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Research Papers: Applications

J. Tribol. 2018;141(3):031101-031101-8. doi:10.1115/1.4041670.

Charged particles are emitted when materials undergo tribological interactions, plastic deformation, and failure. In machining, plastic deformation and shearing of work piece material takes place continuously along with intense tool-chip rubbing contact interactions; hence, the emission of charged particles can be expected. In this work, an in-situ sensor has been developed to capture the emitted positive (positive ion) and negative (electron and negative ion) charged particles in real-time in an orthogonal machining process at atmospheric conditions without the use of coolant. The sensor consists of a Faraday plate, mounted on the flank face of the cutting tool, to collect the emitted ions and the intensity of emissions is measured with an electrometer. Positively and negatively charged particles are measured separately by providing suitable bias voltage supply to the Faraday plate. Ion emissions are measured during machining of three different work piece materials (mild steel, copper, and stainless steel) using a carbide cutting tool. The experimental results show a strong correlation between the emission intensity and the variation in machining parameters and material properties. Increasing material removal rate increases the intensity of charged particle emissions because of the increase in volume of material undergoing shear, fracture, and deformation. It is found that emission intensity is directly proportional to the resistivity and strength of workpiece material. Charged particles emission intensity is found to be sensitive to the machining conditions which enables the use of this sensor as an alternate method of condition monitoring.

Commentary by Dr. Valentin Fuster

Research Papers: Coatings and Solid Lubricants

J. Tribol. 2018;141(3):031301-031301-10. doi:10.1115/1.4041762.

The main objective of this investigation is the evaluation of the performance of hard diamond-like-carbon (DLC) or tungsten carbide (WC) and soft (epoxy composite) dual-coatings on the internal combustion (IC) engine piston rings as a protective coating to reduce their wear. The rings were coated with DLC or WC by physical vapor deposition (PVD) method and then soft polymeric composite coating (epoxy/graphene/base oil SN150) was applied on the hard coating. The tribological tests of the dual-coated piston rings were conducted for 3.6 × 105 cycles at 1500 rpm engine speed and 50% rated load of a diesel engine in order to evaluate the wear performance of the piston rings. Scuffing of cylinder liner and piston rings interface was prevented by the application of polymer composites over the hard-coated rings. DLC hard and soft polymer composite dual coating over the top piston ring was found to have the lowest wear rate 1.69 × 10−12 mm3/N·m compared with the wear rate of dual coatings on the middle and lower rings.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(3):031302-031302-7. doi:10.1115/1.4042075.

The synergistic effect of solid lubricants plays a significant role in wide-temperature-range lubrication, where the combination of lubricious oxide and Ag is the promising solid lubricants. In this paper, the friction and wear performances of Ni3Al with the addition of Ag and V2O5 solid-lubricating composites were evaluated from room temperature to 1000 °C. It was found that Ni3Al matrix composite with the addition of V2O5 has high friction coefficient of 0.3–0.7, while Ni3Al matrix composite with simultaneous addition of Ag and 2 wt % V2O5 has a relatively low friction coefficient of 0.25–0.4 between room temperature and 1000 °C and wear rate with the magnitude of 10−5 mm3/N m at high temperatures. The results revealed that nickel aluminum matrix solid-lubricating composite lubricated by silver and in situ formed silver vanadate at elevated temperature achieves a wide-temperature-range lubrication, which is attributed to the synergistic action of silver and silver vanadate formed at high temperatures.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2018;141(3):031401-031401-11. doi:10.1115/1.4041535.

