18-8 stainless steel is low-cost and corrosion resistant, the strength of 70,000 psi is close to SAE Grade 2 bolts. A-286 Alloy is comparable strength to non-stainless bolts, but at a relatively higher cost. The minimum tensile strength of A-286 screws typically exceed SAE Grade 8.
When a single bolt can’t fail, a certified A286 alloy bolt is often called for. When there is redundancy and margin, more widely available 18-8 is often more convenient and cost effective.
For potential flight project, materials and fastener certifications should always be procured out of an aboundance of caution.
Without extensive review and waiver processses the following materials should not be used on space flight hardware:
Vented Socket Head Screws increase cost and decrease choices but allow venting of blind holes to prevent gas build up in space/vacuum applications when the hole is not otherwise vented (the smallest screw hole that must be vented is somewhat fuzzy, but the fewer unvented holes the better).
Lubricants that are vacuum safe and low-outgassing (see outgassing.nasa.gov), such as Braycote 601 EF can decrease the risk of galling.
For detailed calculations and requirements see NASA Standard 5020. NASA Tech Memo 19960012183 locally: 19960012183 introduces preloaded joints and also has a table of suggested torque values.
The quick and approximate approach for non-critical estimates: Look up fastener clamping load and recommended torque on a table like the Spaenaur Suggested Tightening, locally chart, taking care to use the lubricated column if applicable, and 18-8 bolts approximated as Grade 2 and A-286 approximated as Grade 8. The clamping load of the part should exceed the expected load on the joint being supported by a safety factor. e.g. a kilogram part expecting to see 30 g loads should and a minimum safety factor of three should be held with at least 1 [kg]*3*(30*9.8) m/s/s = 882 Newtons (e.g. >198 lbs-force).
Inserts, such as locking helical inserts, provide a locking mechanism to prevent screws from backing out, can add strength, and increase the tolerable number of cycles of a joint. To prevent galling, mil-spec Nitronic 60, keyed or helical (“helicoils), inserts are required whenever practical. Locking inserts are preferred when the number of cycles is expected to be low.
See NASA standard 5020 section B.5 Best Practices for Adhesive Locking Features.
At a minimum, a small amount of adhesive (i.e. 2216 B/A Gray) applied to the clean interface of the head of a screw to the bearing material is recommended to provide secondary retention.
| Test | Notes | |
|---|---|---|
| Tension margin Yield and Ultimate | ||
| Shear margin Yield and Ultimate | ||
| Shear+Tension Combined factor margin | depends on shear plane, see NASA 5020 or NSTS 08307 | |
| Gapping | for critical but none catastrophic failures the safety factor should be multiplied by 1.2 | |
| Shear out/ Pull out strength | depends on insert, parent material, and fastener | |
| bolt head pull through margin | ||
| Bearing Yield and Ultimate | strength analysis of the joint members (e.g., bearing member or non-fastener material). |