In any type of vehicle – whether a car, motorcycle, construction or farming equipment, or even an airplane — there can be severe consequences if critical fasteners loosen or fail during operation. If this occurs, it not only affects the function of the equipment and likely causes downtime and costs for repair but for the operator and any passengers it can be potentially life-threatening.
Yet this is exactly what happens to vehicles when subjected to vibration, shock, dynamic loading, or thermal stress. The most frequent cause of self-loosening is the side sliding of a nut or bolt head relative to the joint — resulting in related motion occurring in the threads. Typically, the gradual rotation causes a bolted joint to lose its preload (the initial fastener tension when tightened), leading to fatigue failure.
Although many OEMs view fasteners as commodity items, in vehicles the conditions demand superior solutions to prevent fasteners from loosening. During vehicle assembly, approximately two-thirds of the parts and half the labor are related to fastening in one way or another. So, applying the wrong type of fastener can incur a negative impact on the assembly costs, warranties, sales, liability, and the brand.
Fortunately, a new approach to fastener design is promising to resolve issues of loosening due to vibration using a smaller, lighter, and more compact fastener, without the use of adhesives.
OEMs have long used a variety of fastener designs that attempt to stop bolted joint loosening through the use of adhesives or added components that physically hold and restrain the bolt or nut. However, these methods have significant drawbacks.
Locking adhesives attempt to hold fasteners in place once tightened, yet most adhesives will progressively lose effectiveness as the temperature rises. Bolts secured with a single-use, dry-patch adhesive that’s activated when the bolts are tightened tend to add to the assembly costs. With both of these options, if the item is to be removed and re-used, the threads must first be cleaned, which means time, labor, and a price tag.
If relying on mechanical locking approaches, the goal is also to physically prevent loosening. However, this often means adding components that increase the size of the fastener, adding weight and complexity to the component design. For land or air vehicles, “smaller and lighter” affects fuel efficiency, so heavier fasteners are a drawback.
A new, innovative approach that physically prevents vehicular bolt loosening is available and without the conventional limitations of excess weight, complexity, and length. This fastener design, called ForeverLok, employs three items:
1. A central threaded fastener
2. A threaded intermediate fastener
3. A retaining fastener.
Essentially, this fastener system is simple — it holds the nut in place to physically prevent it from loosening. Although there are competitive products on the market that work in a similar way, this design is more compact than the traditional nut and bolt configuration. The locking design does not use special pins, bolts, or tools to install or remove the nut, and only common tools are needed to fasten and unfasten it.
The design allows the fastener to be smaller, lighter, and more compact than a larger fastener while providing a comparable torque value. For example, the torque value of a 1/2-inch fastener design tested greater than the recommended torque value of a 5/8-inch bolt. Additionally, the fastener design is reusable as many times as needed and it can be made of several materials, including titanium, steel, and other metals and alloys. This design also works just as well for plastic fasteners when weight or cost is a prime consideration.
The technology, which is available for licensing, is flexible enough for a manufacturer to create a unique new product based upon it.
The effectiveness of the ForeverLok approach has already been put to the test in three of the most rigorous anti-vibration tests.
In a maximum torque test, involving a 1/2-inch grade eight bolt, the design tested at 159.9 ft/lbs., a 77% increase over the 90 ft/lbs. of torque recommended for a standard ½-inch bolt. The higher torque value allows OEMs and design engineers to use a lighter, smaller fastener to save space and weight.
The ForeverLok design was also tested against the NASM 1312-7 standard, which involves accelerated vibration testing on a fastener system capable of providing a clamp-up load. In the test, the bolt/nut combination is installed in the fixture, and the fixture is subjected to controlled vibration and cycles/times until the assembly loosens. The fastener must not capable of loosening by hand after 30,000 cycles, or about 17 minutes of testing.
The testing of the ForeverLok design was suspended after over 420,000 cycles and four hours of testing with no loss of torque retention. NASM 1312-7 does not require the residual torque value to be reported. The testing facility provided the measurement: the fastener design retained 93.5% of its original torque value.
Against the tougher transverse vibration standard, DIN 25201-4, the design torque retention test results were 89.43%, clearly surpassing the certification standard which requires 80% retention or higher. This standard involves testing the fastener 12 times. The less strenuous DIN 65151 tests the fastener to a less exacting setup and verification standard, testing it once.
Although traditional locking fastener systems are available, OEMs and engineers searching for solutions to critical fastener loosening will find that considering a new design approach can result in lighter, simpler, more reliable fasteners in any type of vehicle.
Multi Piece Fastener L.L.C