Crossed threads. Cracked bosses. Torque-out. Wasted time. Wrong size. If
it weren't for engineering innovation, such might be the epitaphs for threaded
fasteners. Fortunately, specialty designs exist to compensate for almost any
assembly trouble, from delicate electronics to robust satellites.
Like any engineering component, threaded fasteners are limited by what they are designed to do. Yet, perhaps more than any other design element, the humble fastener is expected to perform many tasks, including correct for inconsistent assembly conditions, operator error, and finicky substrates.
What follows are three trailblazing fasteners. They are designed to form strong, low-stress threads in thermoplastics, install quickly over threaded bolts, or repair crossed threads. Each solves a specific problem, yet has potential for application in a wide range of industries.
Screw forms secure threads in thermoplastics
While the trend toward thin-wall design works wonders for reducing overall product weight, mold cycle time, and cost, it places additional demands on fastener performance. The spectrum of material properties and operating criteria involved in a typical application mean engineers must devote careful attention to joint design and testing.
The causes of thermoplastic joint failure can be hard to correct and even harder to predict. Sometimes a failure is downright puzzling. Case in point: A spring-loaded door on a dash-mounted automotive glove-box assembly. The polycarbonite assembly performed correctly in tests and was no problem during manufacture. However, just a few weeks after the cars were purchased, the assembly's bosses were cracking, causing the doors to fly into the rear seat. To make matters worse, the glove-box light would then drain the car's battery.
While it may sound like a bizarre aberration, this application highlights the challenges typical of crystalline and amorphous thermoplastic joints, says Rick Smith, application engineer at fastener manufacturer ATF, Inc., Lincoln-wood, IL. "With the increasing number of fasteners being used to threadform into thermoplastic components, issues such as boss cracking, assembly problems, and loss of clamp load have created the need for new technology," he adds.
The glove-box culprit turned out to be stress-cracking agents and stresses exceeding 2,500 psi. Yet, the solution wasn't a material substitution. Instead, engineers teamed with ATF to replace the fasteners. "There were approximately eleven bosses and fasteners on the glove-box attachment," recalls ATF engineer Bryon Fountain. ATF tests revealed that radial stress on the bosses--which was causing the bursting--didn't show up during installation. "It was only after a period of time that the stresses built up," adds Fountain.
To correct the problem, ATF selected the PT® fastener: A patented steel screw designed specifically for thermoplastics. Three areas of threadform design--the helix angle, the flank angle, and the root of the fastener--are critical to performance and long-term behavior of fasteners in thermoplastics, explains Smith. The PT screw customizes these design elements to the demands of thermoplastics, he says.
For example, the helix angle, or thread pitch of the PT screw is 8 degrees--smaller than most threaded fasteners. This helps the screw resist vibration loosening, and balances the load ratio between the plastic and the screw. Likewise, the PT's flank angle or thread profile is 30 degrees, rather than the 60 degrees typical of conventional plastic screws. This thread profile displaces the same volume of material as a 60-degree flank, but the increased bearing depth of the thread reduces radial expansion and hoop stress on the boss. The smaller lever arms of a 30-degree thread profile also allow the fastener to be installed using a lower drive torque--a useful feature in avoiding over-torque.
Perhaps most unusual is the PT screw's root, which has a hollow or "cored" tip. "Any time a fastener is secured into a thermoplastic joint at assembly-line speeds, the friction generated between the flank of the screw and the plastic causes the plastic material to melt," explains Smith. "This melted material flows down the flank into the root, causing stress concentrations to develop." The PT screw's cored root gives the melted material a place to flow during thread forming, thus reducing radial stress.
ATF engineers are quick to caution that no single screw configuration suits all thermoplastics. "Assembly techniques and boss design also have a profound effect on the fastener's performance," says Smith. Important design factors to consider include hole size and penetration depth, heat generated at different drive speeds, and the effect of changes in part geometry and operating temperature.
Application engineers at ATF have designed PT screws into computer speakers, PC boards, fan housings, and an array of other plastic components. A range of thread pitches, steels, and head types--including Torx® heads from Camcar, Rockford, IL--accommodate design options.
