1,721,026 research outputs found
Measuring the Wear Volume of Cylindrical Bushings with a Roundness Measuring Instrument
No full textDuring accelerated life tests linear pneumatic cylinders fail because the guide bushing in the front head wears out and leakages from sealings become unacceptable. Wear measurements on the inner surface of these bushings are challenging and require a rotary scanning system which is not usually available in standard optical profilometers or stylus profilometers. In this paper a roundness measuring machine has been exploited as a rotary contact profilometer to scan the inner surface of guide bushings which suffered wear loss after endurance tests. Two scanning strategies were compared which exploit the measurement routine usually applied to determine the tolerance of cylindricity and straightness of holes. Results thus obtained were compared with the results of a wear volume analysis performed through a benchmark optical technology available on the market of optical profilometers. The accuracy of the method was investigated at a preliminary stage by increasing the number of profiles acquired by the roundness measuring instrument, and the deviation from the reference value from optical profilometry is presented
Spiral Groove Thrust Bearing Modeling with Finite Difference Method
No full textThe paper aims to simulate the static load capacity of a gas spiral groove thrust bearing for a high-speed compressor. The pressure distribution inside the bearing air film is computed by discretizing and solving the Reynolds equation through a finite difference numerical scheme. The mesh convergency analysis is performed to investigate the accuracy of the solution when the number of nodes is increased inside the computational domain. The effect of the number of grooves on the load carrying capacity is also investigated while maintaining the other geometrical parameters of the bearing. The results are compared with an analytical model available in the literature
Wear tests on PTFE+pb linings for linear pneumatic actuator guide bushings
Full text linkGuide bushings for linear actuators are subject to intense mechanical stress and wear, especially when external radial forces are applied. Carrying out simplified wear tests on the materials used in these bushings is important in making it possible to estimate how many cycles the actuator can perform before the bushing must be replaced. This paper presents the results of tribological pin-on-disk testing on an anti-friction lining consisting of a porous bronze sinter impregnated and coated with PTFE+Pb. Test specimens were worn with a steel ball, and wear and friction coefficients were determined experimentally with increasing loads and speeds
Grinding performance and theoretical analysis for a high volume fraction SiCp/Al composite
No full textSiCp/Al composite is widely used in space shuttle slides, automotive and machine tools. In this paper, ground surfaces of SiCp/Al composite, obtained with different grinding process parameters, are characterized in terms of friction and wear performances under dry and lubricated conditions. The wear mechanism of SiCp/Al composite is firstly found the combination of cohesive and abrasive wear, then a method to calculate the wear volume of SiCp/Al composite is proposed, and a comprehensive wear evaluation system of SiCp/Al composite is established, in which the three indicators of friction coefficient, wear depth, and equivalent wear section area are adopted to evaluate usability performance. The prediction models of the wear indicators were established and the errors between the experimental and predicted results are within 7%. Finally, the NSGA-II multi-objective algorithm is used to optimize surface performance of the SiCp/Al composite, and the optimized grinding process parameters are obtained as the wheel speed of 33 m/s, the table speed of 0.4 m/min and the grinding depth of 9 μm
Uncertainty-based comparison of conventional and surface topography-based methods for wear volume evaluation in pin-on-disc tribological test
No full textPrecise wear evaluation is essential to develop prediction models, applied to design materials, components and to optimise new manufacturing processes. Pin-on-disc is a widespread standardised conventional sliding wear test. Conventional characterisation of resulting wear exploits gravimetric or profilometric techniques. These have inadequate precision and accuracy for applications characterised by low-wear and uneven morphology and are being replaced with high-resolution and information-rich inspections to measure surface topography. Metrological characterisation of topography-based methods still lacks in literature, which prevents the performance comparison with conventional techniques. This paper develops a framework to evaluate topography-based methods’ measurement uncertainty and compare methods’ performances accordingly on experimental wear data on PTFE and Aluminium. Results show that topographic methods improve pin-on-disc characterisation's reliability
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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