NASA 5020A Requirements for Threaded Fastening Systems in Spaceflight Hardware
Over the last decade, we have done numerous structural and thermal simulations of satellites for commercial and ITAR-type clients. Looking back, I’m sort of surprised how close we got with what we thought were FEA best practices for linear dynamics (that is to say, results based on a normal modes analysis from seismic to shock to PSD).
A big advance over the last couple of years has been our approach to fastener modeling. In prior work, fasteners were idealized using beams and rigid links, while nowadays, our preference is to use 6-DOF springs in combination with rigid links. While a bit messy, it provides an efficient methodology to meet the NASA 5020A Technical Specification. The gist of the NASA 5020A specification is to keep joints tightly joined under worst-case conditions with no or limited slip. This is done by combining the effects of fastener preload against that of the applied load or loads with conservative assumptions and factors of safety.
The specification is an algebraic joy to the mathematically inclined simulation engineer. In our project work, we were surprised to learn, that without shear pins, it is quite difficult to get a stand-along fastener to pass the NASA 5020A specification. Furthermore, although best practices often calls out maximum bolt preload, it can also lead to failing the 5020A specification. To provide some background, we’ll show a simple calculation that we did for a recent space-based optical platform.
Our takeaways from this work is that every fastener loves a shear pin and that high bolt preload is not necessarily your friend.
The NASA 5020A specification is written by engineers for engineers. It just takes a methodical read from start to finish. The short version is that it penalizes any threaded fastener that is required to carry axial and shear loads. Hence the trick is to use shear pins and keep the bolt preload low. An example calculation is shown in the following slides.
The NASA 5020A Requirements for Threaded Fastening Systems in Spaceflight Hardware provides calculation procedure and large factors of safety that assures that joints stay locked together during flight conditions. Given its conservativeness it creates design challenges for unsuspecting design and simulation engineers. Our takeaway from this FEA consulting project was that shear pins are usually a necessity since the specification aims to ensure no-slip joint conditions and given its conservativeness, this condition is difficult to achieve without the use of shear pins.
NASA 5020A bolt preload calculation is based on a minimum and maximum approach where the base preload (force) is then modified by an estimated preload variation (G) and then installation parameter (c). An example graph is provided showing the how the bolt preload min and max’s stack up for fastener sizes from 4-40 to ¼-20. The range between min and max provides a quick insight into why meeting the specification can be difficult.
Calculation of the NASA 5020A bolt factor hf is one of the pure formulaic joys of the spec. The bolt factor determines how easily one can meet the margin of safety (MSu) requirements in Eq. 6 of Eq. 7. For example, one wants a high value of P’tu which is more easily obtained with a higher bolt factor. However, at the end of the day, one usual defaults to hf = 0.25 and moves on. One will also note that a low bolt preload (Pp-max) in relation to the bolt’s strength (Ptu-allow) favors a higher margin of safety (MSu).
The specification provides a robust procedure from fasteners to shear pins. For pull-out and bearing loads, it can be a bit murky but guidance is provided if one keeps digging. Our recommendation is to do a first pass based on MS and then start looking at shear pin requirements.
A simplified analysis is provided for the bearing resistance of the potted shear pin per Bruhn, Section D1.10 – Methods of Failure of Single Bolt Fitting and the Allowable Failing Loads. To determine the Bearing Material Failure limit, a nonlinear analysis was performed with the shear pin embedded into the potting material and surrounded by the base. To keep it simple, the analysis work looked at the worst possible shear pin scenario (shear load vs shear pin diameter), results indicated that it would pass.
The NASA 5020A Requirements for Threaded Fastening Systems in Spaceflight Hardware specification journey was a fascinating experience. At the end of the project, we had written several application programming interface (API) scripts that would automatically dump FEA static and PSD fastener forces directly into a spreadsheet. With this link, we could then update the spreadsheet and quickly review the Margin of Safety numbers. Given the number of revisions that was required to optimize the fasteners and shear pins, this effort paid off for our client in schedule and budget. As consulting engineers, this project was a pleasure since we learned something new while helping our client deliver an advance micro-sat that passed CDR with nary a comment about fasteners.