This work employs a three-dimensional (3D) finite element analysis (FEA) to investigate the fretting metallic contact between a deformable hemisphere and a deformable flat block. Fretting is governed by displacement-controlled action where the materials of the two contacting bodies are set to have identical properties; studied first is steel-on-steel and then copper-on-copper. At contact onset, a normal interference (indentation) is applied, which is then followed by transverse cyclic oscillations. A large range of coefficients of friction (COFs) is imposed at the interface. The results show that the maximum von Mises stress is confined under the contacting surface for small COFs; however, that maximum reaches the contacting surface when the COFs are sufficiently large. It is also shown that fretting under sufficiently large COFs forms large plastic strains in “ring” like patterns at the contacting surfaces. Junction growth is found where the contacting region is being stretched in the direction of the fretting motion. At large COFs, pileups show up at the edges of the contact. The fretting loops of the initial cycles are found along with the total work invested into the system. At certain interference, there exists a certain COF, which results in the largest work consumption. The magnitude of the COF is found to produce either partial slip (prone for fretting fatigue) or gross slip (prone for fretting wear). A scheme of normalization is proposed, and it is shown to be effective for the two said materials that have vastly different material properties. Hence, the normalized results may well characterize a range of contact scales (from micro to macro) of various ductile material pairs that behave in an elastic–plastic manner with strain hardening.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031402-031402-11. doi:10.1115/1.4041537.

For elastic contact, an exact analytical solution for the stresses and strains within two contacting bodies has been known since the 1880s. Despite this, there is no similar solution for elastic–plastic contact due to the integral nature of plastic deformations, and the few models that do exist develop approximate solutions for the elastic–perfectly plastic material model. In this work, the full transition from elastic–perfectly plastic to elastic materials in contact is studied using a bilinear material model in a finite element environment for a frictionless dry flattening contact. Even though the contact is considered flattening, elastic deformations are allowed to happen on the flat. The real contact radius is found to converge to the elastic contact limit at a tangent modulus of elasticity around 20%. For the contact force, the results show a different trend in which there is a continual variation in forces across the entire range of material models studied. A new formulation has been developed based on the finite element results to predict the deformations, real contact area, and contact force. A second approach has been introduced to calculate the contact force based on the approximation of the Hertzian solution for the elastic deformations on the flat. The proposed formulation is verified for five different materials sets.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2018;141(3):031501-031501-11. doi:10.1115/1.4041590.

Friction is usually induced when the contacts are in relative motion, leading to mechanical vibration and consequently heat generation. The reduction of these undesirable parameters is possible by the application of greases, which intends to increase the service life of the bearings. The present work incorporates the frictional and vibration behaviors of concentrated point contact lubricated with bare and nanocomposite greases. The nanocomposite greases were formulated by dispersing different categories of nano-additives like reduced graphene oxide (rGO), calcium carbonate (CaCO3), and alumina (α-Al2O3) in bare grease (BG). The formulated nanocomposite greases are tested for film formation, frictional and vibrational response under a limited supply of greases. The use of transparent glass disk better analyses the profile of film thickness to understand the lubrication mechanism of the point contact. The microstructure of nano-additives and the formulated nanocomposite greases were characterized using high-resolution transmission electron microscopy (HRTEM). The presence of different functional groups in nano-additives and the formulated nanocomposite greases were characterized using Raman spectroscopy. The tribological contact operates under 3% and 30% slide-roll-ratio (SRR) for varying rolling speed (0.001–1 m/s) at a load of 30 N (Hertzian pressure, pH = 0.9 GPa). Film thickness, friction and vibration behavior were recorded to focus the tribo-performance, degree of starvation and dynamics of the tribological contact with slip varying from 3% to 30% SRR. The vibration level was refined to 32% with the addition of rGO nanosheets in BG. The incompatibility of α-Al2O3 with the grease structure results in disruption of tribo-dynamics behavior of the point contact.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031502-031502-11. doi:10.1115/1.4041733.

In order to adapt to increasingly stringent CO2 regulations, the automotive industry must develop and evaluate low cost, low emission solutions in the powertrain technology. This often implies increased power density and the use of low viscosity oils, leading to additional challenges related to the durability of various machine elements. Therefore, an increased understanding of lubricated contacts becomes important where oil viscosity–pressure and compressibility–pressure behavior have been shown to influence the film thickness and pressure distribution in elastohydrodynamic lubrication (EHL) contacts, further influencing the durability. In this work, a finite line EHL contact is analyzed with focus on the oil compressibility–pressure and viscosity–pressure response, comparing two oils with relatively different behavior and its influence on subsurface stress concentrations in the contacting bodies. Results indicate that increased pressure gradients and pressure spikes, and therefore increased localized stress concentrations, can be expected for stiffer, less compressible oils, which under transient loading conditions not only affect the outlet but also the edges of the roller.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2018;141(3):031601-031601-9. doi:10.1115/1.4041536.

