Advanced Automation for Space Missions/Appendix 4F
Appendix 4F Review Of Adhesives, Fasteners, And Fitting
There exist a number of alternatives to welding, brazing, and soldering which might be employed in space industry for joining metals and especially nonmetals. The most important of these are adhesives, metals fasteners, interlace fasteners for stitching or stapling, shrink fitting, and press fitting. Each has been considered for space applications in terms of manufacturing processes, required materials, possible compatible substitute techniques, and the degree of automation attainable. In addition, the unique impact of zero-g (more properly, "free-fall"), hard vacuum, and intense radiation is considered for each joining process examined.
4F.1 Glues and Other Nonmetallic Bonding Agents
Adhesives are used to fasten two surfaces together, usually producing a smooth bond. This joining technique involves glues, epoxies, or various plastic agents that bond by evaporation of a solvent or by curing a bonding agent with heat, pressure, or time. Historically, glues have produced relatively weak bonds. However, the recent use of plastic-based agents such as the new "super-glues" that self-cure with heat has allowed adhesion with a strength approaching that of the bonded materials themselves. As a result, gluing has replaced other joining methods in many applications - especially where the bond is not exposed to prolonged heat or weathering.
A large fraction of modern glues are carbon-based petrochemical derivatives. These can be used to bond almost any combination of surfaces, either by direct contact or by fastening both surfaces to a third as with adhesive tapes. Glues can serve as bonding agents in strong structural materials - one of the earliest, and still common, such use is the fabrication of plywood (a wood composite). Other related composites include fiberglass and various fiber-epoxies such as boron-epoxy and carbon-epoxy. Many of these materials make superior stress-bearing components.
Composite structures often are far less massive than comparable metal components and may be used in structural locations. Some of the early plans for beam construction on Shuttle flights call for carbon-epoxy materials. Composites may be the major use of glue/epoxy adhesives in space. For macroscopic bonding, alternatives such as welding, stapling, bradding, stitching, and other fasteners can replace adhesives if necessary. But although composites in theory can be replaced by metal parts it is far more likely that in space metal parts will give way to composites.
The space application of adhesives includes the following considerations:
- Zero-g - Although some adhesives must bond and cure under pressure, variations on clamping could compensate for the lack of gravity. Application of adhesives also should not demand gravity feed, although squirting and injection techniques have been perfected.
- Vacuum - Many resins and glues used on Earth are fairly volatile and deteriorate under vacuum. But some plastics, once cured, no longer are volatile and may continue to be used in vacuo. Silicate-based waxes and bonding epoxies employed in composites are just two examples of currently available vacuum-compatible adhesives.
- Radiation - Most hydrocarbon-based plastics weaken under the influence of infrared and higher-frequency electromagnetic radiation. These would not be suitable for exposed space use without shielding. More research is needed to develop radiation-resistant adhesives and bonding agents.
The application of glues to complex shapes already is automated in many industries, particularly fabric applications. Composite mixing and curing is now done by machines with a high level of reliability. Further automation of these processes should present no unusual difficulties.
4F.2 Metal Fasteners
Metal fasteners are of two kinds - those producing a permanent bond and those requiring either a releasable or a sliding bond. Screws, nuts and bolts, rivets, brads. retaining rings and clamps are examples from the first category. These are used for permanent fastening where stress loads preclude gluing but do not require welding or where the possibility exists of undoing the bond for some future purpose such as repair. Nonpermanent fasteners include quick-release couplers and clamps intended for removal at a specified time, and pins which allow relative movement of fastened parts. Pins are used where conditions of movement are less rigidly constrained than when heavy bearing capability is required.
Metal fasteners must be strong to bear significant loads. In many cases they can be manufactured by powder metallurgical or casting techniques. Iron is a constituent of many types of metal fasteners, although titanium increasingly is coming into use in applications where strength must be balanced against light weight. In most applications where permanent bonding is required metal fasteners are replaceable by some form of welding or soldering. A major consideration here is whether the fabrication of welding rods and the process of welding is a more or less efficient use of available resources and energy than the fabrication and use of fasteners. For nonpermanent bonds there is not much choice except friction/pressure fittings and these run the risk of vacuum welding.
