The mechanical behaviour of a dipole structure is considered when also friction is taken into account, studying its effect on different components and in different conditions. In particular the difference in behavior between a structure with aluminium collars and one with austenitic steel ones was studied

Bajko, M

Fessia, P

Perini, D

CERN Document Server

IEEE TIWSAC~IONS ON APPLIED SUPERCONDUCTIVITY, VOL. 10, NO. I ,  MARCH 2000 81 FEM Computations Concerning the Effect of Friction in Two LHC Main Dipole Structures M. Bajko, P. Fessia, D. Perini CERN, Geneva, Swiaerland Ab8lract - The mchanfcal behaviour of B dipoIe structure Is considered whcn nho friction is taken into account, studying its effect on different componenls and in digercnt conditions. In particular the difference In hehavior between a strucfure with aluminium collars and one with austenitic steel ones was studied. I. INTRODUCTION The Large Hadron Collidcr 111 requires 1232 superconducting dipole magnets with a magnetic length of 14.3 m and a nominal dipole field of 8.3 T. The LHC superconducting main dipole is the result of the precise assembly of high precision components. All forces between the components are exchanged along the contact surfaces and change their strength during c o d  down and magnet operation at nominal ficld, Up to May 98 all the E M  computations performed on the structure of the main dipole did not take into account the friction between the different components, The main reason was that thc available computing power was not enough to develop and solve an adequate model, From May 1998 a new cluster of computers has been installed at CERN making possible this study that was performed in August 1998. The influence of friction on the structure is studied in this article, trying to highlight the different behaviour of a structure with austenitic steel collars and of another one with aluminium collars. 11. THE UPOLE STRUCTURE The dipole cross section is defined by four main components: the coils, assembled with prestress inside the collar cavity; the collars, clamped around the coils to maintain the coil prestress; the iron yoke, split in two parts by a vertical gap; the shrinking cylinder, that provides the driving force io keep the two iron yoke halves in contact. The behaviour of the structure during assembly, cool down and magnet operation and the characteristics of the main components are discussed in detail in [2]. The collars are 3 mrn thick. Two possible configurations are here compared: Dcsign A: collars made of aluminium. Parameters of the structure optimised for that condition. Design B: made of austcnitic s k d .  Parameters of the structure optimised for that condition. Fig. 1 shows a quarter of the structure and the main contact surfaces. . Manuscript mceived JXI 27 September 1W9 III. FINITE LEMENT MODEU Two, 2-D finite clement (FE) models were meshed and the plane-stress option was used for the elements. It is possible to simulate the collar structures by creating two Iayers with a 0.5 nun thick mesh. For the coirs, iron yoke, cylinder, insert and locking rods LL 1-mm thick layer was meshed. The prestress of the coils is imposed by giving an intcrference at the interfaces between the collars and the coil. The geometry is modelled at room temperature and in non- deformed conditions. That is to say the dimensions used in the model are for parts at their nominal size. The areas were meshed using two-dimensional linear elements (plane 42) and contact surfaces with three-dimensionab coratuci elements (contac 52). Further information concerning these elements is available from the ANSYS manuals [3]. Magnet operation is simuhted by loading the coils with the electro-migneiic forces computed with ihe same code. Iron saturation was taken into account. TABLE I MA- FlUTSRm USED FOlt THB FINITE EI.IM%TMODeLS Temp. Young Mod, Therm. Bxp. Coeff. 1 o 3 r ~ 1  * Ex UY LK] [MPa] [MPa] Inner layer i .8 20700 17250 Insulated 1,447 1.95 cables ... 293 .. 138M1 11500 1- .... . ., ...,...,...,...  ..,... Outwlavw 1.8 18900 15750 Insulated 1.52 2.15 cables 293 . .1 ..l..l.l... 12600 .. 10500 ........ ...I.I ........... Rods 1.0206 Collaring 1.8 209 ooo The mesh considers a one-quarter structure, with appropriate boundary conditions for the simulation of two layers of collars. Due to the history dependency of systems where friction is present, it has been necessary to reproduce the story of the magnet from assembly to operation. The three main stages, which have to be described, are assembly, cooling and operation at nominal field. 1 )  Assembly: during assembly the two shrinking cylinder half-shells nre pressed one towards the other to apply the correct pre-stress after welding. In  the !?E model, to simulate 10S1-8223/00$L0.00 0 2000 IEEE 82 this process, the extrcmities of the half shells are dispIaccd till the desired tension is achievcd. When this condition i s  reached, the edges are blocked in position to simulate the welding. This position i s  then kept f ixed during the fallowing steps. 