Excitation of resonance self-oscillations in a system with two sources and nonlinear dissipative force

A single-mass mechanical system with dissipative nonlinearity and two excitation sources, one parametric source created due to the variable rigidity component and the other a force source, is considered. It is assumed that the quasi-optimal structure of the laws of parametric and force excitations is determined based on a model that does not take into account dissipative nonlinearity; this allows one to obtain the structure in a simple analytical form. The system with these excitation sources is analyzed in the harmonic approximation. The possibility of implementation of the unique self-oscillatory mode is demonstrated, as exemplified by a system with turbulent friction.     

Resonance bifurcation during self-oscillations of the rotor blades of a turbocompressor

Bifurcation of the trajectory of the resonance for the blades of an axial rotor, which leads to flutter of the blade cascade, is investigated. For this purpose, a spectral analysis of readouts of the transient period of blade oscillation is performed. As opposed to the resonance with a constant phase difference and excited oscillations of the blades, the shift of the phase of collective synchronous oscillations of the blades during flutter depends on their number and the form of the diametral mode of the blade cascade. The flutter of the blades is associated with one diametral mode of pressure pulsations of the air flow.     

Frictional mutation

Attention focuses on frictional mutation, which has been observed in the wear of frictional components.     

Frictional Coefficient in Hot Drilling

Russian Engineering Research - A method is proposed for determining the frictional coefficient in hot drilling. Methods of determining the frictional coefficient in analogous conditions are...     

On the problem of the excitation of self-oscillations in relaxation friction

With regard to the problem of decreasing the vibration activity of devices that perform the translational motion of the working body along the pathway, new results have been obtained that determine the threshold speed value; the friction self-oscillations occur lower than this value. It has been established that this value increases upon a decrease in the dissipative characteristics of a drive. The dual character of the influence of the high-frequency disturbance, which leads to the positive effect due to the vibration linearization of the dry friction forces, but simultaneously decreases the realized dissipation level of a drive, has been studied. The optimization of the high-frequency excitation parameters has been performed based on an analysis of the combined influence of these factors. The main conclusions have been proved based on the results of computer modeling.     

Frictional losses in shaft bearings

Analytical formulas are obtained for the frictional losses in shaft bearings on the basis of the Amonton-Coulomb law. The analysis takes account of the molecular-adhesion forces, with corresponding improvement in accuracy.     

Frictional Behavior of sodium-lubricated materials in a controlled high-temperature environment

The frictional behavior between sliding-contact specimens was studied over a temperature range of 80 to 1,300°F. Speed and load were about 0.2 mm/sec and 100,000 p.s.i., respectively. Experiments were performed in argon-swept vacuum, both dry and with molten sodium at the specimen interface. Materials studied include molybdenum, tungsten, tungsten carbide, and titanium carbide.It was found that chemisorbed films were produced on the specimen surfaces, and the composition of these was largely determined by the type and duration of environmental exposure. The relationship between the observed stick-slip behavior and the presence of surface films of specific compositions is discussed.     

Application of the method of discrete vortices to compute self-oscillations of a tube in liquid flow

The method of mathematical modeling and the application of the advanced method of discrete vortices to calculate the parallel-sided separated flow around bodies oscillating in the flow are presented. These methods find an application in the investigation of the nature of hydrodynamic mechanisms of the vibration excitation of tube bundles in heat-exchange apparatuses.     

Frictional cladding by means of a flexible tool

Experimental and statistical models are developed for frictional cladding. The parameter ranges corresponding to the formation of coatings with complete coverage and a thickness of up to 15 μm are determined.     

Frictional heating of a half-space with cracks. I. Single or periodic system of subsurface cracks

The paper deals with the two-dimensional problem of frictional heating for an elastic half-space containing one or a system of periodic subsurface cracks. The generation of frictional heating in the region of contact is estimated by the use of sliding speed, frictional coefficient and contact pressure. The problem is reduced to a pair of singular integral equations which are solved numerically. The influence of the location of cracks on the stress intensity factor is investigated.     

Frictional charging of fuel

A method is proposed for reducing the smoke content and toxicity of exhaust gases by frictional charging of motor fuel. Experimental data are presented.     

