EHD Lubrication Research Articles

Effect of the Sliding Element Surface Topography on the Oil Film Thickness in EHD Lubrication in Non-Conformal Contact.

Under hard operating conditions such as high load, low speed and a lack of a sufficient quantity of lubricant, the sliding pairs could suffer serious damage. One of the methods that improves the tribological performance of sliding elements in hard operating conditions is the appropriate surface creation that keeps lubricating substance in cavities. This article presents the results obtained in experimental investigations of the oil film thickness in lubricated non-conformal contact with a different surface topography of the sliding element. The tests were conducted on a ball-on-disc instrument equipped with colorimetric interferometry. Balls of diameter equaled to 19.05 mm were produced from 100 Cr6 steel. To provide hard operating conditions, the glass disc rotated at small speeds in the range of 0.1-0.2 m/s. The tests were carried out at loads of 20 N and 30 N. As a result, in most cases, the highest minimum and average oil film thickness values were obtained when the surface of steel balls was characterized by high negative asymmetry with mainly shallow cavities and some valleys of great depth compared to the height of the peaks. The modified sliding surface that had better performance comprised a comparatively small number of peaks and the curvature of the peaks were large.

Film thickness in miniature ball bearing grease lubricated

In this paper the authors theoretically evaluated the film thickness in a miniature angular contact ball bearing in grease and oil lubricated conditions and correlated with the variation of the electrical resistance experimentally determined by using tribometer CETR UMT-2. For grease the authors determined the film thickness considering base oil viscosity. The rotational speed of the inner race varied between 1 and 500 rpm and the axial load applied on a 7000C angular contact ball bearing was 8.1 N. The IVR and EHD lubrication regimes were analytically evaluated depending on the rotational speed and lubricants for both inner and outer race. Based of the dependence between film thickness and electrical resistance obtained for oil was evaluated the real film thickness obtained by using the grease. In case of oil lubricated condition, the experimental results showed an increase of electrical resistance with the rotational speed caused by increasing of the film thickness according to the theoretical model. For grease lubricated condition, ”a V-shaped pattern” was obtained. High values of electrical resistance at very low rotational speeds were observed. By increasing the rotational speed the electrical resistance for grease decreases until a limit speed and increase continuum over this limit. This behaviour was also reported in literature by ball-disc interferometry that showed at very low speeds that the thickener is dominant and over a speed limit the base oil is dominant in generating the film thickness.

Prediction of oil film thickness in trochoidal pump

A model for predicting the oil film thickness between the contact surfaces of the gerotor pump teeth is given in this paper. Gerotor pumps are rotary pumps with inner trochoidal gearing. The working process of the pump is followed by mechanical losses due to friction that occurs between the coupled surfaces. To reduce these losses and avoid wearing of the teeth’s active surfaces, it is necessary to provide a minimum thickness of the oil film between the contact surfaces. The minimum thickness of the oil film depends on many different factors, but in this paper for its determination, the Hamrock and Dowson empirical equation is used. Previously, the mathematical model that follows the change of relative velocities in contact, sliding velocity, rolling velocity, and contact force at the contact points of the coupled teeth profiles is developed. The influence of different parameters on the lubrication regime has been analyzed, and it has been shown that the trochoidal pump operates most with the limit EHD lubrication regime. The most unfavorable period occurs when the resulting rolling velocity decreases significantly, and this period dominates during coupling. The results of this research provide guidelines for the design of the tooth profile’s shape and the choice of operating parameters of the pump with high reliability.

