In traditional fasteners, the radial clearances between traditional male and female 60-degree vee threads can permit relative sideways or lateral movement when shock, vibration, or transverse loading occurs. Stress concentration at the first few engaged threads increases the probability of shear and may lead to fatigue failure. Temperature extremes can also expand or contract surfaces and materials, potentially compromising joint integrity.
As a result, a variety of locking devices from wires and washers to prevailing torque threads (as well as chemical and drypatch adhesives) are commonly added to prevent loosening. These methods often have more drawbacks than advantages, though, because they do not always hold up under extreme conditions and may not be reusable. Inserts provide a unique challenge, since there is not much material in which to apply a locking feature.
Spiralock 1/4-28 tapped holes are used to mount connections under the aircraft wing (circled in red) on this F-18 Hornet/AIM-120 AMRAAM air-to-air missile. (Source: Spiralock)
With a prevailing torque wire insert, the middle coil becomes a deformed thread; that is where the locking happens. It is often very difficult to screw the insert all the way down, and a wrench may be needed. As expected, the screw locking inserts can make it difficult to increase the part's longevity via reusability.
For new or redesigned applications, the answer may be to implement the 30-degree wedge ramp design offered by Spiralock. The self-locking technology is already used by aerospace leaders such as BAE, Boeing, Honeywell, NASA, Harris, Raytheon, Hamilton Sundstrand, and the US military. It is now being applied to a variety of application-specific inserts in various materials, with the added benefit of providing an easy conversion from traditional 60-degree vee threads to the 30-degree wedge ramp design.
The 30-degree wedge ramp allows the bolt to spin freely relative to female threads until clamp load is applied. The crests of the standard male thread form are then drawn tightly against the wedge ramp, eliminating radial clearances and creating a continuous spiral line contact along the entire length of the thread engagement. This continuous line contact spreads the clamp force more evenly over all engaged threads, improving resistance to vibrational loosening, axial-torsional loading, joint fatigue, and temperature extremes.
This thread form is free running. Once torque is applied, the bolt/screw is drawn into tension, and the clamp load builds within the joint. The locking of the male fastener bolt with the unique internal threads begins the moment the bolt crests engage with the wedge ramp. Therefore, it allows for an easy bolt rundown, with no interference thread, or galling and seizing, as may be the case with other inserts. Furthermore, essentially unlimited reusability is the end result when using these threads and fasteners.
In addition to being utilized for tapped holes and wire inserts, this wedge ramp design has been integrated into many other envelopes, such as key inserts, float nuts, and other military standard or National Aerospace Standard part series that call for a female internal thread. This thread form can offer the same vibration resistance and reusability while bringing higher strength and clamp load capability to softer materials, or in situations that require a secure, self-locking thread.
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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