The initial Zip Lok® Design was born from an attempt to improve upon the Mark IV Structural Panel Fastener made by Alcoa Fastening System. While in the concept process, it became apparent that modifications could be made to create a new primary mission for the fasteners, namely eliminating the possibility of catastrophic fastener/coupler failure in the Aircraft.
The new mechanical locking fastener system was developed using Robert Fullerton's flexible thread technology. The fastener can be securely locked in place but can be easily unlocked by inserting a tool. The tool also controls the torque applied to the fastener, ensuring that it is tightened to the correct value. Each tool in the set has a differently colored handle, indicating the specific torque it is set to apply. This makes it easy for operators to use the right tool for the job, without having to rely on a torque wrench.
The Zip Lok® Coupler (B Nut) can be mechanically unlocked by sliding the locking collar. It can be tightened by hand without sliding the collar to the desired torque. When the collar is released, the coupler is mechanically locked in place.
Mr. Fullerton recently completed a replacement project for the A.L.C.O.A. Mark IV Structural Panel Fastener. The existing fastener, which uses a wire form stud bolt holdout, was found to have a potential issue where it could catch and tear, leading to the need for a complete unit replacement. This problem was brought to Mr. Fullerton's attention due to issues at L3AMI, Oklahoma with the bolt retaining spring in the A.L.C.O.A. fastener.
Testing revealed that the A.L.C.O.A. unit required a much higher force to install and had concerns regarding lead thread entry and bolt alignment, which increased assembly time and the risk of downtime for repairs or replacements. In contrast, the fastener, developed by Mr. Fullerton, can be easily installed by simply locating the bracket and pushing the stud bolt into the fastener to full insertion. A simple turn then fully tightens and locks the stud bolt with accurate preload determination.
Modern aircraft structures rely on preloaded bolts or threaded pins to transfer loads. The preload is a crucial factor that affects the joint's fatigue life. Standard aerospace nuts use deformed thread sections for locking, which prevent the fastener from loosening in a vibration environment but cause considerable scatter in the mating bolts preload. This deformed thread lock has led the airframe industry to look for a replacement that can be "pushed on" over the bolt or threaded pin threads.
Zip Lok® has been designed as a solution to this problem. It has a locking mechanism that was tested by Almay Test Labs and found to tighten under vibration. Tests by NASA engineers have also shown the same results. It helps workers with installation, as it is easier to install in tight locations where the threads are not always visible. Zip Lok® also eliminates the need for other locking devices, such as washers and safety wires, substantially reducing the potential for fatigue failure compared to standard fasteners.
Zip Lok®, has improved on the standard internal female thread by creating a self-locking female thread form with a unique angled wedge. This unidirectional locking feature works with standard 60-degree male thread fasteners and creates a variable compression loading instead of the standard fixed id loading. The change in female root thread angle creates a locking effect not possible with fixed thread id fasteners. When clamp load is applied, the crest of the male thread is drawn tightly against the female wedge ramps, creating a continuous spiral line of contact. As torque increases, the Constrictor effect increases, compressing the thread and eliminating radial clearance that can cause self-loosening under vibration. The reduced root radius in the Constrictor thread increases its fatigue life compared to a standard fixed id thread fastener. Deformation focuses on the tips of the bolt thread, preserving the life of the fastener. This unique thread form is resistant to vibration loosening, reusable, and improves joint fatigue life.
Zip Lok® uses a unique internal locking thread form that provides superior performance compared to other thread locking devices. Unlike other locking devices that lose preload within the first minute of attachment, the Constrictor design of Zip Lok®. is highly resistant to loosening under vibration and in most cases eliminates the need for additional mechanical locking features. This mechanical interlock operates by locking the threads in place and preventing transverse movement during vibration, which is a common cause of nut loosening. Extensive testing by four third parties using three different Constrictor fasteners with diverse dimensions, thread forms, and materials will produce reliable and credible results.
