Lubrication Gap Research Articles

Functionality of Bearings in the Shafts of a Vertical-Axis Wind Turbine

The article contains a description of the design solutions proposed by the authors for a hybrid wind turbine bearing, in which the sliding part takes over the load to the turbine shaft after reaching the shaft rotation speed, ensuring hydrodynamic lubrication of the plain bearing and relieving the rolling bearing. This allows for low starting resistance of the power plant and ensures quiet operation during use. Two conceptual solutions of a hybrid bearing were presented, differing in the shape of the plain bearing journal. A mechanism for automatic switching of the load between a rolling and a plain bearing was developed. A solid simulation model of this mechanism was built in the Autodesk Inventor—Dynamic Simulation software Inventor Professional 2023 environment, and its operation was simulated. The results confirmed the usefulness of using this design in shaft-bearing systems of wind turbines with a vertical axis of rotation. Based on the simulation, the speed at which the thrust roller bearing will be released was determined. Technical parameters of a plain bearing with a spherical journal shape were calculated. The height of the lubrication gap and the shaft rotational speed at which the bearing load capacity index reaches a critical value were determined.

Changes in Surface Topography and Light Load Hardness in Thrust Bearings as a Reason for Tribo-Electric Loads

The article focuses on the findings of endurance tests on thrust bearings. In addition to the mechanical load (axial load: 10 ≤ C0/P ≤ 19, lubrication gap: 0.33 µm ≤ h0 ≤ 1.23 µm), these bearings are also exposed to electrical loads (voltage: 20 Vpp ≤ U0 ≤ 60 Vpp, frequency 5 kHz and 20 kHz), such as those generated by modern frequency converters. In a previous study, the focus was on the chemical change in the lubricant and the resulting wear particles. In contrast, this article focuses on the changes occurring in the metallic contact partners. Therefore, the changes in the surface topography are analysed using Abbott–Firestone curves. These findings show that tests with an additional electrical load lead to a significant reduction in roughness peaks. A correlation to acceleration measurements is performed. Moreover, it is shown that the electrical load possibly has an effect on the light load hardness. An increase in the occurring wear could not be detected during the test series. Also, a comparison with mechanical reference tests is made. The article finally provides an overview of different measurement values and their sensitivity to additional electrical loads in roller bearings.

Influence of Lubrication Cycle Parameters on Hydrodynamic Linear Guides through Simultaneous Monitoring of Oil Film Pressure and Floating Heights

Hydrodynamic linear guides in machine tools offer a high load capacity and excellent damping characteristics, improving stability, precision, and vibration reduction. This study builds on previous research where floating heights were verified with a simulation model limited to measured floating heights. Advancements include incorporating pressure sensors into a fixed steel rail, enabling simultaneous measurement of oil film pressure and floating heights for a comprehensive understanding of lubrication conditions within the lubrication gap. The experimental results explore the effects of different lubrication methods, providing valuable insights into cavitation and lubrication adequacy. The results demonstrate the feasibility of utilizing pressure sensors to measure oil film pressure within the lubrication gap, providing a nuanced understanding of lubrication dynamics. By measuring both floating heights and pressure measurement, distinctions between hydrodynamic lubrication, mixed friction regions, and instances of lubricant deficiency become readily discernible. The variations in real-time oil film pressure and floating heights help to optimize the lubrication cycle for hydrodynamic linear guides, enhancing system performance and longevity.

Laser-Structured Thin Film Sensor Technology for Sliding Bearings in Internal Combustion Engines

Abstract Global issues such as climate change, environmental pollution, and conservation of resources require manufacturers of internal combustion engines (ICE) to achieve substantially reduced emissions of greenhouse gases and pollutants as well as increased engine efficiency and durability. Condition monitoring and predictive maintenance approaches for sliding bearings in ICEs are key tools for increasing engine durability and saving resources by exploiting more of the useful lifetime of a bearing while avoiding critical engine operation caused by bearing wear and failure. These approaches require appropriate measurement technology capable of acquiring informative parameters that reflect the current condition of the bearings while withstanding the high temperatures and mechanical loads encountered inside the engine and accommodating spatial constraints. This paper deals with research work related to advanced sensor technology that is highly integrated into sliding bearings so that information is obtained nearly directly from relevant areas such as the bearing running layer and the lubrication gap. An isolated, sputtered sensor layer with a thickness of a few micrometers is employed in combination with a laser structuring process to form the desired thin film sensor structure below the bearing running surface. While several measurement parameters and corresponding sensor types are conceivable, this paper focuses on temperature and strain measurements that rely on a change in the electrical resistance of the sensor layer material. Promising sensor layouts and positions targeted for use in condition monitoring applications in ICEs are elaborated in detail. Developments and challenges in implementing the sensor technology concept—in particular with regard to the process of manufacturing the sensor as well as the wire contacting—are outlined in depth. The paper concludes by presenting measurement results obtained with this sensor technology at lab scale as well as an outlook toward implementing the instrumented bearings in ICEs.

