Home About us Contact
|
Home About us Contact | ||
|
|
||
Friction
Kinds of Friction Terms modified by Friction Selected AbstractsAPPLICATION OF COEFFICIENT OF FRICTION TO THE SEPARATION OF COCOA HUSK,BEANS MIXTUREJOURNAL OF FOOD PROCESS ENGINEERING, Issue 5 2007O.K. OWOLARAFE ABSTRACT Investigation was carried out on the application of coefficient of friction in the separation of cocoa beans,husk mixture. An inclined plane at angles 20, 25 and 35°rotating at different speeds (215, 250 and 260 rpm) and receiving the mixture at heights 30, 60 and 90 mm, was used for the study. The result indicates that within the range of factors considered, the separation efficiency of the beans from the mixture increases with increase in speed of rotation and height of fall, while it decreases with increase in angle of inclination. Statistical analysis shows that the effects of each of the factors and that of their interaction are significant at 99%. The best separation efficiency (99%) was observed at a speed of 250 rpm, an angle of inclination of 25° and a height fall of 90 mm. PRACTICAL APPLICATIONS Separation of cocoa beans from the husks has been a serious problem in cocoa processing. Handpicking is still being used to separate the mixture of cocoa beans,husk even with the mechanical pod breaker available. This therefore constitutes a tedious operation. An inclined plane mechanism tested for the separation of the mixture in this study can be incorporated into the design of the cocoa pod processor to reduce the drudgery involved in cocoa processing and improve the quality of the product. [source] DUALITY IN OPTIMAL INVESTMENT AND CONSUMPTION PROBLEMS WITH MARKET FRICTIONSMATHEMATICAL FINANCE, Issue 2 2007I. Klein In the style of Rogers (2001), we give a unified method for finding the dual problem in a given model by stating the problem as an unconstrained Lagrangian problem. In a theoretical part we prove our main theorem, Theorem 3.1, which shows that under a number of conditions the value of the dual and primal problems is equal. The theoretical setting is sufficiently general to be applied to a large number of examples including models with transaction costs, such as Cvitanic and Karatzas (1996) (which could not be covered by the setting in Rogers [2001]). To apply the general result one has to verify the assumptions of Theorem 3.1 for each concrete example. We show how the method applies for two examples, first Cuoco and Liu (1992) and second Cvitanic and Karatzas (1996). [source] Reducing the Coefficient of Friction for Fast-Absorbing Gut SutureDERMATOLOGIC SURGERY, Issue 12 2009JONATHAN LEE BINGHAM MD No abstract is available for this article. [source] Precise disturbance modeling for improvement of positioning performanceELECTRICAL ENGINEERING IN JAPAN, Issue 2 2010Masafumi Yamamoto Abstract This paper presents a modeling methodology for unknown disturbances in mechatronics systems, based on disturbance estimation using an iterative learning process. In disturbance modeling, nonlinear frictions are specially handled as disturbances in the mechanisms, which mainly degrade trajectory control performance. Friction can be mathematically modeled by using learned estimation data as a function of the displacement, velocity. acceleration, and jerk of the actuator. This model has the distinctive feature that friction compensation can be achieved with a generalization capability for different conditions. The proposed positioning control approach with disturbance modeling and compensation has been verified by experiments using a table drive system on a machine stand. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(2): 31,39, 2010; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/eej.20928 [source] Nanotribology , Fundamental Studies of Friction and Plasticity,ADVANCED ENGINEERING MATERIALS, Issue 5 2010Roland Bennewitz Nanotribology explores the mechanical properties of materials at small length scales, where deviations from the scaling laws of macroscopic descriptions are observed. Atomic force microscopy is introduced as an important instrument in nanotribology for imaging friction contrasts on heterogeneous surfaces, for quantitative friction studies, and for the observation of single dislocation processes in plastic deformation. Recent experimental results for the frictional properties of carbon-based materials are discussed. Friction studies using microstructured surfaces are presented as an attempt to bridge the gap between nanotribological and macroscopic friction studies. [source] Tribological Studies of a Zr-Based Glass-Forming Alloy with Different States,ADVANCED ENGINEERING MATERIALS, Issue 11 2009Feng Jiang Abstract The tribological characteristics of a glass-forming alloy, Zr52.5Cu17.9Ni14.6Al10.0Ti5.0, in atomic percent (at.