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Conceptual evaluation of three rotordynamical systems
2001 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

A defibrator is a rotating machine that break raw material of cellulose into small fibres. These fibres can then be used to produce paper or panelboards. The fibre creation process is a complicated process to simulate, mainly because it is hard to model these type of three phase flow and its interaction with the rotor. In this thesis three defibrator concepts are dynamically analysed without modelling the fibre creation process. Even if the fibre creation process is not simulated, the dynamical characteristics of each concept can still qualitatively be analysed. It will serve as a part of the conceptual design of a new machine with increased productivity. The first concept (A) is a model of a radial flow machine with an overhung rotor. The second concept (B) is a partly axial flow machine and the third concept (C) is a radial flow machine with bearing assemblies on both sides of the rotor. In order to reduce the number of parameters and to simplify the comparison between the concepts, the same bearing assembly has been used. The rotors are considered stiff and described with four degrees of freedom in the equation of motion (two translational and two rotational). Unbalance is the only force acting on the system. These equations have been solved with three different methods namely state method (real), complex method and numerical incremental method. To compare the concepts three dimensional “design response spectres” have been made by reducing the number of design parameters. These spectres shows suitable design areas for each concept. The thesis shows that the three concepts have quite different dynamical characteristics. For consept A and C only one critical speed can be found. For concept B however, two critical speeds occur when the ratio between the mass moments of inertia becomes equal to one (which happens for long rotors). One interesting result is that running in the subcritical range (as today) generally gives lower amplitude than in supercritical range. So if the mass is increased (which is common when scaling a machine for higher productivity) the critical speed will decrease and the safety margin to unacceptable vibrations may not be fulfilled. Another interesting result is when a symmetric rotor is centered between two identical bearing assemblies of equal overhung, the rotational amplitude response becomes zero for every driving frequency. Analysis of the bearing forces shows that the bearing closest to the rotor are exposed to a much higher force than the other two. The analysis also shows that the bearing forces are lower in subcritical than supercritical range.

Place, publisher, year, edition, pages
2001.
Keyword [en]
Technology, Rotordynamics, Overhung rotors, Defibrators
Keyword [sv]
Teknik
Identifiers
URN: urn:nbn:se:ltu:diva-47147ISRN: LTU-EX--01/198--SELocal ID: 4bb1a2b7-16e7-428a-9034-db83cb825d14OAI: oai:DiVA.org:ltu-47147DiVA: diva2:1020466
Subject / course
Student thesis, at least 30 credits
Educational program
Mechanical Engineering, master's level
Examiners
Note
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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  • apa
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  • en-GB
  • en-US
  • fi-FI
  • nn-NO
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