Study of Tribological Properties of Fullerenol and Nanodiamonds as Additives in Water-Based Lubricants for Amorphous Carbon (a-C) Coatings

The tribological performances of fullerenol and nanodiamonds (NDs) as additives in water-based lubricants for amorphous carbon (a-C) coatings are investigated to avoid disadvantage factors, such as chemical reactions and deformation of particles. The effects of size and additive amount on tribological properties of nanoparticles are studied by rigid nanoparticles within the dot size range. The results show that owing to its small particle size (1–2 nm), fullerenol cannot prevent direct contact of the friction pair at low concentration conditions. Only when the quantity of fullerenol increased to support the asperity contact loads in sufficient concentration did nano-bearings perform well in anti-friction and anti-wear effects. Unlike fullerenol, nanodiamond particles with a diameter of about 5–10 nm show friction-reducing effect based on the nano-bearing effects at ultra-low concentration (0.01 wt.%), whereas particles at higher concentration block the rolling movement, hence increasing the coefficient of friction (COF) and wear. As a result of the effect of difference in size, fullerenol provides a better overall lubrication, but it is hard to reach a friction coefficient as low as NDs even under the optimal conditions.

THEORETICAL BASIСS OF PROVIDING THE TECHNICAL CHARACTERISTICS OF MILITARY AND CIVIL VEHICLES BY JUSTIFICATION OF THE FORM AND PROPERTIES OF THE MATERIALS OF CONTACTING ELEMENTS

Elements of constructions of modern military and civil vehicles usually work in conditions of high contact loads. Аt the stage of their creation, strength studies are carried out using traditional models of contact of bodies of nominal shape. Нowever, the real structural elements have deviations from such models, which are due to design and technological factors: macrodeviation of the shape, surface roughness, strengthening etc. Such perturbations of nominal parameters have a significant effect on the distribution of contact pressure between the elements of military and civil vehicles, however, traditional methods for studying the stress-strain state of contacting bodies do not make it possible to take such factors into account fully, collectively and exhaustively. To eliminate the existing contradiction, a semi-analytical method is proposed, which is based on the development of variational principles and boundary-element sampling. The created models make it possible to take into account the regularities of the influence of shape perturbations and properties of the surface layers of contacting bodies on the stress-strain state. As a result, it becomes possible to justify favorable perturbations by strength criteria. Such models and methods are offered to the work, and on their basis it’s proposed the implementation of research elements of military and civil vehicles for appointment to ensure world class the technical and tactically technical characteristics. Ключові слова: military and civilian vehicles; design and technological factor; stress-strain state; contact interaction; strength

CONTACT INTERACTION OF A TORSION SHAFT WITH A SPLINED BUSH IN ELASTICALLY PLASTIC DEFORMATIONS

Torsion shafts are the main elastic element of the suspension systems of a large number of vehicles. To simulate their reaction to the action of torque, the stress-strain state is analyzed taking into account the contact interaction with the spline sleeve. The features of the distribution of contact pressure between these bodies are established. The nature of stress concentration in the splined hollows of the shaft head is determined. Models and research methods have been developed that make it possible to develop recommendations for design decisions in the design of vehicle suspension systems. The factors are determined that ensure the strength of the torsion shaft at the values of its head diameters close to the diameters of the torsion shaft stem. In the case under consideration, this factor is firstly strength of the torsion shaft head. In particular, it was found that during manufacturing operations there are significant plastic deformations and contact loads in the heads of torsion shafts. This factor is decisive in substantiating the design parameters of torsion shafts. Keywords: torsion shaft; contact interaction; stress-strain state; elastically plastic deformation; suspension system

RESEARCH AND EXPERIMENTAL STUDIES OF STRESS-STRAIN STATE OF DISCRETE-CONTINUAL HARDENED MACHINE PARTS

Research and experimental studies of stress-strain state of discrete-continual hardened machine parts are presented in this work. This hardening method is distinguished by incorporation of numerous hard spots into the surface layer of one of the bodies. Meanwhile the other part is covered by a continuous corundum layer. Correspondingly, a network of microchannels for lubricant is formed between the bodies. Furthermore the contact loads are intensified in the vicinity of the harder material in the discrete zones. As a result the strength and durability of the loaded parts is increased. The technological parameters of the hardening process have great impact on the resulting characteristics. In particular, this concern the shape of the discrete hardening zones and the material properties of the corundum layer. These factors were varied in stress-strain analyses of the contacting bodies. The dependence of the stress-strain state characteristics on the varied parameters was established. The justified recommendations regarding the technological parameters of the discrete-continual hardening have been developed. Keywords: stress-strain state, discrete-continual hardening, contact interaction, finite element method, contact pressure, machine parts

Fatigue Life Prediction of Rolling Bearings Based on Modified SWT Mean Stress Correction

The existing engineering empirical life analysis models are not capable of considering the constitutive behavior of materials under contact loads; as a consequence, these methods may not be accurate to predict fatigue lives of rolling bearings. In addition, the contact stress of bearing in operation is cyclically pulsating, it also means that the bearing undergo non-symmetrical fatigue loadings. Since the mean stress has great effects on fatigue life, in this work, a novel fatigue life prediction model based on the modified SWT mean stress correction is proposed as a basis of which to estimate the fatigue life of rolling bearings, in which, takes sensitivity of materials and mean stress into account. A compensation factor is introduced to overcome the inaccurate predictions resulted from the Smith, Watson, and Topper (SWT) model that considers the mean stress effect and sensitivity while assuming the sensitivity coefficient of all materials to be 0.5. Moreover, the validation of the model is finalized by several practical experimental data and the comparison to the conventional SWT model. The results show the better performance of the proposed model, especially in the accuracy than the existing SWT model. This research will shed light on a new direction for predicting the fatigue life of rolling bearings.

