Page:Advanced Automation for Space Missions.djvu/194

 4F.4 Interlace Fasteners - Stapling

Stapling is similar to stitching except that staple rigidity is important to the load. The staple passes through holes in the material to be fastened and is bent to prevent loaded matter from easily slipping out. Staples almost invariably are made of metal since they must be strong, cheap, and bendable yet fairly rigid. The relative ease and speed of stapling over stitching has led to its increasing use in the fabrics industry, though few large commercial products have direct space applications. Since staples provide a low-cost, low-energy, rapid-fastening capability, they may play a role in various forms of space construction. Beams of thin aluminum or other metals could be stapled rather than welded if desired. Staple bonds are relatively weak but zero-g permits their use in space on flimsy structural members impossible in terrestrial construction.

Stapling is usually done by machine on Earth and this is unlikely to change in space. As for alternatives, if bonded items are metallic, tack welding often can replace stapling. Energy costs increase with bond strength and tear resistance. If bonded items are nonmetallic then welding methods cannot be used, but glues may replace staples if necessary.

4F.5 Shrink and Press Fitting

Shrink fitting is accomplished by heating a part so that a hole in it expands, after which another piece may be fitted, usually under pressure, into that hole. The outer piece then contracts as it cools, creating a tight seal. Some sinter-like bonds may form, but shrink fitting works primarily by friction bonding. It requires thermal energy which press fitting (see below) does not, but less force is needed to achieve the final bond. If the material to be shrink-fitted is metallic, heating may be accomplished by induction.

Press fitting is similar to shrink fitting except that parts are not heated and higher pressures are necessary. Press fitting requires less energy but the bond is weaker. Also, if bonded material has a buckling problem press fitting is not suitable as a joining technique.

Beams made of rigid materials can be joined by fitting, as can many other parts. Gears routinely are attached to shafts by this method. Fitting can produce bonds strong enough for many applications. The great simplicity of these processes strongly urges their automation.

Usually metals are shrink and press fitted, and these materials are relatively abundant in nonterrestrial resources. The energy and materials efficiencies of these techniques make them prime candidates for space applications. Both are preferred to welding where loads are light. Vacuum welding may serve to strengthen bonds. Flames are hard to produce in a vacuum so shrink fitting probably will be accomplished by induction heating if the materials are metallic.

4F.6 References

Annis, James F.; and Webb, P.: Development of a Space Activity Suit. NASA CR-1892, November 1971.