Frictional behaviour of metals on a crossed cylinder apparatus

It is shown that junction growth occurs until the normal stress over the junction falls to a fixed proportion of the yield shear stress of the specimen when slip occurs. An equation is given which permits an approximate calculation of μ.     

Frictional Voltammetry with Copper

This paper presents an experimental study correlating frictional behavior with in situ voltammetry for a unidirectional sliding contact between a hemispherical tipped alumina probe and a flat rotating copper counterface (maximum Hertzian contact pressure of 68 MPa and sliding speed of 10 mm/s). The contact was immersed in an aqueous 0.1 M Na₂CO₃ solution (pH ∼11) where the copper counterface acted as the working electrode in a potentiostat controlled three-electrode cell; a coiled Pt wire was used as the counter electrode and a saturated calomel electrode (SCE) as the reference. Clear and reproducible trends were found between friction coefficient and published data suggesting the onset of particular redox reactions, graphically presented in a frictional voltammetry plot. At anodic potentials primarily associated with the formation of copper(I) oxide (Cu₂O) (V vs SCE ∼−0.25), the measured friction coefficient was in the range μ ∼0.4–0.5. At cathodic potentials primarily associated with the formation of CuO, Cu(OH)₂, and CuCO₃ (V vs SCE ∼−0.10), the friction coefficient transitions to the range μ ∼0.7–1.0. At sustained cathodic potentials associated with reduction of the native copper oxide, Cu₂O, (V vs SCE ∼−0.65), the friction coefficient is observed to fluctuate between μ ∼0.2 and 0.5, arguably a result of exposure of bare copper due to non-uniform reduction (fractional coverage) of Cu₂O.     

Role of Frictional Heating in Rubber Friction

The energy dissipation in the contact regions between solids in sliding contact can result in high local temperatures which may strongly affect the friction. This is the case for rubber sliding on road surfaces at speeds above 1 mm/s. I derive equations which describe the frictional heating for arbitrary (non-uniform) motion, taking into account that some of the frictional energy is produced inside the rubber due to the internal friction in rubber. Numerical results are presented for one limiting case for steady sliding.     

Frictional Properties of a Mesoscopic Contact with Engineered Surface Roughness

Friction between titanium spheres and an artificially structured silicon surface was measured with a friction force microscope. Two spheres with radii of 2.3 μm and 7.9 μm were firmly glued to the tip of the microscope cantilever. A periodic stripe pattern with a groove depth of 26 nm and systematically increasing groove width from 500 nm to 3500 nm was fabricated from a silicon wafer with a focused ion beam. The sphere substrate friction coefficient shows a strong enhancement at a certain groove periodicity, which is related to geometrical interlocking of the two surfaces. This shows that careful modification of the surface roughness can help to control the tribological behavior of mesoscale contacts.     

摩擦焊接初始阶段的摩擦机制及摩擦系数

对影响摩擦焊接初始阶段温度场的摩擦机制及摩擦系数进行了实验研究。结果表明：在摩擦焊接初始阶段，摩擦表面的摩擦机制主要为粘着摩擦，在外缘区域存在着氧化摩擦。文中还通过回归方法建立了ＧＨ２１３２材料摩擦焊接初始阶段摩擦系数与摩擦压力、摩擦速度及表面温度之间的经验公式。     

Chaotic frictional self-oscillations of the elastic layer contacting the moving semispace

It is revealed that the dynamic behavior of the elastic layer becomes chaotic in contact with the rigid semispace. A bifurcation diagram of the process of doubling of the period of self-oscillations in response to the intensity of the friction mechanism is constructed.     

Frictional performance of silica microspheres

We present a fundamental study of the frictional performance of silica microspheres, either 4, 2, or 0.5 μm in diameter, on a silicon substrate. The tribological properties of these rolling systems were measured with a ball-on-flat tribometer at various loads. The frictional performance of the rolling systems is found to be highly dependent on load and sphere diameter. Rolling motion was confirmed by imaging the surface of microsphere systems that have been coated with a thin layer of gold. In addition, the surface energy of the lubrication scheme was altered using coatings containing octadecyltrichlorosilane or (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane precursors.