daxmann, Tbuelna...
This bolt style is an attempt to save weight in older designs and is part of a bolt-style evolution. Correct this is a full size tension-head fastener... but with short threads, so that ONLY a lower strength low-height shear nut [EX: NAS679] or a very low-height shear-tension nut [MS21042, NAS1291] can be installed.
In the 1940s only flush bolt style available was full-depth tension style head with long threads + a washer/nut. This served its purpose for permanent fastener installations that demanded diameters larger than conventional swaged-collar lock-bolts that were size-limited to ~0.375. In the 1950s and early 1960s fastener designers went on a weight savings binge.
This style bolt emerged: could be installed ILO the deep-head, longer-thread bolt with a lightweight nut. Shorter threads + shallow nut = lighter weight installation!
Then someone said ‘why does the head have to be that big for shear application?’... so the next evolution promoted heads ~60% depth of these bolts. The initial concern was the bolts’ head ability to absorb torque… until it was recognized that only low-height light weight shear nuts [W/WO washer] should be installed… which needed lower installation torques, so that the head recess size could shrink proportional to head size. Shallow head + short threads = even lighter weight installation
Then Hi-Shear had an inspirational break-thru! The Hi-Lok pin-collar lock-bolt system: Permanent fastener installation similar to the Bolt-Nut combination… with huge range of threaded pin/collar diameters available… blowing-away the ~3/8”diameter limitations of the swaged-collar lock-bolt design. Also, since the head drive recess went away, the tension-head, or shear-head depths could be slightly reduced: stress concentration of the drive recess [be it +, offset-+ or coin-slot, etc] is high and affects head design/stresses. Also, the patented hex-drive recess in the threaded tail of the fastener allowed the part to be torqued without the Pin spinning-around and made a little hollow-cavity in the thread-tip where stresses are low. Also the ingenious collar system allowed a stub threaded self-locking collar to remain-in-place after the driving-hex torqued it to the required limit… then broke-way! For shear-rated fasteners an aluminum collar was more than adequate; for tension-rated fasteners a similar design steel collar provided a huge increase in tensile capability. Reduced head depth + hollow-tip recess + light weight collar = a substantial weight savings over bolt/nut installations.
Then came an evolution in metallurgy: Steel HT went from 14-KSI to 160-KSI to 180+-KSI… and other alloy emerged such as Titanium, A-286 CRES etc. Bolt strength, heat resistance and strength/weight abilities skyrocketed.
Bolts, nuts, washers, Pins, collars, etc were available in a huge variety of ‘flavors’… other than the vanilla steel and aluminum in-use for decades. VHS steel alloys, Ti-6Al-4V, A286, 17-4PH, I-718, etc for Bolts/Pins [HL or swaged-collar lock-bolt, etc]… and VHS steel alloys, 303-CRES, PH-CRES, Titanium, etc for nuts, collars, washers, etc.
Immediate weight/strength gains occurred when titanium bolts are assembled with light-weight titanium or CRES nuts and washers/collars [Ti alloys are ~66% mass of steel] or aluminum collars [~0.33% mass of steel]. Just imagine how much lighter/stronger older structural aircraft designs would be if modern fasteners could be incorporated!!!… and this is exactly why new vehicle designs rarely use old design fasteners for any reason.
In this brief summary, I have only brought-you up to the early 1990s. Since then refinements to bolts/nuts have included changes to head and drive styles [6-point to 12-point, reduced drive diameters, ring-flange changes, etc] and overall strength/durability improvements due to alloy/temper/manufacturing changes [EX: strength increases from 160-to-180-KSI for titanium bolts, etc]. Changes to threaded pin-collar designs have been even more impressive: lighter pin and collar designs [shave metal here-there, tighten installation parameters, change locking mechanisms, alloy improvements, etc]; and changes to pins for better/smoother fit when installed in light-to-high interference [for improved fatigue protection, etc]… PLUS the changes mentioned above for collars, incorporated into the Pins!
Rut-Ro... Gotta go back to work.
Regards, Wil Taylor
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