OEM product designers and fastener application engineers have struggled to provide a self-locking screw-thread system that is reusable and cost effective. Over the years, several locking systems for threaded fasteners have been developed and implemented in a wide range of commercial products. Most of these locking fasteners depend on some type of interference fit between the male and female thread. This interference is most commonly accomplished by deforming a few threads in the fastener, which is referred to in the fastener industry as an all-metal, prevailing-torque fastener.

Another common approach to thread interference is the use of hard nylon or plastic to slightly impede the movement of the mating thread during assembly and tightening. The impediment can be in the form of a nylon ring on the top of the nut or a plastic plug that is inserted into the threaded region of the bolt.

Although both of these prevailing-torque type fasteners initially appear to be simple and cost-effective solutions to fastener loosening, they have hidden costs, specifically when the issue of reusability is addressed. It is widely accepted by engineers, assemblers, and service technicians that these fasteners are one-time-use locking fasteners. Most service manuals and assembly line procedures require that a new locking fastener be installed if disassembly has occurred for any reason.

Enlarge this picture Fig. 1. Standard 60 Deg V-shaped thread form has gap that makes assembly susceptible to vibration-induced loosening.

While this one-time use may be financially beneficial for fastener distributors and manufacturers, it is an extremely costly problem for OEMs. To the manufacturing engineer and assembly line personnel it may not seem like a significant cost to grab a new fastener out of the bin and reassemble the components, but there can be other logistical problems. With just-in-time bin replenishment by the fastener distributor, chronic reassembly can prematurely deplete the fasteners on-hand and shut down the assembly line.

Initial reassembly costs in the plant can be expensive, but do not compare to the profit-draining costs of warranty and service work performed in the field. Besides the exponential costs of field repairs for service and warranty, the process of getting replacement fasteners to remote corners of the world can be almost impossible.

This does not mean that fastener designers have failed to address reusability. Several secondary-locking devices have been developed and widely used for many years. These devices are routinely mechanical in nature. Typically, mechanical features such as serrations, protrusions, and tabs are incorporated into washers and are termed lock washers. Beyond washers, there are retaining rings, pins, and multiple fasteners used to lock a fastened joint. These additional components can achieve the desired locking requirement but, as the word additional implies, they cause an increased part count. Increased part count escalates costs and inventory. In the field, the additional components can be difficult to retain during the service procedure. Also, proper reassembly and reuse by service technicians cannot be guaranteed.

Enlarge this picture Fig. 2. The preload locking internal thread form uses truncation of the female screw thread to eliminate the gap between the mating threads.

A reusable, cost-effective fastener is not a dream for the fastener industry. There is an existing screw-thread technology to solve this dilemma, the preload locking internal thread form. This has been a successful solution for many demanding fastening applications for more than 25 years. Perfected and patented by Ace Holmes, the fastener design is a simple modified buttress or truncated female thread. It was proven that the major cause of vibration-induced loosening in the standard 60 Deg, V-shaped thread form is the gap between the male and female threads. To easily assemble the male and female threads, there must be clearances between the mating threads. This clearance creates a gap. The gap between the mating threads produces an area where lateral movement will occur under vibration. (See Fig. 1.)

Combined with the shallow flank angle of the V-shaped thread, the threads will begin to progress along the helical angle of the thread and the bolt/screw will lose tension.

To maintain bolted joint integrity, the bolt or screw must remain in tension and act as a spring. Once tension is lost in the male fastener, it is not a question of if the fastener will loosen, but when.

As mentioned earlier, the preload locking internal thread form uses truncation of the female screw thread to eliminate the gap between the mating threads. (See Fig. 2.)

Enlarge this picture Fig. 3. Junkers vibration test rig features a load cell and two transverse moving plates that are clamped between the nut and bolt.

Designed to mate with a standard Class 2A or 3A (metric 6g/6h) male thread, the truncation is created by an additional ramp angle perpendicular to the trailing flank angle of a 60 Deg V thread. The combination of tension on the male fastener, the elimination of the gap, and the steep angle of the ramp style truncation significantly increases resistance to fastened joint loosening.

Any improvement in technology must be tested and proven. Product engineers worldwide have been perplexed on how to test bolted joint integrity in an accurate, cost-effective, and timely manner. When warranty costs are rising due to a threaded fastener loosening, engineers rarely have the luxury of full life-cycle testing on an application with new technologies. They need accelerated testing results.

Reliable and accurate, accelerated bolted-joint testing is another area not well known by product and reliability engineers, but the fastener industry has a solution for this problem. For many years, the Junkers vibration test has been the benchmark for testing threaded-fastener resistance to vibration. Maintaining tension in a screw or bolt is paramount to keeping fasteners from loosening under vibration. Gerhard Junkers’ test is based on this proven theory.

Enlarge this picture Fig. 4. Chart compares the effect of the Junkers test on tension for different fastener assemblies.

The test rig is quite simple. A load cell and two transverse moving plates are clamped between the nut and bolt to be tested. (See Fig. 3.) An eccentric cam mechanism moves the plates at 12.5 Hz for a maximum of 120 seconds. Tension in the bolt or screw is recorded. The time of 120 seconds was chosen because the test is so aggressive that typically after 120 seconds the bolt fatigues and breakage occurs. Bolt failure caused by fatigue proves that this test will surpass the rigorous conditions a product will experience in the field over its lifetime.

Again, proving a technology is critical to acceptance in any industry. As seen in the Fig. 4 chart, the preload locking internal thread form maintains all but a fraction of the initial preload. When compared to a prevailing torque fastener and a secondary locking feature the results are significant.

The aerospace industry in general, and NASA in particular, was an early adopter of this fastening technology. In the early 80s, NASA was searching for a locking screw thread that could not only be implemented into a fastener, but also into a threaded hole. Most importantly, it had to be reusable. Most orbit-bound space vehicles are completely assembled and reassembled three times before being launched into space. This requirement and the extreme operating temperatures eliminated the common approaches to locking fasteners available at the time.

Extensive testing by the Goddard Space Flight Center proved that the preload locking internal thread form can withstand at least 10X sine and random vibration that the Space Shuttle requires without loosening. More importantly, the tests were repeated 60 times on the same nut and bolt.

Applying the preload locking internal thread form does have a few minor limitations. The thread form is unidirectional. Therefore, the bolt must be assembled into the fastener or threaded hole in a certain direction for the thread form to be effective. This requirement can be overcome by the use of hex flange nut or other unidirectional fasteners. As for threaded holes, proper design of threading tools will insure correct thread orientation. The mechanical locking action of this thread form depends on consistent tension on the male fastener. This requires a hard joint where the materials bolted together will not relax and cause loss of bolt tension during use.

Despite those minor limitations, the preload locking internal thread form continues to appear in a wide range of new applications where self-locking features and reusability are desired design characteristics.

For more information, email: KTurowska@spiralock.com