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Self-Locking Fastener Assembly with Compressible Polymer Insert and Dual-Lead Thread Profile

Technology Field: Mechanical Fastening Systems · Language: English · Generated by ClaimForge

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§1 Background of the Invention ▶

Conventional threaded fastener assemblies comprising a bolt and nut are widely used in mechanical and structural applications. However, under cyclic loading and vibration, standard threaded connections are prone to spontaneous loosening due to relative micro-rotation between engaged threads. This phenomenon has been extensively documented and remains a significant source of mechanical failure in aerospace, automotive, and industrial applications. Existing solutions to vibration-induced loosening include thread-locking adhesives (e.g., anaerobic resins), deformed-thread nuts (prevailing-torque designs), and split-lock washers. Thread-locking adhesives require chemical application, are not reliably reversible, and are sensitive to contamination during assembly. Deformed-thread nuts apply a frictional torque through thread interference, but this deformation is permanent and limits reuse to a small number of cycles. Split-lock washers have been shown to provide minimal retention under dynamic loading once the helical spring action is overcome. A need therefore exists for a fastener assembly that provides robust vibration resistance through a purely mechanical, non-destructive mechanism, that is field-serviceable without special tooling, and that can be tuned to a specific locking torque range for the application.

§2 Summary of the Invention ▶

The present invention provides a self-locking fastener assembly that achieves vibration-resistant retention through a compressible polymer locking insert positioned within an annular recess in the nut body. Upon tightening, the polymer insert deforms radially inward into contact with the bolt shank, generating a friction-based retention force that resists rotational loosening without permanently altering any fastener component. In a first aspect, the invention provides an apparatus comprising: a bolt having a shank with a dual-lead helical groove defining two axial load-bearing contact lines; a nut with an internally threaded bore; an annular recess in the nut bore; and a compressible polymer locking insert within the recess. The dual-lead thread geometry distributes axial load across two contact lines, reducing peak thread stress by approximately 40% compared to single-lead profiles of equivalent pitch diameter. In a second aspect, the invention provides a method of securing components against vibration-induced loosening using the assembly described above, including steps of positioning the polymer insert, engaging the nut with the bolt, and tightening to a target torque. The locking torque is tunable across a range of 5–25 Newton-meters by selecting a polymer insert of appropriate Shore A durometer, providing application-specific configurability. The insert is replaceable by hand without specialized tooling, enabling field serviceability throughout the fastener assembly's service life.

§3 Brief Description of the Drawings ▶

Fig. 1 shows an exploded perspective view of the self-locking fastener assembly, illustrating the bolt, nut, and polymer locking insert.
Fig. 2 shows a cross-sectional view of the assembled fastener, depicting the annular recess and the deformed polymer insert in contact with the bolt shank.
Fig. 3 shows an enlarged cross-sectional detail of the dual-lead helical thread profile on the bolt shank, illustrating the two axial load-bearing contact lines.
Fig. 4 shows a graph of locking torque versus Shore A durometer for the polymer insert, illustrating the 5–25 Nm tunable range.
Fig. 5 shows a sequence diagram of the insert replacement procedure, illustrating tool-free extraction and reinstallation.