Both iron and titanium are in abundance on the Moon and each has received much attention as two extraterrestrial resources most likely to be investigated early for extraction and utilization. The manufacture of metal rivets from lunar or simulated lunar resources would be a worthwhile early materials processing experiment for an orbital laboratory. Space applications considerations include:
- Zero-g - Metal fasteners may be lighter in weight because loads may be far less than on the ground.
- Vacuum - Permanent bonds are largely unaffected by vacuum. Vacuum welding will promote tighter joining, a benefit in the case of permanent bonds but a definite hindrance if breakable or sliding bonds are desired. Very low vapor-pressure lubricants (e.g., graphite), surface poisoners, or careful choice of incompatible metals may help to eliminate this problem.
- Radiation - Some metal fastener materials may become more brittle with time in the presence of ionizing radiation.
The fastening of rivets and bolts already has been automated in some terrestrial applications. Extending the techniques of automation to space, and including screws and nuts, clamps and pins, seems to present no special problems.
4F.3 Interlace Fasteners - Stitching
Interlace fastener stitching is a joining process by which pieces of material are interwoven through holes in the parts to be joined. The bond is primarily frictional if the joined pieces are not rigid,primarily tensional if they are rigid. On Earth, mostly fabrics are stitched, though items such as tennis racquets and sieves also require a type of stitching in their manufacture. Stitching material usually has physical properties and adhesive characteristics similar to those of the materials joined. Parts to be fastened must have a series of holes through which the interlace passes. These holes may be native to the material, as in a fabric, or specially drilled, as in wood or metal sheets. (Terrestrial stitching is applied to some processes not immediately obvious, such as the knitting together of thin plywood sheets to form a mold for fiberglass.) The primary space-related utility of interlace fasteners is expected to be in the manufacture of EVA pressure suits. Designs such as the Space Activity Suit (Annis and Webb, 1971) rely on tension instead of atmospheric pressure to counterbalance internal hydrostatic forces using corset-like interlaces to join special fabrics. Stitching materials may be organic or synthetic fibers, glass fibers, or even metals.
The space environment places a few constraints on possible stitching materials, as discussed below:
- Zero-g - Except for holding parts in place during fastening, zero-g presents no special hardships as regards stitching. Indeed, one possible indirect advantage is apparent: The lack of gravity permits finer threads to be pulled from molten material than is possible on Earth, because of the absence of both the catenary effect and the necessity to support threads against their own weight in zero-g.
- Vacuum - Vacuum poses two problems for stitching. First, it is nearly impossible to make an airtight interlace without sealant. Second, most interlace materials are hydrocarbon-based, hence are volatile and easily deteriorate in a vacuum. Fortunately, nonvolatile stitches made of metals or basalt glasses can be found, and there do exist sealants effective in closing small holes against the loss of atmosphere.
- Radiation - The deterioration of interlacing materials caused by hard radiation is a serious problem for hydrocarbon-based stitches, but replacement of these by glass or metal substitutes may eliminate the problem. Radiation-proof coatings should be vigorously pursued as an important topic in space manufacturing research.
The availability of stitching materials is strongly constrained. Hard vacuum and radiation in space render hydrocarbon-based threads infeasible due to volatility and molecular deterioration, and hydrocarbons are also relatively rare in near-lunar space. On the other hand, glass and metal interlaces do not suffer from these problems and are easily accessible on the Moon.
Stitching most efficiently must be done by machines in most applications, and these processes are already largely perfected. Interlacing beam ends do not seem to present any special problems for automation. As for alternatives, gluing can replace stitching in some applications such as the joining of fabrics. Gluing has the advantage of airtightness but the common disadvantage of lesser strength. Tack welding can replace interlacing of metals in many jobs, but a penalty must be paid in higher energy consumption.
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.
Annis, James F.; and Webb, P.: Development of a Space Activity Suit. NASA CR-1892, November 1971.