%, Vit 105), with different microstructural states have been investigated. Friction and wear studies were conducted using a ball-on-flat reciprocating sliding apparatus against an AISI E52100 bearing steel under dry condition. The observed wear resistance in an ascending order is: the deformed, creep-tested, and as-cast states. Wear analyses suggested that the wear processes of glass-forming alloys involved abrasion, adhesion, and oxidation. The differences in hardness, free volume, and brittleness in different states significantly affected the friction and wear behaviors of the glass-forming alloys. [source] Microstructure Evolution and Mechanical Properties of Linear Friction Welded 45 Steel JointADVANCED ENGINEERING MATERIALS, Issue 8 2007J. Ma Linear friction welding (LFW) is an emerging solid-state joining process to extend the current applications of welding. The microstructure evolution and mechanical properties of LFW 45 steel joint were investigated. The microstructures from the weld center to the parent metal were the superfine ferrite+pearlite in the weld center, the deformed fine ferrite + pearlite in the thermomechanically affected zone, the tempered sorbite, troosite and martensite in the heat affected zone. The microhardness of the joint decreased gradually from the parent metal to the weld center. The tensile properties of the joint were significantly improved in both the strength and ductility. The microstructure evolution, microhardness variation and fracture position are attributed to the various thermal histories of different positions. [source] Friction and degradation of rock joint surfaces under shear loadsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 10 2001F. Homand Abstract The morpho-mechanical behaviour of one artificial granite joint with hammered surfaces, one artificial regularly undulated joint and one natural schist joint was studied. The hammered granite joints underwent 5 cycles of direct shear under 3 normal stress levels ranging between 0.3 and 4 MPa. The regularly undulated joint underwent 10 cycles of shear under 6 normal stress levels ranging between 0.5 and 5 MPa and the natural schist replicas underwent a monotonics shear under 5 normal stress levels ranging between 0.4 and 2.4 MPa. These direct shear tests were performed using a new computer-controlled 3D-shear apparatus. To characterize the morphology evolution of the sheared joints, a laser sensor profilometer was used to perform surface data measurements prior to and after each shear test. Based on a new characterization of joint surface roughness viewed as a combination of primary and secondary roughness and termed by the joint surface roughness, SRs, one parameter termed ,joint surface degradation', Dw, has been defined to quantify the degradation of the sheared joints. Examinations of SRs and Dw prior to and after shearing indicate that the hammered surfaces are more damaged than the two other surfaces. The peak strength of hammered joint with zero-dilatancy, therefore, significantly differs from the classical formulation of dilatant joint strength. An attempt has been made to model the peak strength of hammered joint surfaces and dilatant joints with regard to their surface degradation in the course of shearing and two peak strength criteria are proposed. Input parameters are initial morphology and initial surface roughness. For the hammered surfaces, the degradation mechanism is dominant over the phenomenon of dilatancy, whereas for a dilatant joint both mechanisms are present. A comparison between the proposed models and the experimental results indicates a relatively good agreement. In particular, compared to the well-known shear strength criteria of Ladanyi and Archambault or Saeb, these classical criteria significantly underestimate and overestimate the observed peak strength, respectively, under low and high normal stress levels. In addition and based on our experimental investigations, we put forward a model to predict the evolution of joint morphology and the degree of degradation during the course of shearing. Degradations of the artificial undulated joint and the natural schist joint enable us to verify the proposed model with a relatively good agreement. Finally, the model of Ladanyi and Archambault dealing with the proportion of total joint area sheared through asperities, as, once again, tends to underestimate the observed degradation. Copyright © 2001 John Wiley & Sons, Ltd. [source] Hexagonal Surface Micropattern for Dry and Wet FrictionADVANCED MATERIALS, Issue 4 2009Michael Varenberg Hexagonal micropattern mimicking the attachment pads of bush crickets is reported to control dry and wet elastomer friction. This micropattern eliminates completely the stick,slip disturbances when in dry conditions, and prevents hydroplaning when in wet conditions, rendering the surface behavior stable and predictable. [source] Numerical derivation of contact mechanics interface laws using a finite element approach for large 3D deformationINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 2 2004Alex Alves Bandeira Abstract In this work a homogenization method is presented to obtain by numerical simulation interface laws for normal contact pressure based on statistical surface models. For this purpose and assuming elastic behaviour of the asperities, the interface law of Kragelsky et al. (Friction and Wear,Calculation Methods, Pergamon, 1982) is chosen for comparison. The non-penetration condition and interface models for