Stress and Deflection FEA of CARMA Mounts for BIMA ANTENNAS

Transcription

1 Stress and Deflection FEA of CARMA Mounts for BIMA ANTENNAS Thursday, April 7, 2005 This is a report containing the results of analysis performed by for the University of California Radio Astronomy Laboratory under P.O , Rev. N/C. Load cases and general guide lines were provided in a memo entitled Proposal for FEA Finite Element Analysis on new antenna base, dated Thursday, December 02, Analytic methods are described below in section called SUMMARY OF ANALYSIS. SUMMARY OF RESULTS: STRESS: Transporter loads and survival loading at 100 MPH wind at all specified orientations produces stress in the weldment that are below the yield strength of common ASTM A36 steel. The table below shows the peak stress Figure 1 BIMA 6 Meter antenna sitting and safety factor with respect to A36 yield strength for all on new CARMA base stress load cases, A1 through C4. Max Von Mises Stress S.F. The highest stresses seen in the model occur in the shank of the three jacks. The jacks are not modeled to check their adequacy but are modeled to show the operating and handling stress in the weldment. Case Condition psi W/R yield A1 E'xporter stop going down = 2G 13, B1 E'xporter stops going west = 1G 20, B2 E'xporter stops going east = 1G 26, local hot spots B3 E'xporter stops going south = 1G 34, C1 100MPH w ind West 8, C2 100MPH w ind East 8, C3 100MPH w ind South 9, C4 100MPH w ind South-East 5, Figure 2 Tabulated stress results and safety factors W/R A36 steel yield Transportation generally causes more stress than wind survival at 100 MPH. DEFLECTION: Overturning angular deflection at worst case orientation 30 MPH wind peaks at about 5 arc seconds and averages 3 arc seconds RMS ed spatially among all the wind orientations. Torsional rotations (Azimuth) will be smaller. 6 meter BIMA antennas Page 1 of 8

2 SUMMARY OF ANALYSIS: Meshing: The model provided included a device above the new base that would spread the applied loads to the base. First meshing and analysis was conducted this way. however, the load spreader added far too many elements to the FEA model as is shown below in Fig. 3. Default global mesh size of 2.75" produced a 422 thousand degree of freedom (kdof) model that runs Figure 3 Case A1 crude results with bad element aspect ratios 6 meter BIMA antennas Page 2 of 8

3 fast but mesh element aspect ratio is 2.75/.25=11, not good for study accuracy. This ratio should be no higher than 3 for accurate results. See crude initial results. Later accurate results are significantly different. This ratio was reduced to 3 by deleting the load application part and applying the loads to each of the three mounting points as assessed by calculation. Appendix A contains MathCAD generated calculations where each load case is resolved to three point loading at the top of the new base structure. There are many orientations of the reflector and winds. Calculation of all of them severely exceeds the practical expectations of the funding of this effort. Simplification was conservatively achieved by selecting the worst wind loads irrespective of orientation and simultaneously applying them to the model. If these deflections are acceptable to the operation of the antennas, well and good. If not, resources can be allocated to analysis of several representative cases to assess the more accurate worst case pointing error or be spatially averaged to arrive at a more favorable statistical pointing error. DETAILED RESULTS: A1, Antenna assembly mounted on new structure that is lifted by the transporter and then lowered Figure 4 Combined stress due to 2G s downward load in transporter Case A1 6 meter BIMA antennas Page 3 of 8 Figure 5 Combined stress due to transporter stop going West, Case B1

4 and the lowering stops abruptly, as can happen when the lifting control stops this motion carelessly or the load is dropped on the ground. Fig 4 shows the Von Messis (combined) stress distribution on the weldment surface due to this loading. This and all the stress figures are static in this document. However, they can be dynamically viewed by means of using a viewer to display edrawing files (*.eprt) of these figures. Figure 6 Antenna assembly on transporter going West stopping abruptly causing 1G loading, Case B1 B1, Antenna assembly mounted on the new structure that is transported West and stopped abruptly such that 1G is required to decelerate the load. Maximum stress on weldment is 20,500 psi. near top of West mounting hole. 6 meter BIMA antennas Page 4 of 8

5 B2, Antenna assembly is mounted on the new base that is transported East and stopped abruptly such that 1G is required to decelerate the load. Maximum stress on weldment is 26,200 psi near the top of the North and South mounting holes. Figure 7 Antenna assembly going East on transporter stopping abruptly causing 1G loads, Case B2 B3, Antenna assembly is mounted on the new base that is transported South and stopped abruptly such that 1G is required to decelerate the load. Maximum combined stress on the weldment is 34,600 psi, nearly equal to the specified yield of A36 plate. This is the most severe stress seen in all the cases. It is only seen at a small portion of the 1/4" side plates in the upper right of Fig. 8. Figure 8 Antenna assembly going South on transporter stopping abruptly causing 1G loads, Case B3 6 meter BIMA antennas Page 5 of 8

6 C1, Antenna assembly is mounted to the new base. The base is tied down to a foundation and it s jacks also rest on the foundation. Survival wind blows 100 MPH from the West. The dead weight of the antenna assembly shifts down wind to the North and South posts which show the largest stresses that peak at 8,100 psi combined stress. C2, Antenna is as above but the survival wind is going East. The dead weight of the a n t e n n a assembly shifts to the East post w h e r e combined stress peaks at 8,200 psi Figure 9 Case C1, 100 MPH wing, worst orientations Figure 10 Case C2, 100 MPH wind, worst orientations 6 meter BIMA antennas Page 6 of 8

7 C3, Antenna is as above but wind is from the South. The antenna dead weight shifts toward the North post w h e r e combined stress rises to 9,900 psi. Figure 11 Case C3, 100 MPH wind, worst orientations C4, Antenna is as above but wind is from the South-East. The antenna dead weight is shifted to the West and North post where the combined stress peaks at 5,400 psi. Figure 12 Case C4, 100 MPH wind, worst orientations 6 meter BIMA antennas Page 7 of 8

8 DEFLECTION: Angular deflections of this base are anticipated to be the most impact on antenna performance. They are reported here for overturning (Elevation). Azimuth rotations will be smaller. If overturning are of small impact on performance then the other displacements can be ignored. These displacements should be understood in the context of the performance of the whole antenna assembly. Vertical deflections were differenced across orthogonal base lines to assess the angular rotations among the three antenna mounting points. These rotations were then vectorially combined yielding the total tilt. Figure 13 Vertical deflection plot is probed for deflections at the antenna assembly points Case West vert. North vert. South vert.e-w tilt N-S tilt RSS RSS deflection deflection deflection tilt tilt in. in. in. rad. rad. rad. arc sec. D1-4.36E E E-03 30MPH -4.16E E E-03 wind fm -4.29E E E-03 West -4.20E E E-03 avg E E E D2-5.44E E E-03 30MPH -4.98E E E-03 wind fm -5.20E E E-03 East -5.24E E E-03 avg E E E D3-4.86E E E-03 30MPH -4.50E E E-03 wind fm -4.75E E E-03 South -4.66E E E-03 avg E E E D4-5.26E E E-03 30MPH -4.80E E E-03 wind fm -5.07E E E-03 South-East -5.03E E E-03 avg E-05-8E E Tilt results at the four wind directions was then spatially averaged as the root of the mean of the sum of the squares (not fitted). RMS (arc sec) meter BIMA antennas Page 8 of 8