SMA Backup Structure Loads for Rotational Accelerations
William N. Davis , SAO Central Engineering
20 Aug. 1999
An SMA Antenna finite element model was analyzed for rotational
accelerations about azimuth axis with the reflector positioned at zenith(90o)
and 10o elevations. The reflector backup structure (BUS) tube loads were calculated for a rotational acceleration of 3o/sec2. These are the current design values.
BUS tube loads due to a rotational acceleration range from 0.1 to 37 lbs., representing
a maximum of 1.9% of previously calculated loads for combined wind, gravity,
and thermal conditions, by tube type. (Ref.
SMA Technical Memo No. 119 )
Maximum acceleration values are calculated, based on a total worst case combined load
equal to the proof load for each tube type.
Discussion:
An SMA finite element model was run in the zenith and 10o reflector elevation orientations for rotation about azimuth at a rotational acceleration of 3o/sec2.
Axial tube loads are calculated for rotations about the elevation and azimuth axes, at both zenith and near-horizon antenna positions. Note that some of the old data was for 5o elevation, and the difference between 5o and 10o is insignificant to this analysis.
The maximum combined loads for each orientation were subtracted from the proof loads for each tube type, and the acceleration value which would produce the remaining load was calculated. The results are presented in the following tables:
| Load Case | Limiting Acceleration | Table |
| Zenith Position, Acceleration about elevation axis | 131.1o/sec2 | Table 1 |
| Horizon Position, Acceleration about elevation axis | 107.6o/sec2 | Table 2 |
| Zenith Position, Acceleration about azimuth axis | 134.7o/sec2 | Table 3 |
| Horizon Position, Acceleration about azimuth axis | 109.6o/sec2 | Table 4 |
In addition to determining the maximum rotational accelerations, design curves were generated for maximum hard stop stiffness and minimum stroke, as a function of impact rotational velocity, which limit the rotational acceleration to the maximum values. That information is presented in the following figures:
| Load Case | Rotational Inertia | Radius of Hard Stop Line of Action |
Figure |
| Zenith Position, Acceleration about elevation axis | 2.75x105 lb-sec2-in | 31.04 in | Figure 1 |
| Horizon Position, Acceleration about elevation axis | 2.75x105 lb-sec2-in | 26.94 in | Figure 2 |
| Zenith Position, Acceleration about azimuth axis | 4.94x105 lb-sec2-in | 35.60 in | Figure 3 |
| Horizon Position, Acceleration about azimuth axis | 6.52x105 lb-sec2-in | 35.60 in | Figure 4 |