A locking fastener that can be reused

Kevin Peacock, Application Engineer for Spiralock, describes a screw thread fastening solution that is both self locking and reusable.

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 the commercial products we use every day.

Most of these locking fasteners depend on some type of interference fit between the male and female thread.

Most commonly, this interference is accomplished by deforming a few of threads in the fastener.

It is referred to in the fastener industry as "all-metal prevailing torque" fastener.

Another common approach to thread interference is the use of hard nylon or plastic to slightly impede the mating thread's movement 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 which 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.

Although 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 line 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 reusability has not been addressed by fastener designers.

Several secondary-locking devices have been developed and widely used for many years.

These devises are routinely mechanical in nature.

Most commonly, a mechanical feature such as serrations, protrusions, and tabs are incorporated into washers and 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 retain during the service procedure.

Also, proper reassembly and reuse by service technicians cannot be guaranteed.

A reusable, cost-effective fastener is not a dream for the fastener industry.

In fact, there is existing screw-thread technology to solve this dilemma.

The "Preload Locking Internal Threadform" has been a successful solution for many demanding fastening applications for more than 25 years.

Perfected and patented by Ace Holmes, the Preload Locking Internal Threadform is a simple modified buttress or truncated female thread.

It was proven that the major cause of vibration-induced loosening in the standard 60-degree V-shaped threadform is the gap between the male and female threads.

To assemble the male and female threads together easily, 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.

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 loose 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 Threadform uses truncation of the female screw thread to eliminate the gap between the mating threads.

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-degree 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 proved.

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 a vehicle or machinery of any 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.

The fastener industry has a solution for this problem.

For many years, the Junkers vibration test has been the benchmark for testing a fastener's or a screw thread's resistance to vibration.

As mentioned earlier, maintaining tension in a screw or bolt is paramount to keeping fasteners from loosening under vibration.

Dr Gerhard Junkers' test is based on this proven theory.

The test rig is quite simple.

A load cell and two transverse moving plates are clamped between the nut and bolt to be tested.

An eccentric cam mechanism moves the plates at an amplitude of 12.5Hz for a maximum of 120s.

Tension or loss thereof in the bolt or screw is recorded.

Why does the test only run 120s?.

And what happens after that?.

The test is so aggressive that typically after 120s the bolt fatigues and breakage will occur.

Bolt failure caused by fatigue proves that this test will surpass the rigorous conditions your product will experience in the field over its lifetime.

Again, proving a technology is critical to acceptance in any industry.

This is where the Preload Locking Internal Threadform won its loyal fans.

Bolted joints using common threads will lose all the preload in the joint within 30s of test vibration and the prevailing torque fastener will lose almost 75% of the preload in the same joint.

But, the Preload Locking Internal Threadform maintains all but a fraction of the initial preload.

When compared with a prevailing torque fastener and a secondary locking feature the results are astonishing.

One of the Preload Locking Internal Threadform's biggest fans is the aerospace industry, and specifically NASA.

In the early 1980s, 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 Centre proved the Preload Locking Internal Threadform 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 Threadform does have a few minor limitations.

The threadform is unidirectional.

Therefore, the bolt most be assembled into the fastener or threaded hole in a certain direction for the threadform to be effective.

This requirement can be easily 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 threadform is dependent on consistent tension on the male fastener.

This requires a "hard joint" wherein the materials bolted together will not relax and cause loss of bolt tension during use.

Keeping these minor limitations in mind and successful independent testing by the aerospace and academic community, the Preload Locking Internal Threadform continues to steadily gain fans.

This technology is much more common applied than you think.

It is not confined to just aerospace.

The threadform can be found in many applications were reusability is a necessity, including heavy truck and automotive powertrains, deep-hole oil-drilling equipment, high-speed manufacturing equipment and medical devices.

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