High Contact Stresses Research Articles

Fully ceramic versus steel gears: Potential, feasibility and challenges

The most common gear strength calculations performed in accordance with ISO/TC 60/SC 2 include surface durability and root bending strength. Typical geared power transmissions are characterized by high Hertzian contact stresses (surface durability) on the flanks – frequently in the order of 103 MPa – versus almost half an order magnitude lower first principal stresses on the tooth root (tooth bending strength). Therefore, the use of high strength special steel alloys is almost mandatory, especially in high-end applications, mainly to withstand the high contact stress values. Ceramic materials are typically characterized by high compressive strength versus very low flexural strength that at first glance makes them inefficient for gear transmissions since they are unlikely to withstand the tensile root stresses. Nevertheless, ceramic materials possess a number of advantages that makes them potential candidates for geared transmissions, such as their exceptional thermal properties, their low density (weight), their excellent wear characteristics and limited lubrication demands. The present work investigates the main challenges of using high-end ceramic gears and attempts to provide an insight on defining the design envelope of such gears to counter the main disadvantages. The increase of the normal pressure angle beyond the 25° limit set by ISO 6336 is primarily investigated, since in minimizes the tensile tooth root stresses, while also increasing the curvature of the contact surfaces without greatly increasing Hertzian stresses. Comparative assessment of the performance of fully ceramic gear designs versus their conventional steel counterparts is performed through quasi-static FEA simulations to set the basic design requirements for such gears and to promote further investigation of the feasibility of their usage in high power density applications.

Remarkable lubricity of CNTs microspheres as additives in oil lubricant for ceramic components

Severe wear was often caused by adhesive wear and tribo-chemical reaction for ceramic components. In view of this situation, potentials of carbon nanotubes (CNTs) used as oil additives were explored for lubrication of ceramic/steel tribo-components. Microscopic morphology and dispersion stability of the prepared CNTs were characterized. CNTs microspheres were formed and this might because of their aggregation that also lead to the weak dispersion stability. Additionally, nano-fluids containing various concentration of CNTs microspheres were prepared for exploring the optimum concentration, where their lubricating behavior for ceramic/steel tribo-components was investigated to determine the relation between CNTs microspheres and tribological properties. The results indicated that the optimum concentration was 0.2 wt%, where 53.1 % of friction reduction and 50.7 % of wear resistance were achieved at high contact stress of 1.35 GPa. The superior lubricity of CNTs microspheres might be attributed to their rolling movement and formation of protective film.

Stress-dependent subsurface structural transformations of gradient nanograin Ti–6Al–4V alloy and its impact on wear behavior

Gradient nanograin (GNG) metals possess outstanding resistance to sliding friction and wear. The tribolayer plays a major role in the wear performance of metals. However, the current understanding of the tribolayer in GNG metals is still limited. In particular, some GNG metals show low wear resistance under high contact stress. Here, we investigate the wear behavior of coarse grain (CG) and GNG Ti–6Al–4V alloys and its impact on the wear resistance by sliding contact experiments in combination with finite element modeling. We have found wear-induced the oxidation tribolayer with amorphous structures in GNG Ti–6Al–4V alloy for the first and its impact on wear behavior. It is found that the wear damage mainly occurs in the oxidation tribolayer with amorphous structures, which can generate micro-cracks in Ti–6Al–4V alloy. The wear behavior, including wear resistance, wear micro-cracks, and subsurface structural transformations, is stress-dependent, which can be divided into two distinct regimes-elastic and plastic. Coarse grains (CGs) can form the oxidation tribolayer with amorphous structures in elastic and plastic regimes. Compared to CGs, gradient nanograins (GNGs) can suppress the formation of the oxidation tribolayer with amorphous structures and have higher wear resistance in elastic regime, while GNGs form the oxidation tribolayer with amorphous structures more quickly and have lower wear resistance in plastic regime. This work forges the links between stress-dependent subsurface structural transformations and wear resistance for GNGs, and may provide guidance for the wear application of GNG metals.