contact that take into account the surface micro-structure are investigated in detail. A theoretical basis for the three-dimensional contact problem with finite deformations is shortly presented. The augmented Lagrangian method is then used to solve the contact problem with friction. The algorithms for frictional contact are derived based on a slip rule using backward Euler integration like in plasticity. Special attention was dedicated to the consistent derivation of the contact equations between finite element surfaces. A matrix formulation for a node-to-surface contact element is derived consisting of a master surface segment with four nodes and a contacting slave node. It was also necessary to consider the special cases of node-to-edge contact and node-to-node contact in order to achieve the desired asymptotic quadratic convergence in the Newton method. A numerical example is selected to show the ability of the contact formulation and the algorithm to represent interface law for rough surfaces. Copyright © 2003 John Wiley & Sons, Ltd. [source] Friction term discretization and limitation to preserve stability and conservation in the 1D shallow-water model: Application to unsteady irrigation and river flowINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 4 2008J. Burguete Abstract Friction is one of the relevant forces included in the momentum equation of the 1D shallow-water model. This work shows that a pointwise discretization of the friction term unbalances this term with the rest of the terms in the equation in steady state. On the other hand, an upwind discretization of the friction term ensures the correct discrete balance. Furthermore, a conservative technique based on the limitation of the friction value is proposed in order to avoid unbounded values of the friction term in unsteady cases of advancing front over dry and rough surfaces. This limitation improves the quality of unsteady solutions in wet/dry fronts and guarantees the numerical stability in cases with dominant friction terms. The proposed discretization is validated in some test cases with analytical solution or with measured data and used in some practical cases. Copyright © 2007 John Wiley & Sons, Ltd. [source] Friction and wear properties of NBR/PVC compositesJOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2007Xinwu Huang Abstract Acrylonitrial butadiene rubber (NBR)/Polyvinyl chloride (PVC) composites with different PVC content were prepared. The effect of PVC content on the mechanical strength and tribological properties of the NBR/PVC composites was investigated. The morphologies of the worn traces and debris of NBR/PVC composites and worn traces of mating ball were observed using a scanning electron microscope (SEM). It was found that the friction and wear of NBR/PVC was lower than that of NBR without PVC. The NBR/PVC composite with 30% PVC content showed the best synthetic mechanical and tribological properties. The inferior elastic properties and the lesser deformation under the applied load of composites with PVC resulted in hysteric force and adhesion force decrease, which leading to a lower friction and wear of NBR/PVC composites. The frictional failure unit of NBR70/PVC30 composite being smaller should be an important reason of the wear of the composite being lowest. The lubricating effect of PVC played an important role in decreasing the friction coefficient and wear of NBR/PVC composites. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007 [source] Influence of temperature, friction, and random forces on folding of the B-domain of staphylococcal protein A: All-atom molecular dynamics in implicit solventJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2007Anna Jagielska Abstract The influences of temperature, friction, and random forces on the folding of protein A have been analyzed. A series of all-atom molecular dynamics folding simulations with the Amber ff99 potential and Generalized Born solvation, starting from the fully extended chain, were carried out for temperatures from 300 to 500 K, using (a) the Berendsen thermostat (with no explicit friction or random forces) and (b) Langevin dynamics (with friction and stochastic forces explicitly present in the system). The simulation temperature influences the relative time scale of the major events on the folding pathways of protein A. At lower temperatures, helix 2 folds significantly later than helices 1 and 3. However, with increasing temperature, the folding time of helix 2 approaches the folding times of helices 1 and 3. At lower temperatures, the complete formation of secondary and tertiary structure is significantly separated in time whereas, at higher temperatures, they occur simultaneously. These results suggest that some earlier experimental and theoretical observations of folding events, e.g., the order of helix formation, could depend on the temperature used in those studies. Therefore, the differences in temperature used could be one of the reasons for the discrepancies among published experimental and computational studies of the folding of protein A. Friction and random forces do not change the folding pathway