SMA Technical Memo Number 128

SMA Antenna Six Point Tie-Down Evaluation

William N. Davis , SAO Central Engineering

October 27, 1998

Summary:

An SMA Antenna finite element model was analyzed for load variations at the three jackscrew load points with a variation of +/- 24% of the nominal load which was 1/6 of the antenna weight. The results show a maximum deviation in azimuth bearing mounting plane flatness of 0.0007 in. over the entire bearing, compared to an allowable flatness of 0.0015 in. per radial inch. This bearing motion causes a relative motion of the elevation axis (elevation bearings) resulting in a pointing error of 1.41 arc-sec about the cross-elevation axis, and 0.79 arc-sec error about the azimuth axis. It should be pointed out that these deformations are relative to a uniformly loaded six point mount, and they remain unchanged once the antenna is set down, provided the jackscrew loads don't change.

Discussion:

The SMA is attached to the mounting pads via three hard points and three adjustable jackscrews. An analysis was performed on the SMA Antenna model to determine sensitivities to load variation of the three adjustable jackscrew mounts. The base of the mount was supported at the three hard points, and forces were applied individually at each of the three jackscrews. Since the desired result is departure from the nominal case of six equally loaded points, and the model is linear-elastic, the individual load cases to be combined are simply the departures from nominal. The complete antenna is modeled using a total weight of 91200 lbs. The nominal load at each of the six support points is 15350 lbs.

Tests were performed (George Nystrom) to relate applied jackscrew torque to the resulting off-load force. Two sets of data were taken, the first with the jackscrews bearing on dry baseplates, and the second with greased baseplates. In both cases the screws themselves were greased. The antenna was placed on three loadcells at the hardpoints, and measurements were taken as the jackscrews were all equally torqued in 50 ft-lb increments. The results are summarized in Table 1, with a complete set of test data attached in Appendix A.

Table 1 - Jackscrew Preload vs. Torque Test Results Summary
 
Condition Average Load
(lb/ft-lb)		 Standard Deviation
(lb/ft-lb)		 Avg Load/Std Dev
(%)
Dry Baseplates 52.4 12.5 24%
Greased Baseplates 65.4 5.8 8.5%
All Data 57.9 11.9 21%


The analysis is performed by varying the jackscrew loads by +/- 24%. This results in a possible 27 load combinations. The load deviations for 13 cases are presented in Table 2. These 13 cases cover all possibilities within the loading range.

Table 2 - Load Deviation Cases
 
  Applied Load (lbs)
Load Case point 1 point 2 point 3
1 -3684 -3684 -3684
2 -3684 -3684 0
3 -3684 -3684  3684
4 -3684 0 -3684
5 -3684 0 0
6 -3684 0  3684
7 -3684  3684 -3684
8 -3684  3684 0
9 -3684  3684  3684
10 0 -3684 -3684
11 0 -3684 0
12 0 -3684  3684
13 0 0 -3684
 
 

Deformations of the mounting surface of the azimuth bearing around the circumference are shown in Figure 1 for all 13 combinations. The total range of deformations is .00081", and the maximum range for any single case is .00072".


Figure 1 - Azimuth Bearing Deflections with Jackscrew Load Variations of 50% of Nominal


In addition to azimuth bearing mounting surface deformations, relative motion between the two elevation bearings was also examined. The two bearing nodes are separated by 76.77" in the model, and for each combination, relative displacements are converted to pointing motions about the cross-elevation axis and azimuth axis. Nodes 2 and 5 represent the elevation bearings. Deflections and pointing deviations are tabulated in Table 3. The maximum pointing deviations are 1.41 arc-sec in cross-elevation, and 0.79 arc-sec in azimuth.

