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Research Papers: Applications

J. Tribol. 2018;140(5):051101-051101-10. doi:10.1115/1.4039566.

The varying compliance (VC) vibration directly reflects the oscillation intensity of a rolling bearing, and it can be fully revealed in the VC resonance. Moreover, we define the bearing vibration intensity as the bearing vibration information in this paper. Besides the rolling element number of the bearing, the rotor eccentricity is also an inevitable influencing factor for the VC vibration. This paper focuses on the VC resonance characteristics in a ball bearing rotor system. An analytical model is established, and the vibration responses of the system are calculated in a large speed range with the consideration of different ball numbers and different rotor eccentricities. The theoretical results show that the VC vibration is clearer in low-speed range where the VC resonance exist, while it is suppressed in high-speed range. In general, the intensity of the VC resonance decreases with the increase of ball numbers and is not sensible to the rotor eccentricities in low-speed range. Finally, a ball bearing rotor experiment system is setup, the VC resonance is clearly detected, and the high-quality bearing vibration information is obtained. The experimental results qualitatively agree with the theoretical results.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051103-051103-11. doi:10.1115/1.4039929.

An experimental investigation of spur gear behavior was conducted with the aim of quantifying the impact of lubrication methods and conditions on the power losses and contact fatigue lives. Variations of dip and jet-lubrication are defined, and these behaviors were observed as a function of the lubrication conditions. Both types of measurements were performed using the same type of back-to-back test machines and the same spur gear test articles such that their evaluations can be correlated. Power loss experiments were performed under both loaded and unloaded conditions to determine both load-independent (spin) and load-dependent (mechanical) losses. Sets of long-cycle contact fatigue experiments were performed under the same lubrication conditions to determine macropitting lives in a statistically meaningful manner. Results indicate that the spin power losses are impacted by the lubrication method significantly while the mechanical losses are not influenced. Contact fatigue lives from jet-lubricated tests are comparable to those under dip-lubricated conditions ones as long as jet velocities are sufficient.

Commentary by Dr. Valentin Fuster

Research Papers: Coatings and Solid Lubricants

J. Tribol. 2018;140(5):051303-051303-10. doi:10.1115/1.4039796.

The graphite-MoS2 coated on GCr15 bearing steel is prepared through air spraying and its tribological performances are investigated experimentally. Then its coefficient of friction (COF) and wear scar width (WSW) are investigated through the MFT-5000 multifunction tribometer and other test equipments. The experimental results show that the addition of the graphite can effectively decrease the COF and narrow the WSW of the MoS2. There exists a critical applied load for wearing out the surface with the graphite-MoS2 coating. Moreover, there exists an optimal rotational speed of 500 rpm to decrease the COF and WSW of the GCr15 steel.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051304-051304-9. doi:10.1115/1.4039956.

A novel hybrid polymer nanocomposite coating of ultrahigh molecular weight polyethylene (UHMWPE) reinforced with nanoclay (C15A) and carbon nanotubes (CNTs) has been developed to protect metallic mating surfaces in tribological applications. The hybrid nanocomposite coatings were deposited on aluminum substrates using an electrostatic spraying technique. Ball-on-disk wear tests using a 440C stainless steel ball as the counterface were conducted on the coatings under dry conditions to determine the optimum amount of the loadings of the nanofillers and evaluate their tribological performance at different normal loads and linear velocities. Micro-indentation, raman spectroscopy, scanning electron microscopy (SEM), and optical profilometry techniques were used to characterize the coatings in terms of hardness, dispersion of the nanofillers, morphology, and wear mechanisms, respectively. Results showed that the UHMWPE hybrid nanocomposite coating reinforced with 1.5 wt % of C15A nanoclay and 1.5 wt % of CNTs did not fail even until 100,000 cycles at a normal load of 12 N and a linear speed of 0.1 m/s showing a significant improvement in wear resistance as compared to all other coatings evaluated in this study.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2018;140(5):051401-051401-7. doi:10.1115/1.4039530.

This study proposed a physics-based heuristic modeling for the nonlinear constitutive relation of bolted joints based on the Iwan model accompanying with the rough surface contact theory. The approach led to an Iwan distribution function which possesses the tribology-related features of the contact interface. In particular, the break-free force distribution function of the Jenkins elements could be expressed in terms of height distribution of surface asperities. The model considered the contribution of elastically, elasto-plastically as well as plastically deformed asperities to the total tangential loads. Following this, constitutive relations for lap-type bolted joints and the corresponding backbone curves, hysteresis loops, and energy dissipation per cycle were obtained. A model application was implemented and the results were compared with the published experimental results. The proposed model agrees very well with the experimental results when the contact parameters met the actual contact situation. The obtained results indicated that the model can be used to study the tangential behaviors of rough surfaces.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2018;140(5):051501-051501-8. doi:10.1115/1.4039529.