that was observed in the simulations with the Berendsen thermostat, but their explicit presence in the system extends the folding time of protein A. © 2007 Wiley Periodicals, Inc. J Comput Chem 2007 [source] Role of Internal Friction in Indentation Damage in a Mica-Containing Glass-CeramicJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2001Anthony C. Fischer-Cripps The indentation response of a mica-containing glass-ceramic that exhibits shear-driven yield in an indentation test is interpreted in terms of events occurring on the microstructural scale. It is proposed that shear-driven damage within the specimen occurs via internal sliding along cleavage planes within the mica platelets. The sliding surfaces in this case are considered to be atomically smooth so the real and apparent areas of contact coincide. The frictional shear stress is thus independent of the normal forces arising from thermal mismatch stresses and only depends on the work of adhesion of the interface and the scale of the contacts. The scale of contacts for these materials lies within an intermediate zone in which the frictional shear stress arises from the stress required to nucleate dislocation-like discontinuities within the material. This leads to a size effect similar to that experienced by a crack in Mode II loading and is in accordance with previous work in which a connection between such a size effect and the macroscopic response of the material was identified. This work has particular relevance to the design and manufacturing of ceramics in machining, wear, bearings, and coatings applications. [source] Soot,additive interactions in engine oilsLUBRICATION SCIENCE, Issue 1 2010Dairene Uy Abstract Soot is known to cause engine wear. In this work, we focus on how engine oil formulation affects soot-related wear, and how the lubricant-derived anti-wear film changes when soot is present. Friction and wear experiments of fully and partially formulated diesel engine oils (containing basestock, dispersants and viscosity modifiers) are conducted with a ball-on-disk rig in the presence of carbon black (CB) as a soot surrogate. The friction coefficient was largely unaffected by CB dispersed in the oils, but electrically insulating film formation, an indication of the formation of anti-wear films, was decreased. Wear on the disk was found to either remain the same or decrease when CB was present, depending on the oil formulation. An examination of the lubricant-derived films using Raman and Auger electron spectroscopies found that the presence of more abundant amorphous carbon and lesser amounts of anti-wear film components on the surface was associated with higher wear. Copyright © 2009 John Wiley & Sons, Ltd. [source] Influence of Normal Force and Humidity on the Friction and Wear of Unlubricated Reciprocating Sliding Steel/Steel CouplesLUBRICATION SCIENCE, Issue 4 2004D. Klaffke Abstract The friction and wear behaviour of different steel/steel couples was investigated in laboratory tests with unlubricated reciprocating sliding motion. Two different steel balls were tested against two different steel discs in dry, normal, and moist air at room tem-perature. The influence of normal force on friction and wear was studied in the range from 1 to 10 N for all three levels of relative humidity (RH). RH strongly influenced wear behaviour for all four couples, while the friction behaviour was less affected by RH. For all the couples, normal force was found to influence wear rate with a tendency for the wear rate to increase with decreasing normal force. The coefficient of friction also increased with decreasing normal force, but to a much lesser extent than that evaluated for the wear rate. [source] Influence of Temperature and Relative Humidity on the Friction and Wear of Unlubricated Reciprocating Sliding Steel/ Steel CouplesLUBRICATION SCIENCE, Issue 4 2004D. Klaffke Abstract Many tribosystems are subjected to different conditions with respect to temperature and humidity. Reciprocating sliding tests with steel/steel couples were performed using a laboratory test rig in air with varying relative humidity and temperature. During each test the friction force, the total linear wear, the electrical contact resistance, and the acoustic emission were recorded. Tests with self-mated couples of bearing steel (100r6) and of stainless steel (X10CrNiMoNb18-10/X5CrNi18-9) in a ball-on-disc arrangement revealed small effects of temperature and humidity on friction, but a strong effect of the water vapour content on the wear rate of the system. Attempts were made to correlate changes of wear behaviour with different wear mechanisms. [source] Friction and wear effects on a micro/nano-scaleLUBRICATION SCIENCE, Issue 1 2001E. Santner Abstract In this paper are described tribological effects which can be found in micro-tribological systems, and in those macro-systems which can be analysed by micro-methods, e.g., by atomic force microscopy (AFM) or related methods. Micro-tribology systems have friction contacts with loads in the micro/nano-newton range and/or dimensions in the micro/nanometre range. Experiments on the micro/nano-scale