A fail-safe locking device for reel support systems is described which is suitable for use with reels such as those used with wire stranders and similar machines. In the embodiments described, a pintle supporting shaft member is normally biased by a spring to an extended position. A safety plunger is provided which has a stepped exterior surface so as to define a recess or step. The safety plunger is normally biased by a spring to a disabling position to urge a blocking member, such as a hardened steel ball or sphere, into the path of movement of an abutment surface of the shaft member to automatically and positively lock the shaft member and the pintle. In the locked condition, the plunger member and, therefore, the pintle, cannot accidentally release the reel. The shaft member and the safety plunger are provided with bearing surfaces against which a fluid medium under pressure may be applied by an operator of the machine to, firstly, move the safety plunger from the disabling to an enabling position to permit the ball or sphere to drop into the recess or notch and, therefore, out of the path of movement of the shaft member abutment surface. Once freed, the plunger member is urged by the fluid medium under pressure to a retracted or releasing position. Two opposing pintle assemblies may be mounted on a cradle typically used in wire stranders, and pneumatic pressure is used by the operator to release the fail-safe locking system to accept or release a reel from the cradle. Electrical means may be used for monitoring the position of the safety plunger and for disabling the machine or issuing an alarm upon movement of the safety plunger to its enabling position and, thereby, permitting the plunger member and pintle to move to a retracted position. The fail-safe locking device is also shown on other reel-supporting systems such as bobbin supporting shafts.