Failure is not an option when it comes to medical equipment. From critical or sensitive devices like lasers, MRI scanners, knee joints, and implantable pacemakers to instruments as straightforward as stethoscopes, when life is at stake, medical equipment must be reliable.
Because repetitive loads, shock and vibrational loosening must be handled decisively, 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% of the load, permitting stripping or shearing, while subsequent male threads “float” within the female threads.
One of the simplest solutions is an innovative self-locking fastener called Spiralock, developed by Spiralock Corp of Madison Heights, WI. Spiralock is capable of resisting loosening, even under loads and vibrations strong enough to break the fastener. What makes Spiralock unique is a 30º “wedge” ramp cut at the root of the female thread (while traditional fasteners use a 60º thread). Under clamp load, the crests of the threads on any standard male bolt are drawn tightly against Spiralock’s wedge ramp. This not only eliminates sideways motion that causes vibrational loosening, but 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 a proactive design change to increase reliability in the field, the medical/aesthetic laser technology firm Lumenis recently sought to safeguard its precision critical laser alignments. These could potentially become misaligned during rough shipping or handling using standard thread forms, and traditional locking techniques were not well suited to the task.
“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,” said Peter Hines, a Lumenis R&D engineer responsible for laser design.
Hines ruled out chemical adhesives because of their tendency to emit gaseous material which could collect on sensitive optics, degrading their performance. He was dissatisfied with traditional techniques, such as locking washers, which add weight and complexity with less than desired locking ability. Lumenis chose the Spiralock self-locking fasteners.
“We’re using the Spiralock self-locking fastener to help alleviate our concerns regarding a system’s critical precision alignment,” says Hines. “With its shock and vibration resistance, we’re preventing screws from backing out and preserving critical alignment for enhanced reliability.”
By using the Spiralock self-locking thread form, manufacturers can achieve increased productivity, durability, more cost-effective and repeatable manufacturing processes, as well as the ability to retro-manufacture thousands of inventory parts that could end up as very costly scrap.
Production changeovers to Spiralock fasteners are typically quick and seamless, requiring just an exchange of traditional nuts, wire inserts or simply drilling out and re-tapping existing parts stock that have unreliable standard tapped holes.