should be easier to explain by theoretical modelling due to their simpler system structure. An example is discussed of adhesion and friction measurements between AFM tips and clean, flat, solid surfaces in ultra-high vacuum, which shows some of the special aspects of micro/nano-tribology. Surprising friction characteristics on surfaces with an artificial micro-structure can be explained by skilled and careful topographical analysis of the friction path with an AFM. In micro-sensor contacts, ,single wear events' can be detected using AFM analysis of the contact region. For ceramic compounds, different friction levels for the components of the material can be found. The problems, difficulties, and dangers of misinterpretation are also discussed. [source] Torsional Vibration Damping Through Frictional Torsion Damper with Structural Friction and Slide Taken into ConsiderationPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2005Zbigniew Skup Ph. DSC The paper is concerned with a non-linear discrete stationary mechanical system containing a frictional torsion damper. Proper effect of vibration damping in a two-degree-of freedom system can be reached by the right selection of geometrical parameters for given loads, as pre-determined by a mathematical model. Structural friction was considered, as well as small relative sliding of damper's discs cooperating with a plunger. The system vibrates under harmonic excitation. The problem was considered on the assumption of uniform unit pressure distribution between the contacting surfaces of friction discs and the plunger. When the discs are sliding, the friction coefficient varies, depending on relative angular velocity. Friction characteristics were assumed on the basis of the author's own research and experimental testing by other authors. Properties of the material were assumed to be in accordance with classical theory of elasticity. The author analysed the influence of parameters of the dynamic system upon amplitude and frequency characteristics as well as on phase and frequency characteristics. The equation of motion was solved by means of the slowly-varying-parameters method and, in order to compare the results, by means of numerical simulation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Friction and Party Manifesto Change in 25 Countries, 1945,98AMERICAN JOURNAL OF POLITICAL SCIENCE, Issue 1 2009Stefaan Walgrave Political processes are affected by "friction." Due to cognitive limitations and institutional delays, political agendas do not adapt smoothly to real-world impulses; political agendas either ignore them or overreact. The first question this article tackles is whether the same punctuated change process can be observed in party manifestos. Secondly, it examines whether there are differences across political systems and across party lines. Thirdly, the study tries to account for differences in the degree of "punctuatedness" of party manifestos. Drawing on the vast dataset of the Manifesto Research Group, the article shows that party manifestos are indeed characterized by friction and resistance to change; it also establishes that there are considerable differences in frictional patterns between parties and political systems; and it finds that electoral fragmentation, government participation, and electoral volatility are key to understanding these differences. [source] Loss of cartilage structure, stiffness, and frictional properties in mice lacking PRG4ARTHRITIS & RHEUMATISM, Issue 6 2010Jeffrey M. Coles Objective To assess the role of the glycoprotein PRG4 in joint lubrication and chondroprotection by measuring friction, stiffness, surface topography, and subsurface histology of the hip joints of Prg4,/, and wild-type (WT) mice. Methods Friction and elastic modulus were measured in cartilage from the femoral heads of Prg4,/, and WT mice ages 2, 4, 10, and 16 weeks using atomic force microscopy, and the surface microstructure was imaged. Histologic sections of each femoral head were stained and graded. Results Histologic analysis of the joints of Prg4,/, mice showed an enlarged, fragmented surface layer of variable thickness with Safranin O,positive formations sometimes present, a roughened underlying articular cartilage surface, and a progressive loss of pericellular proteoglycans. Friction was significantly higher on cartilage of Prg4,/, mice at age 16 weeks, but statistically significant differences in friction were not detected at younger ages. The elastic modulus of the cartilage was similar between cartilage surfaces of Prg4,/, and WT mice at young ages, but cartilage of WT mice showed increasing stiffness with age, with significantly higher moduli than cartilage of Prg4,/, mice at older ages. Conclusion Deletion of the gene Prg4 results in significant structural and biomechanical changes in the articular cartilage with age, some of which are consistent with osteoarthritic degeneration. These findings suggest that PRG4 plays a significant role in preserving normal joint structure and function. [source] Sensitive skin is not limited to the faceBRITISH JOURNAL OF DERMATOLOGY, Issue 1 2008C. Saint-Martory Summary