共查询到20条相似文献,搜索用时 31 毫秒
1.
Bharat Bhushan 《Wear》1984,93(1):81-99
The cylinder friction and the normal stress distribution were calculated for the general case of a magnetic tape wrapped around a cylinder. The coefficient of friction was found to depend on the radius r of the cylinder and the initial tension T0 according to the relation and to be independent of the wrap angle θ0. On the basis of tests conducted on five commercial magnetic tapes and on a typical tape substrate (poly(ethylene terephthalate)), n was found to be close to 1. Hence r, T0 and θ0 in the ranges used in the tests reported here have no significant influence on the coefficient of friction.For the speed range from 27 to 80 mm s?1, the sliding speed has no influence on the coefficient of friction, but temperature and humidity do have a significant influence. Therefore, temperature and humidity conditions should be selected on the basis of the application of the machine. 相似文献
2.
S.M. Aharoni 《Wear》1973,25(3):309-327
Wear by roll-formation is common among polymers when bulk deformation and “give” exist. The necessary condition for such wear is that the strength of an asperity of a given polymer will be lower than both the strength of the opposing asperity of the other polymer and the interfacial friction force over the relevant surface of both asperities.Ductility, manifested by roll-formation, was observed in high density polyethylene (HDPE), polypropylene (PP), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), Nylon 6, polyethylene terephthalate (PET), polyamide imide(PAI), polybutadiene(PBD), and polymethylmethacrylate(PMMA). Ductility, manifested by the shear of a whole surface layer, was observed, also, in polystyrene(PS).Ductile wear of brittle polymers is shown to be dependent on () and the product of the applied stress and the volume that can undergo cooperative ductile deformation independently from other, adjacent or close, such volumes. 相似文献
3.
T. Hisakado 《Wear》1977,44(2):345-359
An analysis of the electrical contact resistance between two metals with a tarnish film was carried out assuming the asperities to be represented by randomly distributed cones with base angles which vary with the surface roughness, and assuming that the radius of the broken area of the film at the interface of each contact asperity is constant beyond a critical depth of penetration of an asperity.The validity of the proposed theory was confirmed by experimental data of the electrical contact resistance between a silver cone and a silver flat on which carbon films were deposited, and between a silver flat with an Ag2S tarnish and a palladium flat without tarnish. Comparison of theoretical and experimental data shows that the critical depth of penetration Ue of an asperity varies mainly with the surface roughness, the thickness of the tarnish film and the amount of plastic deformation of the contact asperities with films, i.e., where Rmax and σ are the maximum height and the standard deviation (r.m.s. roughness) of the profile ordinates, and j and k are constants dependent on the type of finish, the thickness of the film and the difference in the hardness of the mating surfaces. 相似文献
4.
T. Hisakado 《Wear》1975,35(1):53-61
Conical asperity models with four kinds of apex angle and sandblasted surfaces were separately deformed by being pressed against a relatively hard flat surface. The real area of contact for the work hardened asperity models increased with decreasing apex angle and that for the sandblasted surfaces also increased with increasing surface roughness. The number of contact points and the separation between a sandblasted and a harder flat surface over the pressure range were also measured for comparison with the theoretical results. The experimental results for the sandblasted surfaces agreed with the theory. It is concluded that the real area, the number of contact points and the separation are influenced by the roughness of the softer surface. 相似文献
5.
A.Broese Van Groenou 《Wear》1977,44(2):203-211
Experimental data on the depth dependence of the specific energy of grinding and of single-point scratching does not agree with the predictions of the ploughing model. In this paper the model is extended to take into account the heat generated beneath the scratching point, the subsequent softening of the material and the influence on the specific energy e and the scratch hardness Hs. For pyramidal points ; for spheres , whereas Hs shows a different behaviour which depends on the choice of parameters. The dependence on depth d and speed vs agrees with experimental data on the grinding and scratching of steel. 相似文献
6.
