Lubricant Film Breakdown Research Articles

Investigation of hydrodynamic lubricant film breakdown between the journal and bearing in a high-speed coolant pump of electric vehicles

The high-speed coolant pump is essential for electric vehicle thermal management, ensuring optimal module temperatures. The pump impeller and bearing are combined into a rotor and then fit on a fixed shaft with a clearance, aiming to generate a complete lubricant film between the stationary journal and rotating bearing during pump operations. However, during the pump durability test under off-design conditions, severe wear occurred on both the journal and bearing. A novel simulation method based on computational fluid dynamics and dynamic mesh is developed to investigate the lubrication failure. User-defined functions are programmed to articulate the simultaneous rotation and whirl of the bearing, and the effects of temperature and cavitation are considered. Both the pump and bearing simulations closely align with experimental results. When the radial force of the pump rotor is steady, the bearing only rotates without whirling, and the bearing load capacity can counteract the maximum radial force of the rotor. However, the rotor experiences fluctuating radial forces over time, primarily attributed to elevated pressure levels at the volute tongue caused by the rotor–stator interaction. This phenomenon results in the bearing to rotate and whirl simultaneously. The load capacity generated by the backward whirl of the bearing is greater than that of the forward whirl. Nevertheless, the bearing whirl initiates a decline in the extremum values of lubricant film pressure. Subsequently, both load capacity and minimum film thickness exhibit a continuous decrease, eventually culminating in the breakdown of the complete lubricant film and resulting in contact wear failure. These new findings contribute to proposing measures to reduce wear under unsteady excitations.

A deterministic contact evolution and scuffing failure analysis considering lubrication deterioration due to temperature rise under heavy loads

Nowadays many friction pairs in engineering inevitably work under mixed lubrication regime, during which both hydrodynamic lubrication and asperities contact share the heavy loads. The heat generated by friction, especially by asperities contact friction, will deteriorate the lubrication performance, and then a scuffing failure may happen. In this study, based on a deterministic mixed thermal elastohydrodynamic lubrication (TEHL) model, the lubrication deterioration, the related temperature rise, and the contact evolution are analyzed together. The simulation results show that under a light load, the temperature of the lubricant film initially increases, but it quickly stabilizes to a not too high value. However, under a heavy load, a much higher temperature rise has happened. Meanwhile the lubrication performance is seriously deteriorated, and the contact ratio increases sharply. It indicates that the contact evolution may lead to lubricant film breakdown and successively surface scuffing under heavy loads.

Prevention of seizure in extrusion of aluminium alloy billet by dies having textured surface

Seizure occurring in extrusion of an aluminium alloy billet was prevented by texturing the die surface. A liquid lubricant is entrapped in micro pockets produced on the textured surface, and is flowed out during extrusion to prevent the lubricant film breakdown. TiCN-based cermet and WC-Co dies having high anti-seizure property were employed, and the dull micro pockets were formed on the die surface by shot peening and polishing processes. The anti-seizure property was evaluated from single extrusion and repeated extrusion of an A5056 aluminium alloy billet using a liquid lubricant containing a sulphur additive. Although the roughness of the textured surface was about one order of magnitude larger than that of the polished surface, the anti-seizure property is considerably higher, whereas the difference in extrusion load between both surfaces was comparatively small. It was found that the TiCN-based cermet die having the textured surface has a high anti-seizure property.

A combined numerical and experimental approach for determining the contact temperature in an industrial ironing operation

Tribological conditions in forming operations depend on several parameters such as tool-workpiece interface pressure, surface expansion, sliding length, sliding speed, tool and workpiece materials and the roughness of the parts. Among indirect parameters, the most influential one is the tool-workpiece interface temperature, which directly influences the lubricant performance. Prior to testing new tribo-systems to determine their limits of lubrication, it is therefore important to find the interface temperature. However, measurement of the interface temperature in metal forming is difficult. The present work investigates the determination of the interface temperature in an industrial ironing operation, where severe process parameters lead to lubricant film breakdown and galling after several strokes. The methodology combines finite element simulations and experimental measurements. The overall procedure is based on a steady-state thermal analysis to determine the temperature distribution within the tool and a transient thermo-mechanical analysis of the ironing process when steady-state conditions are achieved. Results show that the proposed methodology applied to a single stroke can effectively and accurately predict the interface temperature in the test tool, thus avoiding the otherwise required thermo-mechanical FEM analyses of hundreds of strokes to reach steady-state. Furthermore, the influence of parameters, such as the predicted steady-state tool temperature, the friction coefficient and the heat transfer coefficient on the contact temperature, is analysed. It is concluded that the frictional heating is the primary cause for the peak temperature. By calibration of the friction coefficient and the heat transfer coefficient to ensure matching of the numerical results and the experimental measurements, a maximum tool-workpiece interface temperature of 158 °C was determined during the forward stroke and 150 °C during the backward stroke.