Wear study and EHD lubrication analysis on connecting rod big end bearings of off‐highway application engine

AbstractThe connecting rod big end bearings are under dynamic lubrication during working cycle, and in most of the time, the con rod is subjected to compressive stress. The conventional method of performing an EHL analysis on a bearing involves development of complex mathematical equations and simplification of actual physical model. This paper presents a methodology to model and simulate the elastohydrodynamic lubrication and wear study of connecting rod big end bearings of off‐highway application engine using the application of computational fluid dynamics (CFD) and computational structural dynamics (CSD) approaches. The pressure field for a full journal bearing operating under laminar flow regime with various eccentricities was obtained by CFD, and fluid pressure distribution and deformation in the bearing liner due to pressure were evaluated using FSI approach. Relevant parameters of lubrication characteristics were analyzed to optimize the eccentricity value. The maximum bearing load value of 21 kN was noticed at TDC position for the optimum eccentricity. The load distribution indicated critical points in the bearing, and the data obtained from bearing load and sliding velocity of journal were used in Archard's wear relation to determine the wear depth along the bearing width. The simulated wear results were compared with three‐cylinder off‐highway application engine con rod big end bearings, which ran for 1000 hours at full load condition, and satisfactory agreement was observed between experiment and simulation values.

Simulation of EHD lubrication for heavy-duty journal bearing based on flexible multi-body dynamics

For heavy-duty journal bearing, the structural flexibility of the shaft and the bearing block has an important influence on the dynamic characteristics of bearing behaviour. Therefore, the deformation and modal resonance generated at work must be considered. In this paper a computational method for solving Elastohydrodynamic lubrication based on flexible multi-body dynamics is introduced. The dynamic substructure condensation method is used to reduce the he shaft and the bearing block, obtain the flexible body model of the crankshaft and the body, reduce the computational solution scale, and improve the computational efficiency.

Dissipative Properties of EHD Lubricant Film

For the case of the failure of the lubricant film at hydrodynamic lubrication a common thermodynamic theory of strength is considered. According to this theory the failure occurs when the internal energy density (potential and thermal components) in the volume of material reaches a constant for a given material. A special case of this theory is considered when only the density of heat (kinetic) component of internal energy is taken into account. Temperature condition determines the limit state for liquid lubricants - mineral oils. When analyzing the regularities of friction at EHD lubrication the state and properties of the oil film at the condition of irregular and hydrostatic compression. The original structural model of oil film at EHD lubrication in the form of the rotary oscillating cells with elastic interactions to each other is proposed. It is similar to the Rayleigh-Benard cells and corresponds to the cellular hypothesis of J. Gibbs for the case of equilibrium and reversible process. It is quite possible that the size of the cells have an order of about nano level. The oil film dissipates energy in the direction of relative motion of bodies. This oil film has the highest dissipative properties.

The Relationship Between Friction and Film Thickness in EHD Point Contacts in the Presence of Longitudinal Roughness

This study investigates friction and film thickness in elastohydrodynamic contacts of machined, rough surfaces, where roughness is dominated by longitudinal ridges parallel to the rolling/sliding direction. A ball-on-disc tribometer was used to simultaneously measure friction and film thickness in rough contacts as well as with nominally smooth specimens for comparison. The studied rough surfaces were selected so that the influence of the root-mean-square roughness and roughness wavelength can be assessed. Friction and film measurements were taken over a range of slide–roll ratios and speeds and with two lubricating oils with different viscosities, hence covering a wide range of specific film thicknesses. The measurements with the nominally smooth specimens show that friction is strongly influenced by thermal effects at high SRRs and that the transition from mixed/boundary to full EHD lubrication occurs at lambda ratios greater than three. At low speeds, the rough specimens are found to generate higher friction than the smooth ones for all the roughness structures considered, and this is shown to be related to the thinner minimum film thickness. Comparison of friction in rough and smooth contacts shows that the total friction in rough contacts can be divided into two components: one that is equivalent to friction in smooth contacts under the same conditions and is dependent on the slide–roll ratio, and the other that is due to the presence of roughness and is independent of the slide–roll ratio under the conditions tested. Further analysis of the minimum film thickness on tops of roughness ridges indicates that even after the full lift-off, an effect of the roughness on friction persists and is most likely related to the local shear stress in the micro-EHD contacts on the top of roughness ridges. At even higher speeds, the difference in friction between the rough and smooth specimens vanishes.

