Pre stressed modal FE Analysis of bolted joint

Transcription

1 Pre stressed modal FE Analysis of bolted joint Prof. Kashinath.H.Munde 1, Mr. Mahesh P. Mestry 2 1,2 Mechanical Engg./ APCOER/ Pune University Abstract Threaded fasteners are one of the most common means of joining parts of assemblies to maintain integrity of isolated elements of mechanical structures and machines used in various applications due to their high reliability, strong load bearing capacity, easy maintenance and inspection and its capability of being mantled and dismantled using simple tools at low cost. Mounting of a bolted joint usually consists of application of relative rotation between screw and nut until installation of desired preload. Bolted structures in their service life, subjected to transverse and impact loads which may result in reduction in preload on bolted joint which causes periodic retightening of threaded fasteners and high maintenance cost. Objective of this paper is to analyse variation of natural frequency with different preload conditions using pre stressed modal analysis. FEA is serving as good tool hence it is used here with Ansys 14.5 as a software tool to get required results. Keywords FEA, Ansys 14.5, Modal Analysis, Pre stressed, Joints I. INTRODUCTION In most of the machines and structures, for joining different parts, threaded fasteners are one of the most common means of joining parts of assemblies.bolted joints are used widely to connect parts or plates as they can withstand huge amount of pressure and torque without any damage to the parts being tightened. Another reason for this is low cost as compared to other types of joints. Under different applications it is found that bolted joint undergo dynamic loads i.e. shock, vibration and varying thermal loads. Due to which Failure of joints generally occurs due to fatigue or loosening. Even partial loosening can reduce the fastener preload and thereby increase the dynamic loads acting on the fastener, leading to increased likelihood of fatigue failure. Hence to avoid premature joint failure in assembly design It is crucial to understand the causes of loosening. FEA is one of the best numerical methods to analyse behaviour of bolted joint under vibration. II. LITERATURE REVIEW Junker in 1969 identified fundamental aspects of loosening. Hence to find minimum shear force required which causes loosening and it helped to understand mechanism of loosening, and apply them to the design of more complex structures such as a bent frame. After that, more research is done to increase performance of bolted fasteners under impacts and vibration. Some of the important work done is given below. ZhenZang, Mengbing Liu, Zang king Su &Yi Xiao[1] in their work provided easy method for identification of bolt loosening & quantitative estimate of residual torque using WED and VM approach. DOI: /IJRTER ILVDG 225

2 P. Langer, A. Hoppe, C. Guist & S. Marburg[2] Presented simple method for modelling bolted joint in Abaqus by introducing local non linearities only reducing computational cost. Modal Analysis of rectangular plate with lap joints to find natural frequencies & mode shapes was carried out by M. Bhusnar & S. S. Sarawade[13] Shriram Dravid, Karthikeya Tripathi & Manoj Chouksey studied role of washers in controlling loosening of full threaded bolted joint & observed that Failure can be avoided by examining tightening torque required.[5] Variation of clamping length is studied by Ravinder Kumar [7] and it was useful in identification of parameters responsible for loosening. It suggested correct locking device for particular application. Naoya Nishimura, Katsuhiko Murase, Toshio Hattori &Takeru Watanabe [10] showed that the critical relative slippage decreases with a decrease of the bolt axial tension. J.A. Sanclemente & D.P. Hess [11] performed parametric study of threaded fastener loosening due to cyclic transverse loads to develop statistical model to define optimum conditions for preload, modulus of elasticity, diameter, and lubrication, tight fit and fine threads. N.G. Pai & D.P. Hess [12] studied Influence of fastener placement on vibration-induced loosening and helped to decide optimum location of fasteners. III. FINITE ELEMENT ANALYSIS Finite Element Method is one of the best methods for pre stressed modal analysis. 3.1 Preparation of 3D model In this work, 3D specimen model of single lap bolted joint is prepared in solid works. It is shown in below figure. Fig-1: Single Lap bolted All Rights Reserved 226

3 Fig-2: 3D model of Single Lap bolted joint. 3.2 Importing the prepared model in Ansys 14.5 Prepared 3D model is imported in Ansys14.5 in design modeler of Ansys. 3.3 Meshing the model 3.4 Application of boundary conditions One end of Bolted joint is applied with fixed support in static structural module of Ansys 14.5.Other end is kept free to behave joint as cantilever beam. Frictional contact is applied in connections for thread and nut mating interface to get real bolted structure Initially preload of 100N is applied to get equivalent stress (Von Misses stress) in static structural module Results of above static structural module are inputted to pre stressed modal analysis module to get frequencies for mode shapes Same procedure is followed for analysis by applying preload of 500 & 1000N. 3.5 Solution & Results Graphs according to variation of frequency & Mode shapes for each frequency can be obtained in modal analysis. IV. CALCULATION OF PRELOAD In the model, the tightening torque applied to the bolt is converted to a pretension force. This pretension force is nothing but preload The relationship between the given tightening torque and the arising pretension force is described by, T = K F D Where, T = Pre-torque F = Preload or Pre-force D = Nominal diameter of the thread. K= Torque All Rights Reserved 227