The present paper deals with an investigation of film formation in compliant lubricated contact. Despite these contacts can be found in many applications of daily life including both biological and technical fields, so far little is known about the lubrication mechanisms inside the contacts. The main attention is paid to the effect of kinematic conditions on central film thickness. For this purpose, fluorescent microscopy method was employed. Experiments were realized in ball-on-disk configuration, while the ball was made from rubber and the disk was from optical glass. The contact was lubricated by glycerol and polyglycol to examine the effect of fluid viscosity. The measurements were conducted under pure rolling and rolling/sliding conditions. The entrainment speed varied from 10 to 400 mm/s and constant load of 0.2 N was applied. Experimental results were compared with two theoretical predictions derived for isoviscous-elastohydrodynamic lubrication (I-EHL) regime. It was found that the thickness of lubricating film gradually increases with increasing entrainment speed, which corresponds to theoretical assumptions. Against expectations, evident influence of slide-to-roll ratio (SRR) on film formation was observed. In the last part of the paper, some limitations of this study are discussed and several recommendations for further methodology improvement are suggested.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051502-051502-11. doi:10.1115/1.4039567.

Improved fuel efficiency and reduced emissions are key drivers for modern drivetrain systems. Therefore, in recent years, dry sumps with air–oil mist lubrication have been used for efficient transmission design in order to reduce the churning losses. With dry sumps, appropriate cooling measures should be implemented to dissipate the generated contact heat in an efficient manner. This paper integrates a tribological model with three-dimensional (3D) thermofluid analysis in order to predict the heat generated in the lubricated meshing gear contacts and its dissipation rate by an impinging oil jet in air–oil mist environment. Such an approach has not hitherto been reported in literature. The results show that the generated heat under realistic conditions cannot be entirely dissipated by the impinging oil jet in the air–oil mist transmission casing. Numerical results are used to derive extrapolated regressed equations for heat transfer purposes for time-efficient analysis. These conform well with the detailed numerical results.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051503-051503-6. doi:10.1115/1.4039552.

A new method for solving the shear stress and the effective viscosity of Eyring shear-thinning fluid in thermal elastohydrodynamic lubrication (EHL) was proposed and applied to two models. Model 1 is the thermal EHL model with one-direction velocity, and model 2 is the spinning thermal EHL model in which the velocity varies with coordinates. Comparisons between the new and the existing method were carried out. Results show that only replacing the shear strain rate of model 1 with that of model 2, the shear stress and the effective viscosity of model 2 for Eyring shear-thinning fluid can be obtained. For model 1, results obtained with the two methods are the same. The new method can be qualified and applied into model 2. It is proved that the new method has higher efficiency for shear-thinning fluid than the existing method. Therefore, the new method is more efficient and can be used for spinning Eyring shear-thinning thermal EHL.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2018;140(5):051601-051601-9. doi:10.1115/1.4039412.

The steady-state described by running-in attractor in the perspective of nonlinearity, is closely dependent on the running-in parameters. To study the dependence of running-in attractor on system parameters, pin-on-disk friction tests were performed. A suitable contact between pin and disk was ensured by a self-adaptive pin holder, standard block, and self-adapting amendment with sandpaper. Range analysis of correlation dimension, predictability, and entropy shows that running-in attractor is system dependent, which is manifested by the dependence of nonlinear parameters of the attractor on the running-in parameters. Further results indicate that the correlation dimension and entropy increase with load and velocity, but decrease along with initial roughness of a harder counterface, and predictability shows an inverse variation tendency with correlation dimension and entropy.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051602-051602-23. doi:10.1115/1.4039410.

Tribological properties, i.e., wear rate, coefficient of friction (COF), and roughness of worn surfaces of an Al-Mg-Zn-Cu alloy and its composite reinforced with 20 wt % Al2O3 particles developed by stir-casting method have been studied and compared under two-body abrasion considering four independent control factors, i.e., load, abrasive grit size, sliding distance, and velocity each at three different levels. Design of test conditions and analyses of output responses have been performed employing standard Taguchi L27 orthogonal array, signal-to-noise ratio, analysis of variance technique, and regression method. Irrespective of wear conditions, composite exhibits lower wear rate and reduced COF with reference to base alloy owing to the load bearing ability and better wear resistance capability of Al2O3 particles. Roughness of worn surfaces of composite is, however, found to be higher over base alloy due to nonuniform abrasion in case of composite that generates the protruded Al2O3 particles on contact surfaces as the surrounding soft matrix is easily removed. For all three tribo-responses of both materials, the most influential factor is identified as grit size followed by load and then, grit size-load interaction except for the roughness of worn surfaces where the influence of sliding distance is also considerable. Linear regression models with excellent predictability have been developed for all tribo-characteristics separately for base alloy and composite. The predominant mechanisms of abrasion are identified as plowing and microcutting for base alloy, but delamination for composite.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051603-051603-7. doi:10.1115/1.4039411.

