Threads are a Perfect Fit
By Kevin Peacock
General Aerospace Aerospace fasteners OEMOEM product designers and aerospace engineers unlock the advantages of vibration-resistant, reusable threads
Preload-Locking Internal Thread Form
So what do engineers in aerospace and other critical applications use to combat this problem? There is a style of female screw thread that distributes loads more evenly and is therefore safely reusable. Referred to as the Preload-Locking Internal Thread Form, the different thread geometry eliminates the shortcomings of the traditional female thread and improves the performance of the male thread. The difference is that the traditional “V” shape of the thread has been modified to include a third flank perpendicular to the trailing flank of the “V”-shaped thread.
This flank, sometimes referred to as the wedge angle or ramp angle, creates unified loading across all the mating threads (Figure 4). By reducing the high loading on the first few threads, significant yielding does not occur and the threads can be reused repeatedly. Also, the Preload-Locking Internal Thread Form can take advantage of the earlier mentioned thread enhancements. The thread form can also be manufactured by the cold form method and the shape of the thread allows for mating with male UNJ threads without modification.
Out-of-this-World Testing
One of this technology’s biggest fans is the aerospace industry, specifically NASA. In the early 1980s, NASA was looking for a locking screw thread that could not only be implemented into a fastener but into a threaded hole as well, and it had to be reusable. Most orbit-bound space vehicles are completely assembled and dissembled three times before they are launched into space. This coupled with extreme operating temperatures eliminated the conventional approaches to screw threads.
Testing by the Goddard Space Flight Center proved the new thread form can withstand at least 10X sine and random vibration the shuttle requires without loosening. More importantly, the tests were repeated 60 times on the same nut and bolt.
With successful independent testing by the aerospace and academic community, it has continued to gain fans over the years. This technology is more common than you think. The thread form can be found in many applications were thread strength and reusability is a necessity such as heavy truck and automotive powertrains, deep hole oil drilling equipment, high-speed manufacturing equipment and medical devices.
Kevin Peacock is an application engineer for Spiralock Corp. in Madison Heights, Mich. He has more than 20 years experience in fastener and machinery design.
