the handwheel continuously in one direction, and to close it again the motion of the handwheel is simply reversed; either closing or opening the breech by hand occupies about 6 seconds. Supposing the breech closed, the handwheel when rotated gives motion to the link G through the worm wheel S and crank F. By this means the tooth B is moved from its extreme left position to the right, and so disengages the breech screw A from the threads in the gun; the rack A2 on the breech screw then comes into gear with the pinion E and draws the breech screw out of the gun into the carrier ring C, which finally swings on the axis pin and clears the breech opening. While the opening is being performed the firing lock L is operated by the cam groove A3; this puts the firing mechanism, either electric or percussion, to safety by withdrawing the firing needle, extracts the fired tube and leaves the primer chamber open for a fresh primer. All these operations are performed in the reverse order on closing.
Fig. 34.—Hotchkiss Q.F. Breech Mechanism.
With both these types of mechanism the de Bange system of obturation, with the pad only slightly coned, is used.
Fig. 35.—Nordenfeldt Q.F. Breech Mechanism.
With smaller guns the mechanism is simpler, as less power is required for opening the breech. Thus, with the 6-in. B.L. gun Mark IV., introduced about 1885 (fig. 42) the breech is opened in three separate operations—(a) the cam lever, which also locks the breech, is raised into the vertical position and pulled over to the left; this disengages the screw threads; (b) the cam lever is folded down so that the cam acting on the rear face of the gun releases the de Bange obturator, and the screw is then pulled by hand through the carrier ring out of the breech; (c) the carrier ring and breech screw are revolved together to the right, clear of the breech opening.
Fig. 36.—Eccentric Screw, Breech Mechanism.
In a modern 6-in. gun fitted with de Bange obturator all these operations are combined and the mechanism (fig. 43) worked by a horizontal hand lever which is moved from left to right through an angle of about 200°. The hand lever A moves a link B connected to a pin C on the breech screw D and disengages the screw from the gun; a small lateral movement is then given to the axis pin of the carrier so as to allow the obturator pad E to swing out of its seating; when this is quite free, the whole mechanism revolves on the axis pin and thus clears the breech opening. The firing lock F is actuated at the same time and ejects the fired tube G. A new tube is inserted while the gun is being loaded, so that immediately the breech is closed the charge can be fired without loss of time. In the old mechanisms the breech had to be closed first, and the firing tube inserted after.