Continuous Strip Reduction Test Simulating Tribological Conditions in Ironing

Abstract Laboratory testing of tribo-systems for sheet metal forming applications must ensure similar conditions with the tribo-parameters that are commonly utilized in real production in order to generate data that is meaningful for industry. The main parameters to consider are the tool and workpiece materials, surface roughnesses, normal pressure, sliding length, sliding speed, interface temperature and lubrication. This paper proposes a new Strip Reduction Test (SRT) for industrial ironing processes that is capable of replicating the highly severe tribological conditions that are experienced during both the forward stroke and the backward retraction of the punch. The new SRT tool design is implemented in a new Universal Sheet Tribo-Tester (UST), which can run multiple tests continuously from a coil. The test is capable of simulating various process parameters such as reduction, drawing speed, tool temperature, sliding length and quantifying the onset of breakdown of the lubricant film and subsequent galling after several strokes not only when emulating the forward strokes but also the backward strokes. Preliminary tests disclose promising results as regards the identification of lubricant film breakdown by detecting changes in measured force, surface roughness and/or torque values.

Effect of surface roughness on lubricant film breakdown and transition from EHL to mixed lubrication

The ability of elastohydrodynamically lubricated (EHL) contacts to separate surfaces represents essential design requirement ensuring long life and low friction of machine components. Nevertheless, it is strongly affected by surface roughness that tends to decrease formed lubricant film. Surface roughness related effects are highly scales dependent. Dimensions in longitudinal and transverse directions influence roughness deformation while dimensions in roughness height can have significant effects on lubrication process. The prediction of the roughness features effect on lubricant film in the case of possible lubricant film breakdown still represents the challenge even for nowadays power full numerical simulation tools. At the same time some simple measures as the lambda ratio does not provides realistic estimation of lubrication regimes. However, there is a strong demand for such a prediction especially close to the transition to the mixed lubrication regime. In this paper possible approach based on the combination of experimental and numerical data is discussed. It is suggested that quantified experimental data could bridge possible gaps in the ability of the theory to provide the sufficient fast estimation of lubrication capabilities of engineering surfaces in the transition region between EHL and mixed lubrication.

Lubricant Film Breakdown and Material Pick-Up in Sheet Forming of Advanced High Strength Steels and Stainless Steels when Using Environmental Friendly Lubricants

Increasing focus on environmental issues in industrial production has urged sheet metal stamping companies to look for new tribo-systems in order to substitute hazardous lubricants such as chlorinated paraffin oils. The efficiency of chlorinated paraffin is due to the fact that the lubricant reacts chemically with the tool and workpiece material forming thin films, which adhere strongly to the surfaces and reduce the tendency to metal-metal contact and material pick-up. Production tests of new, environmentally benign tribo-systems are, however, costly and laboratory tests are preferred as a preliminary simulative method to investigate alternative tribo-systems. The present paper presents a case study where an industrial process, consisting of deep drawing with two subsequent re-drawings, was selected and four potential new tribo-systems were tested including different workpiece materials, i.e. AHSSs and stainless steels. The performance of the tribo-systems was analyzed in the laboratory by means of a newly developed simulative test as well as in an industrial production process. The results obtained show a good agreement between the laboratory test and the industrial production process regarding the tribological performance, i.e. tendency to material pick-up and galling, of the evaluated tribo-systems. Moreover the SEM analysis shows that different workpiece materials result in different types of material pick-up.

Thermal Analysis of Bending Under Tension Test

The tribological conditions in deep drawing can be simulated in the Bending Under Tension test to evaluate the performance of new lubricants, tool materials, etc. Deep drawing production with automatic handling runs normally at high rate. This implies considerable heating of the tools, which sometimes can cause lubricant film breakdown and galling. In order to replicate the production conditions in bending under tension testing it is thus important to control the tool/workpiece interface temperature. This can be done by pre-heating the tool, but it is essential that the interface temperature during testing is similar to the one in the production tool. A universal sheet tribo-tester has been developed, which can run multiple tests automatically from coil. This allows emulating the temperature increase as in production. The present work performs finite element analysis of the evolution and distribution of temperature in the bending under tension test by making use of boundary conditions and calibration values directly measured from experiments. The overall methodology combines 2D and 3D models of the bending under tension test with steady state and transient thermal and thermo-mechanical procedures. Results show that the proposed methodology applied to a single stroke can effectively and accurately predict the interface temperature in the test tool, thus avoiding the otherwise required thousands of thermo-mechanical FEM analyses of temperature development during testing before thermal steady state has been reached.