Transferability of the scuffing load capacity of gear oils determined on spur gears to hypoid gears

AbstractThe scuffing load carrying capacity is a decisive design criterion in the development of gears. In addition to the gear geometry, the entire tribological system of the gearbox must also be taken into account. Besides the loads and speeds occuring during the operation of the gears, the lubricant used and its additives have a significant influence on the scuffing load carrying capacity. In particular, hypoid gears show an unfavorable combination of high sliding velocities and high contact stresses in tooth contact with regard to the failure mode scuffing. To ensure safe operation, highly additivated gear oils are generally used in hypoid transmissions. Since it is generally not possible to determine the influence of the lubricant on the load carrying capacity of gears on the basis of physical or chemical oil data alone, experimental test methods are necessary. Due to the high scuffing load carrying capacity of highly additivated hypoid oils, the limits of many test methods are reached. In order to ensure testing highly additivated hypoid oils as close as possible to their application a new hypoid scuffing test was developed. However, since spur gear scuffing test methods can be carried out more quickly and with less resource and effort, the applicability of the spur gear scuffing test S‑A10/16.6R/90 to high additivated hypoid oils is investigated. The comparability of the two test methods and the transferability of the scuffing load carrying capacity of a gear oil determined on spur gears to hypoid gears is evaluated in scope of this paper.

Self-Adaptive Macroscale Superlubricity Based on the Tribocatalytic Properties of Partially Oxidized Black Phosphorus.

The high-flash heat generated by direct contact at asperity tips under high contact stress and shear significantly promotes the tribocatalytic reaction between a lubricating medium and a friction interface. Macroscale superlubricity can be achieved by using additives with good lubrication properties to promote the decomposition and transformation of a lubricating medium to form an ultralow shear interface during the friction process. This paper proposed a way to achieve self-adaptive oil-based macroscale superlubricity on different tribopairs, including steel-steel and steel-DLC (diamond-like carbon), which is based on the excellent lubricating performance of black phosphorus with active oxidation and the catalytic cleavage behavior of oil molecules on the surface of oBP. This work potentially expands the industrial application of superlubricity.

Tribocatalytically-active nickel/cobalt phosphorous films for universal protection in a hydrocarbon-rich environment

High-contact stresses generated at the sliding interfaces during their relative movement provide a unique combination of local heating and shear- and load-induced compression conditions. These conditions, when involving the sliding of surfaces with certain material characteristics, may facilitate tribochemical reactions with the environment, leading to the formation of a protective, damage-suppressing tribofilm directly at the contact. Here, we employ the electrodeposition process to design a coating composed of a hard cobalt-phosphorous matrix with the inclusion of tribocatalytically-active nickel clusters. The coating is optimized in terms of its relative composition and mechanical characteristics. We demonstrate the excellent tribological performance of the coating in the presence of a hydrocarbon environment, both in the form of a liquid lubricant and as a hydrocarbon-saturated vapor. Characterization of the wear track indicates that the origin of such performance lies in the formation of a protective carbon-based tribofilm on the surface of the coating during sliding. These results contribute to the advancement of knowledge on material transformations in the contact, thus providing a robust and versatile approach to addressing tribological challenges in mechanical systems.

Uncover rock-climbing fish's secret of balancing tight adhesion and fast sliding for bioinspired robots.

The underlying principle of the unique dynamic adaptive adhesion capability of a rock-climbing fish (Beaufortia kweichowensis) that can resist a pull-off force of 1000 times its weight while achieving simultaneous fast sliding (7.83 body lengths per second (BL/S)) remains a mystery in the literature. This adhesion-sliding ability has long been sought for underwater robots. However, strong surface adhesion and fast sliding appear to contradict each other due to the need for high surface contact stress. The skillfully balanced mechanism of the tight surface adhesion and fast sliding of the rock-climbing fish is disclosed in this work. The Stefan force (0.1 mN/mm2) generated by micro-setae on pectoral fins and ventral fins leads to a 70N/m2 adhesion force by conforming the overall body of the fish to a surface to form a sealing chamber. The pull-off force is neutralized simultaneously due to the negative pressure caused by the volumetric change of the chamber. The rock-climbing fish's micro-setae hydrodynamic interaction and sealing suction cup work cohesively to contribute to low friction and high pull-off-force resistance and can therefore slide rapidly while clinging to the surface. Inspired by this unique mechanism, an underwater robot is developed with incorporated structures that mimic the functionality of the rock-climbing fish via a micro-setae array attached to a soft self-adaptive chamber, a setup which demonstrates superiority over conventional structures in terms of balancing tight underwater adhesion and fast sliding.