3D EHD lubrication and wear for piston ring-cylinder liner on diesel engines

The three-dimensional EHD lubrication and wear loss of piston ring-cylinder liner components for five rings in the firing diesel engine was studied by using VC++ programs. The gas blowby, variable density and viscosity of lubricant, the surface roughness, friction heat flux and the normal elastic deformation are all considered in the EHD lubrication analysis algorithm. Meanwhile, the wear loss of the cylinder liner was also calculated when the diesel was started under cold starting condition in which the wearing was more serious than that in warm starting condition. The simulated results were compared with the wear measurement of the cylinder liner, and they matched well. It was found that the hydrodynamic friction force was larger than the asperity friction force, and the wear distribution of the barrel piston ring was very different from that of the rectangular piston ring. The distribution trend of the nominal oil film thickness was similar to the normal elastic deformation, but opposite to the wear loss of the piston rings. The wear loss caused by the first piston ring was the biggest and played a dominant role in the wear loss of cylinder liner. The greatest wear loss of the cylinder liner occurs at the time near top dead centre.

A Study on Plastic Deformations due to Contact Fatigue Wear on a Cermet Coating Deposited by Thermal Spraying Methods

This study presents the contact fatigue wear behavior of the cermet NiAlSi coating deposited by plasma spraying on alloyed steel. Wear is the failure cause for many component parts of the 4-stroke engine. For example the cylinders are subjected to wear caused by the sliding of the surfaces in EHD lubrication working conditions. Using the AMSLER test machine the rolls have been put to fatigue wear test for 5 hours in conditions of mixed friction. In order to make the results evident one carried out microscopic electronic scavenging analyses using the electronic microscope type QUANTA 200 3D DUAL BEAM.

Computer Simulation of the Coupled Analysis of Piston Secondary Motion and EHD Lubrication Based on Flexible Multi-Body Dynamics

A coupled model of piston secondary motion and mixed hydrodynamic lubricated was established based on flexible multi-Body dynamics. The model included elastic deformation and roughness of the two surfaces. The effects of rough surface compared to absolutely smooth surface on piston secondary motion mixed lubrication are discussed. The results show that roughness of the surface can make the fluid a better way to lubricate the surface and decrease the secondary motion, but in the high load asperity contact will occur.

Slip Analysis of Wind Turbine Angular Contact Ball Bearing

Wind machine high speed shaft bearing in high speed and light load conditions,it is prone to produce skidding.Skidding damage often becomes one of the main damage forms.According to EHD lubrication theory,this paper establishes the interacting model among rolling-elements,which includes gyroscopic and centrifugal effects,to study the skidding characteristics of angular contact ball-bearings.The model will enable engineers to improve bearing reliability at the design stage.

Full Shafting-Based Elastohydrodynamic Lubrication Simulation for Main Bearings of Marine Diesel Engine

To predict and analyze accurately the lubrication characteristics and its influencing factors of main bearing for large low-speed two-stroke marine diesel engine, based on EHD lubrication model of dynamically loaded bearing, coupling simulation between EHD and MBD for main bearing of marine diesel engine was carried out. Systemic models were established separately considering the independent crankshaft of diesel engine and the full shafting of propulsion power plant. Main bearing load, peak oil film pressure, oil film pressure distribution, MOFT and orbital path in one working cycle under rated working conditions were investigated, and simulation results were compared based on both models. The results show that, if the impact of full shafting was considered, the lubrication characteristics of No.8 main bearing changed significantly, the lubrication characteristics of other main bearings were similar as the simulation results of the independent crankshaft model.

Influence of machine tool setting parameters on EHD lubrication in hypoid gears

The thermal elastohydrodynamic analysis of lubrication is applied to investigate the influence of machine tool setting for pinion tooth finishing on lubrication in hypoid gears. The calculation is based on the simultaneous solution of the Reynolds, elasticity, energy, and Laplace’s equations. The full thermal EHD lubrication analysis is applied, therefore, the oil viscosity variation with respect to pressure and temperature and the density variation with respect to pressure are included. The real shape of the gap existing between contacting tooth surfaces is treated, based on the gear teeth geometry defined by the gear processing method. By using the corresponding computer program, the influence of machine tool setting applied for pinion teeth processing on maximum oil film pressure and temperature, EHD load carrying capacity, and on power losses in the oil film is investigated. The obtained results are presented and discussed.