Research on two-phase thermo-hydrodynamic lubrication performance considering bubble dynamics

Gas-liquid two-phase fluid in the lubrication gap exist in many important engineering tribo-pairs. Based on the Reynolds equation, the bubble dynamics equation, and the full energy equation for two-phase fluids, the thermo-hydrodynamic lubrication problem in the lubrication gap is investigated. The evolution of the fluid phase composition with operating time and its impact on the thermodynamic properties are studied using the developed model. The mechanisms by which sliding velocity, clearance, surface dilatational viscosity, surface tension, and initial gas volume fraction affect two-phase fluid lubrication behavior are evaluated. The results show that hydrodynamic pressure-driven bubbles eventually condense in the dispersion area and have a major impact on the temperature distribution. Velocity and clearance affect the gas-phase expansion region and initial formation location in the steady state, respectively. The pressure amplitude, bubble expansion, and horizontal and vertical gas phase expansion in the dispersion zone were affected differently by surface dilatational viscosity and surface tension. It becomes difficult to preserve the initial gas-phase composition inside the pressure convergence zone when bubble migration is taken into account, which may be crucial for establishing gas-liquid two-phase lubrication.

Lubricating characteristics and sealing performance of mixed TEHD analysis of piston-cylinder interface in the piston pump

PurposeIn high-pressure pumps, due to the interaction of asperities on the upper and lower surfaces, the piston–cylinder interface suffers severe lubrication and sealing problems during mixed lubrication. This study aims to establish a mixed thermo-elastohydrodynamic (EHD) model for the lubrication gap to determine how working conditions affect the lubricating characteristics and sealing performance of the interface.Design/methodology/approachA mixed thermo-EHD lubrication model is established to investigate the lubricating characteristics and sealing performance of the interface between the piston and cylinder. The model considers piston tilting, thermal effect, surface roughness and bushing deformation. The interface lubricating characteristics and sealing performance under different working conditions are calculated by the proposed numerical model.FindingsA higher inlet pressure contributes to an increase in the minimum film thickness. Increased shaft speed can significantly reduce the minimum film thickness, resulting in severe wear. Compared to roughness, the impact of the thermal effect on the interface sealing performance is more significant.Originality/valueThe proposed lubrication model in this study offers a theoretical framework to evaluate the lubricating characteristics and sealing performance at the lubrication gap. Furthermore, the results provide references for properly selecting piston-cylinder surface processing parameters.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-03-2023-0072/

Lubrication mechanisms of dispersed carbon microspheres in boundary through hydrodynamic lubrication regimes

HypothesisCarbon microspheres have been shown to reduce friction and surface wear at relatively low speeds and high applied loads (i.e., within the boundary lubrication regime). We hypothesize that in dilute colloidal lubricating systems there is an interplay between the size of the carbon microspheres and the lubrication gap size, which determines the dominant lubricating mechanism of the system. ExperimentsA 60 wt% aqueous glycerol solution was used as the base lubricant and compared to various carbon particle-based lubricant formulations ranging in particle concentrations from 0.05 to 0.30 vol%. The tribological properties of the various lubricant formulations were tested on a pin-on-disk tribometer. A simplified Stribeck plot was produced to understand the changing mechanism of lubrication over a wide range of conditions. FindingsThe Stribeck curves show that the carbon microspheres assist lubrication by a rolling mechanism primarily in the boundary lubrication regime. A 0.20 vol% carbon-based lubricant formulation showed the best friction reduction compared to the base lubricant. Increasing speed increases the lubricating gap between the friction pair beyond the size of the particles, thereby nullifying the rolling mechanism of the particles. We introduce a modified specific film thickness parameter to determine the lubrication regime in a particle-lubricant system.

