840,374. Locks. LOWE & FLETCHER Ltd. March 27, 1957 [March 28, 1956], No. 9636/56. Class 44. In a lock of the kind in which movement of a bolt 14 between locking and releasing positions is controlled by one or more tumblers 18 which are displaceable by means of a key between locking and releasing positions, in the former of which it or they prevent the bolt from being moved to its releasing position and in the latter of which it or they allow the bolt to be so moved, the tumbler or tumblers is or are positively retained against movement from its or their locking position or positions by a stop member 28 itself displaceable between tumbler retaining and tumbler releasing positions by the key in the latter position the tumbler or tumblers being then displaceable to its or their releasing position or positions and the key being prevented by the stop member from withdrawal from the lock until the stop member regains its tumbler retaining position. As shown the lock is a padlock having a case 10 and a cover plate (not shown) of the general form described in Specification 667,955. A slot 17 is formed in both the lock case and cover plate which is closed by a C-shaped head 16 of the bolt when the bolt is in its locked position. The bolt is guided in its sliding movement by a pin 19 which engages an elongated slot 22 in the tail portion of the bolt; the pin 19 also acts as a pivot for a stack of the tumblers 18 each of which is connected to a leaf spring 20, the springs bearing against an abutment pin 21 so that the tumblers are rotated in a clockwise direction as shown. Each of the tumblers is provided with an open ended slot at the end remote from the pivot pin 19 and when the slots are aligned by a key they receive'a projection 23 on the tail of the bolt and the bolt can then move to its releasing position. When the key is out of the lock the tumblers are locked in the position in which they prevent movement of the bolt by the stop member 28 which is pivotally mounted upon a pin 29. The stop member is urged to a position in which a lug 34 on an arm 32 engages abutment faces 35 of the tumblers by a torsion spring 30. When the key is inserted into the lock initial rotation of the key in an anticlockwise direction first rotates the stop member so that the lug 34 releases the tumblers, it then rotates the tumblers so that their slots are aligned and then engages the bolt and moves it to its releasing position. The stop member is provided with a second arm 36 which prevents complete rotation of the key so that it cannot be aligned with the keyhole by continued rotation and hence it cannot be withdrawn from the lock. The stop member 28 also prevents the key being withdrawn from the lock by reverse rotation until the bolt is in its locking position since the lug 34 on the stop member abuts the tumblers and thus it cannot move back to its initial position; hence reverse rotation of the key engages part of the stop member so that the key cannot become aligned with the keyhole. A guard plate 37 is also provided in the lock; this is mounted on the pivot pin 19 and it is arranged so that one or more of the key bits must pass underneath it when the key is rotated to move the stop member and tumblers; this plate also acts to prevent withdrawal of the key from the lock. Instead of providing a spring 30 for returning the stop member to its tumbler locking position a connection can be provided between the stop member and the tumblers or between the stop member and the bolt. The arm 36 on the stop member can be omitted if desired.