The mechanical properties of crystalline phase of glass ceramics are critical. This study aimed to evaluate wear resistance of different crystalline-reinforced dental chairside computer-aided design/computer-aided manufacturing (CAD/CAM) glass ceramics. Materials of feldspar (Vita Mark II, VM), leucite (IPS Empress CAD, EC), lithium disilicate (IPS e.max CAD, EX), lithium disilicate enriched with zirconia (Vita Suprinity, VS), and enamel were embedded, grounded, and polished, respectively. Samples were indented with a Vickers hardness tester to test the fracture resistance (KIC). Two-body wear tests were performed in a reciprocal ball-on-flat configuration under artificial saliva. The parameters of load force (50 N), reciprocating amplitude (500 μm), frequency (2 Hz), and the test cycle (10,000 cycles) were selected. Specimen microstructure, indentation morphology, and wear scars were observed by scanning electron microscope (SEM), optical microscopy, and three-dimensional profile microscopy. EX, VS, and EC demonstrated significantly higher KIC values than the enamel, while ceramic materials showed smaller wear depth results. Cracks, massive delamination, and shallow plow were seen on the enamel worn scar. Long deep plow, delamination, and brittle cracks are more common for VM and EC, and short shallow plow and smooth subsurface are the characteristics of EX and VS. Greater fracture toughness values indicated higher wear resistances of the materials for the test glass ceramics. The CAD/CAM glass ceramics performed greater wear resistance than enamel. Feldspar- and leucite-reinforced glass ceramics illustrated better wear resistance similar to enamel than lithium disilicate glass ceramics, providing amicable matching with the opposite teeth.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031602-031602-6. doi:10.1115/1.4041644.

The interaction observed between two surfaces in contact with one another is part of a number of physical processes, such as wear. In this paper, we present a numerical study of the asperities between two surfaces in contact with each other. The real contact area between two surfaces varies due to the multiple roughness scales caused by the stochastic nature of asperities. In our research, we employ a tribological system comprising two partitions: C1 is the contact set (CS), where the two surfaces are in direct contact with each other, and C2 is the noncontact set, where the two surfaces are not in contact with each other. Here, we have developed a new numerical model to describe the CS using ε-entropy to prove the existence of a minimum value for entropy in sliding contact scenarios. In this system, the lower and upper bounds of entropy are determined through the Kolmogorov approach using the aforementioned model. Using this model, we conclude that the ε-entropy value is bound between ln 2 and 2·ln 2 for a tribological system comprising two partitions. Additionally, we conclude that a correlation between the stochastic tribological contact behavior and the rate of entropy change is the key parameter in thermal nonequilibrium scenarios.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031603-031603-13. doi:10.1115/1.4041587.

A new wear prediction method of tooth surfaces of involute gears based on a real tooth surface model and a modified fractal method is developed. The real tooth surface model of an involute gear pair is introduced, and microgeometry feature detection of tooth surfaces is achieved by monitoring variations of normal vectors of each discrete data point of the real tooth surface model. To predict wear progression of tooth surfaces of a gear pair, an abrasive wear analysis model and the modified fractal method are used to analyze contact performance and its changes with accumulation of surface wear. The abrasive wear analysis model can analyze wear depths of gear tooth surfaces with sliding distances, local contact pressure, and directions of wear progression based on Archard's model. The modified fractal method is proposed to calculate instantaneous contact stiffness and estimate elastic and plastic deformation regions based on an asperity contact model. Microgeometry features of tooth surface asperities can be described as the basis of an asperity contact model and allow tooth contact analysis of real tooth surface models with their local microgeometry feature changes due to plastic deformations. Feasibility and effectiveness of this wear prediction method were verified by comparing predicted results of gear surface wear progression with gear wear test results.

