28,546 research outputs found
Fatigue failure analysis of vibrating screen spring by means of finite element simulation: a case study
Vibrating screens are often used in the mining industry to separate mineral particles by size. In many designs, spring arrays are used to provide the system with the necessary stiffness for screens to vibrate in a controlled manner. Naturally, these springs are subjected to varying loading cycles, which can cause their premature fatigue failure. This behavior has been studied by means of finite element analysis and compared with data obtained from a real case scenario, in which a helical spring failed. The 3D computational model was developed using the geometric characteristics and material properties of a fractured spring, as well as the loading characteristics of a specific vibrating screen. The meshing and the simulation tasks were performed in the general purpose software ANSYS Mechanical. Given the nature of the helical springs and the high-cycle loading conditions, for the fatigue analysis it was determined that a stress-life approach with constant amplitude and non-proportional loading best fit the investigated phenomenon. In solving the nonproportional loading case, stress values of two static scenarios were required to determine the upper and lower limits. Then, to perform the fatigue calculations a solution combination was used. In addition, in order to correct the effect of mean stress and calculate the stresses component respectively the Goodman and Von Mises theories were employed. Simulation results showed that spring would present failure below the second turn of the coil when working with the full nominal load during nearly forty million cycles. These results strongly agreed with the data extracted from a vibrating screen where fractured spring had been working. Fatigue analysis also predicted that the nominal load should be reduced to 90% in order for the spring to meet the minimum life requirements before failure occur
On the heating of source of the Orion KL hot core
We present images of the J=10-9 rotational lines of HC3N in the vibrationally
excited levels 1v7, 1v6 and 1v5 of the hot core (HC) in Orion KL. The images
show that the spatial distribution and the size emission from the 1v7 and 1v5
levels are different. While the J=10-9 1v7 line has a size of 4''x 6'' and
peaks 1.1'' NE of the 3 mm continuum peak, the J=10--9 1v5 line emission is
unresolved (<3'') and peaks 1.3'' south of the 3 mm peak. This is a clear
indication that the HC is composed of condensations with very different
temperatures (170 K for the 1v7 peak and K for the 1v5 peak). The
temperature derived from the 1v7 and 1v5 lines increases with the projected
distance to the suspected main heating source I. Projection effects along the
line of sight could explain the temperature gradient as produced by source I.
However, the large luminosity required for source I, >5 10^5 Lsolar, to explain
the 1v5 line suggests that external heating by this source may not dominate the
heating of the HC. Simple model calculations of the vibrationally excited
emission indicate that the HC can be internally heated by a source with a
luminosity of 10^5 Lsolar, located 1.2'' SW of the 1v5 line peak (1.8'' south
of source I). We also report the first detection of high-velocity gas from
vibrationally excited HC3N emission. Based on excitation arguments we conclude
that the main heating source is also driving the molecular outflow. We
speculate that all the data presented in this letter and the IR images are
consistent with a young massive protostar embedded in an edge-on disk.Comment: 13 pages, 3 figures, To be published in Ap.J. Letter
A new anti-neutrino detection technique based on positronium tagging with plastic scintillators
The main signature for anti-neutrino detection in reactor and geo-neutrino
experiments based on scintillators is provided by the space-time coincidence of
positron and neutron produced in the Inverse Beta Decay reaction. Such a
signature strongly suppresses backgrounds and allows for measurements performed
underground with a relatively high signal-to-background ratio. In an
aboveground environment, however, the twofold coincidence technique is not
sufficient to efficiently reject the high background rate induced by cosmogenic
events. Enhancing the positron-neutron twofold coincidence efficiency has the
potential to pave the way future aboveground detectors for reactor monitoring.
We propose a new detection scheme based on a threefold coincidence, between the
positron ionization, the ortho-positronium (o-Ps) decay, and the neutron
capture, in a sandwich detector with alternated layers of plastic scintillator
and aerogel powder. We present the results of a set of dedicated measurements
on the achievable light yield and on the o-Ps formation and lifetime. The
efficiencies for signal detection and background rejection of a preliminary
detector design are also discussed.Comment: 18 pages, 10 figure
Foreign ownership, firm performance, and the geography of civic capital
It is well established in the literature that foreign affiliates are subject to a series of governance and assimilation costs that deteriorate their performance. This is particularly relevant for firms which have been recently acquired by foreign investors. We employ the variation in civic capital across Italian provinces as an exogenous determinant of these governance costs. We derive the testable implication that there should be a clean evidence of a negative effect of foreign ownership on performance in areas where civic capital is low. As the level of local civic capital increases, this reduces the scope for internal transaction costs, and makes the governance of foreign affiliates easier, and their performance better. We take this prediction to the data and find confirmation of our conceptual framework. Our analysis underlines the importance of the geographic heterogeneity of informal institutions when analyzing the effect of foreign ownership on firm performance.
Electrical resistivity of the Ti4O7 Magneli phase under high pressure
We have measured resistivity as a function of temperature and pressure of
Ti4O7 twinned crystals using different contact configurations. Pressures over
4kbar depress the localization of bipolarons and allow the study of the
electrical conduction of the bipolaronic phase down to low temperatures. For
pressures P > 40 kbar the bipolaron formation transition is suppressed and a
nearly pressure independent behavior is obtained for the resistivity. We
observed an anisotropic conduction. When current is injected parallel to the
principal axis, a metallic conduction with interacting carrier effects is
predominant. A superconducting state was not obtained down to 1.2 K, although
evidences of the proximity of a quantum critical point were noticed. While when
current is injected non-parallel to the crystal's principal axis, we obtained a
logarithmic divergence of the resistivity at low temperatures. For this case,
our results for the high pressure regime can be interpreted in the framework of
interacting carriers (polarons or bipolarons) scattered by Two Level Systems.Comment: 9 Revtex pages, 12 EPS figures included, submitted to The European
Physical Journal B. Contact author: C. Acha (e-mail address: [email protected]
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