Locking of threaded members is accomplished by a wide variety of engineering solutions. Generally, in terms of separate locking devices or by built-in locking features such as friction generating means.
In regard of space flight vehicles, threaded joints are subject to severe vibration loads during launch, maneuvering, and reentry. This requires fastening systems which are capable to join structural members and attach accessories or equipment in a secure manner. However, manned spacecraft and especially payload components will be subject to installation activity during orbital or interstellar flight maintenance, repair, or mission modification. This, in turn, requires fast separation and engaging of the concerned fasteners, yet providing performance characteristics for high reliable and safe joints.
The further described Advanced Thread Locking Mechanism (ATLM) has been developed to combine the merits of both, safe joining technique and a fast installation process. The ATLM uses a freewheel which is securely installed in one part of the threaded member and releasable coupled with the engaged counterpart. While screwing the threaded members together, once coupled, the freewheel will allow free mating of the threaded members including torquing to the desired value. The moment, the pair of threaded fasteners is forced to unscrew (by intended or unintended occurrence of torque in the undoing direction) the freewheel does lock instantly. Disengaging the coupling between the members of the threaded joint takes a separate release action.
Owing to the nature of the ATLM, there are a number of design variants ready for implementation.
Threaded fasteners in ATLM design are highly recommended for mechanical joints subject to fastening and unfastening during Extra Vehicular Activity (EVA) in course of space flight operations. This is justified through:
Threaded members mate free running, thus, torque values preset at the wrenching tool are not influenced by varying prevailing (a.k.a. self-locking, running) torque.
No extra locking hardware or retaining compounds are required.
ATLM fasteners produce stable retention (locking) torque with no limitation for the number of screw cycles; i.e., unlimited use of fastener in respect of the self-locking capability.
Anti friction coatings on threads have no influence on locking torque.
Locking of the threaded members takes place automatically - no assembly errors possible.
Integrated tool design for installation / torquing and unlocking / removal of the fastener.
Capture of unscrewed fastener by the tool - no inadvertent loss of hardware resulting in space debris.
Relative to screw locking, no assembly inspection procedures are required.
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