Self-Locking Design Safeguardsthe Reliability of Medical Lasers

The Project: Prevent warranty issues by eliminating the possibility of laser misalignment during shipping or handling.

The Solution: Use a self-locking fastener designed to resist loosening even under heavy loads and severe vibrations.

By Lyndon Lattie

Many medical lasers from Lumenis, a global developer and manufacturer of surgical devices, use self-locking fasteners to ensure reliability and product safety.

Failure is not an option when it comes to medical equipment. From critical or sensitive devices such as lasers, MRI scanners, knee joints, and implantable pacemakers to instruments as straightforward as stethoscopes, medical equipment must be reliable when the quality of life is at stake.

In the medical field, there is little room for the $15 billion per year spent by 25 of the largest U.S. manufacturers on warranty claims, and there is little room for processing those claims that eat up 2.5 percent to 4.5 percent of revenues for companies across all industries. The medical industry must have higher aims than other fields, and customer satisfaction and warranty problems associated with fastener failure must be prevented or quickly and economically handled.

Due to the unique nature of medical equipment applications, fasteners such as C-clips and specialized retaining rings can prove too cumbersome and costly. Because repetitive loads, shock, and vibrational loosening must be decisively managed, traditional fasteners susceptible to self-loosening rotational movement, stripping, and shearing are not always appropriate. Testing, in fact, has found that the first two threads of traditional fasteners can carry as much as 80 percent of the load, permitting stripping or shearing. As a result, the subsequent male threads “float” within the female threads.

Medical equipment manufacturers are successfully attacking these problems with a variety of new technologies. Sometimes the most innovative solutions are also the simplest. Consider the case of Lumenis Ltd., a global developer and manufacturer of laser and light-based devices for medical, aesthetic, ophthalmic, dental, and veterinary applications with U.S. headquarters in Santa Clara, CA. Lumenis wanted to rule out the possibility of laser misalignment during rough shipping or handling. To safeguard its precision critical laser alignments, the company rejected standard thread forms and selected a self-locking fastener designed to resist loosening even under loads and vibrations strong enough to break fasteners.

“We wanted to increase the design’s robustness to better withstand the shock and vibration of shipping as well as carting from room to room in the field,” explained Peter Hines, an R&D engineer responsible for laser design at Lumenis. He said the self-locking design alleviated concerns about misalignment. “With its shock and vibration resistance, we’re preventing screws from backing out and preserving critical alignment for enhanced reliability.”

Hines had ruled out chemical adhesives because of their tendency to emit gaseous material, which can collect on sensitive optics and degrade their performance. He also was less than satisfied with traditional techniques such as locking washers, which add weight and complexity with less than desired locking ability.

The self-locking device used in this application is called a Spiralock. Its design was created by re-engineering the standard (female) internal thread form and adding a unique “wedge ramp” at the root of the thread. Unlike traditional fasteners that use a 60-degree thread, the Spiralock has a 30-degree wedge ramp cut at the root of the female thread. This unidirectional internal thread form mates with standard 60-degree male thread fasteners.

The wedge ramp allows the bolt to spin freely relative to the female threads until clamp load is applied. Under clamp load, the crests of the threads of standard male bolts are drawn tightly against this wedge ramp. This eliminates not only the sideways motion that causes vibrational loosening, but it also distributes the threaded joint’s load throughout all engaged threads—a claim supported by a Massachusetts Institute of Technology research study. The load percentage on the first engaged thread is significantly lower than traditional thread forms, which further reduces possible bolt failure and improves product performance.

In addition to medical lasers, self-locking fasteners hold various components together in artificial limbs, heart pumps, and MRI machines. Due to their vibration resistant, they are being considered for cardiovascular devices such as pacemakers and implantable defibrillators as well as for dental and orthopedic surgical instruments and CT scanning applications.

Lyndon Lattie is a field sales engineer specializing in medical applications at Spiralock Corp., Madison Tech Center, 25235 Dequindre Rd., Box 71629, Madison Heights, MI 48071. He can be reached at slinfo@spiralock.com or 800-521-2688.

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For additional information on the products and technologies discussed in this article, see Medical Design Technology online at www.mdtmag.com and Spiralock Corp. at www.spiralock.com.