Multiscale Wear Simulation in Textured, Lubricated Contacts

Specific surface textures may reduce the friction and increase the lifting forces in lubricated contacts. For the detrimental operating condition of mixed friction, wear is induced by the solid contact. In this study, a methodology for wear calculation in textured, lubricated contacts is presented that considers the wear-induced surface topography evolution. Based on the Reynolds differential equation, the mass-conserving cavitation model according to Jakobsson, Floberg, and Olsson (JFO), a wear-dependent asperity contact pressure curve and the wear equation according to Archard, wear in a wedge-shaped, textured lubrication gap was calculated. The results show the wear behavior of textured lubrication gaps. Based on the wear simulations, the tribological behavior of the textured surfaces compared to smooth surfaces is discussed. It is evident that textures, which improve the tribological performance in the hydrodynamic lubrication regime, are not necessarily associated with low wear values in a lubrication condition in the mixed friction regime. The analysis of the wear-dependent parameters initially showed a ‘recovery’ of the tribological system with increasing wear until the performance decreased again after a specific reversal point. This behavior is attributed to the relative position of the surface textures in the lubrication gap.

Breakdown voltages in thrust bearings: Behavior and Measurement

This paper investigates breakdown characteristics in the bearings of variable speed electric drive trains. The analyses reveal that breakdown phenomena previously known from high-voltage technology appear to also occur in the bearing contact, though at significantly lower voltages. By loading the bearing with AC voltage of different frequencies at various mechanical load points, five states, which range from partial discharge through arc discharge to a completely conductive channel, emerge. Consistent with the concept that the magnitude of the breakdown voltage depends on the lubricant film height, the contact force, and the frequency of the AC voltage, a thicker lubricant film results in a higher breakdown voltage, analogously to a capacitor with a larger plate spacing, which requires a higher potential difference to be punctured. Furthermore, the sinusoidal frequency of the AC load correlates with the breakdown voltage. The breakdown tends to occur earlier at low frequencies, which is illustrated by the fact that at low frequencies, capacitive behaviour can only be observed at higher speeds, i.e., at a larger lubrication gap, while no voltage can be built up over the bearing at lower speeds. An increase of the contact force for the same lubricant film height lowers the breakdown voltages. The reduction of the breakdown voltage by the contact force may result from an increase of the relative permittivity of the lubricant with higher pressure and likewise of the contact area; the increased area may also increase the number of surface-roughness peaks where an increase in the electric field occurs.

Sensor integrating plain bearings: design of an energy-autonomous, temperature-based condition monitoring system

Plain bearings are increasingly being used under mixed friction, resulting in a higher risk of wear-induced failure. Therefore, the relevance of condition monitoring is increasing, which can enable the early detection of wear inducing conditions. The presented work focuses on the development of an energy-autonomous, temperature-based condition monitoring system for plain bearings. The components of the condition monitoring system are to be integrated directly into the machine element and operating data to be transmitted wirelessly.The fundamental monitoring approach pursued, is the in-operando calculation of the lubrication gap height via the standardized Gümbel-curve based on the temperature map in the plain bearing. If the gap height undercuts a critical threshold, the plain bearing will operate in mixed friction. Thus, representing a potentially wear-causing operating point. The energy required for operating the system is harvested thermoelectrically within the plain bearing via thermo-generators, which use the temperature gradients in the plain bearing during operation.In this work, the simulative evaluation of the harvestable energy will be presented. The usable temperature difference will be determined for varying positioning of the energy harvester. The results obtained allow the subsequent manufacture of a first Sensor integrating Machine Element (SiME)-prototype. In this context a feasible setup regarding the types and number of sensors as well as a microcontroller for processing the measured data will be defined with respect to the harvestable energy. Furthermore, the principle for temperature-based gap height determination will be introduced, applied for different operating points and compared to simulative results.

Tribological behaviour of AISI 4140 and WC-Co carbides during dry condition, using cryogenic media, and sub-zero metalworking fluids at high contact stresses

