Times have changed, and the way OEMs deal with the nuts, bolts, taps, fasteners and ultimately the reliability of their products is changing, too. Today, buyers can't access most nuts and bolts because they're typically sealed within complex electronic systems. Products are so complicated consumers simply expect them to work and won't accept those that don't. With today's hectic schedules, there's no patience for forced maintenance or downtime.
It used to be that the last thing considered in designing a product was how to hold it together. Most of the time they deferred to what had been done before and today that's not enough. It's well documented that warranty claims are an issue in many industries and this is often due to fastener problems leading to components coming loose from products.
The nuts and bolts of product design can also determine the cost-effectiveness of a component during its product life cycle. Consumers can no longer be expected to get the toolbox and regularly tighten loose screws and bolts as standard maintenance on products ranging from lawnmowers and bikes to appliances and boat engines.
The reliability challenge is tougher on OEMs today as advanced products, such as a 70,000-rpm dental drill, require ever lighter, stronger, higher speed, higher rpm and higher temperature designs to meet market competition and consumer tastes. Much of this reliability challenge stems from screw thread design, which was standardized during World War II to promote compatibility and has remained virtually unchanged since.
Under ideal conditions, standard male and female threads would fit together perfectly. In the real world, however, under the constraints of mass production and JIT delivery, this seldom happens.
“A fastener may seem like a small thing but can make the difference between aircraft power being online or off,” says Darin Morman, Hamilton Sundstrand's manager of generator engineering in Rockford, IL. “The issue was that prevailing torque nuts can gall or freeze on threaded bolts during installation, leading to intermittent power availability if the nuts' rundown torque exceeded the final torque limit before clamping.”
In an effort to avoid any intermittent power disruption or maintenance under severe shock, vibration and thermal variation prevalent in aerospace, Hamilton Sundstrand tested and adopted an alternative thread form designed to address not only fastener loosening and stripping under high load and vibration but also galling as well as slippage due to thermal expansion and contraction.
While secondary locking devices have been developed in an attempt to address this shortfall, at best they simply prevent the threads from catastrophically coming apart. Because none actually holds the clamp load in the joint, they are susceptible to vibration, fatigue or temperature-related joint failure.
Though engineers traditionally look to lock washers, prevailing torque fasteners, adhesives or other secondary locking devices, these may be inappropriate or have considerably higher total costs over the product life cycle. For example, split washers, lock washers and lock wires add extra weight and complexity to component design. This increases the chance 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.
Crimping threads after screwing require extra steps and can prevent reusability. Similarly, prevailing torque fasteners can damage threads and 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 specialized tools.
Most locking adhesives, in turn, 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.
“To survive the vibration and high temperatures of launch, we required the most reliable locking engagement thread,” says Dan Harpold, a NASA scientist who worked on the Cassini-Huygens mission across 750 million miles of space. “Screws had to remain tight without opportunity for retightening. With conventional threading, however, screws loosened up and baked out under testing.”
Among the tests carried out were a series of about 12 high temperature “bake outs,” where screws and their matching internal thread forms were heated from room temperature to 300C to simulate temperature-induced thread loosening.
“The Spiralock thread form retained a tight seal at 300C,” says Harpold. “Once torqued down properly, the screws stayed put in the threads, which helped us meet our flight schedule. Not one has come loose that I'm aware of.”
Securing bolts with single-use drypatch adhesive, activated when the bolts are tightened, can also 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.
At the heart of the matter is a basic design problem with the standard 60-degree thread form: 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.
To address these reliability concerns while reducing component weight and enabling re-usability, engineers have turned to the first thread innovation since World War II, 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.
The innovative locking fastener thus compensates for variations in manufacturing tolerance and process due to mass production with its locking thread actually inside the joint. This eliminates the need for secondary locking devices or procedures and can significantly reduce costly warranty claims and potential liability. The locking fastener has been validated in published test studies at leading institutions including MIT, the Goddard Space Flight Center, Lawrence Livermore National Lab. and British Aerospace. It has been used in the manufacture of thousands of applications to solve design challenges in a wide range of industries. 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.
Several 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. 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.
Though the company is celebrating its 80th anniversary this year, it's still innovating with manufacturers in mind. Of interest to engineers seeking the added strength and reduced weight of softer metals like magnesium or aluminum is a new type of wire thread insert the company is developing. Besides providing greater component reliability in high vibration, rpm and temperature environments, the wire thread insert which has no tang yet supplies thread locking in the joint itself, will offer threaded joints additional strength, secure locking and multiple reusability. This will be especially helpful for diesel and gas engine manufacturers using softer metals prone to fatigue or stripping.
Proactive manufacturers understand they're in the service business, meaning their products must operate nearly flawlessly. For just as consumers and contractors can't afford to have their tools and equipment in repair shops when there's work to do, manufacturers can't afford to issue recalls, face potential liability or suffer market share erosion. Nor can manufacturers of large equipment, such as that used in construction, agriculture, oil drilling, food processing or rail applications, afford to fly field reps all over the country for repairs.
From top-end design down to the nuts and bolts holding the product together, consumers now expect and demand trouble-free reliability.
For detailed test data, including comparative graphic loading characteristics or photoelastic analysis/load vector comparison animation, e-mail slinfo@spiralock.com.
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