Ongoing efforts to increase fuel efficiency, tactical mobility, and payload capacity in aerospace design have driven design engineers to research numerous strategies that reduce mass through the extensive use of lightweight materials such as composites, aluminum, and plastics.
However, use of these lightweight materials can create other compromises. 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.
Spiralock's Drive Notch inserts, including its new 2-56 DN insert, feature a patented 30-degree wedge ramp formed into a unique five-sided wire. (Source: Spiralock)
Assemblies that combine lightweight materials create additional challenges. Considering aluminum as an example. Cold forming threads directly into this material or utilizing a wire insert are common practices, yet most composites are too brittle to be tapped. Shear strength and concentration of forces need to be carefully reviewed in order to confirm that specific loads and vibration environments can be tolerated. Structural joints in particular rely on high strength to develop sufficient pretension in the fasteners used to assemble them.
In most structural joints using lightweight materials, the parent material needs to be reinforced through the use of a wire or ultrasonic insert. The implementation of inserts allows higher joint tension and extended reusability, compared with a hole tapped directly in the softer materials.
Tapping into aluminum with a small thread increases the risk of galling, seizing, and thread shear. This is particularly true when considering reusability of the threaded hole, since smaller threads are much easier to ruin in soft material.
One solution is to choose one the variety of thread reinforcement options being utilized in aerospace applications: wire thread inserts for aluminum or soft materials, potted blind inserts for composites, and ultrasonic or molded inserts for plastics. However, the majority of these inserts still fall short of addressing the limitations and potential for vibration-induced thread loosening that are inherent in the standard 60-degree thread form.
Fortunately, advances in fastener insert technology protect against fastener loosening or failure in the field, with the additional benefits of reducing assembly time, maintenance costs, and even overall weight.
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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