This paper aims at an improved understanding of the influence of different cooling and lubrication conditions on the tribological behavior at extremely high contact pressures. A novel tribometer was used to investigate the tribological behavior of the material pairing AISI 4140 Q + T and WC-Co carbides, which is common in cutting processes. Regarding the cooling and lubrication conditions, dry condition, liquid nitrogen (LN2), carbon dioxide snow (CO2), and two different types of sub-zero metalworking fluids (sub-zero MWF) were investigated. The tribological behavior is analyzed on the basis of the thermo-mechanical load in the tribological contact, the coefficient of friction, and the surface topography of the friction partners. Furthermore, the influence of the hydrodynamic lubrication gap height, resulting from different types of sub-zero metalworking fluids, on the lubricating effect is investigated. The analysis shows that the use of cryogenic media resulted in a substantial reduction of the thermal load. However, the coefficients of friction as well as the topography of the friction partners indicate that a lubricating effect can only be achieved up to a contact pressure of roughly 2100 MPa. In contrast, both types of sub-zero MWFs led to a high lubricating effect even at contact pressures of up to 3600 MPa. By using both types of sub-zero MWFs at a supply temperature of -30 °C a high cooling effect was also achieved. A higher hydrodynamic lubrication gap height can be beneficial for the lubricating effect depending on the type of sub-zero MWF. In this context, however, further friction-reducing effects such as adsorption films need to be considered.

Research on Linear Actuators for Active Foil Bearings.

Active foil bearings are a kind of gas foil bearing. They contain actuators which allow for modification of the bearing sleeve size and the shape of the lubrication gap. Rotor vibrations can be actively controlled by these changes. It is possible, among other things, to reduce the starting torque, control the vibration amplitude at different speeds and improve operational safety. Prototypes of active foil bearings are being developed based on different mechanical concepts. This paper provides an analysis of the linear mechanisms that are the base, and they are crucial for such developments. In the literature, there is a lack of characteristics of these actuators tested under real operating conditions of an active foil bearing. This article aims to fill this gap. New test rigs have been developed and used to precisely investigate the possibility of using actuators in active foil bearings. Since their geometry and control methods differ, the measurement systems have been adapted accordingly. The actuators studied were piezoelectrics, shape-memory alloys (SMAs) and stepper motors. Each of them was characterized by different operating characteristics. The results obtained allow for a comparison of the actuators. This approach is especially useful for the design and analysis of active foil bearings.

Structural design and magnetic field analysis on magnetic fluid lubricated bearings

A structural model of a bearing is proposed based on a general standard bearing structure and magneto-fluidic lubrication, and the magnetic field of the bearing lubrication gap is analyzed under the action of an applied magnetic source. Results show that the magnetic field can form a magnetic circuit in the bearing structure, and a significant magnetic flux density is formed in the contact area of the rolling element of the bearing. The magnetic induction intensity in the contact area of the rolling element of the thrust bearing can provide magnetic fluid lubrication, and the magnetic induction intensity in the contact area of the rolling element of the point contact bearing is greater than that of the linear bearing. The magnetic inductance of the rolling elements in the rolling element contact area of a double row point contact bearing is greater than that of a single ring. The double permanent magnet ring structure has a more advantageous magnetic field effect than a single ring, and the magnetic inductance in the rolling element contact area of both rows of a double row point contact bearing is relatively the same.

Experimental Investigation of a Large Tilting-Pad Journal Bearing—Comparison of a Flooded and Non-Flooded Design

In tilting-pad journal bearings (TPJB), power loss corresponds to the internal friction in the shearing of the oil. Besides the lubrication gap, intermediate spaces between the pads account for a notable amount of frictional losses. Against the background of increasing demands for efficiency and sustainable use of resources, the reduction of power loss takes a key position in the further development of bearings. In our research, we compare two bearing lubrication concepts of a five-pad TPJB. Our objective is to work out the influence of different lubrication methods and bearing housing designs on the bearing operation characteristics. We conduct experimental testing of a 500 mm TPJB in two different bearing configurations with respect to the lubrication concept: an oil-flooded and non-flooded bearing design. In the flooded bearing design, oil is supplied via spray-bars and axial seals ensure the inter-pad spaces to be completely filled with oil. The non-flooded design comes without axial seals but oil drain channels to avoid oil accumulation in the bearing. In the latter design, oil is fed in via leading edge grooves (LEG). For the non-flooded bearing design, the experimental data show that the unloaded pads are not completely filled with oil and therefore, no pressure build-up occurs. The absence of additional load on the lower pads compared to the flooded design results in an increase of minimum film thickness. With the non-flooded design, power loss at high speeds is reduced to almost half. As a result, the efficiency of the entire turbomachinery application can be considerably improved.

