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Article
Viscosity Effect on Stiffness of Non-conventional (Five Tilted Pads) Journal Bearing

Authors: Mohammed Oawed Atteaa Alhassany --- Ali Khalid Aldulaimy
Journal: Tikrit Journal of Engineering Sciences مجلة تكريت للعلوم الهندسية ISSN: 1813162X 23127589 Year: 2018 Volume: 25 Issue: 3 Pages: 53-57
Publisher: Tikrit University جامعة تكريت

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Abstract

In this tribological study, we highlight the effect of lubricating oil viscosity in the Multi-pads hydrodynamic journal bearings generate important improvement in characteristics of stiffness and stability in the high speed turbomachines. Depending on viscosity of oil film (three values) variation for five tilted pads bearing, each pad is pivoted and is facilitated to be tilted with small angles, by using Matlab program, we calculate the oil film thickness for convergence layer. We applied Reynold's equation and solved it's numerically by using finite difference method with 5 nodes technique to find the pressure distributed on each node in the mesh of tilted pad, then calculate stiffness coefficients. Results show that there is clear effect on stiffens with viscosity change. The increase in value of Krr (for n = 0.3) between viscosity (0.04 Pas. s) and viscosity (0.058 Pas. s) is14.33 MN/m, while the increase in Krr value between viscosity (0.058 Pas. s) and viscosity (0.087 Pas. s) is 11.37 MN/m. the increase in value the of Kss (for n = 0.3) between viscosity (0.04 Pas. s) and viscosity (0.058 Pas. s) is5.921 MN/m, while increase in Kss value between viscosity (0.058 Pas. s) and viscosity (0.087 Pas. s) is9.55 MN/m respectively. the increase in value of Ksr (for n = 0.3) between viscosity (0.04 Pas. s) and viscosity (0.058 Pas. s) is 8.95 MN/m, while the increase in Ksr value between viscosity (0.058 Pas. s) and viscosity (0.087 Pas. s) is 14.41 MN/m respectively. the increase in value of Krs (for n = 0.3) between viscosity (0.04 Pas. s) and viscosity (0.058 Pas. s) are 5.08 MN/m, while the increase in Krs value between viscosity (0.058 Pas. s) and viscosity (0.087 Pas. s) is8.19 MN/m respectively. The values of the dominate principal coefficients Krr is greater than that of Ksr, also The values of the principal coefficients Kss is greater than that of cross coupling Krs for all values of viscosity that studied. From this result, we can conclude the side effect of cross coupling coefficients (Ksr ,Krs) can be overcome by great values for principal coefficient (Krr, Kss) respectively, so we can get good improvement instability for this bearing by variation the viscosity. After that, we regarded to use high viscosity lubricant in multi-pad journal bearing to improve the performance and stability by controlling the stiffness coefficients.


Article
The Effect of Pads Number and Their Tilt Angles on the Bearing Stiffness
تأثير عدد الوسادات وزوايا ميالنها على معامل نابضية المسند

Author: Ibrahim Ali Muhsin
Journal: Tikrit Journal of Engineering Sciences مجلة تكريت للعلوم الهندسية ISSN: 1813162X 23127589 Year: 2018 Volume: 25 Issue: 4 Pages: 6-10
Publisher: Tikrit University جامعة تكريت

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Abstract

In this work a plain (conventional) and a tilted bad bearings are analyzed and studied (both have the same dimensions and work conditions). Three, four and five pads models are studied. The pads are fixed circumferentially in a manner each two close pads are separated with an angle that is the same for all the close pads.Each pad is pivoted in its trailing edge and is facilitated to be tilted around this pivot with small angles. This study was achieved for three values of tilting angles. These angles are, 0.01o, 0.011o and 0.012o. Two dimensions' form of Reynolds equation is used and solved numerically then the stiffness coefficients had been calculated. A comparison had been done between the stiffness coefficients values for the two models (tilted pads and conventional bearings). The results of this comparison was found that (for n = 0.4), are, +28%, +275%, +270 % and -100% for Ksr, Krr, Kss and Krs respectively. It was noticed that the values of the principal coefficients Krr and Kss are increased significantly while the cross coupling coefficient, (Krs) is decreased significantly and (Ksr) is increased slightly. And this is a positive sign for stability improvement. Also it was found that increasing the tilt angle (delta) cause an increase of the stiffness coefficients values. These increases (for n = 0.4 and changing the tilt angle from 0.010 to 0.011 then to 0.012 respectively) are, Krr=+50%, +47%, Kss = +40%, +32%, Ksr = +33%, +42%, and Krs = +15%, +20%. And since increasing the values of the cross coupling coefficients make the bearing stability worse, so increasing the tilt angle is not apreferable method for improving stability. Changing the number of pads is a better solution and it was found that three pads bearing is the best among them.

في هذا البحث تمت دراسة مسند تقليدي وآخر ذو وسادات قابلة لألمالة والضبط حول حافاتها األمامية. حيث ان كال المسندين لهما نفس األبعاد وظروف العمل. تمت دراسة مسند ذو ثالثة وأربعة وخمسة وسادات. في كل منها تكون الوسادات مثبته محيطياً على جسم المسند وتنحصر بينها فضاءات محيطية متساوية وفي نفس الوقت الزاوية o المركزية ألي وسادة تكون مساوية للزاوية المركزية ألي فضاء. أنجزت هذه الدراسة لثالث قيم لزاوية إمالة الوسادات. هذه القيم هي , 0.01 o و 0.011 o 012.0 . حيث تم معادلة رينولدز والتي حلت عدديا باستخدام تقنية الفروقات المحددة ومن ثم تم إيجاد معامالت النابضية لكلي المسندين (التقليدي وذو الوسادات) وكانت النتيجة كما ً استخدام يلي: لنسبة المركزية مقدارها )4.0 = n )فأن قيم معامالت النابضية األربعة Ksr ,Krr, Kss وKrs على التوالي للمسند ذو الوسادات تزيد بمقدار %28%,+275%,+270 + و%100 -على التوالي مقارنة بقيمها للمسند التقليدي . من النتائج أعاله يمكن مالحظة زيادة قيم معامالت النابضية األساسية ) Kss, Krr )ونقصان ملحوظ لمعامل النابضيةالمستعرض Krs وهذا مؤشر على تحسن أستقرارية المسند ذو الوسادات على الرغم من الزيادة القليلة للمعامل Ksr .كذلك وجد أن قيم هذه المعامالت للمسند ذو الوسادات قد o زادت بزيادة زاوية أمالة الوسادات وكما يلي: بتغيير زاوية أمالة كل وسادة من الى 0.010 o الى 0.011 o ثم 012.0 ولقيمة نسبة المركزية مقدارها )4.0 = n )فأن كل معامالت النابضية األربعة قد زادت وكما يلي: Krr قد زاد بمقدار %50% ,+47 +على التوالي، Kss قد زاد بمقدار ,%32% +40 ، +Ksr قد زاد بمقدار % 33 ,%42 .Krs قد زاد بمقدار %15%, +20 . +مادامت زيادة زاوية إمالة الوسادات يؤدي الى زيادة كل معامالت النابضية بنوعيها، األساسية ) Kss, Krr )والمستعرضــة )Ksr، Krs )لذا فأن زيادة زاوية أمالة الوسادات التعد الطريقة المفضلة لتحسين استقراريه المسند ويبقى أسلوب زيادة عدد الوسادات هو الحل األمثل لتحسين األستقرارية وبالتحديد استخدام المسند ثالثي الوسادات.

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