(Ti,Mo)C, TiB2, and Mo2B particles reinforced Fe-based composite coatings were fabricated by laser cladding process. The effects of Molybdenum (Mo) on the microstructure and wear properties of the coatings were investigated. The results show that block-like or cuboidal TiB2, Mo2B and flower-like (Ti, Mo)C ceramics reinforcements were formed in the coatings. The size of reinforcements reduced with the increasing of FeMo70. However, cracks were found in the coating, while the addition of FeMo70 exceeded 9 wt %. The laser cladding coating presented a good wear resistance with a 9 wt % addition of FeMo70. With the increasing of FeMo70, the coatings enhanced the capability of resisting microcutting, microplowing, and surface plastic deformation.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051604-051604-8. doi:10.1115/1.4039525.

Wear experiments are performed to explore dynamic states changes of friction noise signals. A new characteristic parameter, moving cut data-approximate entropy (MC-ApEn), is adopted to quantitatively recognize dynamic states. Additionally, determinism (DET), one key parameter of recurrence quantification analysis, is applied to verify the reliability of recognition results of MC-ApEn. Results illustrate that MC-ApEn of friction noise has distinct changes in different wear processes, and it can accurately detect abrupt change points of dynamic states for friction noise. Furthermore, DET of friction noise rapidly declines first, then fluctuates around a small value, and finally increases sharply, which just corresponds to the evolution process of MC-ApEn. So, the reliability of wear state recognition on the basis of MC-ApEn can be confirmed. It makes it possible to accurately and reliably recognize wear states of friction pairs based on MC-ApEn.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051605-051605-8. doi:10.1115/1.4039528.

The current design of materials against wear considers hardness as the sole material property. As a result, the brittleness associated with increased hardness leads to severe damage. The purpose of this research is to understand the nature of conflicts between hardness and toughness of a new alloy composite. First, we designed Al-Cu-Fe alloys containing crystal structures of λ, β, and quasi-crystalline i-phase. These and their combination with others lead to a set of alloys with various hardness and fracture toughness. Experimental study was carried out using a noble and hard tungsten carbide (WC) ball against sample disks. The WC ball did not produce any wear. The wear rate of those alloys was found to be dependent not only on their hardness, but also the toughness, an alternative to the well-accepted Archard-based equations.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2018;140(5):051701-051701-8. doi:10.1115/1.4039408.

Reduced oil supply flow rates in fluid film bearings can cause cavitation, or lack of a fully developed hydrodynamic film layer, at the leading edge of the bearing pads. Reduced oil flow has the well-documented effects of higher bearing operating temperatures and decreased power losses and is commonly referred to as starvation. This study looks at the effects of oil supply flow rate on steady-state bearing performance and provides increased experimental data for comparison to computational predictions. Tests are conducted on a five-pad tilting-pad bearing positioned in a vintage, flooded housing with oil supply nozzles. Pad temperatures, sump temperature, journal operating position, and motor input power are measured at various operating speeds ranging from 2000 to 12,000 rpm and various oil supply flow rates. Predicted results are obtained from bearing modeling software based on thermoelastohydrodynamic (TEHD) lubrication theory. A starved flow model was previously developed as an improvement over the original flooded flow model to more accurately capture bearing behavior under reduced flow conditions. Experimental results are compared to both flow models. The starved bearing model predicts significantly higher journal operating positions than the flooded model and shows good correlation with the experimental data. Predicted pressure profiles from the starved bearing model show cavitation of the upper unloaded pads that increase in severity with increasing speed and decreasing oil supply flow rate. The progressive unloading of these top pads explains the rise in shaft centerline position and helps further validate the starvation model.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051702-051702-12. doi:10.1115/1.4039721.

