Engineers and managers who design and develop products for the medical marketplace or buy components and equipment understand that in any type of medical device — whether implant, ventilator, imaging equipment, or mobile tools pushed by cart — there can be severe consequences if critical fasteners loosen or fail during operation. If this occurs, such failure can be not only potentially life threatening but also costly in terms of liability risk, warranty claims, and reputation.
Even the latest medical and biomedical innovations, such as imaging equipment, sensors, electronics, materials, and mechanical components, as well as test and measurement hardware or systems, must unfailingly remain held together by fasteners and functional in any and all circumstances when incorporated into medical devices.
Once artificial hips and knees are surgically implanted inside the body, for example, the devices must not fail and are not replaceable without another invasive procedure. Ventilators, whether used during surgery or long term, must also support breathing and respiration without fail for patients to sustain their lives when they are unable to do on their own.
Yet the critical fasteners in such medical devices can fail when subjected to vibration, shock, dynamic loading, or thermal stress. Vibration of medical equipment during use is one of the main causes of fastener self-loosening and can be a particular problem for high-vibration devices like computed tomography (CT) scanners and MRIs. In fact, while imaging technology produces images differently, most utilize rapidly spinning components, which shake and vibrate.
The vibration causes 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 leads to progressively greater fastener loosening.
Any shock or jolt to a medical device can also prompt fasteners to loosen. When mobile equipment like ventilators or diagnostic equipment is moved around healthcare facilities, the units can be bumped, jostled, or dinged, which can lead to fastener loosening and machine failure.
Medical implants such as artificial hips and knees are also be subjected to dynamic loading, forces that change over time due to standing, walking, and other movement. Thermal stress caused by cyclical changes in temperature can also contribute to loosening of fasteners.
Although many OEMs view fasteners as commodity items, the medical industry — due to its critical nature — requires superior solutions. 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.
In addition, this fastening approach is helping medical design engineers and product managers to streamline design and reduce manufacturing costs. This combination of enhancing medical device reliability and quality with cost reduction is all the more important as the industry faces greater competitive and regulatory pressures in the marketplace.
Solutions for Preventing Loosening
Medical OEMs have utilized a variety of fastener designs that attempt to prevent bolted joints from 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. However, locking adhesives, of course, are inappropriate for any internally used medical device because the compounds are biologically incompatible. Such adhesives may even be ill advised for use with any device that could contact the patient externally due to the potential for patient sensitivity to the chemical compounds utilized.
Besides this, the adhesives progressively lose effectiveness as temperature rises. This can lead to fastener loosening in any high temperature environment or when medical devices are used for long periods of time. 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 medical devices, “smaller and lighter” is critical for implants and can affect portability, so heavier fasteners are a drawback.
Now, however, an original, innovative approach physically prevents 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 a traditional nut and bolt configuration. The locking design does not use special pins, bolts, or tools to install and 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-in. fastener design tested greater than the recommended torque value of a 5/8-in. bolt.
In addition, the fastener design is reusable as many times as needed. The fasteners can be made of many materials, including titanium, steel, and other metals or 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 medical manufacturer to create its own unique new product based upon it.
The effectiveness of the fastener has already been subjected to three of the most rigorous anti-vibration tests. In a maximum torque test, involving a ½-in. grade eight bolt, the design tested at 159.9 ft/lbs., a 77 percent increase over the 90 ft/lbs. of torque recommended for a standard ½-in. 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 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. However, the testing facility provided the measurement: the fastener design retained 93.5 percent of its original torque value.
Against the tougher transverse vibration standard, DIN 25201-4, the design torque retention test results were 89.43 percent, clearly surpassing the certification standard, which requires 80 percent 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 most types of medical devices.
This article was written by Del Williams, a technical writer based in Torrance, CA. For more information, visit here .