In any type of vehicle – whether car, motorcycle, airplane, military tank, farming machine or construction equipment, 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 causes substantial 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 the nut or bolt head relative to the joint, resulting in related motion occurring in the threads. The gradual rotation causes a bolted joint to lose its preload (the initial fastener tension when tightened) and subsequently lead to fatigue failure.
Although many OEMs view fasteners as commodity items, in vehicles the conditions demand superior solutions to prevent fasteners from loosening. In vehicle assembly alone, 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 have a negative impact on assembly costs, warranties, sales, liability and even the overall brand image.
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.
Solutions for Preventing Loosening
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 restrain the bolt or nut from loosening. However, these methods have significant drawbacks.
Locking adhesives attempt to hold fasteners in place once tightened, yet the adhesives progressively lose effectiveness as temperature rises. Bolts secured with a single use, dry patch adhesive that is activated when the bolts are tightened also add to assembly costs. With both options, if the item is to be removed and re-used the threads must be cleaned first at great cost in time and labor.
With mechanical locking approaches, the goal is to physically prevent loosening. However, this often means adding components that increase the size of the fastener and add weight and complexity to component design. For land or air vehicles, “smaller and lighter” affects fuel efficiency, so heavier fasteners are a drawback.
Now, however, an original, innovative approach physically prevents vehicular bolt loosening without the traditional limitations of excess weight, complexity and length.
The fastener design, called ForeverLok™, involves three items: a central threaded fastener, a threaded intermediate fastener and a retaining fastener.
Essentially, the fastener system holds the nut in place to physically prevent it from loosening. Although there are competitive products on the market that work in a similar fashion, 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/remove the nut, and only common tools are needed to fasten/unfasten.
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 ½-inch fastener design tested greater than the recommended torque value of a 5/8-inch bolt.
In addition, the fastener design is reusable as many times as needed. The fasteners can be made of many materials such as titanium, steel and other metals/alloys. The 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 their own 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 ½-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 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. For this test, the fastener must not be able to be loosened by hand after 30,000 cycles, approximately 17 minutes of testing. Testing of the ForeverLok design, however, was suspended after over 420,000 cycles and 4 hours of testing with no loss of torque retention. NASM 1312-7 does not require the residual torque value to be reported. However, 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.