NASA 5020A Requirements for Threaded Fastening Systems in Spaceflight Hardware
Over the years, we have done a number of satellite analysis projects for commercial and those other government agencies. Looking back, I’m sort of surprised how close we got with what we thought were FEA best practices for linear dynamics (i.e., normal modes and PSD analysis). The big advance over the last couple of years has been in our approach to fastener modeling. In prior work, fasteners (bolts, screws, what-ever) were idealized using beams and rigid links (Nastran RBE2) while nowadays, our preference is to use six DOF springs (Nastran C-Bush) in combination with rigid links. While a bit messy, it provides an efficient methodology to meet the NASA 5020A technical specification.
The gist of this specification is how to calculate, whether or not, the fastener will fail given: bolt preload, with and without shear pins and joint slippage. It is a tall order and the specification is a algebraic joy to the mathematically inclined simulation engineer. Fastener failure is dominated by the designer’s choice of bolt preload. Interesting enough, the NASA specification favors low bolt preload. It sounds odd, but if pushed, one can avoid NASA 5020A fastener failure by lowering the bolt preload. The reason for this is due to the relationship between bolt preload and the applied tensile load. There is no free lunch and regardless of the initial bolt preload, the applied load adds to the overall bolt tensile load. The specification favors hand calculation but with some FEA modeling, one can improved upon the hand calculations and eke out a bit more headroom. If you would like to read more, the NASA 5020A specification can be downloaded here.