Topics: Wear , Stress , Gears , Fractals
Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031604-031604-14. doi:10.1115/1.4041764.

The present work was primarily conducted to study the wear behavior of as-received and severely deformed Al-15%Mg2Si in situ composites. The severe plastic deformation was applied using accumulative back extrusion (ABE) technique (one and three passes). The continuous dynamic recrystallization (CDRX) was recognized as the main strain accommodation and grain refinement mechanism within aluminum matrix during ABE cycles. To investigate the wear properties of the processed material, the dry sliding wear tests were carried out on both the as-received and processed samples under normal load of 10 and 20 N at room temperature, 100 °C, and 200 °C. The results indicated a better wear resistance of processed specimens in comparison to the as-received ones at room temperature. In addition, the wear performance was improved as the ABE pass numbers increased. These were related to the presence of oxide tribolayer. At 100 °C, the as-received material exhibited a better wear performance compared to the processed material; this was attributed to the formation of a work-hardened layer on the worn surface. At 200 °C, both the as-received and processed composites experienced a severe wear condition. In general, elevating the temperature changed the dominant wear mechanism from oxidation and delamination at room temperature to severe adhesion and plastic deformation at 200 °C.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031605-031605-9. doi:10.1115/1.4042001.

The effects of thermal oxidation of a biomedical titanium alloy (Ti–6Al–7Nb) on its morphology, structure, mechanical properties, and sliding friction and wear against alumina were investigated. It was found that at 600 °C, the surface of the alloy was characterized with a thin inhomogeneous oxide scale. Increasing the temperature of oxidation to 700 °C and 800 °C allowed obtaining homogeneous layers, which fully covered the examined surfaces. By contrast, the oxide scale obtained at 800 °C was composed of big oxide particles with a developed surface. Thermal oxidation process allows a fourfold increase in the hardness of Ti–6Al–7Nb alloy. It was shown that the oxide scale on the examined alloy efficiently enhances its resistance to sliding wear against alumina.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031606-031606-10. doi:10.1115/1.4041894.

The aim of the study was to investigate the friction and wear phenomena of 3 mol % yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) ceramics with the inclusion of copper oxide (CuO) in large area conformal contact geometry. The pin-on-disk tribometer was used to conduct the dry sliding test using CuO/3Y-TZP as pin and alumina as counter surface. The coefficient of friction (μ) for CuO-added 3Y-TZP was decreased by ∼38% compared to pure 3Y-TZP due to formation of protective tribo film to the substrate. In addition, the experiments also showed that the specific wear rate (k) was reduced by ∼54% with the inclusion of CuO in to 3Y-TZP matrix. The different phases of the zirconia, copper, and yttria as well as the phase transformation before and after sliding test were identified by X-ray diffraction (XRD) analysis. Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDS) analysis revealed the existence of CuO in the patchy layers in the worn-out surface of the tested CuO/3Y-TZP sample leading to lower coefficient of friction and improve the wear resistance against alumina counterface in conformal contact geometry. Severe wear mechanism was the dominating factor due to the local plastic deformation of the large number of asperities since the pair of contact was conformal.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2018;141(3):031701-031701-9. doi:10.1115/1.4041460.