Background, Sensitive skin (or reactive or hyper-reactive skin) is defined as skin that reacts by erythema and/or subjective symptoms (pricking, burning, pain, pruritus etc.) to stimuli that are not pathogens in themselves (e.g. wind, heat, cold, water, cosmetics, stress). This phenomenon is very frequent, occurring in about 50% of the European population. Objectives, Sensitive skin is always reported on the face. The aim of our study was to determine if it can occur in other localizations. Methods, We have performed this study in two centres. One was a department of dermatology in a university hospital while the other one was a centre for cosmetological studies. A questionnaire was given to women aged > 15 years. The questions were: Do you have sensitive skin? If yes, in which localization? What are the symptoms and triggering factors? Results, Four hundred subjects were included in the study (200 in each centre). The two populations were similar in terms of age, sex, and most of the results. The mean age was 40 years. Eighty-five per cent of the 400 subjects declared that they had sensitive skin on the face, and 70% had sensitive skin in another area: hands (58%), scalp (36%), feet (34%), neck (27%), torso (23%) or back (21%). Triggering factors included cold (66%), heat (28%), stress (61%), sun exposure (51%), wind (42%), water from a shower (29%) or a swimming pool (40%), soaps (42%), cosmetics (28%) and pollution (18%). Friction from clothes was reported in 28% of cases. Sensitive skin was observed as redness in most cases along with various subjective symptoms. Conclusions, The proportion of subjects presenting with sensitive skin is probably overestimated. However, the main result of this study is that sensitive skin is not restricted to the face but rather it is also present at other localizations, mainly the hands, and often the scalp and feet. [source] Liquidity in Asset Markets With Search FrictionsECONOMETRICA, Issue 2 2009Ricardo Lagos We develop a search-theoretic model of financial intermediation in an over-the-counter market and study how trading frictions affect the distribution of asset holdings and standard measures of liquidity. A distinctive feature of our theory is that it allows for unrestricted asset holdings, so market participants can accommodate trading frictions by adjusting their asset positions. We show that these individual responses of asset demands constitute a fundamental feature of illiquid markets: they are a key determinant of trade volume, bid,ask spreads, and trading delays,the dimensions of market liquidity that search-based theories seek to explain. [source] Financial Frictions and Risky Corporate DebtECONOMIC NOTES, Issue 1 2007Doriana Ruffino We offer clarifications on Cooley and Quadrini (2001) regarding financial frictions and risky corporate debt pricing. Even in a frictionless world, the promised rate on corporate debt is not identical across firms and across capital structures and it is not equal to the risk-free rate. Frictions are unnecessary for credit spreads to arise. Only if the macroeconomy is in actuality risk free or risk neutral do interest rates on corporate debt reflect default probabilities. To the extent that the firm's entire financial structure is traded, a bias in credit spreads introduces an exploitable arbitrage opportunity. Re-establishing no-arbitrage, firm dynamics move in the opposite direction to Cooley and Quadrini's. [source] Unemployment and Employment Protection in a Unionized Economy with Search FrictionsLABOUR, Issue 2 2008Nikolai Stähler Higher employment protection reduces job creation as well as job destruction. However, in most models, wages are bargained individually between workers and firms. Using a conventional matching model in which a monopoly union sets wages, I show that employment protection can unambiguously increase unemployment. Interestingly, I find that tightening the restrictions on redundancies and dismissals may even increase the probability of dismissal. [source] Impulse-based dynamic simulation in linear timeCOMPUTER ANIMATION AND VIRTUAL WORLDS (PREV: JNL OF VISUALISATION & COMPUTER ANIMATION), Issue 4-5 2007Jan Bender Abstract This paper describes an impulse-based dynamic simulation method for articulated bodies which has a linear time complexity. Existing linear-time methods are either based on a reduced-coordinate formulation or on Lagrange multipliers. The impulse-based simulation has advantages over these well-known methods. Unlike reduced-coordinate methods, it handles nonholonomic constraints like velocity-dependent ones and is very easy to implement. In contrast to Lagrange multiplier methods the impulse-based approach has no drift problem and an additional stabilisation is not necessary. The presented method computes a simulation step in O(n) time for acyclic multi-body systems containing equality constraints. Closed kinematic chains can be handled by dividing the model into different acyclic parts. Each of these parts is solved independently from each other. The dependencies between the single parts are solved by an iterative method. In the same way inequality constraints can