It should be recognized that some locking features are not a means of maintaining preload but rather are safety devices principally intended to resist rotational loosening and prevent loss of fasteners. Examples of the selection of locking features are provided in Appendix B.
4.6.1 Preload Independent Locking Feature
[TFSR 16] Regardless of the magnitude of preload, each threaded fastening system in spaceflight hardware shall incorporate a minimum of one locking feature that does not depend upon preload to function.
[Rationale: Previous spaceflight missions have been affected by fastening system hardware that loosened and became liberated, causing loss of joint integrity or foreign object debris that compromised completion of mission objectives.]
Devices such as jam nuts may not be suitable locking features to satisfy this requirement if they require preload to function effectively or if a change in preload could compromise the locking performance. A redundant locking feature may be advisable for some joints as described in Federal Aviation Administration (FAA) Advisory Circular (AC) 20-71, Dual Locking Devices on Fasteners, December 8, 1970.
An estimate for the self-loosening moment can be calculated as the product of the preload times the thread pitch divided by 2 . For reference, consult “Threaded Fastener Secondary Locking Requirements” (D. P. Hess, Journal of Failure Analysis and Prevention, Vol. 17, Number 4) .
4.6.2 Mechanical Locking Feature
[TFSR 17] A mechanical locking feature shall be used on any bolt subject to rotation in operation.
[Rationale: Bolts subject to rotation during operation (not including installation or removal of the bolt) may produce forces that would overcome the effectiveness of a prevailing torque or adhesive locking feature. A pure mechanical feature such as safety wire has been demonstrated to be effective at preventing liberation of fastened joint members.]
A bolt is considered to be subject to rotation if it serves as an axis of rotation producing relative motion between the bolt and one or more components attached to the bolt. Friction-type locking features include prevailing torque and adhesive locking features that rely on friction to resist loosening or provide retention. On a bolt subject to rotation in operation, the relative motion between parts may produce forces capable of overcoming the locking friction, so friction-type locking features are not suitable except when they are used in conjunction with a mechanical locking feature such as a cotter pin or safety wire. For reference, consult FAA AC 20-71 and FAA AC 23.607-1, Self-Locking Nuts on Bolts Subject to Rotation, August 24, 1984. If safety wire is used, it is advised to prevent the failure of the safety wire and allow permissible bolt rotation by wiring the nut and bolt to each other exclusively. In this situation, do not safety wire the bolt and the nut to an adjoining structural member. For guidance on safety wire design, see (SAE) International AS567K, Safety Cable, Safety Wire, Key Washers, and Cotter Pins for Propulsion Systems, General Practices for Use of.
4.6.3 Liquid Locking Compound Process Control
[TFSR 18] When using locking adhesives, whether as thread-locking compounds or staking materials, installation processes shall be developed and validated prior to implementation to ensure adhesives cure, adhere, and function as expected.
[Rationale: Lack of proven process controls has been shown to compromise the performance of liquid locking compounds. If process controls are not developed that enable the full functionality of the compounds, the expected locking characteristics can be compromised.]
Requirements for staking of nonstructural fastening systems used on electrical printed wiring assemblies (PWA) are found in NASA-STD-8739.1, Workmanship Standard for Polymeric Application on Electronic Assemblies.
4.6.4 Locking Feature Verification
[TFSR 19] The presence and performance (locking moment) of locking features shall be verified using the methods of Table 4, Locking Feature Torque Verification.
[Rationale: Locking features reduce the likelihood of fastening system components becoming foreign object debris that could affect flight or system performance. Mechanical locking features such as safety wire and safety cable are preferred in external environments where liberation of elements could create critical or catastrophic hazards.]
| Locking Feature Type | Method |
|---|---|
| Mechanical (including, but not limited to, cotter pins, safety wire, and safety cable) | Visual inspection after installation |
| Prevailing Torque (including, but not limited to, deformed thread features and non-metallic pellets, strips, or patches) | Torque measurement during installation(1) |
| Liquid Locking Compounds (including, but not limited to, adhesives, sealants, thread lockers, and chemical thread pre-coats that are applied to the threads and cured prior to joint assembly) | Torque measurement on witness specimens using the same materials and processes, and processed at the same time, as the hardware being verified(2) |
| (1)Verification of prevailing torque locking features by torque measurement may be omitted with prior approval of the delegated NASA Technical Authority if loss of fastener does not credibly cause a catastrophic hazard and if other controls are utilized to minimize the likelihood of assembly with inadequate locking features. Examples of such controls are fastener lot acceptance testing of locking feature performance, locking feature surveillance with periodic measurement of prevailing torque on like fasteners in flight hardware assemblies, technician training to recognize when no prevailing torque is present, etc. The rationale for this approach needs to properly consider fastener reuse if fastener reuse is permitted in the flight assembly. Preloaded joint analyses cannot assume a specified torque 'above running torque' if prevailing torque is not measured. Relaxation of this verification requirement is generally intended for secondary or tertiary structures such as P-clamps. | |
| (2)For adhesive locking features that are not sensitive to the threaded fastening system materials or configuration, cure samples processed at the time of application/processing may be used for verification. | |
Compounds that are pre-applied and dried without curing before installation and with no bonding between mating threads are not considered adhesives and should not be used in applications that could credibly result in severe injury, loss of life, or loss of vehicle.
Proper installation processes are located in specifications such as National Aerospace Standard NASM 33540, Safety Wiring, Safety Cabling, Cotter Pinning, General Practices for, and SAE AS567K.
Example verification methods for each of the locking feature types listed above are located in various sections of Appendix B.