the fixed sleeve 7 in circumferentially equally spaced relation to respectively receive therein lock balls 11 having a diameter larger than the wall thickness of the fixed sleeve 7 annular lock recesses 12 with which these lock balls 11 may engage are formed in the outer peripheral surface of the outer cylinder 3b pressing protrusions 13 are provided by which said lock balls 11 may be simultaneously pressed inwardly that is towards the lock recesses 12 which extend from the inner peripheral surface of the lock ring 9 thus the pressing protrusions 13 press the lock balls 11 when the lock ring 9 is turned to its locked position l as shown and release the lock balls 11 when turned to its unlocked position u accordingly the fixed sleeve 7 which retains the lock balls 11 the lock ring 9 provided with the pressing protrusions 13 and the lock recesses 12 in the spring retaining cylinder 3 constitute a lock device which is a central feature of the present invention the lock ring 9 is connected to a turning device 14 which is capable of turning the ring to the aforementioned locked position l and the unlocked position u the turning device 14 comprises a turning spring 15 retained between a lug 9a disposed on one side of the lock ring 9 a.