A rotor supported by gas bearings vibrates within the clearance. If the static imbalance of the rotor is large, even if the rotation speed is low, large amplitude vibration is generated by the centrifugal force. This is a serious problem because the risk of bearing damage increases. In order to solve this problem, an externally pressurized gas journal bearing with asymmetrically arranged gas supply holes has been developed. This type of bearing has a large load capacity as compared with the conventional symmetric gas supply bearing because pressurized gases are supplied to the loaded and counter-loaded side bearing surfaces via asymmetrically arranged gas supply holes. The bearing has a new gas supply mechanism in which gas is supplied from the rotor through inherent orifices. The characteristics of the developed bearing are beneficial from the viewpoint of using the bearing in rotational-type vibration exciters. In other words, this rotor has a large static imbalance. Numerical calculations of the characteristics of this bearing were performed, and the resulting characteristics were compared with those of a conventional symmetric gas supply journal bearing. The bearing load capacity of the developed bearing is considerably larger than that of conventional symmetric type bearings. The load capacity increases owing to the asymmetry of the gas supply holes. In the controlled gas supply pressure condition, rotor radial vibration during rotation can theoretically be zero. A test rig and gas control system to realize vibration reduction was constructed. A rotational test under the gas pressure control condition was conducted using a large unbalanced rotor taking advantage of this property. The control program was constructed using matlab and simulink. The devices were driven by a digital signal processor. The magnitude of the unbalance of the rotor is 13.5 × 10−3 kg m. The bearing diameter and length were 60 and 120 mm, respectively. The rotational vibration amplitude decreased at a high rotational frequency under the proposed bearing configuration, although the amplitude increases monotonically with the frequency in the conventional bearing. When the gas supply pressure was controlled synchronously with the rotation frequency modulation of the large unbalanced rotor, the amplitude of the vibration amplitude was greatly reduced. The rotor of the test rig was safely supported by this bearing, and effective data for practical operation were obtained.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031702-031702-9. doi:10.1115/1.4041511.

The ultrasonic technique is very effective in measuring lubricant film thickness in a noninvasive manner. To estimate the film thickness with reflection signals, two main ultrasonic models are often applied in cases of different film thicknesses; they are the spring model for thin films and the resonant model for thick films. However, when measuring oil film thicknesses distributed in a wide range, there is an inherent blind zone between these two models. This problem is especially prominent in online monitoring because the abrupt variation of film thickness is highly correlated with the occurrence of abnormal conditions. To address this issue, we further proposed a method using the phase spectrum of reflection coefficient which can cover a wide range of film thicknesses. The slight variation of reflection signal in the blind zone can then be identified and bridged the measurement gap between those two traditional models. A calibration rig was used to verify the theoretical analysis and the results indicated that the developed model is capable of providing reliable ultrasonic measurement of lubricant film thicknesses in a wide range.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031703-031703-12. doi:10.1115/1.4041720.

Energy efficient operation of fluid film bearings demands savings in delivery flow while also managing to reduce the temperature in the fluid film and bearing pads. To achieve this goal, tilting pad journal bearings (TPJBs) implement a variety of oil feed arrangements, use pads with highly conductive material and engineered back surface, and also end seals to keep (churning) lubricant within the bearing housing. This paper introduces a novel model for the mixing of flow and thermal energy transport at a lubricant feed port and which sets the temperature of the lubricant entering a pad leading edge. Precise estimation of this temperature (and inlet oil viscosity) largely determine the temperature, and the current model aids to deliver improved temperature predictions in conditions that limit a conventional model, including the ability to impose an actual lubricant supplied flow, specifically when the bearing is operating in either an over-flooded or a reduced flow conditions. An empirical groove efficiency parameter regulates the temperature of the above-mentioned flows to represent conventional and direct lubricant feeding arrangements as well as end-sealed (flooded) or evacuated bearing configurations. Predicted temperatures are compared against published test data for two bearings while revealing the advantages of the novel model, in particular for operation under lubricant starvation. This paper delivers recommendations for the feed port efficiency parameter for various types of oil supply configurations. This parameter, not being a function of bearing operating conditions, allows for proper sizing of pumps, oil reservoirs, and heat exchangers in lube oil delivery systems for a packaged-unit machine.

Commentary by Dr. Valentin Fuster

Research Papers: Lubricants

J. Tribol. 2018;141(3):031801-031801-6. doi:10.1115/1.4041538.

Grease plays important roles in reducing frictional loss and providing protection of rubbing surfaces. In this research, we investigated the effects of α-zirconium phosphate nanoparticles as additives in grease on the galling behavior of a pair of steels (4130 against P530). The results showed that the addition of 0.5 wt% of nanoparticles in petroleum jelly could reduce the friction for 10% and the area being galled for 80%. In terms of particle sizes, the 1 μm sized particles have profound influence in galling reduction. This is due to the increased contribution of van der Waals forces in the stacked layers of those particles. Under shear, those particles are exfoliated, resulting in low friction and more surface coverage to protect surfaces from galling.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2018;141(3):032201-032201-12. doi:10.1115/1.4041461.

