For a 0.25-28 UNJF thread fastener, the prevailing torque locking standards [6-11] list an allowable  3.5 in-lb (0.40 N-m) minimum and 30 in-lb (3.4 N-m) maximum prevailing torque. Since the self-loosening  moment for the thread is 13.6 in-lb (1.54 N-m), meeting the minimum 3.5 in-lb (0.40 N-m) is not enough to  counter the self-loosening moment.  If a prevailing torque locking feature is used with a measured prevailing torque of 20 in-lb (2.3 N-m), then  the locking moment is 20 in-lb (2.3 N-m) which is in excess the self-loosening moment.  Data [12] exists from dynamic transverse load tests with NAS1004 0.25-28 UNJF threaded fasteners.  Minimum and maximum pitch diameter are 0.2243 (5.697 mm) and 0.2268 inch (5.761 mm) for an  average of 0.2256 inch (5.730 mm). The dynamic transverse force is 200 lb (890 N) and the preload is  2400 lb (10.7 kN). Therefore, the loosening moment from this external transverse load is shown in Eq. 11.  (200)(0.1128) 22.6ext l front t backM F r F r−= = = =  in-lb (2.55 N-m) (11)  This 200-lb (890-N) external transverse force is the amount that acts on the fastener. The dynamic  transverse test machine [12] used is designed with bearings between the clamped components of the  joint. This intentionally puts the applied external transverse force on the fastener which makes the test  severe. In practice with a typical joint, additional friction between the clamped components of the joint  would counter the external transverse load. This joint friction force would need to be subtracted from the  external load to determine the transverse force on the fastener that is used in the previous equation. An  estimate of this joint friction force is the product of clamping force and coefficient of friction at the joint  interface.    At a preload of 2400 lb (10.7 kN), the test fasteners were subjected to a dynamic external transverse load  of 200 lb (890 N), and a total loosening moment shown in Eq. 12.  13.6 22.6 36.2l self l ext lM M M− − = + = + = in-lb (4.09 N-m) (12)  A set of tests [13] for a variety of secondary locking features provide measured locking moment data for  0.25-28 UNJF (similar to M6) thread fasteners. Specifically, the following locking moment data was  obtained: 16-23 in-lb (1.8-2.6 N-m) for metal and nonmetal prevailing torque locknuts, 15-18 in-lb (1.7-2.0  N-m) for medium strength Loctite, 55-65 in-lb (6.2-7.3 N-m) for high strength Loctite, 50-60 in-lb (5.6-6.8 N-m) for cotter pins with castle nuts, and 36-42 in-lb (4.0-4.7 N-m) for 0.032-inch (0.813-mm) Inconel safety lock wire through bolt heads. This data indicates only high-strength Loctite, cotter pins, and safety lock wire provide sufficient locking to overcome the loosening moment due to combined self-loosening and the severe external transverse loading. However, in the absence of the external load, all of the locking features tested provide locking moments in excess of the self-loosening moment. Conclusions  This paper provides some guidance for specifying secondary locking features in threaded fasteners to  provide sufficient locking and prevent loosening.  A mechanism for loosening of threaded fasteners is provided and defined in terms of a self-loosening  moment inherent to threaded fasteners and an external load loosening moment.  Equations for loosening moments are defined. Secondary locking feature moments are quantified in  terms of these loosening moments.  An example is provided. Some sample data for measured locking moments is given for one thread size.  NASA/CP—2018-219887 220