Stylus measurements of the microroughness of rotating disks and their significant correlation with hydrodynamic drag measurements were studied. The roughest disks (Ra ≈ 16 μm) were found to have drag coefficients about 30% greater than those of the smoothest disks (Ra ≈ 0.14 μm). For a Reynolds' number of 1.5 × 106, the following empirical relationship between the drag coefficient C, the average roughness Ra and a parameter designated the peak count wavelength λpc was obtained: where .Other surface parameters and functions were measured in addition to Ra and λpc; however, it seems that knowledge of an amplitude-sensitive parameter and a wavelength-sensitive parameter is adequate for characterizing increases in the drag of rotating disks due to surface roughness. Ra is the preferred amplitude-sensitive parameter because it is the most widely used. We propose λpc as the wavelength parameter because it is sensitive to the larger profile features rather than to the fine structure.However, there is a need for all workers in the field to standardize on measuring the same parameters so that the results of one group can be easily related to the work of other groups. 相似文献
7.
M. Gotoh 《International Journal of Mechanical Sciences》1977,19(9):505-512
It is shown from various view points that many of the disadvantages of the conventional theory based on a quadratic yield function can be satisfactorily removed by the use of a yield function of fourth order. Incremental equivalent strain is defined by , and cannot generally be expressed simply by the strain increment components d?ij. In contrast with the conventional theory, coefficients in the yield function f cannot be determined from the r-values only in uniaxial tensile tests, but yield stresses in these tests and for example in an equi-biaxial tension for the same are also required. This fact ensures that the curve for arbitrary loading is uniquely determined by the uniaxial tension curve in the rolling direction (R.D.), and thus such an intrinsic difficulty of the conventional theory as dependence of the curve on types of loading does not arise. Some formulae for the determination of the coefficients in f are given. Relationships between types of earing in axi-symmetrical deep-drawing and the coefficients of f are examined in detail and it is emphasized that only very special cases are included in the conventional yield function and thus use of it is very limited. 相似文献
8.
G. Repp 《Mechanism and Machine Theory》1974,9(2):191-201
In its first part the paper considers, from an advanced point of view, the essential properties of Joukowski's conformal mapping . In general, the point corresponding to a real point Z (z=x+iy) is uniquely determined, with the exception of the fundamental points situated at the origin O and the circular points (Fig. 1). Conversely, a real point arises from two real points Z, Z′ and two conjugate imaginary points Z″, Z3? are related by the involutary Moebius transformation V: z.z′ = 1.The circle k (1.8), determined by its complex center coordinate M≠0 and radius r≠|m|, is carried by the cubic transformation J into a rational bicircular quartic (1.10), but the same curve is also the mapping of a second circle k′ (1.11), connected with k by V (Fig. 4). The singular foci and correspond to the centers M and N of k and k′. The proper point-pair D, D′ common to k and k′ is mapped onto the proper double point of .The second part deals with the kinematic generation, at first by means of an antiparallelogram, of such a quartic , determined by and an arbitrary simple point . The foci are the fixed pivots of the four-bar linkage , and the position of the coupler triangle is to be found as the mirror image of the triangle with respect to the perpendicular bisector of (Figs. 5 and 6). According to the theorem of Roberts, there exist two additional kite linkages generating the same quartic . The third fixed pivot corresponds to in the reflection (Fig. 6).The essential dimensions of all generating linkages are given by formulae (2.2–2.7). 相似文献
9.
Squeeze film step bearings lubricated with a micropolar fluid were analysed. It was found that the load capacities increase as the micropolar parameter increases or as decreases. The load capacity also decreases as the step height increases. 相似文献
10.