Prior Fullerton Fastener designs (Zip Nut®) have achieved the following test results, and there is no reason not to expect the same for this series.
1. 1/4-28-UNF (2B) AL6061-T6 anodized outer body and 303 CRES Thread segments tested by an aerospace customer - Mil-Std 1312 Test 7: satisfied customer self-locking nut requirements for satellite construction.
2. 1/2-20-UNF (2B) A-286 CRES outer body and 15-5 CRES thread segments. Extensive Testing by Almay Research & Testing Corp., Los Angeles, CA. Mil Std 1312, and Test 7.
It satisfied customer standards for joining the structural truss of the International Space Station.
Tested by Consolidated Laboratories, Inc. Covina, CA--Passed random vibration test specified for Space Station hardware by customer.
3. 1/2-13-UNC (2B) ASTM 4140 outer body and thread segments tested by Almay Research & Testing Corp., Los Angeles, CA. Mil Std 1312, Test 7; no Unit exhibited any sign of loosening for up to 30,000 cycles.
The testing results of Zip Lok® will demonstrate that the joints using these fasteners will distribute loads much more evenly compared to those using standard 60-degree threads. The first engaged thread in the joint will experience a lower load percentage (approximately 24% vs 65%) with five lines of engagement, thanks to the uniform thread loading provided by Zip Lok®’s constrictor thread form.
The reduction in load concentration at the first engaged thread will improve joint reliability by reducing the risk of line stripping. Additionally, Zip Lok®’s Constrictor thread form will generate a higher radial thread loading than standard threads, utilizing the material's hoop
strength and decreasing the risk of shear failure.
The Fullerton Torque Driver® Tool Set (Patent Pending) will come from the manufacturer in sets with pre-set maximum torque set at the factory. Each torque is identified by a different colored handle. Each unit will have shaft ends that will permit adapting different ends, ie hex, star, etc.
The Fullerton Torque Driver® will permit the application of the Fullerton RNE ( as well as other fasteners) to a pre-described torque (for example the ultimate torque required for the 1/4 in. fastener is 100 in.lbs. the maximum torque required for the 3/8 in. Is 240 in.lbs.
The tool will automatically "cycle" when the maximum torque is achieved, thus eliminating stress from the fastener as well, as the application surface structure.
Zip Lok®’s constrictor thread form has unique wedge ramps at its tip and root that create a locking capability by redirecting a significant portion of assembly forces from axial to radial direction. Compared to standard 60-degree threads, Zip Lok®’s threaded joints require 10-20% more torque to generate the same tension. This increased torque requirement varies based on the materials and coatings used, and the proper torque needed to achieve the desired tension is determined by the torque/tension relationship in the joint.
Bolted joints are generally loaded (tensioned) by applying torque to the nut, the bolt head, or both. "Torque" is simply the measurable means for rotation, which, with the spiral inclined plane of screw threads, creates an axial load against the bolt and the joint once the Male (nut or bolt), torqued to the required tension just as a standard nut or bolt.
Relating a specific torque force to the resulting tension on the bolt is typically a problematic issue for the engineer. However, the results follow:
1. 1/2-20-UNF (2B) 303 CRES outer body and thread segments. Extensive testing by R.S. Technologies, Ltd of Farmington, Michigan; Torque/ tension relationship of multiple units Tested found to be within a statistically acceptable range.
2. 3/4-10-UNC ASTM 4140 outer bodies and thread segments. Testing by R.S. Technologies, Ltd of Farmington, Michigan; Torque/ tension relationship of multiple units tested found to be within a statistically acceptable range.
3. 1/2-13-UNC ASTM 4140 outer body and thread segments. Testing by a customer confirmed a thread friction increase over standard nuts of 10-20%. Tests also verified that the use of commercially available lubricants such as molybdenum, cadmium, and Decrement® fully compensated for the increase and brought the torque/tension relationship into that of a standard nut of the same size.
Fullerton Design has worked with numerous customers in the design and material analysis to ensure that the Flexible thread product meets the customer's tension requirements.
