Are worm gear reducers "self-locking"?

Are worm gear reducers "self-locking"? The answer is: yes.
It is said that a worm gear set to be self-locking or irreversible when the gear can not drive the worm. And such condition is obtained, if the lead angle of the worm is less than the friction angle, and as a result the efficiency for reversed driving is zero. The friction angle for static conditions will vary with such factors as surface finish and lubrication. Based upon the generally accepted value of static coefficient of friction equal to 0.15, the friction angle would be approximately 8°. However, the friction angle of worm gear reducer decreases rapidly with the start of motion, moreover, vibrations from nearby sources quite often upset the static condition of a locked set of gearing a sufficient amount to reduce the friction angle to a point where motion occurs. Sometimes, it called "stiction". These unpredictable factors make it advisable to resort to a brake rather than based on the self-locking characteristics of the worm gearing.