Four Things Aerospace Engineers Must Know About Locking Fasteners To Secure Their Products And Careers

By Del Williams

In the world of aerospace components, let's face it, fasteners aren't the first thing engineers think about. They're not exciting, they're not sexy. They're commonly treated as routine. But choose the wrong fastener for the application and the best laid engineering plans and products can fall apart.

Aerospace engineering poses unique challenges whether for avionics, electronics, sensors, or other mission critical applications. Components may be required to withstand extreme shock, G-force, vibration, or thermal stress, often repeatedly with little or no opportunity for re-tightening. Such conditions can make adhesives or locking devices such as prevailing torque fasteners-- which can harm threads or produce foreign-object-damaging burrs or shavings-- unacceptable.

Moreover, galling or thread seizing can throw off the torque-tension relationship in fasteners, especially in metals such as titanium or stainless steels. Yet fasteners for every component must be as simple, small, light, and reliable as possible while retaining the possibility of re-use.

In today's ultra-competitive, litigative, cost-conscious aerospace environment, engineers need every advantage to set themselves apart from the competition and prosper. What follows are four things engineers must know about locking fasteners secure both their products and their careers.

1) Focus on retaining fastener tension, not torque assembly methods. Bolt tension, which causes the bolt to stretch, is what actually keeps a joint together. Yet 90% of the torque applied to a bolted joint goes not into fastener tension, but into overcoming friction. This is one reason that breakaway torque is not a reliable measure for determining joint integrity or tension.

Though friction is necessary to hold a bolted joint together, excess friction can damage threads or cause galling, known as thread seizing, especially among fasteners made of titanium, stainless steel and aluminum. Achieving the proper joint tension is challenging.

Testing with standard threads has shown that for a given torque, bolt tension can vary as much as 50%. This is why torque recommendations are guidelines, not "cast in stone" values. Many factors affect bolt tension when torque is applied including hardness, surface finishes, material types, plating, lubricants, tightening speed, thread fit, and surface pressures.

Even when sophisticated manufacturers use torque wrenches and other tools in assembly, the only way to determine the ideal torque for an application is through testing. This can be effectively done by measuring bolt stretch manually or ultrasonically, by using a load cell to measure bolt tension at a certain torque/angle, or by other means.

2) Treat your fasteners with the same care you would your most critical component. No matter how clever the engineering or robust the components, the end product is only as reliable as its weakest link -- which, too often, are the fasteners holding everything together. While fasteners are often viewed as commodities, they're more than this --especially in any critical application where failure could be costly or disastrous. In these cases, the basics of strength, size, material and service requirements must be reliably and efficiently handled; and locking fasteners are often called for.

But how do you choose the right locking fastener type for the job when tradeoffs are involved? Engineers must understand the relative advantages and disadvantages of different fastener types.

3) Choose the right type of locking fastener for the job. Among the various types of fasteners and tooling, some offer specific advantages for certain applications. For instance among fasteners, wire inserts can add strength or aid in repairing stripped out threads in soft materials. Clinch nuts are good for adding grip length and thread engagement when used with thin materials. Threaded inserts help create a stronger, metallic interface for fasteners in thinner, weaker materials. And hex flange nuts can add strength to the fastener or spread the load over an increased surface by adding bearing face surface. Some elaborately machined fasteners such as spanner nuts, collar nuts, or captive washer nuts can also be useful in specialized engineered applications.

Optimizing thread finish Tooling such as taps, gauges, thread milling cutters, or threading inserts also have their place depending on the capabilities of a manufacturer's processing equipment. A carbide insert, single point cutter, or a thread milling cutter will give the best thread finish and tool life, for example. This can be important in aerospace applications, where a highly polished surface finish can help prevent galling.

Thread milling or thread turning provide better thread quality than tapping because tapping shears away more material under less favorable conditions. In milling or turning, less material is sheared away in a more open environment. Because the sheared chips flow more freely and the coolant circulates better, there's less surface contact at any one time and therefore less heat and friction buildup. Better chip management translates into better surface finish, less galling, easier assembly and longer tool life.

