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Fastener Analysis

MidSTAR will use #10-32 A-236 bolts throughout. The specs for those bolts are shown below. Shear loads will be taken by friction rather than pins. This decision is based on the large size of the bolts and the high preload to be used, combined with the high coefficient of friction of aluminum-on-aluminum. Even in the event of absolutely no friction, #10 bolts still have adequate shear strength to take the shear forces on their own. These bolts are larger and stronger than required for a spacecraft structure as small as MidSTAR, so all the FOS results were relatively high, and well above the required FOS of 1.25. Some results are shown in the Factor of Safety/Margin of Safety Table. The number of bolts being used to hold each component together and each payload box to the spacecraft are shown in the fastener spreadsheet.

Bolt Specs

From NAS1351 spec for #10-32 A-236

For fastener analysis calculations, the following assumptions were made:

  1. The bolts will be pre-loaded (torqued) to 25% of their yield strength (which is 3200 lb - the actual percentage will be 50%)
  2. The electronics boxes are like bricks - they don't flex
  3. The box center of gravity is at the box's geometric center
  4. The tension force (the force trying to lift the box off it's mounting plate) is carried equally in all fasteners.
  5. The shear force (the force trying to slide the box along it's mounting surface) is also carried evenly in all fasteners and that force acts in the long direction of the box (this is the worst case for tension)
  6. 15g's of load is on the box in each direction, applied simultaneously

  7. The tension load is a combination of the 15g's acting perpendicular to the mounting surface and the overturning moment caused by the 15g's acting parallel to the mounting surface trying to tip over the CG. Since the CG is up from the mounting surface by half the height of the box, when the lateral load is applied it creates an overturning moment. That moment is reacted by some of the bolts pulling down on one end of the box and the mounting surface pushing up on the other.
  8. The preload on the boxes is always constant, making the static friction always constant, unless the load on the box exceeds the preload.

Pictures of sample boxes

The actual numbers for MidSTAR are:

The following links and references were helpful with Fastener Analysis.