Electrical brushes which were made of silver, copper and aluminum foils of 12.5 and 25 μm thickness and were composed of 15–195 individual foils, were tested in purified argon on a polished copper rotor at a speed of 13 m s?1. Brush pressures varied between 3.1 × 103 and 2.8 × 104 N m?2 and current densities were up to about 700 A cm?2 (about 4500 A in?2).The observed dependence of the voltage drop across the brushes as a function of the current densities agreed closely with Holm 's contact theory as applied to foil brushes. The film resistivities were found to be near for copper and silver and to be about for aluminum. The projected performance of foil brushes based on these results is very favorable and the future commercial use of foil brushes appears to be possible.The total loss, electrical and mechanical, through the brushes is independent of current density if the brush pressure is chosen to minimize the total loss. If so, the loss depends only on the brush speed, the hardness of the softer of the two materials involved (i.e. of foil and rotor or slip ring), the coefficient of friction and the film resistivity.Microscopic surface examinations of rotor and brushes show that the brush surface is smoothed through running the brush, whereas the rotor remains almost unaffected or is mildly roughened, as long as no arcing takes place. Arcing causes considerable surface roughening on both the brush and the rotor surface and debris is thus deposited on the rotor; this can score the brushes. Further experiments are required to determine the rate of brush wear. 相似文献
11.
Coulomb damping is often relied upon to control the response of nuclear boiler tube banks acted upon by fluctuating eddy forces from turbulent coolant flow. The required energy dissipation is affected by relative sliding movements between the clamped joints of the tube and support structure. These movements Z are in general related to the centre span deflection y which is coupled to the exciting forces by the relation where the αn are termed coupling coefficients. Specific models of the damping are developed for direct Coulomb damping (n = 1) and second-order damping (n = 2) under conditions where a constant friction force, i.e. static loading, is assumed to act. The effects of partial slip across the joint are examined, and this gives rise to a damping contribution at amplitudes that lie below the threshold yp for gross slip at the joint. In general direct Coulomb damping gives rise to a non-linear type of damping and also changes natural frequency with amplitude. Second-order damping tends asymptotically to a constant loss factor at large amplitudes but in general would be comparable with the hysteretic damping.The penalty from frictional damping is the occurrence of an associated wear rate which can be expressed by where K is the specific wear rate, ξD is the energy dissipated as wear and μ the friction coefficient. Since K and μ are defined by environmental and material properties compatible with nuclear environments, the design should be optimized to minimize the energy dissipation. 相似文献
12.
13.
Previously published measurements of the cell diameters d of dislocation cells underneath copper surfaces exposed to particle erosion were evaluated in terms of the subsurface stresses τ to which they correspond. These were compared with the elastic stresses expected underneath spherical indenters impacting on the surface with different speeds. The inferred stresses differ markedly from theoretical predictions, not only in regard to the dependence on speed and angle of impact but even in regard to their decay along the z axis, the direction normal to the surface. Instead of τ decreasing as z?n with n continuously rising from 0 at a shallow depth to 2 at large depths, as predicted from elastic theory, the stresses follow a z?1 dependence throughout the measured range. All data are satisfactorily explained by the relationship (using MKS units), where v stands for the velocity of the impacting particles, namely rather irregularly shaped alumina particles 50 μm in diameter, and α is the angle of impact. No theoretical explanation has so far been found to account for this result. It is noted, however, that previously a z?1 decay of stress had already given excellent results in a theoretical interpretation of the subsurface shear strain underneath a surface subject to sliding wear. Further, sin α is the direction cosine for the normal force component, whereas a velocity dependence of is found from elastic theory for the depth scaling of the elastic stress due to a spherical indenter. Suggestions are made for further experiments. 相似文献
14.
A detailed comparison of cavitation erosion performance in tap water for five alloys in a vibratory (no-flow) system and a Venturi (flow) system was made. The effects of temperature variation (80 – 200 °F), Venturi throat velocity (34 – 49 m s?1) and vibratory horn double amplitude were studied. Correlations between maximum erosion rate (maximum mean depth of penetration rate (MDPRmax)) and incubation period IP, and the material mechanical properties Brinell hardness and ultimate resilience . (where UTS is the ultimate tensile strength and E is the elastic modulus), were examined. Only moderate success was achieved in correlations between “erosion resistance” MDPRmax?1 and IP and these mechanical properties. However, a good correlation was found between MDPRmax and IP, pertinent to both facilities, of the form MDPRmax?1 = aIPn, where n is near unity (0.94). The cavitation intensity, as measured by MDPRmax, was found to be 10–20 times greater in the vibratory system, depending on horn amplitude and material. This ratio varies between 5 and 30 if individual materials are considered separately, being greatest for 1018 carbon steel and least for 316 stainless steel. This indicates the important differences in form between these cavitating regimes and the imprecision of material comparisons made in both regimes. 相似文献
15.