4 From this, preload can be calculated if pre torque is known. Again in this case, it is observed that if the same tightening torque is applied for no of times of ranges preload generated is not constant but it is varied within particular range, hence it is difficult to predict the exact value of preload. V. ANSYS RESULTS 5.1 Frequency Analysis under 100N 6 mode shapes are considered and analyzed for preload of 100N.From Modal analysis,following results were obtained. 5.2 Frequency Analysis under 500N Mode Frequency [Hz] Table 1: Frequency response under 100N Fig-4: Frequency response under All Rights Reserved 228

5 Mode Frequency [Hz] Table 2: Frequency response under 500N 5.3 Frequency Analysis under 1000N Fig-5: Frequency response under 1000N Mode Frequency [Hz] Table 3: Frequency response under 1000N VI. CREATION OF MODE SHAPES Mode shape under each preload is created for only 3modes i.e. 2 nd,4 th and 6 th in order to check only trend and variation under different All Rights Reserved 229

6 6.1 Mode shapes under 100N International Journal of Recent Trends in Engineering & Research (IJRTER) Fig 6: 2 nd mode shape deformation Fig 7: 4 th mode shape All Rights Reserved 230

7 Fig 8: 6 th mode shape deformation 6.2 Mode shapes under 500N Fig 9: 2 nd mode shape All Rights Reserved 231

8 Fig 10: 4 th mode shape deformation Fig 11:6 th mode shape All Rights Reserved 232

9 6.3 Mode shapes under 1000N International Journal of Recent Trends in Engineering & Research (IJRTER) Fig 12: 2 nd mode shape deformation Fig 13: 4 th mode shape All Rights Reserved 233

10 Fig 14: 6 th mode shape deformation VII. RESULTS AND DISCUSSION Observation of mode shapes shows that there is gradual increase in natural frequency of each mode. Again frequencies obtained under different preload with ten times variation do not change values of frequencies during each mode. Also, in general as the frequency goes on increasing it tries to converge though different preloads are generated. From mode shapes it is observed that maximum total deformation occurred is at free end of bolted joint and minimum at fixed end of joint. Also, as no rigid connection is considered between two plates, there is some amount of rotational shift about axis of bolt. Bending of both the plates is predominant than rotational displacement when mode shape reaches to maximum frequency. Again effect of variation in plate thickness, configuration of joint etc can be considered and there is scope for further research. VIII. CONCLUSION It can be concluded that small Variation in preload does not causes noticeable change values of natural frequencies in each mode. As it is observed that maximum total deformation occurred is at free end of bolted joint and minimum at fixed end of joint, probability of failure can be predicted. Again as preload is function of tightening torque, as there is bending of plate during maximum frequency, optimum tightening torque is required to avoid failure due to bending. ACKNOWLEDGMENT I wish to acknowledge my guide Prof. K.H.Munde, Prof. G.E..Kondhalkar and Mr. Ashish Pawar sir for encouragement, guidance and support for completion of above All Rights Reserved 234

11 REFERENCES I. ZhenZang, Mengbing Liu, Zang king Su, Yi Xiao Continuous Monitoring of Residual Torque of Loose Bolt in A Bolted Joint, 2017, Science Direct II. P. Langer, A. Hoppe, C. Guist, S. Marburg, Bolted Joints: Eigen frequencies from non-linear FE-models and III. experimental modal analysis, 2017, Science direct Jianhua Liu, Huajiang Ouyang, Zhiqiang Feng, Zhenbing Cai1, Xuetong Liu, Minhao Zhu, Study on selfloosening of bolted joints excited by dynamic axial load, 2017,Tribology International IV. Jianhua Liu, Huajiang Ouyang, Jinfang Peng,Chaoqian Zhang Pingyu Zhou,Lijun Mac, Minhao Zhuan, Experimental and numerical studies of bolted joints subjected to axial excitation, 2016, Elsevier V. Shriram Dravid, Karthikeya Tripathi, Manoj Chouksey Role of Washers in Controlling loosening of Full Threaded Bolted Joints, 2014, ICIAME, Science Direct VI. Febli lhuda, Itsuro Kajiwara, Naoki Hosoya, Shozo (Kawamura), Bolt loosening analysis and diagnosis by noncontact laser excitation vibration tests, 2013, Science direct VII. Ravinder Kumar, Causes & prevention of loosening in pre stressed bolts, 2013, mechanica Confab VIII. Naoya Nishimura, Katsuhiko Murase, Toshio Hattori, Takeru Watanabe, Loosening Evaluation of Bolt-Nut Fastener Under Transverse Cyclic Loading,2013,Engineering Transactions IX. Anirban Bhattacharya, Avjit Sen, Santanu Das, An investigation on the anti-loosening characteristics of threaded fasteners under vibratory conditions, 2010, Science direct X. Toshio Hattori, Minoru Yamashita, Hiroki Mizono, Tomohiro Naruse, Loosening & sliding behavior of nut bolt fatsener under transverse loading, 2010, EDP Sciences XI. J.A. Sanclemente, D.P. Hess, Parametric study of threaded fastener loosening due to cyclic transverse loads, 2007, Science direct XII. N.G. Pai, D.P. Hess, Influence of fastener placement on vibration-induced loosening, 2003, Science direct XIII. M. Bhusnar, S. S. Sarawade, Modal Analysis of rectangular plate with lap joints to find natural frequencies & mode shapes, 2016, All Rights Reserved 235