DEPARTMENT OF THE AIR FORCE SMC REGULATION 800-23
Headquarters Space and Missile Systems Center (AFMC)
Los Angeles Air Force Base, California 90009-2960
Locking Features for Threaded Devices
This regulation establishes guidelines and procedures and assigns responsibilities for use of locking features on threaded devices in SMC program office acquisitions.
1. Policy All SMC space programs which use threaded devices for space and launch vehicle applications shall employ locking features as stipulated within this regulation.
2. Background Threaded devices are used in many areas of launch and space vehicles. These devices include fasteners (nuts and bolts,) separation nuts, adjustment devices, separable fluid fittings, electrical connectors, etc. When subjected to launch vibration or other disturbances, these devices tend to loosen, resulting in loss of preload, loss of adjustment, loss of fasteners, leakage, or outright separation of the joined parts. Positive locking features have been implemented on many space programs to eliminate this problem.
a. Preferred Devices: Preferred locking devices have positive locking features such as bent tab washers, cotter pins, safety wire (lockwire) which prevents any rotation between threaded parts by applying a tightening torque to the threaded parts (not "champagne cork" type), self-locking threads, or self locking provisions by means of plastic material contained in the nut, bolt, or screw.
Self-locking nuts are preferred to bolts or screws that contain plastic material for use as a locking device. Through bolts with locknuts are preferred to threaded inserts. When self-locking features are used, the screw length shall be sufficient to extend at least two threads through the locking device under worst case tolerances.
Safety wiring and cotter pins shall be in accordance with MS-33540.
b. Other Acceptable Devices: Other acceptable locking features include locking compounds applied to threads, epoxy compounds applied after torquing is complete, and "champagne cork" type safety wire. "Champagne cork" safety wire is applied around or over the threaded parts, rather than through the threaded parts, and may be used only for applications where some relative rotation of the threaded parts can be tolerated or small amounts of rotation are not detrimental. Locking compounds shall not be used in areas where excess compound could migrate to surfaces which must remain free to move.
Self-locking devices which depend upon an interference fit between metallic threads may be used but shall be avoided in applications where particulate contamination may cause damage or degradation to the equipment or vehicle.
c. Drawings: Contractor prepared assembly drawings shall clearly depict the safety wiring method and configuration used. When self-locking features are used, an allowable range of run-in torque, and the maximum number of reuses that would still ensure an adequate lock, shall be specified in contractor prepared assembly drawings. Self locking nuts or screws with a plastic insert should only be used once.
4. Reponsibilities and Procedures
a. For SMC launch and space vehicle programs each program office shall:
1. Ensure that all threaded devices use preferred locking features in accordance with provisions of paragraph 3.
2. Ensure that where preferred locking features cannot be used, other acceptable locking features are implemented in accordance with provisions of paragraph 3.
3. Ensure that spring type or star type lock washers, or thread friction due to preload only, are not relied upon as a locking feature.
4. Require contractor compliance with this regulation for space and launch vehicle threaded devices during design, manufacture and test.
5. Review contractor compliance with this regulation from proposal evaluation through delivery of the space or launch vehicle.
6. Require contractor notification of any non-compliance.
b. Instructions for applying and executing this regulation are provided by SMC Regulation 540-15 (Systems Engineering Policy). Any program director who does not apply and execute this regulation, as required by SMCR 540-15, shall immediately request a waiver from the Commander.
Edward P. Barry, Jr.
Lieutenant General, USAF