§4 Detailed Description of the Preferred Embodiments ▶

With reference to Fig. 1, the self-locking fastener assembly 100 comprises a bolt 102, a nut 104, and a compressible polymer locking insert 106. The bolt 102 includes an elongate shank 108 having a dual-lead helical groove 110 formed along at least a portion of its length. The dual-lead groove 110 defines first and second thread leads distributed circumferentially such that two distinct axial load-bearing contact lines 112a, 112b are presented to the engaging nut threads at any given angular position. As shown in the cross-sectional view of Fig. 2, the nut 104 defines an annular recess 114 formed in the inner bore 116. The polymer locking insert 106, comprising a compressible elastomeric ring, is seated within the annular recess 114 prior to engagement with the bolt 102. Upon tightening of the nut 104 to a target torque as claimed in claim 1, the locking insert 106 deforms radially inward under the compressive load applied by the bolt shank 108, generating a friction-based retention force FR that resists relative rotation of the nut 104 with respect to the bolt 102 under vibratory loading. Fig. 3 illustrates the dual-lead thread profile in detail. Unlike a conventional single-lead thread, which presents a single helical contact line of contact width W1, the dual-lead profile presents two contact lines 112a, 112b each of width W2, where W2 is at least 0.5 mm measured in the axial direction. The load sharing across two contact lines reduces peak thread stress by approximately 40% as claimed in claim 1.2, thereby improving fatigue resistance of the bolt and nut under cyclic loading. The polymer material of the locking insert 106 is selected from the group of compressible elastomers, including nitrile rubber (NBR), fluoroelastomer (FKM), and silicone, as recited in claim 1.6. The Shore A durometer of the selected elastomer determines the radial deformation under a given tightening torque and, consequently, the locking torque generated. As shown in Fig. 4, a durometer range of 40–90 Shore A provides a locking torque range of 5–25 Nm across practical tightening conditions, consistent with claim 1.3. An alternative embodiment employs the assembly in a Method of securing components as claimed in claim 2. In this embodiment, the locking insert 106 is replaced after a defined service interval by applying a radially outward extraction force not exceeding 50 N, without heating or chemical dissolution, as recited in claim 2.4. The replacement insert is installed into the annular recess 114 by hand pressure. This field-serviceability distinguishes the invention from thread-locking adhesives, which require chemical removal, and from prevailing-torque nuts, which are typically single-use by design.

§5 Claims ▶

1. A self-locking fastener assembly, comprising: a bolt including a shank having a dual-lead helical groove formed thereon, wherein the dual-lead groove defines first and second thread leads distributed circumferentially to provide two axial load-bearing contact lines; a nut having an internally threaded bore configured to mate with the dual-lead helical groove of the bolt; an annular recess formed in an inner bore of the nut; and a locking insert comprising a compressible polymer material positioned within the annular recess, the locking insert configured to deform radially inward upon engagement with the bolt shank to generate a friction-based retention force resisting rotational loosening of the nut.
2. The fastener assembly of claim 1, wherein the locking insert is removably disposed within the annular recess such that the locking insert is replaceable without the use of specialized tooling.
3. The fastener assembly of claim 1, wherein the dual-lead thread profile is configured to distribute axial load such that peak thread stress is reduced by at least 35% compared to a single-lead thread profile of equivalent pitch diameter.
4. The fastener assembly of claim 1, wherein the locking insert has a Shore A durometer in the range of 40–90, wherein selection of durometer determines a locking torque in the range of 5–25 Newton-meters.
5. The fastener assembly of claim 2, wherein the locking insert is retained within the annular recess by an interference fit, the interference fit being dimensioned to permit manual extraction without mechanical assistance.
6. The fastener assembly of claim 3, wherein each of the two axial load-bearing contact lines has a contact width of at least 0.5 mm measured in the axial direction of the bolt.
7. The fastener assembly of claim 1, wherein the compressible polymer material comprises a thermoset elastomer selected from the group consisting of nitrile rubber, fluoroelastomer, and silicone.
8. A method of securing components against vibration-induced loosening, comprising: providing a bolt having a shank with a dual-lead helical groove defining two axial load-bearing contact lines; engaging a nut having an internally threaded bore with the dual-lead helical groove of the bolt; positioning a compressible polymer locking insert within an annular recess in the nut prior to engagement; and tightening the nut to a target torque such that the locking insert deforms radially inward into contact with the bolt shank, generating a friction-based retention force sufficient to maintain the nut position under vibratory loading.
9. The method of claim 8, further comprising: removing the locking insert from the annular recess after a service interval; and installing a replacement locking insert into the annular recess without applying a torque-setting tool.
10. The method of claim 8, wherein the target torque is selected from the range of 5–25 Newton-meters based on a Shore A durometer of the compressible polymer locking insert.

§6 Abstract ▶

A self-locking fastener assembly and method for resisting vibration-induced loosening in mechanical connections. The assembly comprises a bolt having a dual-lead helical groove that distributes axial load across two contact lines, reducing peak thread stress by approximately 40% compared to single-lead designs. A compressible polymer locking insert, seated within an annular recess in the nut bore, deforms radially inward upon tightening to generate a friction-based retention force without permanently altering any fastener component. Insert durometer selection provides a tunable locking torque range of 5–25 Nm. The insert is field-replaceable without specialized tooling.

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