Securing Fasteners in Safety Critical Aerospace and Defense Applications – SME
Safety-critical aerospace manufacturing applications demand secure strings that won’t loosen. Manufacturers now have an excellent, comprehensive solution via the latest self-locking threads for thread milling and tapping that feature a wedge ramp angle on the thread’s flank.
Whether it is the screws used to assemble a piece of budget furniture or those that keep F-15 Eagle fighter jets from flying apart during Mach 3 maneuvers, all have the same basic V-shape thread profile, despite their varying quality levels. Some are machined using “single-point” threading techniques on CNC lathes, although many more are rolled, chased, or whirled using special threading heads, especially where higher quantities are required.
But because these fasteners accomplish nothing without a threaded counterpart to screw into, it’s equally important to describe this side of the machining equation. For the majority of all threaded holes, this is accomplished using one of two cutting tools: a thread mill, or a tap. Single-point threading is also an option (on COMPUTER NUMERICAL CONTROL lathes plus multitasking machines, at least), but given the widespread availability of thread mills and cut or even form taps—not to mention their own greater productivity—there are relatively few occasions where single-pointing of internal threads is necessary.
Regarding line tolerances, note that all strings, whether inner or external, are made to a specific class of fit plus must be precision machined and measured based on those specifications. For example , a 1/4-20 UNC 2A is a free-fitting, coarse twine screw, while a 1/4-20 UNC 3A is considered the medium fit, higher quality thread and is therefore machined in order to slightly closer tolerances (it’s the 2 and 3 immediately before the B in each of these examples that will determines the class associated with fit). These screws’ female counterparts are usually designated 1/4-20 UNC 2B and 1/4-20 UNC 3B, respectively (the B means internal carefully thread; an A signifies external). As with exterior threads, each must be inspected during production using gages of the exact same class. The same holds true for metric threads, albeit making use of different terminology.
Solutions for Loose Threads
Adhering to the guidelines with regard to thread manufacturing, their different classes of fit, and proper tolerancing plus gaging when machining screw threads is the first step toward making strong, dependable fasteners. However , even the highest quality screw, nut, or bolt can come loose unexpectedly (often at the worst possible time) when subjected to vibration and other forces.
That is because threaded fasteners are intended to come apart. It’s an unfortunate truth that all machinery along with other mechanical assemblies must be constructed in a manner that allows them to be serviced, refilled, and repaired. If not, they could simply be welded together, eliminating the need for threads entirely.
One common approach to securing these fasteners is with a pair of washers—one flat, the other split—known as a spring or lock washer. When properly torqued, these washers make the mess much less likely to come loose than one without. The flat washer also prevents damage to the surface of the mating component. Unfortunately, this arrangement is not failure proof. As many manufacturers have found, vibration and dynamic loading tend to loosen even the most tightly torqued screw, lock washing machine, or nut.
Because of this, some recommend applying a liquid adhesive or a “dry patch” to the strings during assembly to prevent loosening, which is why any mechanic is familiar with using Loctite and other commercially available adhesives. They’re also familiar with the fact that these adhesives are both messy and expensive, while using them makes screws challenging to remove, and the threads require thorough cleaning before reassembly— driving up time and costs even further.
Other producers have tried solving the problem by arming screws plus bolts along with tooth-like serrations on the head’s bottom face, thus forcing it to bite into the workpiece. Nevertheless , these specialty fasteners aren’t much more secure than the lock washer approach, and they damage the workpiece.
Still, others specify the use of safety, tie, or lock wires. These are inserted through a drilled hole in the mess head and then secured to another fastener or attached to the particular workpiece. Again, this not only adds cost, but technicians must be trained on the tying procedure; worse, the fastener is still free to come loose—the wire’s primary role is to keep it from falling out, possibly damaging the equipment. In other words, safety wires do not guarantee that torque is maintained.
There are also jam nuts, sometimes called double nut products, which are cumbersome to use and make it difficult to attain the proper torque. Tab washers, as their name implies, have multiple tabs that the installer must bend upwards after set up, taking time. Similarly, castle nuts need insertion of a cotter pin through a hole in the enthusiast and its mating bolt or even threaded stud. And finally, a few manufacturers make nuts containing nylon inserts. These absorb the oscillation that loosens fasteners, but cannot withstand high torque and are thus limited to non-safety critical applications.