An improved gear surface of helical gears for pumping action is obtained mainly by theoretical investigation, and partly by test results. This is a further study following the research on gears[1]. A new theory is presented here by selecting characteristic function sequences which is monotone increasing. The larger n becomes, the larger rate of mass flow increases at the same time, thus the optimum tooth profile of gears (consequently the optimal gear surface) can be determined under the assumption that the minimum available cutting pressure angle (θONO be given in advance. 相似文献
16.
An exploratory investigation was conducted concerning materials and their properties for use in aircraft brakes. The overall objective of the program is to develop improved brake materials primarily from a safety point of view. In this investigation a study was made in order to determine what improvements could be made in present brake materials. Primary consideration was given to the heat dissipation. Used brake pads and rotor disks, taken from aircraft at overhaul were analyzed to provide additional information on material behavior.A simplified analysis was conducted in order to determine the most significant factors which affect surface temperature. Where there are size and weight restrictions the specific heat and maintaining the contact area appear to be the most important factors. A criterion is suggested for determining the number and thickness of brake disks, within the limited space available in a wheel, to provide an optimal compromise between effective surface temperature of the disk and weight. This criterion is that the quantity ( should be approximately equal to 1 where α is the thermal diffusivity, tmax is the time at the end of the braking cycle and l is half the disk thickness.For lower surface temperatures maximum benefit can be obtained by using materials of high specific heat (and density) and by maximizing the contact area. 相似文献
17.
Recent research into elastohydrodynamic (EHD) traction has shown that under high pressure and isothermal conditions the flow properties of typical lubricants follow the Eyring relation between shear stress τ and shear rate /.γ: where η is the Newtonian viscosity and τ0 a representative stress. The maximum in the traction curve arises from shear heating, and Crook's thermal analysis for a Newtonian lubricant has been modified to apply to an Eyring lubricant.In an EHD contact the pressure and hence the viscosity η vary from inlet to outlet, but it is shown that under the conditions of maximum traction it is sufficiently accurate to use the average values of η and τ0 associated with the average values of pressure and temperature through the length of the contact. A simple formula can then be derived for the maximum traction coefficient in terms of the properties of the fluid (viscosity, pressure and temperature indices, and the representative stress τ0 and the operating conditions (contact pressure, speed and film thickness). 相似文献
18.
19.
A surface roughness parameter in Hertz contact 总被引:2,自引:0,他引:2
The influence of surface roughness on the accuracy with which the Hertz theory of elastic contact predicts the contact pressure and contact area between a sphere and a plane is examined theoretically and experimentally. Statistical theories of surface contact suggest that the influence of surface roughness is governed primarily by a single non-dimensional parameter α defined by where σ is the combined roughness of the two surfaces, R is the radius of the sphere and a0 is the contact radius for smooth surfaces given by the Hertz theory. Experimental measurements of contact area correlate well with this parameter. Provided that the value of α is less than about 0.05, errors in the application of the Hertz theory due to roughness of the surfaces are not likely to exceed about 7%. 相似文献
20.
A theory of friction during the steady state sliding of metals is developed on the basis of the upper-bound method, and it focuses on the energy consumed by the relative motion of materials. The sublayer's deformations, the apparent Coulomb (Amontons) coefficient of friction for moderate loads and the constant friction factor for excessive loads are determined as functions of the nominal normal pressure , the interface friction factor m0 and the wedge angle α1. Furthermore, a criterion for steady state sliding is offered through the optimization process. The results appear to be consistent with a number of published observations. 相似文献