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The nonlinear friction disturbance of the moving joint surface of the feed system can lead to the residual vibration of the system, prolong the stability time of the system and reduce the motion precision and machining precision of the machine tool. This paper aims to concern the vibration between joint surfaces caused by nonlinear friction.

The model is established from the micro and macro scale based on the LuGre model. The friction characteristics of the moving joint surface are explored. The friction experiment of GCr15 pin and 45 steel disk is designed and the influence of lubrication condition, speed, acceleration and normal load on friction characteristics are studied.

Among the drive speed, damping and stiffness, the negative gradient effect of friction, which is characterized by the difference of static and dynamic friction coefficient Δµ, is the main cause of friction vibration between moving joint surfaces. Sufficient lubrication, a proper increase of speed and acceleration, a reasonable reduction of normal load can reduce the negative gradient effect, which can weaken the vibration caused by the nonlinear friction and improve the friction characteristics of the moving joint surface.

In the past studies, more attention has been paid to revealing the relationship between the relative speed and friction, while the acceleration is often ignored. The negative gradient effect of friction is improved in this paper by changing the contact conditions. Research findings of this paper effectively improve the friction characteristics of the moving interface and provide the basis for restraining the nonlinear vibration between the moving interfaces.

The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-11-2019-0476/

This study aims to research the influence mechanism of microtextured geometric parameters of dry gas seal end face on the tribological behavior under dry frictional conditions.

The microtexture was processed using laser processing, while the diamond-like carbon (DLC) film was applied through magnetron sputtering; the experimental platform of friction vibration was established, the frictional and vibrational properties of different geometric parameters were tested; the data signals of vibrational acceleration and frictional torque were collected and processed using data acquisition instrument. The entropy characteristic parameters of 3D vibrational acceleration were extracted based on wavelet packet decomposition method. The end-face topography was measured with ST400 three-dimensional noncontact surface topography instrument.

The geometry of pits plays a key role in influencing friction performance; the permutation entropy and fuzzy entropy of the vibration acceleration signal changed with variations in microtextured parameters. A textured surface with appropriately size parameters can trap debris, enhance the dynamic pressure effect, reduce impact between the friction interfaces and improve the frictional vibrational performance. In this research, microtextured surface with Φ150 µm-10% and Φ200 µm-5% can effectively reduce friction and vibration between the end faces of a dry gas seal.

DLC film improves the hardness of seal ring end face, and microtexture improves the dynamic effect; the tribological behavior monitoring can be realized by analyzing the characteristics of vibration acceleration sensitive parameter with friction state. The findings will provide a basis for further research in the field of tribology and the microtexture optimization of dry gas seal ring end face.

The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-12-2023-0389/

The purpose of this study is to establish an effective method for characterizing the tribological properties of friction brakes during continuous braking because they have direct influences on the reliable operation of transport vehicles and industrial equipments.

First, tribological tests were carried out with the X-DM type friction tester, and changing curves of friction coefficient and temperature were obtained. Second, a novel tribological characteristic parameter set characterizing the tribological properties of brake pair in continuous braking was extracted from some important experimental data such as friction coefficient, wear rate and temperature. Finally, the influence of law and mechanism of braking number on dynamic tribological parameters was studied through continuous braking experiments.

The extracted tribological characteristic parameter set includes two subsets: dynamic characteristic parameter subset and overall characteristic parameter subset, which is composed of ten parameters: dynamic parameters of friction coefficient (including average, trend coefficient and stability coefficient), dynamic wear rate, dynamic average temperature, dynamic temperature rise, overall average friction coefficient, overall wear rate, overall average temperature and overall temperature rise.

Conclusively, the novel tribological characteristic parameter set is more comprehensive and objective, and it can provide a theoretical basis for the study of tribological properties in continuous braking.

Bearing friction moments are important factors that affect the vibrations of rotor systems. The bearing friction moments are related to the dimension parameters, lubrication conditions and manufacturing errors of support bearings. This work studies the effects of the bearing friction moments on the vibrations of rotor systems.

The rotor is separated into several shaft elements for formulating a flexible rotor. The time-varying friction moment (TFM) is affected by the time-varying contact loads. The vibrations of FRS from the TFM and Palmgren's friction moment (PFM) calculation methods are compared. Moreover, the effects of the rotor offset and radial clearance on the frequency-amplitude characteristics of FRS are studied.