Effects of couple stresses and surface roughness on the film thickness of heavily loaded rollers

PurposeThis paper aims to study the effects of couple stresses and surface roughness on the minimum film thickness of heavily loaded rollers and to discuss these by following Grubin's approach and Crook's approximations.Design/methodology/approachA generalised form of Reynolds equation for rough surfaces with lubricant as couple stress fluid is derived. This equation is then used to study the combined effect of couple stresses and surface roughness on the roller bearings under heavily loaded conditions. EHD minimum film thickness expressions is obtained by following Grubin's approach and Crook's approximation and it is studied numerically.FindingsIt is found that, as the chain length of the additive molecules increases, the elastohydrodynamic minimum film thickness increases. Also, as the mean height of roughness asperities increases, the elastohydrodynamic minimum film thickness increases for the transversal roughness and it decreases in the case of longitudinal roughness.Research limitations/implicationsThese effects are studied theoretically by the mathematical equations in heavily loaded roller bearings.Practical implicationsReduction of the film thickness in the EHD lubrication between the rollers can be compensated by the use of lubricants containing additives of molecules of size. As a result the bearing performance can be improved.Originality/valueThis research paper provides a closed form of the expressions for the bearings in EHD lubrication and is studied with regard to couple stress parameter. This paper helps to manufacture better bearings.

Effect of Surface Roughness on Mixed EHD Lubrication Characteristics

The effect of surface roughness on the mixed EHD lubrication characteristics has been studied in this paper by using a numerical approach developed recently by Zhu and Hu (1), (2). This approach is capable of handling measured rough surfaces under very severe operating conditions, and simulating the entire transition from the fill film EHD, mixed, down to boundary lubrication. In the present study a set of sample cases in a circular contact is analyzed. Three typical machined surfaces are used, representing different types of topography and orientation. The original RMS roughness (before deformation) is adjusted to 8 different levels, from 0.1 μm up to 1.5 μm, in order to cover a practical range for industrial applications. Results show that in the typical mixed EHL the maximum pressure, contact load ratio, friction, flash temperature as well as maximum subsurface shear stress, are all increased significantly as the roughness increases. The effect on the average film thickness (or gap), however, is relatively limited. Presented at the 2nd World Tribology Congress in Vienna, Austria September 3–7, 2001

Effects of Rough Surface Topography and Orientation on the Characteristics of EHD and Mixed Lubrication in Both Circular and Elliptical Contacts

Effects of rough surface topography and orientation on the characteristics of EHD and mixed lubrication are investigated in this paper by using a numerical approach recently developed by the present authors. This approach is capable of handling measured engineering surfaces under very severe operating conditions, and simulating the entire transition from the full film EHD continuously down to boundary lubrication. In the present study, three typical machined surfaces are employed for cases in circular contacts: a shaved surface with nearly isotropic topography, a transversely ground surface and a longitudinal ground surface. For elliptical contacts (ellipticity 4.0), 3-D sinusoidal wavy surfaces with different wavelengths in x- and y-directions are used. More than 80 cases have been analyzed, covering a very wide range of operating conditions, including full film EHD, mixed and boundary lubrication. The results indicate that at low λ ratios, the average film thickness in rough surface contacts is often slightly greater than that in smooth surface contacts. Generally speaking, the surface topography and orientation may have quite noticeable effects on the asperity contact area and load sharing, but only a limited (in many cases insignificant) influence on the average film thickness (or gap) in both the circular and elliptical contacts. Presented at the 56th Annual Meeting Orlando, Florida May 20–24, 2001

Experimental Evaluation of EHD Film Shape and Its Comparison With Numerical Solution

Colorimetric interferomentry has been applied to the study of EHD lubrication of point contacts under pure rolling conditions to obtain lubricant film shapes with high accuracy and resolution. An RGB CCD camera together with an extensive image processing software has enabled real time evaluation of chromatic interferograms. The classical numerical isothermal solution of EHD lubrication of point contacts has been used for the comparison with three-dimensional representations of film thickness distributions obtained from experiments. A good agreement was found between experimental and numerical EHD film shapes by comparing lubricant film profiles and positions of minimum film thickness. Both experimental results and numerical solution confirm the ratio between central and minimum film thickness to change significantly with operating conditions. [S0742-4787(00)00404-5]

