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Four Things Engineers Must Know About Locking Fasteners

Tue, 06/27/2006 - 11:32am

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By Del Williams

In the world of engineered components, 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.


In today's ultra-competitive and cost-conscious manufacturing 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 to keep their products and careers securely in the fast lane.

1.     Focus on retaining fastener tension, not torque assembly methodsBolt tension, which causes the bolt to stretch, is what actually keeps a joint together. Yet, 90 percent 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 freezing, especially among fasteners made of stainless steel, aluminum, titanium, and some alloys — 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 percent. This is why torque recommendations are guidelines, not "cast in stone" values. Many factors affect bolt tension when torque is applied including the hardness of all parts, 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 suitable 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 componentNo 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 needed.

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

3.     Choose the right type of locking fastener for the jobAmong 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 such as aluminum, plastic, and wood. 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.

Tooling, such as tapping, 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.

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 chipsflow 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 and stainless steels as well as in aluminum, brass and copper. 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 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 threading agent 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.

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 may be inappropriate or have 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. They also increase the chance that something may go wrong during assembly or maintenance, as well as complicate 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, maintenance or rebuilds, they can pose a quality assurance risk.

Since prevailing torque fasteners can damage threads, these often prevent reusability while raising labor and maintenance 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 important, 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 risk 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.

For engineers beset by design, budget, and competitive challenges, keeping these four tips about locking fasteners in mind can help them keep their products and careers securely in the fast lane for the long haul.

More information is available at www.spiralock.com or slinfo@spiralock.com, or by calling 800-521-2688.

Del Williams is a technical writer who writes for Spiralock.

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