In aerospace, wire insert applications have ranged from heat sinks and electronic chassis to satellite connectors, circuit board clamps, and avionics box enclosures.
To encompass the widest range of use, the wire inserts are available in standard sizes from #2-56 through 7/16-20 in tanged or tang-free Drive Notch, and from M3 through M16 in tanged only. The tang-free Drive Notch wire inserts conform to NAS1130 dimensionally, but the internal thread form has this patented thread profile.
The inserts are made of stainless steel. If a hole were tapped into a soft material, that hole would be able to withstand a certain amount of installs before the threads started to shear and gall. If the hole were tapped in aluminum, that thread would have a strength of about 30psi. A wire insert made of 300 series stainless steel has a strength of 200ksi, which is a vast difference and greatly diminishes any concern about these threads ever galling or shearing.
Spiralock tapped holes are used in the gas turbine engine in this BMG-109 Tomahawk missile. (Source: Spiralock)
This self-locking fastener has been validated in published test studies at leading institutions, including MIT, the Goddard Space Flight Center, Lawrence Livermore National Laboratory, and British Aerospace. It has been used in extreme fastening applications with virtually no chance of recall, from the main engines of NASA's space shuttle to the Saturn Cassini orbiter and the Titan Huygens probe.
The self-locking fastener's ability to retain clamp load under extreme shock, vibration, and temperature with less weight, complexity, and maintenance makes it a natural choice for aerospace. It retains clamp load even with exotic materials, such as composites and titanium, which is important in reducing component weight.
NASA was among the first to deploy this thread when designing the main engines of the shuttle orbiter. Each of the three main engines developed 400,000 pounds of thrust and terrific vibration. But the space agency also wanted a 15-cycle reuse capability per fastener. In its own testing, NASA determined the fasteners in holes threaded with this unique thread form did not back off or loosen when subjected to 10 times the shuttle-specified vibrations, and they stayed that way 10 times longer than called for. These tests found that the wedge-ramp-thread fasteners delivered 50 uses with no loss of clamp load. Every shuttle carried no fewer than 757 of these fasteners.
For the Cassini-Huygens mission across 750 million miles of space, NASA used this thread form to resist vibration and temperature-induced thread loosening on mass spectrometer instrumentation to measure the atmospheres of Saturn and Titan. A total of several hundred fasteners in the Cassini orbiter and the Huygens probe had to maintain vacuum-tight sealed cavities from final assembly and testing through launch until the end of the seven-year mission.
Robert Farhat is an aerospace technical sales engineer for Spiralock.
Thanks for the feeback, bobjengr. That's even scarier. Sounds like no one's paying enough attention, either to potential failures on individual planes, or to the entire system.
Hello Ann, I asked the very same question and sometimes they did not. There were times, granted not many, when emergency landings had to be made due to cowlings or flight surfaces coming loose and vibrating uncontrollably during flight. Of course, this can affect the airworthiness of the plane and consequently provide exceptional drag. It was always amazing to me how uninvolved some pilots were relative to pre-flight inspections. The "walk-arounds" recommended were sometimes cursory at best. My experience was during Viet Nam and there were so many aircraft coming and going at the Ogden Air Material Area (OAMA) it was impossible to say what part belonged to what aircraft. They were labeled with a date and placed in a special bin. Then you wait for a phone call.
This statement: "A secondary issue arises: finding safe and reliable methods of fastening assemblies that can protect against fastener loosening while minimizing assembly and maintenance costs. These methods must provide complete assurance of joint integrity under the severe conditions of shock, vibration, and thermal cycling common in aerospace environments" brought to mind Apollo 13 - if I recall correctly the explosion was caused by a defective part off the assembly line. I would be a lot more interested in maximizing safety then in minimizing the costs of the fasteners...
Robert--very interesting post. I think fastener technology has greatly improved over the last 20 or 25 years. During my "tour of duty" in the Air Force, we would sweep the runways three times per day for components that actually fell off the aircraft. It was amazing to me the parts we found. Our sweeper had the capability of lifting a part weighing up to 100 pounds--and we found them. Cowling, hundreds of screws and bolts, nuts, etc. you name it. Believe it or not, we never had an accident, to the best of my knowledge, as a result of components vibrating off but, I certainly don't know why not. The technology has definitely advanced since those days--thankfully.
This technology is a far cry from your Helicoil of yore. If you click the Spiralock link you will see the 30 degree "vibration stoppers", etc. Someone put some effort into this.
The only issue that I have experienced with threaded inserts is the special tap needed for the OD of the insert. I have been in shops where they might only have one Helicoil tap for a certain ID thread. Production stops when the guy that keeps the tap in his tool box is on vacation.
I use steel inserts in two cases. One where a bolt will be removed repeatedly, or I have to fix a stripped out hole. I think the later is where most inserts are used. In many cases the insert has a stronger holding potential versus the original thread. I only wish I could get some of the more exotic sizes cheaper.
I'm not sure anything in this article is new as much if not all of it has been known for 50-100 yrs!!
In composites one doesn't cut threads for either bolts or inserts if one is smart but instead molds them with epoxy, etc in place giving good holding and locking in many cases. If a sandwiched material one hollows out the foam/etc core and fill it with epoxy to spead the load, then insert the bolt, insert, etc as needed.
In other plastics drilling a smaller hole then screwing a hot bolt, insert into it gives the needed strength in many cases.
If higher loads than the local material can handle glue on a reinforcement piece with the threads built into it.
As for working loose there are many types of thread lockers out there.
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