The TFM method is more consistent with the actual operation mechanisms. The rotor offset and radial clearance can significantly affect the nonlinear vibrations of FRS. This work provides a new reference and research method for the vibration analysis of rotor systems considering the friction effects.

The elastohydrodynamic lubrication (EHL), elastic hysteresis and differential sliding are considered. A flexible rotor system (FRS) dynamic model considering the TFM is proposed. The vibrations of FRS from the TFM calculation method and empirical calculation formula are compared. The effects of the rotor offset and radial clearance on the frequency–amplitude characteristics of FRS are studied.

A free-piston engine (FPE) is an unconventional engine that abandons the crank system. This paper aims to focus on a numerical simulation for the lubricating characteristics of piston rings in a single-piston hydraulic free-piston engine (HFPE).

A time-based numerical simulation program was built using Matlab to define the piston motion of the new engine. And a lubrication mode of piston rings was built which is based on the gas flow equation, hydrodynamic lubrication equation and the asperity contact equation. The piston motion and the lubrication model are coupled, and then the finite difference method is used to obtain the piston rings lubrication performances of the FPE. Meanwhile, the lubrication characteristics of the new engine were compared with those of a corresponding conventional crankshaft-driven engine.

The study results indicate that compared with the traditional engine, the expansion stroke of the HFPE is longer, and the compression stroke is shorter. Lubrication oil film of the new engine is thicker than the traditional engine during the initial stage of compression stroke and the final stage of the power stroke. The average friction force and power of the hydraulic free piston engine are slightly lower than those of the traditional engine, but the peak friction power of the FPE is significantly greater than that of the traditional engine. With an increase in load, the friction loss power and friction loss efficiency decrease, and with a decrease in equivalence ratio, the friction power loss reduces, but the friction loss efficiency decreases first and then increases.

In this paper, only qualitative analysis was performed on the tribological difference between conventional crankshaft engine and HFPE, instead of a quantitative one.

This paper contributes to the tribological design method of HFPE.

No social implications are available now, as the HFPE is under the development phase. However, the authors are positive that their work will be commercialized in the near future.

The main originality of the paper can be introduced as follows: the lubrication and friction characteristics of the new engine (HFPE) were investigated and revealed, which have not been studied before; the effect of the HFPE’s special piston motion on the tribological characteristics was considered in the lubrication simulation. The results show that compared with the traditional crankshaft engine, the new engine shows a different lubrication performance because of its free piston motion.

The purpose of this paper is to use a combination of numerical simulation and experiment to evaluate the performance of laser surface texturing (LST) in the field of gear lubrication, and to more accurately predict the lubrication characteristics of different surfaces.

The method used in this paper is developed on the basis of the deterministic solution of the three-dimensional (3D) mixed elasto-hydrodynamic lubrication (EHL) model and the model parameters are corrected by friction test. The film pressure, film thickness and friction coefficient of different micro-textured tooth surfaces are predicted on the basis of accurate 3D mixed EHL models.

The results demonstrate that the micro-texture structure of the tooth surface can increase the local film thickness and enhance the lubricating performance of the tooth surface without drastically reducing the contact fatigue life. The stress distribution and friction characteristics of the tooth surface can be optimized by adjusting the micro-texture arrangement and the size of the micro-textures.

A new evaluation method using a 3D hybrid EHL model and friction test to predict the lubrication characteristics of LST is proposed, which can effectively improve the processing economy and save time.

The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-11-2020-0423

Granular lubrication is a new lubrication method and can be used in extreme working conditions; however, the obstacle of force transmission characteristics needs to be urgently solved to fully understand the mechanical and bearing mechanisms of granular lubrication.

A flat sliding friction cell is developed to study the force transmission behaviors of granules under shearing. Granular material, sliding velocity, granule size and granule humidity are considered in these experiments. The measured normal and shear force, which is transmitted from the bottom friction pair to the top friction pair via the granular lubrication medium, reveals the influence of these controlling parameters on the force transmission characteristics of granules.

Experimental results show that a low sliding velocity, a large granule size and a low granular humidity increase the measured normal force and shear force. Besides, a comparison experiment with other typical lubrication styles is also carried out. The force transmission under granular lubrication is mainly dependent on the force transmission path, which is closely related to the deconstruction and reconstruction of the force chains in the granule assembly.