An efficient method for designing high-performance planetary roller screw mechanism with low contact stress

The high contact stress of the first few teeth of the planetary roller screw mechanism can be effectively alleviated by convex–to–concave contact mode. The load distribution model is constructed based on the convex/concave tooth profile model, and the maximum contact stresses at the screw-roller and roller-nut interfaces are simultaneously considered to construct a multi-objective optimization model (MOOM). However, complicated arc contact will inevitably make the MOOM involves many nonlinear constraints (NCs). Therefore, a feasibility identification-based NSGA-II is used to locate and further search the complicated feasible regions brought by many NCs for searching the best structural parameters. The results show that the contact stresses over thread teeth are decreased based on the designed multi-objective modeling and optimization framework.

Tribological behavior of graphene/DLC coatings after AO and UV irradiation in vacuum environment

Friction and wear remain as the primary modes of energy dissipation in transmission system. Here, the uniform coating of graphene on the diamond like carbon (DLC) surface is achieved with the effect of argon protection and low-temperature heating of the substrate. The generation of graphene nanoscrolls is induced by the dual function of high-contact stress at the friction interface and frictional heat. The results show that the Graphene/DLC coatings maintain a stable low friction coefficient (average friction coefficient < 0.03) for a long time in atmospheric, vacuum (Before and after AO and UV irradiation) environments.

Driveline Oscillation Damping for Hybrid Electric Vehicles Using Extended-State-Observer-Based Compensator

Driveline oscillation is a significant concern in the context of hybrid electric vehicles (HEVs), because it can adversely affect the vehicles’ sustainability. The reason for this is that the oscillation not only diminishes the longevity of components due to high mechanical contact stress but also results in poor driving comfort, which in turn reduces customer satisfaction. To address the issue of driveline oscillation effectively, two critical challenges, namely the time-varying torque load and driveline backlash, need to be tackled. To this end, this study constructs a control-oriented model of a second-order system plus a dead zone for the driveline backlash. An extended state observer is designed in order to estimate the unmeasurable load torque. As such, an extended-state-observer-based compensator is proposed to suppress driveline oscillations for HEVs. To evaluate the control and observation performance of the proposed extended-state-observer-based compensator, simulation and engine-in-loop experiments are conducted. Results obtained in the time and frequency domains reveal that the proposed control scheme substantially reduces driveline oscillation.

Facile formulation of graphene oxide modified commercial polyurea grease and its enhanced tribological capability under extremely high contact stress

Grease lubricants have been widely applied to the rubbing components to increase the operation stability and reliability. In this study, graphene oxide modified polyurea grease was prepared by formulating different amounts of exfoliated expandable graphite (EG) into commercial polyurea grease with combined mechanical agitation and grinding processes, tribological characteristics of graphene oxide modified polyurea grease were subsequently evaluated under extremely higher contacting stress. Among all the formulated polyurea grease specimens, polyurea grease with 2.0 wt.% graphene oxide displays better capabilities of minimizing friction and wear. In particular, the wear loss reduces considerably by 80% meanwhile the averaged friction coefficient decreases to 0.10, as referenced with existing polyurea grease. The improved tribological behavior is proposed to be associated with the in‐situ produced graphene oxide solids that enhances extremely lower shearing ability and promotes the fatigue resistance of grease, as well as its desired chemical compatibility with resulting iron sulfide tribofilm. Particularly, graphene oxide works as the dispersed precipitate within the dense iron sulfide/oxide layer induced from ZDDP that covers on the steel substrate.