The stability of oil-free high-speed turbo-machinery can be effectively improved by increasing the damping characteristic of the gas foil bearing (GFB). Novel hybrid bump-metal mesh foil bearings (HB-MFBs) have been previously developed. Prior experimental results show that the parallel combination of bump structure and metal mesh not only can improve the structure stiffness but also provide better damping property compared with the bump-type foil structure. To investigate the dynamic behavior of floating HB-MFBs and promote its application, this study measured the dynamic force coefficients of HB-MFBs on a rotating test rig. The vibrations of HB-MFBs with different mesh densities (40%, 32.5%, and 25%) and a generation І bump-type foil bearing (BFB) with similar size are measured under static and impact loads to estimate the bearing characteristics. Static load test results show that the linear stiffness decreases when the air film is generated (from 0 rpm to 20 krpm) but increases gradually with speed (from 20 krpm to 30 krpm, and 40 krpm). Moreover, the dynamic force coefficients of HB-MFBs indicate the significant influence of metal mesh density on bearing dynamic characteristics. The growth in block density increases the dynamic stiffness and damping coefficients of bearing. The comparison of HB-MFB (32.5% and 40%) and BFB emphasizes the good damping characteristics of HB-MFB.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2018;140(5):052201-052201-17. doi:10.1115/1.4039428.

Numerical and experimental analyses were carried out to investigate the dynamic characteristics of liquid annular seals with helical grooves in the seal stator. In the numerical analysis, the governing equations were the momentum equations with turbulent coefficients and the continuity equation, all averaged across the film thickness and expressed using an oblique coordinate system in which the directions of coordinate axes coincided with the circumferential direction and the direction along the helical grooves. These governing equations were solved numerically to obtain the dynamic characteristics, such as the dynamic fluid-film forces, dynamic coefficients, and whirl-frequency ratio (WFR). The numerical analysis included the effect of both fluid inertia and energy loss at the steps between the helical groove and the land sections. In the experiments, the dynamic fluid-film pressure distributions, which were induced by a small whirling motion of the rotor about the seal center, were measured to obtain the dynamic characteristics. The equivalent numerical results reasonably agree with the experimental results, demonstrating the validity of the numerical analysis. The value of the tangential dynamic fluid force induced by the rotor whirling motion decreased with increasing the helix angle γ. Consequently, the values of the cross-coupled stiffness coefficient and WFR decreased with increasing γ and became negative for large γ. In general, pump rotors rotate with a forward whirling motion under normal operating conditions. Hence, the negative value of WFR for helically grooved seals contributes to rotor stability by suppressing the forward whirling motion of the rotor.

Commentary by Dr. Valentin Fuster

Research Papers: Tribochemistry and Tribofilms

J. Tribol. 2018;140(5):052301-052301-12. doi:10.1115/1.4039527.

An attempt has been made to develop and study the properties and behavior of structure-based aluminum composite. Aluminum (AA6063)-based composites were fabricated by stir casting technique (also known as liquid metallurgy route) by varying weight percentage of metallic-based copper nitrate Cu (NO3)2 with fixed proposition of ceramic-based silicon nitride (Si3N4) reinforcement. The mechanical and corrosion properties and tribological behavior of composite were studied. Further, the sample microstructure and characterizations were investigated by scanning electron microscope (SEM) and X-ray diffraction (XRD) technique. The composite with fixed weight proportion of ceramic and higher metallic reinforced samples shows higher tensile strength, improved corrosion resistance, and higher hardness behavior. Due to higher hardness nature, the tribological properties of composite such as wear rate and coefficient of friction have been reduced. Moreover, the impact strength of composite decreased due to combination of ceramic and metallic reinforcement. In addition to the above study, design of experiment (DOE) was adopted to optimize the major wear test parameters such as percentage of reinforcement, applied load, sliding distance, and sliding speed. Finally, analysis of variance (ANOVA) was carried out to identify the most significant test parameter and its interaction affecting wear behavior and its coefficient of friction of composite sample.

Commentary by Dr. Valentin Fuster

Discussion

J. Tribol. 2018;140(5):055501-055501-1. doi:10.1115/1.4039797.

The author proposes an intriguing novel fluid rheology model [1] which can address both gas and cavitating liquid bearings depending on the choice of a single adjustable gas mass fraction number λ. His model appears to be readily incorporated into a finite element representation of the Reynolds equation, and his numerical results appear to agree well with previously published numerical and experimental results for thrust bearings under steady load and speed.

Commentary by Dr. Valentin Fuster

Closure

J. Tribol. 2018;140(5):056001-056001-1. doi:10.1115/1.4039798.

The author of paper [1] would like to thank Professors Boedo and Booker for the relevant discussion. Their main concern is about the lack of reference pertaining to cavitation modeling in the state of the art. Indeed, the paper of Kumar and Booker [2] and the associated references are not cited. The author would like to apologize for this oversight. Since the topic of paper [1] is liquid and gas lubrication, including cavitation, it was not possible to be exhaustive on all these topics.

Commentary by Dr. Valentin Fuster

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