A drawback of polymer materials is their low thermal conductivity which affects the operating temperature of polymer gears. The mechanical properties of a polymer gear are critically dependent on the maximum operating temperature. In order to improve thermal behavior of polymer gears, a hybrid polymer gear concept is suggested which consists of a polymer gear tooth with a metallic insert to promote heat evacuation from the meshing surface. The material selection based on finite element method (FEM) simulations showed that an aluminum insert performed better than copper and steel for a hybrid polymer gear. The results show that an aluminum insert increases the mass by 9% in comparison with a standard polymer gear but it decreases the maximum operating temperature by 28%. Insert geometries of different complexity were studied and their influence on operating temperature assessed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):032202-032202-7. doi:10.1115/1.4041763.

In this work, layered ternary Cr2AlC powders with high purity and a size of 0.5–1 μm were synthesized by solid-state reaction method. NiCr–Cr2AlC composites have been prepared by spark plasma sintering (SPS) process. The composites' tribological properties were evaluated against alumina ball under dry sliding condition from room temperature to 600 °C. Compared with unmodified NiCr alloy, Cr2AlC addition has an effect on reduction of friction coefficient of NiCr–Cr2AlC composites at the temperatures up to 400 °C. Especially, in comparison with NiCr alloy, the wear rates of NiCr–Cr2AlC composites significantly decrease from 10−4 mm3/(N·m) to 10−5–10−6 mm3/(N·m) from room temperature to 600 °C (except for 200 °C). The NiCr–20 wt % Cr2AlC composite exhibited excellent tribological properties with a friction coefficient of 0.3–0.4 and a wear rate of about 10−6 mm3/(N·m) from 400 °C to 600 °C. Through the analysis of scanning electron microscope (SEM) and X-ray photoelectron spectroscope (XPS), it is clarified that effective improvement of tribological properties of NiCr–Cr2AlC composites is attributed to a glaze layer consisting of NiO, Cr2O3, Al2O3, and NiCr2O4, which is formed by tribo-oxidation during wear process.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):032203-032203-8. doi:10.1115/1.4041719.

Hydraulic turbines and centrifugal pumps at times show low frequency vibrations when installed with upstream step (band) clearance seals with a narrow clearance facing the incoming external flow. When implementing a downstream step clearance seal, one with the narrow clearance located at the seal exit, the same machine does not show the same problem. This paper presents both theoretical and experimental analysis on the leakage and dynamic force coefficients of both upstream and downstream step clearance seals. The predicted and measured results show that an upstream step clearance seal produces a significant negative direct stiffness (K < 0) that could cause a static instability. On the other hand, a downstream step clearance seal generates a positive direct stiffness (K > 0) that is beneficial to a rotor system. Both the upstream and downstream step clearance seals show positive direct damping and virtual mass coefficients.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):032204-032204-13. doi:10.1115/1.4041895.

This paper studies the stiffness characteristics of preloaded duplex angular contact ball bearings. First, a five degrees-of-freedom (5DOF) quasi-static model of the preloaded duplex angular contact ball bearing is established based on the Jones bearing model. Three bearing configurations (face-to-face, back-to-back, and tandem arrangements) and two preload mechanisms (constant pressure preload and fixed position preload) are included in the proposed model. Subsequently, the five-dimensional stiffness matrix of the preloaded duplex angular contact ball bearing is derived analytically. Then, an experimental setup is developed to measure the radial stiffness and the angular stiffness of duplex angular contact ball bearings. The simulated results match well with those from experiments, which prove the validity of the proposed model. Finally, the effects of bearing configuration, preload mechanism, and unloaded contact angle on the angular stiffness and the cross-coupling are studied systematically.

Commentary by Dr. Valentin Fuster

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