at the end of the gear tests [Hunt TM. Handbook of wear debris analysis and particle detection in liquids. UK: Elsevier Science; 1993.] and are analyzed by Direct Reading Ferrography (DR3) in order to evaluate and compare the wear particles concentration indexes of both lubricants.An energetic model of the FZG test gearbox is developed, integrating the mechanisms of power dissipation and heat evacuation, in order to determine its operating equilibrium temperature. An optimization routine allows the evaluation of the friction coefficient between the gear teeth for each lubricant tested, correlating experimental and model results.For each lubricant and for the operating conditions considered, a correction expression is presented in order to adjust the friction coefficient proposed by Höhn et al. [Höhn BR, Michaelis K, Vollmer T. Thermal rating of gear drives: balance between power loss and heat dissipation. AGMA Technical Paper; October 1996. pp 12. ISBN: 1-55589-675-8.] to the friction coefficient exhibited by these lubricants. The influence of each lubricant on the friction coefficient between the gear teeth is discussed taking into consideration the operating torque and speed and the stabilized operating temperature.     

Influence of ball precision on the assembly of radial thrust ball bearings

The influence of ball precision on the assembly of ball bearings is theoretically studied.     

Static characteristics of conical hydrodynamic bearings lubricated by turbine oil

Formulas for the static characteristics of conical hydrodynamic bearings of three types are considered, with variable thermophysical properties of the lubricant.     

Lubrication of hydroelectric turbine thrust bearings with a diester-based synthetic lubricant

A diester-based lubricant was found to reduce friction substantially in the thrust bearings of a hydroelectric turbine, compared with a conventional petroleum oil. Bearing temperatures were immediately reduced by 10–11 °C and bearing failures were eliminated. Revenue savings estimated at $10,000,000 per year have been made due to an increase in power production, and the elimination of repair work and downtime costs.¹     

A model for coulomb torque hysteresis in ball bearings

The frictional torque resisting rotation of an angular contact ball bearingg builds up from rest to its steady value through a small but finite angle of rotation. A similar transient torque is observed when the direction of rotation is reversed so that, in a bearing which undergoes oscillating rotations of small amplitude, the resistance torque traces out a hysteresis loop with angle of rotation. Following Dahl [AFO 4695-67-C-D158, Aerospace Corp. (1968)], the shape of the loop is expressed in terms of two parameters: (i) the initial and reversal gradient s and (ii) the steady-state torqueT_s. Theoretical expressions are derived for both s and T_s. s depends upon the elasticity of the bearing materials but not upon the frictional conditions at the contact interface whereas the opposite applies to T_s. Both depend upon the geometry of the bearing, particularly the conformity between the ball and race. Good correlation has been found between theoretical predictions and experiments carried out on three bearings of significantly different geometry.     

Frictional torque numbers for ball cup and journal bearings

Plastic bearing material wears in ball cup and journal bearings. Contact areas in the ball cup and the journal bearing increase. The frictional torque needed to rotate the ball or journal also increases.When the coefficient of friction is assumed to be constant during wearing out, the frictional torque increases to a maximum of 1.273 times the frictional torque at zero wear.     

Friction modifiers in engine and gear oils

Reducing friction is an important target for any lubricant oil formulator. There are several ways, such as utilisation of multi‐grade oils with low viscosity at low temperature, or use of friction modifiers, to reduce friction in automotive engines and transmissions and thus save fuel. A good means to obtain an energy‐saving lubricant is by the addition of a friction‐reducing additive in a high‐range multigrade oil. This paper presents some considerations on the action mechanism of friction modifiers and the results obtained in engine and gear oils with two new nitrogen‐, sulphur‐, and boron‐containing additives.     

Static and dynamic characteristics of externally pressurized circular step thrust bearings lubricated with couple stress fluids

The combined effects of couple stresses, fluid inertia and recess volume fluid compressibility on the steady-state performance and the dynamic stiffness and damping characteristics of hydrostatic circular step thrust bearings are presented theoretically. Based on the micro-continuum theory, the modified Reynolds equation and the recess flow continuity equation are derived by using the Stokes constitutive equations to account for the couple stress effect resulting from a lubricant blended with various additives. Using a perturbation technique, results in terms of steady-state load-carrying capacity, oil flow rate, stiffness and damping coefficients are presented. A design example is also illustrated for engineering and industrial applications.     

An experimental investigation of wear and fatigue life reduction of boundary lubricated spherical roller thrust bearings

Wear and fatigue life reduction in boundary lubricated spherical roller thrust bearings has been investigated. Previously presented results from measurements show that there is a significant change in the washer profile due to wear. This change in surface profile greatly affects the fatigue-life of the bearings. A fractional factor experiment series was used to study how the contacting surfaces of the bearing were affected by the rotational speed, load, hardness of the contacting surfaces and type of oil. The results show that the fatigue life and amount of wear on the washer surfaces were affected by the type of lubricant and rotational speed. A change of the load or a change of the hardness of the contacting surfaces did not influence the wear or the fatigue life of the contacting surfaces.     

A theoretical and experimental study into the steady-state performance characteristics of industrial air lubricated thrust bearings

The effect of pocket length on the steady-state performance characteristics of a single orifice industrial aerostatic bearing has been studied experimentally. For a given air gap the pocket length is found to have an important influence on stiffness. A further theoretical and experimental study of the performance of the twin orifice version of the bearing has been made. Load capacity and film stiffness are found to improve at the expense of flowrate. There is considered to be satisfactory agreement between predicted and observed performance.     

One-dimensional porous journal bearings lubricated with micropolar fluid

One-dimensional journal bearings lubricated with a micropolar fluid were analysed. It was found that the load capacity increased and the coefficient of friction decreased as the micropolar parameter characterizing the concentration of substructures in the lubricant due to the presence of additives increased. The load capacity decreased and the coefficient of friction increased as the parameter characterizing porosity increased.