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A Self-Locking, Reusable, Screw Thread Fastening Solution

The Preload Locking Internal Thread Form is said to prevent the unnecessarily high logistical, service, and warranty costs of typical locking thread forms.

By Kevin Peacock
Application Engineer
Spiralock Corporation
Madison Heights, Michigan

For years, OEM product designers and fastener application engineers have struggled to provide a self-locking screw-thread system that's reusable and cost effective. Although locking systems for threaded fasteners have been developed for everyday commercial products, most of these locking fasteners have significant drawbacks that are effectively addressed by an innovative self-locking, reusable screw thread fastening solution.

Drawbacks of Typical Locking Thread Forms

Most locking thread forms depend on some type of interference fit between the male and female thread. This interference is typically accomplished by deforming a few of threads in the fastener, such as in the "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 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 cost-effective solutions to fastener loosening, they have significant hidden costs when reusability is required because they are essentially "single use" fasteners. Most service manuals and assembly line procedures require new locking fastener installation if disassembly has occurred for any reason.

While fastener distributors and manufacturers may appreciate selling "single use" fasteners, this can be a costly problem for OEMs. To the manufacturing engineer and assembly line personnel, it may seem insignificant to grab a new fastener from a bin to reassemble components, but this can pose costly 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 they don't 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 may not be viable.

Some secondary-locking devices have been developed and used for years. These devices are typically mechanical features, such as serrations, protrusions, and tabs, which are incorporated into washers and called "lock washers."  Retaining rings, pins, and multiple fasteners can also be used to lock a fastened joint, but these additional components increase part count, which escalates cost and inventory. In the field, additional components may not be on hand to perform service procedures, and their proper reassembly and reuse by service technicians cannot be guaranteed.

One existing screw-thread technology, however, can help solve this dilemma. The "Preload Locking Internal Thread Form" is a simple modified buttress or truncated female thread that has proven its effectiveness in a range of demanding fastening applications for more than 25 years. Because the major cause of vibration-induced loosening in the standard 60-degree "V' shaped thread form is the gap between the male and female threads, there must be clearance between the mating threads in order to easily assemble the male and female threads. But this clearance creates a gap between the mating threads and produces an area where lateral movement can 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 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?"

To address this inherent thread-loosening problem, the Preload Locking Internal Thread Form 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.

Testing Joint Effectiveness

Any improvement in technology must be tested and proven, but product engineers 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 the loosening of a threaded fastener, engineers rarely have the luxury of full life-cycle testing on a vehicle or machinery. They need accelerated test results, which a test called the Junkers Vibration Test can provide.

Maintaining tension in a screw or bolt is essential to prevent fasteners from loosening under vibration. And for years, the Junkers Vibration Test has been the benchmark for testing the resistance of a fastener or a screw thread to vibration. 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 12.5 Hz for a maximum of 120 seconds, and tension or tension loss in the bolt or screw is recorded. Why does the test only run 120 seconds? The test is so aggressive that after 120 seconds, the bolt typically fatigues and breakage occurs. Bolt failure caused by fatigue proves that this test will surpass the rigorous conditions most products will experience in the field over their lifetimes. When tested on the Junkers Vibration Test, the Preload Locking Internal Thread Form maintains all but a fraction of the initial preload, while prevailing torque fasteners and secondary locking features quickly lose their tension.

One of Preload Locking Internal Thread Form's biggest fans is the aerospace industry, 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. And it had to be reusable, since most orbit-bound space vehicles are completely assembled and reassembled three times before launch. 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. The tests were repeated 60 times on the same nut and bolt.

While the Preload Locking Internal Thread Form solves thread loosening and reuse concerns, it has a few minor limitations. As the mechanical locking action of this thread form depends on consistent tension with the male fastener, this requires a "hard joint," where the materials bolted together will not relax and cause bolt tension loss during use. Because the thread form is uni-directional, the bolt must be assembled into the fastener or threaded hole in a certain direction for the thread form to be effective. However, this is easily accomplished by using hex flange nuts or other uni-directional fasteners. For threaded holes, proper design of threading tools will ensure correct thread orientation.

Amid these minor limitations and successful independent testing by the aerospace and academic communities, the Preload Locking Internal Thread Form continues to gain fans.  This technology is commonly applied in a range of applications and industries where reusability is necessary, including heavy truck and automotive power trains, deep-hole oil-drilling equipment, high-speed manufacturing equipment, and medical devices.

Spiralock Corporation, based in Madison Heights, Michigan, provides fastening solutions for the transportation, aerospace, medical, oil and gas, and machinery industries. For detailed test data, including comparative graphic loading characteristics or photo-elastic analysis/load vector comparison animation, visit Spiralock at www.spiralock.com email: slinfo@spiralock.com; or call (800) 521-2688.

Kevin Peacock is an Application Engineer for Spiralock Corporation. He has a Master's Degree and over 20 years of experience in fasteners and machinery design.