Progress in Bearing Performance of Advanced Nitrogen Alloyed Stainless Steel, Cronidur 30

The bearing rig tests performed in this study, demonstrate superior bearing performance of Cronidur 30 steel over conventional bearing steels. In these tests the L10 life of Cronidur 30 steel as calculated by the DIN/ISO 281 method was 80 times the unfactored L10 life under full lubrication conditions. In boundary lubrication conditions, the Cronidur 30 steel demonstrated the L10 life capability typical of EHD lubrication conditions, whereas the other steels showed a drastically reduced lives. In tests with predamaged races and boundary lubrication conditions, Cronidur 30 demonstrated 8 times the calculated L10 life, whereas the conventional steels exhibited further debit in lives as compared to the boundary lubrication testing whithout predamage. The improved performance of Cronidur 30 steel over conventional bearing steels is attributed to its unique compositional formulation and microstructure that results in provision of balanced properties in the alloy – hardness, toughness and corrosion resistance. Fortschritte bei der Lagerlebensdauer durch den Einsatz des stickstofflegierten nichtrostenden Stahles Cronidur 30 Die Prüfstandsversuche mit Wälzlagern, über die hier berichtet wird, ergaben mit dem Stahl Cronidur 30 ein erheblich besseres Laufverhalten als mit herkömmlichen Wälzlagerstählen. Unter elasto-hydrodynamischer Schmierung ergab sich im Vergleich zur L10-Lebensdauer, die ohne Berücksichtigung von Faktoren nach DIN/ISO 281 errechnet wurde, eine 80-fach längere Lebensdauer. Unter Mangelschmierung erreichten Lager aus Cronidur30 immer noch Lebensdauern wie andere Wälzlagerstähle unter Vollschmierung. Die Lebensdauern dieser Stähle sinkt unter Mangelschmierung drastisch ab. In Versuchen mit absichtlich vorgeschädigter Laufbahn und Mangelschmierung erreichten Lager aus Cronidur30 immer noch die 8-fache rechnerische L10-Lebensdauer, während die Lebensdauer der Lager aus anderen Stählen weitaus stärker absank. Die Überlegenheit des Cronidur30 über herkömmliche Wälzlagerstähle wird zurückgeführt auf die ungewöhnliche Stahlzusammensetzung und Mikrostruktur, die in einem ausgewogenen Verhältnis zwischen Härte, Zähigkeit und Korrosionswiderstand resultieren.

Finite Element Mass-Conserving Cavitation Algorithm in Pure Squeeze Motion. Validation/Application to a Connecting- Rod Small End Bearing

A simple analytical approach using conditions of conservation of the mass is proposed for the 1-D “negative squeeze” lubrication problem in order to calculate the cavitation boundary position during oscillatory motion of two plates. The same geometrical case is analyzed using Bonneau’s finite element code. Good agreements between analytical and numerical results support validation of Bonneau’s algorithm. As an example of application of this algorithm to squeeze motion case, the EHD lubrication of an elastic connecting-rod small end bearing is analyzed. Influences of the shaft elasticity and lubricant piezoviscosity are presented. [S0742-4787(00)02101-9]

Characteristics of a New Type of Cylindrical Worm-Gear Drive

A new type of cylindrical worm-gear drive is presented. The worm is ground by a grinding wheel whose profile consists of two circular arcs, and the obtained worm profile is concave. The teeth of the gear are processed by a hob whose generator surface is identical to the worm surface. The aim of this paper is to introduce this new type of worm gearing and to compare the operating characteristics of the new type and of the commonly used worm-gear drives on the basis of tooth contact analysis, load distribution calculation, and thermal elastohydrodynamic lubrication analyses. The results obtained show the advantages of the new type of worm gear. The results of the load distribution calculation and EHD lubrication analysis are also used for the optimization of the design parameters of the new type of worm-gear drive.