G. Dalmaz

Film Thickness in Starved EHL Point Contacts

This paper presents a numerical study of the effects of inlet supply starvation on film thickness in EHL point contacts. Generally this problem is treated using the position of the inlet meniscus as the governing parameter; however, it is difficult to measure this in real applications. Thus, in this paper an alternative approach is adopted whereby the amount of oil present on the surfaces is used to define the degree of starvation. It is this property which determines both meniscus position and film thickness reduction. The effect of subsequent overrollings on film thickness decay can also be evaluated. In the simplest case a constant lubricant inlet film thickness in the Y direction is assumed and the film thickness distribution is computed as a function of the oil available. This yields an equation predicting the film thickness reduction, with respect to the fully flooded value, from the amount of lubricant initially available on the surface, as a function of the number of overrollings n. However, the constant inlet film thickness does not give a realistic description of starvation for all conditions. Some experimental studies show that the combination of side flow and replenishment action can generate large differences in local oil supply and that the side reservoirs play an important role in this replenishment mechanism. Thus the contact centre can be fully starved whilst the contact sides remain well lubricated. In these cases, a complete analysis with a realistic inlet distribution has been carried out and the numerical results agree well with experimental findings.

Starved Film Thickness: a Qualitative Explanation

Numerically calculated film thickness can accurately predict measured film thicknesses under fully flooded conditions. However, as more detailed information concerning real life applications becomes available, the validity of the “fully flooded” assumption becomes less tenable. Starved films are much thinner than those formed under fully flooded conditions and it is of great practical importance therefore to be able to predict the ensuing lubrication level. Previous researchers have investigated the relation between the position of the inlet meniscus and starved film thickness. This paper combines a hydrodynamic lubrication approach with a physical boundary condition: the oil inlet film thickness distribution on the surface. Three different cases are investigated; a constant and a harmonic oil inlet film thickness distribution, and the effect of repeated passes. The results are compared qualitatively with experimental observations of starved EHL.

A Numerical Study of Friction in Isothermal EHD Rolling-Sliding Sphere-Plane Contacts With Spinning

This paper presents a study of the spinning influence on film thickness and friction in EHL circular contacts under isothermal and fully-flooded conditions. Pressure and film thickness profiles are computed with an original Full-System FEM approach. Friction was thereafter investigated using a classical Ree-Eyring model to calculate the longitudinal and transverse shear stresses. An analysis of both the velocity and shear stress distributions at every point of the contact surfaces has allowed explaining the fall of the longitudinal friction coefficient. Moreover in the transverse direction, spinning favors large shear stresses of opposite signs, decreasing the fluid viscosity by non-Newtonian effects. These effects have direct consequences on the friction reduction that is observed in presence of spinning. They are expected to further decrease friction in real situations due to shear heating.Copyright

Power Loss Prediction in High-Speed Roller Bearings

Abstract Friction between the various elements in a rolling bearing results in power loss and heat generation. Therefore, an estimation of the rolling bearing power losses is necessary to refine lubrication techniques and to optimize machine component design. A new model which predicts and locates power losses in a high-speed cylindrical roller bearing, operating under purely radial load, is presented. Its new features come from the consideration of both cage action and the effect of lubricant film thickness in the computation of bearing kinematics at equilibrium. Lubricant rheological properties are used in order to calculate hydrodynamic and elastohydrodynamic forces in each lubricated contact. This model considers cage and roller kinematics to be unknown. These are obtained by solving the equations of motion for each bearing element. The computation and the location of power losses are given by the friction forces and the sliding speeds among the various bearing elements, i.e., contact between roller and inner or outer ring, roller edge-race flange, roller-cage pocket and cage-ring pilot surface. The authors compare first the values of the calculated power loss with experimental data to assess the programs predictive capability. Afterwards the model is used to estimate and locate power losses in a well-lubricated high-speed roller bearing. Results show that the total power loss varies strongly with the rotational speed, the lubricant inlet temperature and the oil flow through the bearing. Nevertheless, it is less sensitive to radial load. Power loss results are also given as a function of the bearing internal geometry.

First Traction Results of High Spinning Large-Size Circular EHD Contacts from a New Test Rig: Tribogyr

This article presents a new test rig named Tribogyr and the first results obtained so far. This device allows EHL friction tests in rolling-sliding under high spinning conditions applied to large-size circular contacts. Spinning is imposed thanks to a unique design that includes a tilting facility of one of the two spindles. After a brief description of the test rig structure and of the specimen geometry, the sensors implementation and the device capabilities are reported. The parameter used to quantify the spin in this study is then defined. It only takes into account the rotational speed normal to the contact surface and the contact radius. Then, the friction coefficient and the temperature measurements are presented. The data are at first plotted versus the classical slide-to-roll ratio and compared to numerical results obtained with and without spin. The spin parameter then is introduced to replace the slide-to-roll ratio as the varying parameter. This representation allows reporting all the results in the same figure, where the influence of the operating conditions like the entrainment speed, the tilting angle, and the slide-to-roll ratio on friction and on temperature is much more visible. It is discussed that the observed temperature difference increase is generated by an additional shearing introduced by spinning despite an observed reduction of friction.