Surface stresses and flow modes on contact surface in a combined double cup extrusion process

The main goal of this study is to examine the influence of geometrical parameters in double cup extrusion process (DCEP) on the surface stress profiles such as surface expansion, contact pressure, sliding velocity and distance. The flow modes, sliding over or sticking onto the punch surface, were also investigated to see the possibility of the metal-to-metal contact or the lubricant film breakdown along the punch and workpiece interface. Two major design factors chosen for analysis include the reduction in area in backward direction (RAB) and the wall thickness ratio (TR). A sequential operation has been also simulated to compare the surface stress profiles and flow modes with those in a combined operation.

Testing and modelling of new tribo-systems for industrial sheet forming of stainless steels

Sheet metal forming of stainless steels is known to be tribologically demanding. To ensure satisfactory production without pick-up and galling, lubrication with environmentally hazardous chlorinated paraffin oil is normally required and in the most severe cases combined with ceramic tool coatings. The present paper presents an attempt to clarify the challenges of replacing chlorinated paraffin oils with new environmentally friendly oils in an industrial production, by investigating these oils in combination with alternative tool materials and tool coatings. A number of tribology systems have been screened in the laboratory by strip reduction testing and the best tribo-systems have been tested in a progressive production tool at the Danish company Grundfos A/S. The combination of a TiAlN coating with one of the environmentally friendly lubricants shows good performance in the simulative test as well as the production test with no sign of galling even at maximum production speed. Lowering production speed, and thereby the interface temperature, allows less efficient tribo-systems to function satisfactory, i.e. without galling. The tool/workpiece interface temperature is simulated for the strip reduction test as well as the production test in order to estimate the critical interface temperature for lubricant film breakdown. Simulation results show good agreement with experimental measurements of tool temperature close to the interface.

A transient finite element cavitation algorithm with application to radial lip seals

The presented elastohydrodynamic (EHD) formulation allows accurate analysis of lubricant film breakdown and reformation during the operation of lubricated devices. The algorithm is based on finite-element discretization and the JFO model. Active and non-active lubricant film zones are determined for non-stationary operating conditions. The model was validated through comparisons with previously published studies. The presented results show the shape of rupture boundaries, with a sample of plots demonstrating the efficiency of the algorithms in solving complex problems, such as the EHD lubrication of lip seals. The analysis of thickness, pressure and filling rate fields allows the optimisation of lip seal element design and demonstrates the efficiency of the lubricant feeding device.

Bending Under Tension Test with Direct Friction Measurement

A special bending-under-tension (BUT) transducer has been developed in which friction around the tool radius can be directly measured when drawing a plane sheet strip around a cylindrical tool pin under constant back tension. The front tension, back tension, and torque on the tool pin are all measured directly, thus enabling accurate measurement of friction and direct determination of lubricant film breakdown for varying normal pressure, sliding speed, tool radius, and tool preheat temperature. The transducer is applied in an experimental investigation focusing on limits of lubrication in drawing of stainless steel, showing the influence of varying process conditions and the performance of different lubricants.

Comparison between different test methods for evaluation of galling properties of surface engineered tool surfaces

In forming operations, galling is a tribological process associated with pick-up of formed material by the surface of the tool and subsequent damage of the work material surface. In lubricated operations, it is associated to lubricant film breakdown. Currently the main focus in development of forming tools is on improving their galling resistance, mainly by including new forming tool steels, improving surface finish, and applying surface treatments and coatings. However, in order to qualitatively rank the galling resistance of material couples, a proper test method for galling evaluation is needed. During the years several methods for determining the galling resistance have been reported in the literature. In the present investigation three different test methods were compared and evaluated as to their ability to assess the galling tendencies of coated and un-coated forming tool steel. The study included the pin-on-disc configuration, a modified cylinder-on-cylinder test with a spring loading system, and a new load-scanning test method, which involves two crossed cylinders. It was proved that the load-scanning test rig is both a very simple and the most suitable method to evaluate galling properties of tool steels. It gives the capability to compare different tool steels, tool surface finishes and treatments, and hard coatings in terms of friction level, amount of material transfer, and critical load for galling initiation.

Prediction of limits of lubrication in strip reduction testing

Pick-up and galling due to lubricant film breakdown is a severe limitation in cold forming of tribologically difficult metals like stainless steel and aluminium. The present paper describes a method of combined experimental and numerical analysis to quantify the limits of lubrication in a dedicated simulative strip reduction test. The limit of lubrication is quantified as the threshold drawing length before galling occurs. A numerical model of the test is established calculating tool/work piece interface temperatures and normal pressures. Identifying a critical maximum value of the interface temperature the results show good agreement between numerically predicted and experimentally observed threshold drawing lengths at different test conditions.