be integrated in the simulation process in order to handle collisions and permanent contacts with dynamic and static friction. Copyright © 2007 John Wiley & Sons, Ltd. [source] Synthesis and tribological properties of laminated Ti3SiC2 crystalsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 8 2010Qiong Wu Abstract Laminated Ti3SiC2 crystals are prepared of Ti, Si, C and Al powders by the method of hot isostatic pressing with NaCl additive in argon at 1350 °C. The laminated morphology of Ti3SiC2 is presented through the SEM and TEM observations. The results of high resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) patterns combined, it can be seen that the layers are of Ti3SiC2 crystals. The growth mechanism of Ti3SiC2 crystals, controlled by two-dimensional nucleation, is also explained. The tribological properties of Ti3SiC2 crystals as additives in HVI500 base oil are investigated by a UMT-2 ball-on-plate friction and wear tester. The study shows that under determinate conditions, the friction coefficient of the base oil containing Ti3SiC2 crystals is lower than that of pure base oil, and it decreases with the increase of mass percent of Ti3SiC2 nanolayers when its proportion is lower than 5wt. %. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] About 200 years after Mohs , Nanoscratching LiB3O5CRYSTAL RESEARCH AND TECHNOLOGY, Issue 11 2008P. Paufler Abstract Though simple scratch hardness tests after Mohs are still used today, the development of diamond nanoscratching equipment offers new possibilities to meet demands of modern nanotechnology. Preceding approaches to assign hardness values to materials are briefly reviewed, and scratch hardness is related to indentation hardness. Taking single-crystalline LiB3O5 as an example, the dependence of scratch morphology on the direction of scratching is demonstrated quantitatively. The coefficient of friction depends on normal load and varies between 0.25 and 0.37. Moreover, it is oscillating during scratching thus reflecting processes at nanoscale. Dislocation etch pits were observed due to scratching. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Reengineering Academic Teams Toward a Network Organizational Structure,DECISION SCIENCES JOURNAL OF INNOVATIVE EDUCATION, Issue 2 2007Emmanouil Kaldis ABSTRACT This article examines student teamwork in the academic field from a structural perspective. Student teams are often prearranged and then left to organize themselves and get on with their work, without any further structural support; this, however, can become a negative experience on teamwork. A varied contribution among team members often occurs and unavoidably leads to friction and reduced performance. The aim of this project is to explore the main problems in academic teamwork and investigate tools that provide relevant solutions. We present the concept of network organizational structure and discuss how this can improve collaboration and communication. The main tools to achieve a structural transformation from the more traditional form of team organization to the fairer network form, and their implications are discussed. [source] Control of flexor motoneuron activity during single leg walking of the stick insect on an electronically controlled treadwheelDEVELOPMENTAL NEUROBIOLOGY, Issue 3 2003Jens Peter Gabriel Abstract In the present study, motoneurons innervating the flexor tibiae muscle of the stick insect (Cuniculina impigra) middle leg were recorded intracellularly while the single leg performed walking-like movements on a treadwheel. Different levels of belt friction (equivalent to a change in load) were used to study the control of activity of flexor motoneurons. During slow leg movements no fast motoneurons were active, but a recruitment of these neurons could be observed during faster leg movements. The firing rate of slow and fast motoneurons increased with incremented belt friction. Also, the force applied to the treadwheel at different frictional levels was adapted closely to the friction of the treadwheel to be overcome. The motoneurons innervating the flexor tibiae were recruited progressively during the stance phase, with the slow motoneurons being active earlier than the fast (half-maximal spike frequency after 10,15% and 50,60% of the stance phase, respectively). The resting membrane potential was more hyperpolarized in fast motoneurons (64.6 ± 6.5 mV) than in slow motoneurons (,52.9 ± 5.4 mV). However, the threshold for the initiation of action potentials was not statistically significantly different in both types of flexor motoneurons. Therefore, action potentials were generated in fast motoneurons after a longer period of depolarization and thus later during the stance phase than in slow motoneurons. We show that motoneurons of the flexor tibiae receive substantial common excitatory inputs during the stance phase and that the difference in resting membrane potential between slow and fast motoneurons is likely to play a crucial role in their consecutive recruitment. © 2003 Wiley Periodicals, Inc. J Neurobiol 56: 237,251, 2003 [source] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||