Zip Lok®s are tested for load specifications specified by the customer or industry standards by Fullerton Design. The load test involves loading a fastener to a matching stud. The company has a record of no fastener, coupling, or tool failure in axial load tests. Test results for Axial Tensile Strength, 96-Hour Stress Durability, and Maximum Operating Pressure are available upon request.
Due to its design characteristics, Zip Lok® is highly suitable for automated robotic manufacturing, as it eliminates concerns related to thread lead, alignment, thread condition, and mixed material construction. This thread form can be applied to all female thread fasteners under critical conditions, including blind applications, where speed and accuracy are important.
The added advantage of Zip Lok®’s thread form is that it is easy to manufacture and assemble, with individual segments easily distinguishable to avoid misplacement in the spiral pattern
Location: Offutt Air Force Base, Nebraska:
Crew, 27, no injuries. (Aircraft was on "take off" was aborted.
Cause: The retaining "B" nut connecting a metal oxygen tube to a junction fitting, came loose releasing pure oxygen to the atmosphere.
Damage: $62.4 million repair
1 Fatality.
Location: Tupelo, Mississippi
Cause: Fuel line “B” Nut (fuel line) to engine manifold came loose.
Damage: Destroyed
No Fatality.
Location Texas,
Cause: four fasteners came loose.
Damage: a 20-foot section of wing lost.
Fatalities 104
Silk Air, Indonesia, (new Aircraft)
Cause: 26 fasteners were missing.
Damage: Destroyed
Fatalities 14.
Cause: 47 fasteners were missing from Horizontal Stabilizer.
Damage: Destroyed
2 Fatalities- 3 severe.
Cause: forward aft shaft missing due to fastener failure (missing nuts)
Damage: Destroyed
Naha Airport,
Cause: fastener on Slat-can Assembly released, causing the entire down-stop assembly to detach and fall onto the slat-can body during the Aircraft's movement to parking.
The moving metal parts ruptured the fuel tank compartment, causing a catastrophic fire.
Since the passengers were in the process of "Deplaning," there were no serious injuries.
Cause: fastener coming loose due to vibration.
Damage: Destroyed
Boulder City, Nevada,
Only the pilot was on board and sustained no injuries:
Cause: the main fuel supply line "B" nut fitting came loose, causing loss of power.
Damage: major
1 Fatality.
Location: Tupelo, Mississippi
Cause: Fuel line “B” Nut (fuel line) to engine manifold came loose.
Damage: Destroyed
In short, the use of Zip Lok® can save lives.
Zip Lok® is the "push on" solution the airframe industry has been waiting for. It performs well under vibration, helps workers with installation in tight locations where threads may not be visible, and eliminates the need for other locking devices, such as washers and safety wire. Previous iterations of the design have been proven to save lives and prevent catastrophic failures in flight. Learn more about Zip Lok® aircraft panel fasteners here.
The global market for hydrogen fuel cells is expected to grow from $7.5 billion in 2022 to $19.5 billion in 2027 at a growth rate of 21%. As hydrogen becomes more widely used as a vehicle fuel, there will be a need for a fuel transfer coupler that can be used by the public, similar to a gas pump. The Zip Lok® Hydrogen Coupler, which was originally conceived for use on the proposed Moon Base, meets the high application and safety requirements needed for everyday use. The coupler is a simple and efficient device that will enable the safe transfer of hydrogen fuel to vehicles, without the fear of escaping gases. Learn more about Zip Lok®'s fuel coupler applications here.
Astronaut Steven L. Smith, Hubble Space Telescope (HST) servicing mission 3A payload commander, retrieves a Pistol Grip Tool (PGT) power tool while standing on the mobile foot restraint at the end of the Remote Manipulator System (RMS). Here Smith is using a flexible thread system to quickly make repairs in one of the most complex environment - An application where Zip Lok® thrives. Read more about Zip Lok®'s potential space applications here.