Quantification of galling in sheet metal forming by surface topography characterisation

One of the major problems in forming of stainless steel sheet is galling due to lubricant film breakdown leading to scoring and bad surface quality. In a Danish research programme new lubricants substituting the normally applied chlorinated paraffin oils are being developed and tested for this purpose. In order to determine the limits of lubrication of these new lubricants, as well as commercial ones already available on the market, two sheet forming tests have been developed. Quantification of the degree of galling is done by roughness measurements on the workpiece surface. In a strip reduction test, this is done by 2D profilometry at strip locations corresponding to different sliding lengths, whereas a deep drawing test is based on 3D roughness mapping of local areas.

Scuffing and lubricant film breakdown in FZG gears Part II. New PV scuffing criteria, lubricant and temperature dependent

The analysis of experimental and theoretical results, from constant temperature scuffing tests with FZG type A gears lubricated with base oils, show a good agreement between the scuffing loads and the scuffing lubricant film thickness, meaning that a close relation exists between scuffing failure and lubricant film breakdown, for a base oil tested in the FZG rig. This relation is exploited, associating the classic PV scuffing criteria with the lubricant film thickness in scuffing conditions. Correlating experimental and theoretical results, a more general PV scuffing criteria is developed which depends on the lubricant properties and lubricant temperature. The application of the new scuffing criteria to FZG-A/8.6/90 standard tests predicted the corresponding scuffing loads with excellent accuracy.

Scuffing and lubricant film breakdown in FZG gears Part I. Analytical and experimental approach

This paper presents the influence of the base oil viscosity and of the pitch line velocity on the scuffing load of a spur gear. Standard FZG A/8.6/90 tests, and constant temperature tests for several pitch line velocities, were performed, for three mineral paraffinic base oils, with different viscosities. These tests were complemented by the analytical evaluation of the most important operating parameters, such as the oil bath temperature and viscosity, the lubricant film thickness and the contact temperature along the gear contact path, for each load stage considered. The analysis of experimental and theoretical results shows a good correlation between the scuffing loads and scuffing temperatures and the lubricant film thickness, suggesting that a close relation exists between scuffing failure and lubricant film breakdown, for a base oil tested in the FZG rig.

Observations in Transiently Loaded EHL Contacts under Pure Sliding Conditions

An elastohydrodynamically lubricated contact is experimentally investigated during transient loading and pure sliding motion. This is a combination of motions where lubricant film failures sometimes occur. The experimental setup consists of a Ø70 mm ball segment mounted on a pendulum. The ball segment can be rotated while it impacts a lubricated glass disc. The deformation of the surfaces in the contact region is studied by means of optical interferometry and high-speed video recording equipment. Two polyo-α-olefin-type lubricants with viscosities of 94 and 2600 mm2/s are used in the experiments. The squeeze velocity, i.e., the normal approach velocity, is approximately 0.06 m/s, giving an impact time of about 20 ms. The sliding velocity is varied from 0 to 34 m/s. Some of the observed phenomena are that lubricant film breakdown occurs at high sliding velocities while full film is maintained at no or low sliding velocity and that the traditional horseshoe-shaped fringes are found even for this load situation. At very high sliding velocities, the interferograms are distorted from their normal circular shape to a pear-like shape.

Failure of thin film lubrication — A detailed study of the lubricant film breakdown mechanism

The mechanism of breakdown of the lubricant film in a concentrated steel contact has been studied by examining friction-time diagrams as functions of sliding speed (ranging from 0.0007 to 0.7 m s −1) and normal force on the contact. It was found that, even at the lower speed of 0.0007 m s −1, the load-carrying capacity of the contact still depends strongly on lubricant viscosity. Over the whole range of speeds, lubricant film breakdown can be explained in terms of a major disturbance in the steady state condition, which is characteristic of (partial) elastohydrodynamic lubrication. In that condition, oxidation of microasperities which enter the surface zone keeps abreast of metal-to-metal junction formation.

Lubrication of Sliding/Rolling Contacts Using a Disc Machine of Novel Design

A new type of disc machine has been developed which may be used for evaluation of the load-carrying properties of lubricants. Commissioning tests revealed that it possesses some advantages over more conventional machines, notably in terms of operating range and flexibility. Examination of critical temperature and film thickness failure criteria suggested that both approaches may be used to explain the occurrence of lubricant film breakdown. The machine has shown its ability to differentiate between oils of different load-carrying properties, and encouraging agreement with the IAE gear machine, when operated under similar conditions, has been obtained.