Product Focus: Life in the FASTENERS Lane

In the world of engineered components, fasteners aren’t the first things engineers think about and 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. Following are four details engineers must know about locking fasteners to keep their products and careers securely in the fast lane.

First, focus on retaining fastener tension, not torque assembly methods. Bolt tension, which causes the bolt to stretch, is what keeps a joint together. Yet 90 percent of the torque applied to a bolted joint goes into overcoming friction and 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. 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 so torque recommendations are only guidelines. To determine the ideal torque for an application through testing, measure bolt stretch manually or ultrasonically by using a load cell to measure bolt tension at a certain torque/angle.

Second, treat your fasteners with the same care you would your most critical component. The basics of strength, size, material and service requirements must be reliably and efficiently handled; and often call for locking fasteners.

Third, choose the right type of locking fastener for the job. For instance, 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 weaker materials. Elaborately machined fasteners such as spanner 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. For example, a carbide insert, single point cutter or a thread milling cutter will give the best thread finish and tool life. Thread milling or thread turning provides 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, the sheared chips flow more freely, the coolant circulates better, there’s less surface contact and therefore less heat and friction buildup. Better chip management can translate 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, 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: no metal chips are generated and no cutting edges wear down. Cold form tapping is popular in softer, more malleable materials. 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 because 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.

Fourth, consider lifetime cost including design, assembly, warranty and liability. While many engineers gravitate toward lock washers or prevailing torque fasteners, these may be inappropriate or have considerably higher total costs over the product lifecycle. Split washers and lock washers, 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. Prevailing torque fasteners can damage threads and are prone to galling and require more effort to ratchet down using special tools.

Locking adhesives 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. Bolts secured with single-use, drypatch adhesive — activated when the bolts are tightened — can similarly add to assembly, maintenance or warranty costs 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 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 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 up to 50 times with the 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. Production changeovers to this fastener often require just an exchange of traditional nuts, wire inserts or simply drilling out and re-tapping existing parts stock that have unreliable standard tapped holes.

These tips can help engineers keep their products and careers securely in the fast lane for the long haul.