Influence of Lubricant Film Cavitation on the Vibration Behavior of a Semifloating Ring Supported Turbocharger Rotor With Thrust Bearing

Abstract This contribution investigates the influence of outgassing processes on the vibration behavior of a hydrodynamic bearing supported turbocharger rotor. The examined rotor is supported radially by floating rings with outer squeeze-film damping and axially by thrust bearings. Due to the highly nonlinear bearing properties, the rotor can be excited via the lubricating film, which results in subsynchronous vibrations known as oil-whirl and oil-whip phenomena. A significant influence on the occurrence of oil-whip phenomena is attributed to the bearing stiffness and damping, which depend on the kinematic state of the supporting elements, the thermal condition, and the occurrence of outgassing processes. For modeling the bearing behavior, the Reynolds equation with mass-conserving cavitation regarding the two-phase model and the three-dimensional (3D) energy as well as heat conduction equation is solved. To evaluate the impact of cavitation, run-up simulations are carried out assuming a fully (half-Sommerfeld) or partially filled lubrication gap. The resulting rotor responses are compared with the shaft motion measurement. Also, the normalized eccentricity, the minimum lubricant fraction, and the thermal bearing condition are discussed.

Lubricated rolling over a pool

Experiments are conducted to explore the rolling of a cylinder over a pool of viscous fluid. The speed, width and loading of the cylinder are varied along with the initial depth and length of the viscous pool. Depending on the conditions, the cylinder will either ride on a lubrication film or remain in solid contact with the underlying substrate. For the former situation, a lubrication theory is presented that describes the pressure underneath the cylinder and the thickness of the film. The theory approximates the flow by the one-dimensional Reynolds equation with the addition of one term, with an adjustable parameter, to account for the flux of fluid to the cylinder sides. Once this parameter is calibrated against experiment, the theory predicts peak lubrication pressures, gap sizes and film thicknesses to within approximately ten per cent. For lubricated rolling, the film splits evenly between the cylinder and substrate downstream of the nip. The printer's instability arises during the splitting process, patterning the residual fluid films on the substrate and cylinder. If the pool length is less than the cylinder circumference, the fluid adhering to the cylinder is rotated back into contact with the substrate, and when there is sufficient adhered fluid a lubrication film forms that can again be modelled by the theory. Conversely, if there is insufficient adhered fluid, no contiguous lubrication film is formed; instead, the pattern from the printer's instability ‘prints’ from the cylinder to the substrate.

Deformations of a piezo‐actuated bearing ring in journal bearings

AbstractAn elastically deformable bearing ring is investigated. The piezo‐actuated shaping of the elastic bearing ring as well as the journal motion are coupled and determine the height of the lubrication gap which takes a crucial part in the theory of hydrodynamic lubrication. The influence of the active‐dynamic shaping of the bearing ring on the rotor dynamic behaviour has been investigated in [1], especially for a two‐lobe journal bearing. The present investigations are aiming at the active‐static shaping of a bearing ring, i.e. a finite number of piezo actuators is used to cause deformations of the bearing ring. Moreover, the realisation of different shapes by the introduced piezo actuators is examined, whereas the desired shapes are not necessarily limited to two‐lobe geometries.

Instability and self-propulsion of active droplets along a wall

In experiments, chemically active drops most often swim along a rigid wall, the impact of which on self-propulsion is in general completely overlooked in models. Using linear stability analysis, we demonstrate here that the proximity to a rigid surface promotes droplet propulsion as a result of the localization of the strongest interfacial flows within the thin lubrication gap separating the droplet from the wall.

Multiple frequency vibration of the micro lubricating gap geometry between cylinder block and valve plate in an axial piston pump

Multiple frequency vibration of the micro lubricating gap geometry between cylinder block and valve plate in an axial piston pump

Thermal elastohydrodynamic lubrication simulation model for the tooth guidance contact of a single tooth gearbox under mixed friction conditions

This article presents a simulation model for thermal elastohydrodynamic lubrication of the tooth guidance (piston/cylinder) contact of a single tooth gearbox. A finite-element analysis provides the boundary conditions for the thermal elastohydrodynamic lubrication simulation. The finite-element analysis under dry conditions allows calculating the macroscopic deformed contact surfaces for the elastohydrodynamic simulation model. The Hertzian deformation should be calculated in the presence of a lubricant and not with a dry finite-element analysis. A recently presented method, based on the finite-element submodeling technique, is extended to remove the Hertzian deformation of the macroscopic deformed lubrication gap from the dry finite-element analysis. A mixed friction with measured rough surfaces using the averaged flow model and mass conserving cavitation is implemented as well. An example shows how optimization of the gearbox by application of the simulation model is possible.