Table 3 - Elevation Bearing Deflections

            cross-elev azimuth
combination node dx (in.) dy (in.) dz (in.)   arc-sec arc-sec
1 2  -3.833E-05  -1.711E-04  2.561E-06       
 5 -3.896E-05 -1.698E-04 2.782E-06  -0.004 0.002 
        
2 2 2.086E-04 -2.344E-04 5.611E-04    
 5 3.541E-04 2.771E-05 5.604E-04  -0.704 -0.391 
        
3 2 4.556E-04 -2.976E-04 1.120E-03    
 5 7.470E-04 2.251E-04 1.118E-03  -1.405 -0.783 
        
4 2 3.529E-04 2.708E-05 -5.561E-04    
 5 2.064E-04 -2.328E-04 -5.568E-04  0.698 0.394 
        
5 2 5.999E-04 -3.617E-05 2.551E-06    
 5 5.994E-04 -3.538E-05 7.546E-07  -0.002 0.001 
        
6 2 8.471E-04 -9.944E-05 5.611E-04    
 5 9.924E-04 1.621E-04 5.583E-04  -0.703 -0.390 
        
7 2 7.443E-04 2.253E-04 -1.114E-03    
 5 4.516E-04 -2.959E-04 -1.116E-03  1.400 0.786 
        
8 2 9.910E-04 1.620E-04 -5.561E-04    
 5 8.450E-04 -9.848E-05 -5.589E-04  0.700 0.392 
        
9 2 1.238E-03 9.876E-05 2.543E-06    
 5 1.238E-03 9.896E-05 -1.273E-06  -0.001 0.002 
        
10 2 -6.382E-04 -1.349E-04 9.170E-09    
 5 -6.384E-04 -1.344E-04 2.028E-06  -0.001 0.001 
        
11 2 -3.913E-04 -1.982E-04 5.586E-04    
 5 -2.453E-04 6.308E-05 5.596E-04  -0.702 -0.392 
        
12 2 -1.443E-04 -2.615E-04 1.117E-03    
 5 1.478E-04 2.606E-04 1.117E-03  -1.403 -0.785 
        
13 2 -2.470E-04 6.326E-05 -5.586E-04    
 5 -3.931E-04 -1.974E-04 -5.576E-04  0.700 0.393 
        
     max abs.1.405 0.786 

Position deviations for the tertiary mirror location are included in Table 4. The tertiary mirror has a maximum displacement of .00131" and maximum rotation of 1.73 arc-sec about any axis.
 
 

Table 4 - Tertiary Mirror Positional Deviations
 
case dx (in.) dy (in.) dz (in.) rx (arc-sec) ry (arc-sec) rz (arc-sec)
1 -4.168E-05 -1.704E-04 2.817E-06 0.004 0.002 0.077 
2 2.961E-04 -1.031E-04 5.884E-04 0.705 -0.388 -0.376 
3 6.339E-04 -3.579E-05 1.174E-03 1.406 -0.778 -0.828 
4 2.942E-04 -1.027E-04 -5.839E-04 -0.699 0.391 -0.373 
5 6.320E-04 -3.532E-05 1.739E-06 0.002 0.001 -0.826 
6 9.697E-04 3.203E-05 5.873E-04 0.703 -0.389 -1.279 
7 6.302E-04 -3.483E-05 -1.171E-03 -1.402 0.781 -0.823 
8 9.677E-04 3.251E-05 -5.849E-04 -0.701 0.391 -1.276 
9 1.306E-03 9.985E-05 6.603E-07 0.001 0.001 -1.729 
10 -6.737E-04 -1.351E-04 1.078E-06 0.001 0.000 0.903 
11 -3.359E-04 -6.782E-05 5.867E-04 0.703 -0.390 0.450 
12 1.866E-06 -4.832E-07 1.173E-03 1.404 -0.780 -0.003 
13 -3.378E-04 -6.733E-05 -5.856E-04 -0.701 0.390 0.453 
 

Conclusions:

The axial displacements caused by variation in the jackscrew applied forces are well within manufacturers' tolerances for the azimuth bearing. Pointing variations are calculated and presented here for evaluation by the SMA optical working group and/or other people involved with antenna pointing.