2) Cool down: in order to get the correct displacements during cool down it hns been necessary to introduce, for each material, the curves of thermal contraction and of the Young modulus as function of temperature, This has been done for the following materials: stcel or aluminium for the collars, steel for the shrinking cylinder, iron for the insert and the yoke. For the Poisson coefficient a value of 0.3 has been assumed constant with the tcmperature and equal for all the materials. For the other materials the properties are reported in Tab. 1. 3) Rump up in field: the current is injected into the cables and the field rises. Because of the Lorentz forces the force exchanged along the vertical gap decreases while those between collars and yoke increase. Fig. 1. Cross section and main contact surfaces of the LHCmain dipole. Apart fcom the effect of friction, from the following results it appears that the two structures behave quite differently, The difference i s  dctermined by the presence of an open yoke gap in design A (gap size 1 mm before assembly and 0.6 mm after assembly) while in design B (gap size 0.3 mm before assembly) the yoke gap is already closed at room temperature. In particular for design A the closure of the inner gap corresponds to D. change in slope at 90 K for the curve of the mating force along tho inclined gap. Because of the differential thermal contraction between collars and yoke, design A shows higher contact forces between the collars and the yoke. Also in the evolution of the shrinking cylinder stress, the difference between an open gap and a closed one is clear. In the first one, in fact, part of the energy i s  lost closing the gap, while in the second, the force i s  increasing throughout the cool down. 1v. COMPUTATION ALONG A COOLING, WARMINO, COOLING Friction was applied at the interface yokelcylinder and dong the inclined contact. The value of the friction coefficient (p) WRS taken 0.1 for both the contacts [3,4]. CYCLE This value of p was used in the following computations in which a complete cool down (293K+I.8K), warm up (ln8K+293K), and partial cool down (293K+140K) have been simulated to study possible memory effect of the structures. It can be generally assessed that no differences are present between the first and the second cool down cycles. The curves of the cool down arc: parlially overlapped in Fig 2 to Pig 4. 1) Effeecr of friction on the yoke muting forces: the effect of tiiction along the cylindcr-yoke contact and along the inclined contact is a reduction of the forces at cold on the outer and inner gap. For design A (Fig 2) the reduction is  about 120 Nlmm (-29%) €or the outer gap force and about 50 N/mm (-7% ) for the inner gap mating force. In the design B (Pig 3) the reduction is respectively 80 Nlmm ( -5%)  and 10 N/mm (-2%). The amplitude of the hysteresis cycle and the reduction of the transmitted forces is bigger for the design A. Since larger matting forces mcan more stable mechanical structure, the computations with friction are therefore a good approximation in case of steel collars but too optimistic in case of aluminium collars. F z E U1 1 1  I m --A- voketndlned vrtthfr I \  \U\ 1 4 - v o k e o u t s r n o 1 r  I I I\ Fig. 2. Design A. Behavior of thc mating forces nlong the gap. The arrows show the cool down and warm up. Large differences are present belwwn the siiuntion with friction and wilhout friction iw ' ibo - ab0 . 2jo ' pa TBrnpemture Fig. 3 &sign E. Behavior of the mating forces along the gap. The arrows show ihe cool down and warm up. As it can be see b e  difference between the two situations (friction /no friction) i s  8 0  small that the curves are pmclicall y overlapped 2 )  Effect of friction on the shudder forces: apart [room a remarkable hysteresis cycle for design A, there is no big influcncc of friction for both the designs. 3) Effect of fricrioa on the cylinder tensile stress: in Figs. 4 and 5 the evolution of the shrinking cylinder stresses arc shown in two different sections: along the mid-planc and a1 80" with respect to the mid-plane. For dcsign B, it is possible to observe that the amplitude of the hysteresis loop is sinal1 (5 MPa) and that the higher stress is at the mid-plane with 40-50 MPa more than at 80". For design A, the difference between thc highest and towest stress is almost the same, but the amplitude of the hysteresis cycle is much wider (4OMPn). A higher pre-slress in the cylinder increases the mechanical stability of the slructurc. Again the situation for the design B is very similar in the two cases: with and without friction. The computations without friction give a bad estimation of rcali ty for design A. 220 200 190 180 140 120 1W 80 60 40 0 50 io0 1% 203 250 300 TBIIIpefalUIE M Fig. 4. Design A. Behavior of the shrinking cylinder' s tcnrile stress during cool down, wann up and pnrtid cool down. Values ;It On, 80" with respect to the magnet mid plnne. mo K "  i, Q 0 Fig. 5. Design B. Behavior of the shrinking cylinder' s tensiIe stress during cool down, warm up and padial cool down. Vnlues at O*. 