A comprehensive study on Hertzian contact stress behaviour of engineering thermoplastic gears using 3D finite element analysis

Two-dimensional Finite Element Analysis is frequently utilized to determine the behaviour of gear contact stress due to its computational time and higher efficiency. Based on the gear tooth-to-face width ratio, either plane stress or plane strain was selected in the investigations. However, the existing investigations failed to explain the above selection criteria. Thus, the present work attempted to do detailed contact stress investigations for all thermoplastic gears using 3D FEA and compared them with 2D FEA. Based on the criteria, the face widths of 5 and 20 mm were selected during the 3D analyses, replicating the plane stress and plane strain conditions of the 2D analysis. Contact stress variations with respect to face width were extracted at 10 distinct locations. For both 5 and 20 mm gears, higher contact stresses were obtained on gear edges, whereas lesser contact stresses were in the centre regions of the gear. For all the analysed thermoplastic gears, 5 and 20 mm face-width gears exhibited plane stress conditions. Subsurface and surface stress was detected during 3D and 2D analysis for both gears. For PP, the maximum contact stress occurred at the lowest depth value of 0.15–0.155 mm, and PC exhibited maximum contact stress at 0.275–0.28 mm depth. A dimensionless ratio of proportional limit – to – young’s modulus was introduced to predict the maximum contact stress depth for the thermoplastic gears. And it was observed that the ratio is directly proportional to the maximum contact stress depth value.

Analytical and Numerical Contact Stress Analysis of Spur Gears

The damage that occurs on the teeth flank of cylindrical gears is a complex phenomenon and depends on many factors. The most common cause for these damages is the high contact stress of the meshing gears. Although this contact pressure, by itself, cannot be a criterion for determining the durability of the gears, a good correlation has been found between the contact or Hertz pressure and the damage that occurs on the tooth flank. This paper analyzes the influence of the pressure angle α on the contact stress. Analytical calculation according to the ISO 6336-2:2006 and finite element method (FEM) was used for the analysis. Four cases were analyzed with a change in the pressure angle, i.e., cylindrical spur gears and pressure angles of 17.5°, 20°, 22.5° and 25°. In the results, it was noted that by increasing the pressure angle, the contact stress decreases. It can also be concluded that by increasing the pressure angle, the difference in the results between the analytical and FEM analysis, also increases.

Silicon wafer as a feasible candidate for tribological characterization of thin coatings under high contact stress?

Silicon (Si) wafer is generally used as the substrate for structural analysis and residual stress measurement of thin coatings. A few studies can be found for mechanical and tribological characterization wherein Hertzian contact stress never exceeded 1 GPa. Si, being single crystal, pure, and having low surface roughness (Ra) is an ideal potential candidate for thin coating analysis. However, Si wafer breaks immediately under higher contact stress. The current study aims to push the contact stress limit to test thin coatings on Si wafer especially for friction and wear testing. To serve the purpose, different bias DLC coatings (−120 V, −80 V, −40 V, Zero V, +40 V, +80 V, +120 V) were deposited on Si wafer employing closed field unbalanced magnetron sputtering (CFUMS) system. Using conveniently available experimental approach, Hertzian contact stress in tribological tests has been pushed up to 2.6 GPa successfully on DLC coated Si wafer. Thus, it is feasible to test thin coatings on Si wafer at high contact stress which will provide useful and consistent friction and wear data such that further test using the steel substrate will be reduced by a significant margin. A thorough mechanical, tribological and structural (Raman) analysis is presented. It was found that −120 V possessed the highest hardness (H: 17.6 GPa), modulus (E: 184 GPa) and residual stress (4.5 GPa). The same coating produced the lowest sp2/sp3 ratio, wear rate using contact stress (0.092 × 10−13 mm4/N) and wear/friction energy (0.071 × 10−13 mm3/J2) in comparison to other coatings used in the study. Counter body analysis for −120 V shows the minimum scratch area depicting friction between transfer film and DLC hence the lowest coefficient of friction (0.061) was obtained. Moreover, surface topography is explained with the aid of atomic force microscope (AFM) images and surface roughness (Ra) data. AFM revealed craters and bumpy like surfaces for positive and negative biased samples respectively.