Now there is a successful solution to secure nails. It involves modifying the conventional 60° form by cutting a locking “ramp” on the internal thread, securely and predictably locking the fastener in place while making it easy to put together and disassemble multiple times.
History in the Making
In 1979, the particular Detroit Tool Co. made a simple modification to a standard thread user profile. Dubbed Spiralock, it soon became quite popular for use in mission-critical applications simply by military plus aerospace manufacturers, and eventually the medical industry.
With a conventional 60° thread form, the mating components make contact along their respective thread flanks. Unfortunately, this particular creates a small amount of radial clearance between the fastener’s major diameter and that of the threaded opening; when put through vibration, this clearance enables just enough transverse motion for the fastener in order to gradually loosen.
A Spiralock-threaded hole, on the other hand, is made with the 30° “ramp” or wedge form adjacent to the thread’s root and oriented perpendicular to the direction of stress. This wedge surface engages with the crests located at the fastener’s major diameter—as the particular screw is tightened, and it forms a continuous and concentrated point of contact along the thread’s whole length.
The particular wedge ramp design helps prevent any slanted movement, thereby locking the fastener safely in place whilst also producing the line much stronger. Studies show that while a traditional tapped or thread milled hole concentrates as much as 75 percent from the load on the first two threads, 1 with a wedge lock design carries the load evenly across the thread’s entire length.
There are other reasons for the particular design’s success. Aside from eliminating the need for adhesives, dry patches, safety wire, and other costly, often unsatisfactory methods of acquiring fasteners, it’s much easier to repeatedly assemble plus disassemble components. There are no more concerns over adhesive residue, threads are less prone to fatigue and subsequent stripping, and because the locking mechanism will be contained within the female thread, standard UN and ISO screws or even threaded studs can be used. This makes it both easy and cost-effective to implement.
Line Locking Device Options
Unfortunately—and somewhat ironically, considering the safety-critical nature of these fasteners—no ANSI/ASME or DIN standards exist for the sand wedge ramp style. Many in the manufacturing industry adopted what was then a proprietary thread type. When the patent ran out in 1994, Detroit Tool sold the Spiralock name to Stanley Engineered Products, a manufacturer plus distributor associated with threaded fasteners, rivets, cleaners, and other hardware items.
Since that time, the sand iron ramp line form has continued to gain popularity, and for good reasons. Unfortunately, manufacturers choosing to adopt it have long been locked into a single supplier, one that does not manufacture cuttings tools but instead sells them to its customers on a private label basis.
Emuge-Franken, the manufacturer of high-performance taps, thread mills, end generators, drills, as well as other rotary equipment, recognized this particular and obtained the rights to produce taps, line mills, and inspection gages for the pitching wedge ramp form. Because it wasn’t allowed to use the Spiralock name, Emuge filed its own trademark —Self-Lock— plus began producing thread-locking trimming tools and accompanying gages at the plant within Germany.
Although Spiralock plus Self-Lock are usually identical in thread type and function, they are not interchangeable. This is due to some slight dimensional differences, but it means that manufacturers need to select the correct gages whenever producing and measuring these threads plus must consequently decide on a single brand or the other.
Success without standards
Because there are no standards regarding locking wedge ramp-style twine forms, OEMs who specify these forms in their product designs should first confirm that in-house machine shops and suppliers have a comprehensive, well-supported platform intended for producing these types of threads.
Top on the list for this platform are high-quality, readily available, plus cost-effective shoes and carefully thread mills. The supplier should also offer a broad assortment of these cutting tools, with the geometries and coatings needed to tackle different materials productively. In addition , manufacturers need precision gages with which in order to measure threaded holes throughout production, and even though neither ANSI/ASME nor NOISE standards can be found for sand wedge ramp nails, these gages should be traceable and certified to known, unambiguous sources.
Last but not least, taps, thread mills, and indeed almost all cutting equipment perform best when gripped in high-precision, extremely rigid toolholders. If these toolholders come from the same supplier, engineering life becomes simpler and production more efficient. The same can be said for a supplier that will stands behind its products and can offer the technical support needed for productive machining, a statement that’s especially true for internal threading, one of the more demanding machining programs.