spacehardwarehandbook-public

Useful References

Material Selection

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.

no-go materials:

Without extensive review and waiver processses the following materials should not be used on space flight hardware:

Style selection

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).

Lubricant

Lubricants that are vacuum safe and low-outgassing (see outgassing.nasa.gov), such as Braycote 601 EF can decrease the risk of galling.

Torque

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).

Torque wrenches

Inserts

Nitronic 60 locking helical inserts are suggested for threading into aluminum. Inserts, such as locking helical inserts “helicoils”, provide a locking mechanism to prevent screws from backing out, can increase strength, and the tolerable number of cycles of a joint.

Staking

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.

Analysis Checklist

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).