Effect of the volume of resected discoid lateral meniscus on the contact stress of the tibiofemoral joint: A finite element analysis

BackgroundPartial meniscectomy is commonly performed for symptomatic patients with discoid lateral meniscus (DLM) if conservative treatment fails. However, the development of knee osteoarthritis and osteochondral lesion are detrimental postoperative complications. This study aimed to evaluate the effect of the volume of resected DLM on the contact stress of the tibiofemoral joint using a finite element analysis. MethodsSubject-specific finite-element models of the knee joint of a patient with DLM were developed from computed tomographic and magnetic resonance images. To evaluate the effect of partial meniscectomy on the contact stress in the lateral tibiofemoral joint, six knee models were created in the study (the native DLM, and five partially meniscectomized DLMs (according to the preserved width of the meniscus: 12 mm, 10 mm, 8 mm, 6 mm, and 4 mm)). ResultsAs the volume of resected DLM increased, higher contact stress was applied to the lateral tibiofemoral joint. Greater contact stress was applied to the preserved lateral meniscus than to the native DLM. ConclusionsFrom a biomechanical viewpoint, the native DLM was the most protective against lateral tibiofemoral contact stress in comparison to partially meniscectomized DLMs.

Macroscale Superlubricity Induced by MXene/MoS2 Nanocomposites on Rough Steel Surfaces under High Contact Stresses.

Toward the goal of achieving superlubricity, or near-zero friction, in industrially relevant material systems, solution-processed multilayer Ti3C2Tx-MoS2 blends are spray-coated onto rough 52100-grade steel surfaces as a solid lubricant. The tribological performance was assessed in a ball-on-disk configuration in a unidirectional sliding mode. The test results indicate that Ti3C2Tx-MoS2 nanocomposites led to superlubricious states, which has hitherto been unreported for both individual pristine materials, MoS2 and Ti3C2Tx, under macroscale sliding conditions, indicating a synergistic mechanism enabling the superlative performance. The processing, structure, and property correlation were studied to understand the underlying phenomena. Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy revealed the formation of an in situ robust tribolayer that was responsible for the performance at high contact pressures (>1.1 GPa) and sliding speeds (0.1 m/s). This report presents the lowest friction obtained by either MoS2 or MXene or any combination of the two so far.

Influence of the Tibial Tunnel Angle and Posterior Tibial Slope on "Killer Turn" during Posterior Cruciate Ligament Reconstruction: A Three-Dimensional Finite Element Analysis.

This study aimed to evaluate the influence of various posterior tibial slopes (PTSs) and tibial tunnel angles (TTAs) on "killer turn" in posterior cruciate ligament (PCL) reconstruction by using three-dimensional finite element analysis (FEA). The study models were created using computed tomography images of a healthy young Asian male. Using SolidWorks, PCL grafts and tibial bone tunnels at different tibial drilling angles (30°, 45°, 60°) were developed. Anterior opening wedge high tibial osteotomy (aOW-HTO) was performed to evaluate the influence of the PTS (+8°, +4°, native, -4°, -8°). An FEA was performed utilizing the ANSYS software program. In the same PTS model, the peak of the equivalent Von Mises stress in PCL grafts decreased as the angle of the TTA increased. In the same TTA model, the peak of the Von Mises in PCL grafts decreased as the PTS angle increased. The "high-contact stress area" (contact stress greater than 10 MPa) was diminished when the TTA and PTS were increased. aOW-HTO was used to steepen the PTS, and a larger TTA may reduce the stress at the "killer turn" during PCL reconstruction. In conclusion, the study findings suggest that using aOW-HTO to steepen the PTS and a larger TTA may reduce the stress at the "killer turn" during PCL reconstruction. The usefulness and safety of this surgical procedure need to be evaluated in future clinical studies.