Speedy threads push on and twist off
You can't properly install a nut and bolt without twisting the nut several times, right? Wrong. A new spring-loaded fastener designed by Thread Technology, Inc., Chantilly, VA, installs with a push and one full rotation. The ZipNut® prevents crossed threads and disconnects as easily as a conventional nut.
The nut is the unique part of the ZipNut design, and it works with any standard bolt, as long as the thread types are compatible. The ZipNut looks like a conventional nut on the outside. But inside, it's comprised of several pieces. The housing encloses three separate segments that form the internal threads of the nut when assembled. Two springs hold the segments in place and force them to a minimum diameter. An internal ramp holds the segments together or apart, depending on the insertion direction of the bolt.
The ZipNut was originally designed for lug nuts on race cars. Engineer/inventor Robert Fullerton later licensed the design to Thread Technology. The ZipNut soon caught the attention of NASA engineers. In space, the lack of gravity makes using a wrench to generate torque difficult. So when astronaut Jeffrey Hoffman installed a handrail on the Wide Field and Planetary Camera for repairs to the Hubble Telescope in '94, he used eight ZipNuts. When astronauts service the Hubble again this year, they will use 25 more.
For the design of the Space Station, McDonnell-Douglas engineers specified ZipNuts for robotic assembly of a truss and antenna. Honeywell Satellite Systems chose ZipNuts for the gimbals of every solar panel of the Station. And Boeing engineers are evaluating them for connecting payloads to their "Express Rack" system and for attaching components to the Space Station's permanent air lock.
The ZipNut fastener demonstrates potential benefits for down-to-earth applications, too. For example, fast installation assists firefighters in Maryland and Virginia to make connections--especially with corroded fire-hydrant bolts. Engineers at Superior Products, Cleveland, are also exploring applications for pressurized gas connections. For maintenance of nuclear facilities, the ZipNut would allow less "stay time" in high-radiation areas, says Joseph Regalo, manufacturing manager at Flexalloy, Inc., Cleveland.
Flexalloy is further developing the ZipNut, and plans to market it to the nuclear and construction industries and the Big Three automakers. Although a commercial piece-price has yet to be determined, "there are a ton of application possibilities for the ZipNut," says Regalo. Look for it on the market at the end of the year.
Self-tapping bolt cuts its own threads
It's an age-old problem: you're threading a fastener and--ouch!--you cross the threads. The fastener is ruined, but worse: the hole is also destroyed. Instead of reaching for a drill to correct the problem, you can reach for an Ultra-Lok self-tapping bolt from the Ultra-Lok Fasteners, Busy, KY.
Heat-treated Ultra-Lok bolts create their own threads as they twist into a pre-drilled hole. "By using this product on stripped or damaged threads, the tools, time, labor and effort--all expensive items in repair and assembly processes--are reduced," says Vincent Altamuro, President of Robotics Research Consultants, Toms River, NJ. "The bolts eliminate the need for tapping tools, or 'taps,' for each size hole involved. They also eliminate the separate step of using taps to cut the threads for each bolt to be assembled into each hole."
A milled slot in the bolt shaft serves as a reservoir for the shavings created as it cuts threads to match its profile. The patented reservoirs let the bolts make a tight fit in their holes, says inventor Blake McKinney, Chief Engineer at Ultra-Lok.
After working for 12 years at Caterpillar, Inc. and running his own business, McKinney was convinced there was a better solution to a recurring fastener problem. "The fierce vibrations in big trucks cause bolts to loosen or strip out of the aluminum bell housing," he explains. The repair called for removing the transmission and replacing the bell housing, or installing inserts--both expensive and time-consuming jobs. "What intrigued me was the fact that there was nothing on the market that would work."
"One morning, I had standard bolts two sizes too large to fit in the damaged hole machined down, with a slot and a hole cut in them. The machinist made about a dozen and I put them in a truck," he recalls. The truck was running day and night; McKinney monitored it for 40,000 miles. "I had no problems with it," he says. Later, when they did a clutch replacement, mechanics removed the bolts and put the same bolts back in, and they worked fine, he adds."So those first bolts that originally cost about $40 saved about $1,800."
McKinney eventually made several design changes, and recently began volume production of the grade-eight fastener in English sizes. Applications include automotive and truck transmission pans, heavy equipment, construction, lawn mowers, and marine engines.