These findings reveal the force transmission mechanism of granular lubrication and can also offer the helpful reference for the design of the new granular lubrication bearing.

The purpose of this paper is to numerically investigate the flow characteristics and extend the data of friction factor and Reynolds number product of hydrodynamically developing laminar flow in three-dimensional rectangular microchannels with different aspect ratios.

Using a finite-volume approach, the friction factor characteristics of Newtonian fluid in three-dimensional rectangular ducts with aspect ratios from 0.1 to 1 are conducted numerically under no-slip boundary conditions. A simple model that approximately predicts the apparent friction factor and Reynolds number product f_appRe is referenced as a semi-theoretical fundamental analysis for numerical simulations.

The accurate and reliable results of f_appRe are obtained, which are compared with classic numerical data and experimental data, and the simple semi-theoretical model used and all comparisons show good agreement. Among them, the maximum relative error with the classic numerical data is less than 3.9 per cent. The data of f_appRe are significantly extended to other different aspect ratios and the novel values of f_appRe are presented in the tables. The characteristics of f_appRe are analyzed as a function of a non-dimensional axial distance and the aspect ratios. A more effective and accurate fourth-order fitting equation for the Hagenbach's factor of rectangular channels is proposed.

From the reliable data, it is shown that the values of f_appRe and the model can be references of pressure drop and friction factor for developing laminar flow in rectangular channels for researchers and engineering applications.

The serious friction caused by the fluctuation of friction occurs when start-up and will reduce the positioning accuracy of the servo axes of high precision machine tools, the purpose of this paper is to study the friction fluctuation characteristics of friction coefficients between interfaces under different working conditions.

HT200 and 45# materials were experimentally studied by friction and wear testing machine UMT-3, the variation of friction coefficient under different working conditions (different start-up conditions, the variation of lubrication state area and different roughness) were measured.

The results show that the larger start-up acceleration shortens the pre-sliding time of the interface friction, makes the friction coefficient decrease faster, reduces the mixed lubrication area of the contact surface and makes the contact surface reach the stable lubrication state quickly. It can be concluded that the larger roughness surface will lead to the larger mixed lubrication area, the larger static friction coefficient and the larger drop between static and dynamic friction coefficient and easy to cause friction vibration.

The results reveal the friction fluctuation rule of the metal interface during the different start-up process, which is of guiding significance to reveal the lubrication principle and mechanism of the mechanical interface.

The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-11-2019-0482/

The sensorless external force estimation of robot manipulator can be helpful for reducing the cost and complexity of the robot system. However, the complex friction phenomenon of the robot joint and uncertainty of robot model and signal noise significantly decrease the estimation accuracy. This study aims to investigate the friction modeling and the noise rejection of the external force estimation.

A LuGre-linear-hybrid (LuGre-L) friction model that combines the dynamic friction characteristics of the robot joint and static friction of the drive motor is proposed to improve the modeling accuracy of robot friction. The square root cubature Kalman filter (SCKF) is improved by integrating a Sage Window outer layer and a nonlinear disturbance observer (NDOB) inner layer. In the outer layer, Sage Window is integrated in the square root Kalman filter (W-SCKF) to dynamically adjust noise statistics. NDOB is applied as the inner layer of W-SCKF (NDOB-WSCKF) to obtain the uncertain state variables of the state model.

A peg-in-hole contact experiment conducted on a real robot demonstrates that the average accuracy of the estimated joint torque based on LuGre-L is improved by 4.9% in contrast to the LuGre model. Based on the proposed NDOB-WSCKF, the average estimation accuracy of the external joint torque can reach up to 92.1%, which is improved by 4%–15.3% in contrast to other estimation methods (SCKF and NDOB).

A LuGre-L friction model is proposed to handle the coupling of static and dynamic friction characteristics for the robot manipulator. An improved SCKF is applied to estimate the external force of the robot manipulator. To improve the noise rejection ability of the estimation method and make it more resistant to unmodeled state variable, SCKF is improved by integrating a Sage Window and NDOB, and a NDOB-WSCKF external force estimator is developed. Validation results demonstrate that the accuracy of the robot dynamics model and the estimated external force is improved by the proposed method.