Effects of carriage flexibility on friction force in linear ball guides

This paper presents the effects of carriage flexibility on the friction force in linear ball guides, which includes hydrodynamic rolling friction, elastic hysteresis friction, slip friction, and drag friction. To this end, we developed a computational model for the friction force in linear ball guides that accounts for the carriage flexibility. The model was validated through experiments, and the results prove that it provides more accurate friction-force estimates than the conventional model under the assumption of a rigid carriage. Subsequently, we examined the effects of external load, preload, and speed on the friction force. Among several friction components, hydrodynamic rolling friction makes a major contribution to the total friction force. Ball contact loads, which significantly vary with carriage flexibility, were found to influence the hydrodynamic rolling, elastic hysteresis, and slip friction forces. The proposed model considering carriage flexibility in linear ball guides is expected to find use in the design and operation of linear-ball-guide systems.

Methods for determining the bearing capacity and extreme pressure properties of lubricants used in heavy-loaded tribodynamic couplings of metallurgical machines and units

To solve contact problems related to the study of the interaction between a lubricated or non-lubricated friction pair, we used standard equipment - friction machines and tribometers. However, the use of factory-made research equipment does not always allow us to accurately answer the questions concerning the tribotechnical characteristics of friction and wear processes under specific operating conditions. A review of the literature on the technical capabilities of testing machines showed that their main drawback is the inability to programmatically set the modes of acceleration of the tribological system and tracking slippage in real-time. The study revealed the need to develop a methodology for investigating the extreme pressure properties of liquid lubricants in the tribocoupling acceleration mode, which is caused primarily by the effect of high contact loads acting in the contact areas of friction pairs, e.g. gears, rolling bearings, etc. when technological machines and equipment start and reach the operating speed. The paper considers a modernized friction machine capable of programmatically setting acceleration modes and investigating tribodynamic processes of lubricated frictional couplings. We developed a technique that makes it possible to evaluate the efficiency of a liquid lubricant during acceleration and its effect on the wear of a lubricated contact based on the use of the modernized friction machine.

An analytical model of static stiffness for linear rolling guideway considering the structural deformations of carriage

To solve the deformations and stiffnesses changes of linear rolling guideway under different loads, and to provide reference for the structural design of the carriage, this paper presents an analytical model of three-degrees-of-freedom static stiffness for linear rolling guideway considering the structural deformations of the carriage. In this study, first, the contact loads and elastic deformations between balls and raceways caused by external loads and preload were obtained by Hertz contact theory. The elastic deformations between balls and raceways were described by the change of the curvature centers of the carriage raceways. Next, according to the constraints and loading conditions of the carriage, the elastic beam theory was introduced, and the structural deformations of the carriage under the contact loads were equivalent to the deformations of two cantilever beams. Then, the external loads and the displacements of the carriage were derived by the static equilibrium conditions of the carriage. At last, the proposed equivalent model of carriage structure deformations was validated by comparing the calculated deformations of the carriage with those from a commercial program under various loading conditions, and the accuracy of the static stiffness model proposed in this paper was further verified by the experimental results in the reference. The results show that the calculated structural deformations of carriage matched the deformations calculated by a commercial program well. Also, with relative errors of 4.4–17.5%, the calculated stiffnesses using the model proposed in this paper more closely matched the measured stiffnesses. Clearly, there is a better match between the calculated results of the proposed model and the measurements than with the conventional rigid model.

Comparison between Synthetic Oil Lubricants for Reducing Fretting Degradation in Lightly Loaded Gold-Plated Contacts

This paper presents a comparison between the performances of two chemistries of synthetic oil lubricants, polyalphaolefins (PAOs) and perfluoropolyethers (PFPEs) when applied on gold-plated electrical contacts operating at contact loads of 9.8 cN and experiencing fretting-induced degradation. Performance assessment was done using the contact resistance and coefficient of friction behavior and the surface’s response to fretting in the presence of different types of lubricants within the two chemistries. It was found that the PAOs improved the fretting performance of the lightly loaded contacts, and statistically, were at least fifty times more reliable for a longer duration of fretting cycles than the PFPEs, suggesting their suitability for low contact load applications. At low loads, PFPEs underwent contact separation due to hydrodynamic lubrication, and the behavior was more observable among the PFPEs having higher kinematic viscosities. On the contrary, viscous PAOs had improved fretting performance and delayed time to contact failure than less viscous PAOs. The applied lubricant film thickness also contributed to the contact’s performance, and it was found that increasing the thickness of the PFPE films advanced contact failures, while the PAO film postponed contact’s time to failure.