80" with respect Io the inngnct mid plwc. V. COMPUTATION WITH ELECTROMAGNETIC Fonc~s The computations werc re-done adding also the effect of electromagnetic forces up to 8.5 T in 5 steps. I) Ejfect of friction mi  ihe yoke mtingforce: ihe effect of friction during operation is quite small for dcsign (inner gap + 3%, outer gap - 8%). For design A the outer gap mating force shows the same reduction already present after cool down. For the inner gap the mating force with friction is bigger than the one without friction (+ 54%, + 60 Nlmm). This means that although the gap is still closed, the structure is moving at least partially. In  this particular case the friction acts like a break helping to keep ihc gap closed. See Figs. 6 and 7.  a5 0 Pnsld m f m m  IN Fig. 6. Design A. Behavior of the gap maiing forces during asscinbly, cool down and magnet operation. I a s s e m b l y  1203 I I I I Pig. 7. Design B.  Behavior of the gap mating forces duriiig assciiibly, cool down nnd mngnet opcrrtion, 2) Eflect of friction on the shoulders' farces: undcr 1hc crfcct of the clectromagnetic forces the collars arc pushed against the yokc. No effect of friction on the strength of forces transmitted to thc yoke by the shoulders is observed. 3) Effect of friction mi the cylinder delasile stress: when the gap is slill closcd there is no important variation of the cylinder stress. The d k c t  of the friction is thc onc left after cool down. See Figs 8 and 9. 84 In [5] the probability that a magnet meets the design requirements taking in account the effect of the components toleranccs has been computed. The same computations performed again considering friction would give a slightly different picture for design A. The probability to have the inner gap closed would slightly increase while the one concerning the outer gap would show a reduction, For design B no evident change would be remarked. .,............... 1 1 . ........................ 1 .,,.,. I I 183 160 140 120 iw m e3 0 1m 150 m 160 143 140 120 120 100 1 1 ’1 €0 60 a 40 40 Tenpvetm IN fig. 10. Comptdson between the measured cylinder’s tensile strms of the MBLlAJ2 and the computed one. The computations show the cool-down, warm-up, C O O l - d 4 W  cycle. After that the curves present good matching. This proves that the choice of a friction coefficient of 0.1 should not be too far from the reality# VII. GENERAL REMARKS AND CONCWSIONS Fig. E. Design A. Behavior of the cylinder’ s tensile stress during assembly, cool down and magnet operation. It can be generally assessed that design A, which requires an open yoke gap (0.6 mm) before cool down, presents a higher sensitivity to friction than design A (closed gap). The effects of friction along the cylinder and on the inclined contact between the yoke and insert are relevant for both cases and could affect the stability of design A. Further studies, not reported in this article, show that the friction along the shoulders does not seem to affect the structures. Fram the literature, the computations and the measurements it seems that a coefficient of friction of 0.1 is a good approximation of the reality. These results indicate that for the purpose of computation, friction should be included in design A, but can be ignored in design B. I_ ’ ma g 290 E 200 1 160 100 9 50 Fig. 9. Design B .  Behavior of the cylinder’s tensilo slress during assembly, coot down and magnet operation. VI. EXPERIMENTAL CHECK On the graph of Pig, 10 the measured curve of the shrinking cyhder’s stress during the coo1 down of the IO-m modeI MBLlAJ2 on Znd Juky 1997 is reported. The magnet had aluminium collars, The starting points of measurements and’ computations are brought to match. The curves are slightly different because the measured curve describes a structure with racetrack collars with insert inside the collar and no inclined contact. The difference is evident after some points along the curves where the decrease of stress in the structure with racetrack collars is larger. AKNOWLBDGMHNTS The authors would like to thank 0. Spigo and J. Lucas for the suggestions and the data provided, and the group LHC-MTA for the measured data, REFERENCES [ I  1 “The Large Hadron Collider” Concepiual Design CERNIAC195-OS (LHC) 20 October 1995 [21 P.Fessia, D. Perini, R. Vuillermet, C.  Wyss “Compmiive study o f  different designs of the mechanical structure for the LHC main dipoles” LHC Report, to be published 131 ANSYSTM Manuals Smnson Analysis Systems, Inc [43 K. Moos, U. Clair, et al. ‘The measiirement of friction coefficient to [5 ]  M. Bajko. P. Fessia, D. Perini: “Statistical studies of the robustness of the 1.8 K for LHC magnets” LHC Note 303 LHC main dipole mechanical structure”. 

FEM Computations Concerning the Effect of Friction in Two LHC Main Dipole Structures

Abstract

Similar works

Full text

Available Versions

CERN Document Server