Thread milling cutters are particularly good for jobs where larger thread diameters are required. They'll save handling, set-up and machining time by threading right in a company's machining center with improved thread quality.

Where tapping is necessary, cold forming tapping is preferable since the process generates threads by displacing material rather than by cutting. Therefore no metal chips are generated and no cutting edges wear down. Cold form tapping is popular in softer, more malleable materials like some steels, stainless steels, and aluminum. Due to displacement of material to create the threads in cold form tapping, however, a dimensional allowance must be made on the drilled hole prior to tapping.

When tapping, it's also a good idea to avoid deep blind holes. The reason for this is that once a tap gets into the bottom of the hole, there's a lot of heat buildup and little available coolant. To minimize this concern, it's best to use the minimum thread necessary for the design process.

A number of online tools can help engineers find the best locking fastener or tooling for the application. Among these are an online Tap Selection Tool; Torque Calculator; Drill and Hole Size Calculator; and Tap Troubleshooting Guide on the Technology page at www.spiralock.com. The Tap Troubleshooting Guide, for instance, can help walk engineers through an issue such as when a go-gage does not go, which can be a sign that a tap is wearing down or getting poor tool life. It can also help engineers spot when too much tension is being generated in the tapping process.

4) Consider lifetime cost including design, assembly, warranty and liability. While many engineers gravitate toward lock washers, prevailing torque fasteners, adhesives or other common choices, these can be inappropriate in an aerospace setting, in addition to having considerably higher total costs over the product lifecycle.

Split washers, lock washers, and lock wires, for instance, add extra weight and complexity to component design. This increases the chance that something may go wrong during assembly or maintenance and complicates inventory control. Other mechanical locking features, such as brackets, can also prove costly and tedious to use on components with multiple bolts. If not properly fastened during assembly, they can pose a quality assurance risk.

Since prevailing torque fasteners can damage threads, they often prevent reusability while raising labor, maintenance, and quality inspection costs. Due to high resistance during assembly, they are prone to galling and require more effort to ratchet down using special tools.

Locking adhesives, for their part, progressively lose effectiveness as temperature rises. In high volume, their use typically requires a large capital expense to purchase and program robot applicators. And when re-application is necessary, cleaning the threads of affected components takes added time and labor before re-application is possible.

Bolts secured with single-use, drypatch adhesive - activated when the bolts are tightened - can similarly add to assembly, maintenance, or warranty costs. This is because, once used, the bolts must be replaced for any necessary rebuilds or maintenance. Affected internal threads must also be cleaned before new bolts with drypatch adhesive can be applied, adding to time and labor costs.

Most importantly, however, these and other locking fasteners do not address a basic design problem with the standard 60-degree thread form: that the gap between the crest of the male and female threads can lead to vibration-induced thread loosening. Stress concentration and fatigue at the first few engaged threads is also a problem, along with an increased probability of shear, especially in soft metals, due to its tendency toward axial loading. Temperature extremes can also expand or contract surfaces and materials, potentially compromising joint integrity.

Engineers, however, have successfully attacked these challenges while reducing component weight and enabling re-usability with the innovative Spiralock locking fastener. This re-engineered thread form adds a unique 30-degree wedge ramp at the root of the thread which mates with standard 60-degree male thread fasteners.

The 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.

The Spiralock 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 thousands of applications to solve design challenges in a wide range of industries including aerospace/military, automotive, heavy truck, medical, food processing, agriculture, construction, rail, and oil drilling. Production changeovers to this fastener are typically quick and seamless, often requiring just an exchange of traditional nuts, wire inserts or simply drilling out and re-tapping existing parts stock that have unreliable standard tapped holes.

For aerospace engineers beset by design, budget, and competitive challenges, keeping these four tips about locking fasteners in mind can help them secure both their products and their careers.