Starvation Phenomena in E.H.L. Point Contacts: Influence of Inlet Flow Distribution.

In comparison to the fully flooded Elastohydrodynamic (EHL) regime, it is difficult to predict the complete film thickness distribution under starved lubrication conditions. Starvation of the contact results in an overall reduction, and a shape modification, of the film in the contact. Existing models cannot fully predict these changes, as the boundary conditions used do not represent physical reality. Experiments show that the film thickness distribution in the Hertzian zone depends on the amount and distribution of the lubricant in the inlet zone. The area where the pressure build-up starts; the inlet meniscus, is a result of the supply condition rather than the fundamental physical parameter determining starvation. The aim of this paper is to present numerical EHL results based on the amount of lubricant locally available on the surfaces. The results of two different simulations of the inlet film are presented. The first model assumes a constant inlet oil film and gives “classical” film shapes when sufficient oil is available. As the oil quantity decreases, variations in film thickness are reduced, eventually tending to the dry contact shape. The second approach adopts a more realistic inlet oil distribution, pairing film depletion in the centre of the track with a polynomial distribution elsewhere. A complete modification of the film shape is observed when the width of the central depletion increases and when the oil quantity in the central part decreases. The minimum thickness no longer appears in the side lobe area but in the central region; the sides remain abundantly lubricated. The numerical results are compared to film distributions obtained experimentally and good agreement is found.

Simulating angular ball bearing lubricated elliptical contacts Film thickness and traction measurements

Abstract A high performance barrel and plate apparatus was built to study film formation and traction by simulating the real situation of a lubricated elliptical contact in an angular ball bearing under general kinematic conditions. Simultaneous measurements of load, speed of each surface, traction, and film thickness by optical interferometry can be performed. The sapphire disc plate and the steel barrel are driven independently at constant controlled speeds. Small relative sliding, lateral sliding and spinning near pure rolling conditions can be imposed by controlling barrel shaft angle contact location. Tests were performed at ambient temperature for a small barrel whose principal radii are 1.34 mm and 9.7 mm, for applied loads which generate Hertzian pressures up to 2 × 10 9 N/m 2 , and for a low viscosity mineral oil. Typical experimental results show that under elasto-hydrodynamic conditions, the centre film thickness is slightly below the values calculated from classical elastohydrodynamic theories and that oil starvation occurs at high speeds. Traction curves versus slide/roll ratio are presented for different loads and under spinning and lateral sliding conditions.

The Evolution of Lubricant Film Defects in the Starved Regime

Surface damage or failure of rolling element bearings is often due to inadequate lubricant replenishment of the contact. Our understanding of the mechanisms of starved elastohydrodynamic lubrication and the behaviour of thin fluid films has advanced significantly in recent years and, thus, provides a basis for a fundamental study of different bearing failure modes. In this paper a possible explanation for the surface damage observed in grease lubricated spherical roller bearings operating at low temperatures is advanced. This type of bearing failure is characterized by ‘brown bands’ that form identical patterns on every element of the bearing: the roller set, inner and outer ring. In this case damage is attributed to a single defect in the lubricant film, which is then propagated around the bearing. Such a defect might be initiated by local scraping, and removal, of the lubricant film by a bearing cage. It is the propagation of the film defect and the role of lubricant replenishment that is studied in this paper. In the starved regime insufficient lubricant replenishment of the defect results in a locally depleted film which, combined with surface roughness, leads to surface damage and possibly brown band formation. The numerical work studies the influence of a local lubricant defect on the central film thickness in the contact. An established starvation model is used to explore the relationship between oil inlet film thickness and the maximum defect size that can be replenished. In parallel experimental work a local defect has been deliberately created in the lubricant film in a rolling contact. The behaviour of the defect has been studied under different levels of lubricant replenishment. This work confirms qualitatively the effects predicted by the numerical study.

Traction and film thickness characteristics of traction fluids in high speed elastohydrodynamic contact

Abstract The performances of continuously variable transmission have been investigated experimentally. The aim of this approach was to determine simultaneously traction and film thickness under isothermal elastohydrodynamic conditions. The experiments have been carried out with four traction fluids under various ranges of pressure (0.5 – 2 GPa) and speed (0.5 – 12 m/s). The profile of lubricant and film thickness separating the surfaces between a steel ball and a sapphire plate was determined by the use of a well-known interferometry technique. For high rolling speed and high pressure, the modification of the usual optical arrangement with a new system allows the observation of the interference pattern and thus the measurement of oil film thickness with a good quality.

Tribogyr: A New Test Rig for High Spinning EHL Large Size Contacts

Only few studies have been published on EHD contacts that experienced simultaneously rolling, sliding and spinning motions. This paper describes a new test rig especially designed to simulate such operating conditions while imposing normal load and speeds as those found in real life mechanisms. Then, experimental results are presented and discussed.Copyright