On the mechanism of rock fracture with tools made of hard alloys and polycrystalline superhard materials

When studying the process of rock fracture with tools equipped with polycrystalline superhard materials (PSHM) and hard alloys, the main regularities of the fracture mechanism, despite the significant differences in physical and mechanical properties of the used tool materials, are assumed to be identical. This approach not only reduces the efficiency of using the created rock-destroying tools, but also limits the scope of application of polycrystalline superhard materials. Experimental studies using various methods of obtaining information shown significant differences in the mechanism of rock fracture by hard alloys and polycrystalline superhard materials. The study of the zone of the pre-destroyed surface showed that when cutting rocks with PSTM, the process of destruction is carried out not only by the entire polycrystalline plate, but also by ridged diamond formed on the cutting edge and rear surface of the tool. At the same time ridged diamonds, when embedded in the rock, create high contact stresses and an additional network of microcracks interacting with microcracks formed due to the embedding of the entire cutting edge of the plate. The impact of two independent indenters simultaneously increases the zone of the pre-destruction layer in the rock mass, which leads to a more significant decrease in its strength and, as a consequence, to the intensification of the process of rock destruction by the PSTM tool. In case of wrong choice of PSTM application area and operation modes, errors in tool design and insufficient cooling, ridges are not formed on the cutting edge and back surface of the polycrystal. As a result, the polycrystalline insert works as a carbide insert and the efficiency of the PSHM tool is sharply reduced. The use of knowledge obtained as a result of the conducted research makes it possible to create tools equipped with diamond-hard-alloyed inserts, the wear resistance of which is dozens of times higher than that of similar tools made of hard alloy. For example, effective tools equipped with diamond-hard-alloyed inserts have been created and widely introduced into practice for rotary drilling of boreholes, degassing wells, anchoring of mine workings, saw stone cutting, drilling of abrasive permafrost soils and others. Keywords: polycrystalline diamonds cutters (PDC), hard alloys, pre-destruction zone, rock breaking tools, rock destruction.

Contact Stress-Induced Wear Mechanism Transitions of PcBN/Al2O3 Under Vacuum and Air Conditions

Abstract Since the millennium, incremental breakthroughs in aerospace have attracted widespread attention from countries around the world on deep space exploration. Technological innovations in ceramic and superhard materials have also played a key role in deep space exploration. Inspired by this, a tribological ball-disk experiment of polycrystalline cubic boron nitride (PcBN) sliding against aluminum oxide (Al2O3) was implemented in air and vacuum conditions, in order to evaluate the friction and wear properties of PcBN based on drilling in the deep space environment. The results prove that the coefficient of friction (CoF) is interrelated with load and wear conditions, where CoFs gradually decrease with load growth in both air and vacuum. When the loads keep increasing, however, the wear mechanisms finally change under the high Hertz contact stress and lead to the CoF lift. Detailed characterizations were made to verify the tribological behaviors of the microscopic surface and chemical composition. Finally, by analyzing the surface topographies and chemical residues, it is certain that the wear mechanisms change due to the high Hertz contact stress. As a result, abrasive wear and adhesive wear turn to furrow wear in air and three-body wear in vacuum. These results can influence actual work in deep space by reducing large stress loads to avoid the impact of severe vibrations on precision instruments during work and improving cutting removal efficiency by selecting the appropriate loading.

Thermal model of cone drill bit bearing with air flushing

Cone drill bits are used in various-purpose drilling in difficult geotechnical conditions, as well as in drilling-and-blasting in open-pit mining. The reliability of the drill bits depends on the rock-breaking source and on the bearing support. One of the causes of low reliability of a drill bit is the failure of the retaining ball bearing due to destruction of roller path and journal of the bearing under high contact and bending stresses. Based on the general provisions of similarity theory, a physical model of the retaining bearing of a roller bit was created. The test data of the bearing on a friction and wear testing machine are presented as the dependences of the bearing temperature, friction moment and clearances on time. The dependence of the bearing temperature on the thermal power is determined. In extreme conditions of lubricant shortage, the contact stresses in the bearing rings and balls and the temperature significantly exceed the permissible values. The methodology is proposed to be used in development of a cone rock bit drilling tool. The article discusses the method of determining the moment of friction of the bearing of a cone drill bit. The design of bearings without retainers features increased clearances. To determine the moment of friction under these conditions, a simple design of the test bench is proposed, which includes a load and a counterweight connected by a thread thrown over a rolling bearing. The calculations to determine the reduced moment of friction of the rolling bearing in the conditions of the test bench are described. The study was carried out under the state contract between the Ministry of Science and Higher Education of the Russian Federation and the Ural State Mining University.