Rudimentary Treatise on the Construction of Locks

by A. C. Hobbs; George Dodd

fig. 58.

In other locks a spring action of this kind would greatly facilitate the picking, inasmuch as it would afford the gentle uniform pressure desired upon the levers. In other locks, therefore, the bolt is caused to move, and the stump to enter the gatings, by the direct contact of the key with the bolt, instead of by a spring; but as the key, while moving the stump into the gatings, is also altering its position under the levers, a slight tremulous motion of the levers is thereby occasioned, which no care in manufacture can obviate. This tremulous motion is aggravated by the circumstance that, as the keyhole is open to inspection, it is necessary to make all the levers fit flush with one another when down, in order to avoid affording any clue to the shape of the key from the positions of the levers; but as the various steps of the key, being of different lengths, describe different arcs, the curves of the levers when raised are of necessity in error to them all. The result of these combined faults is that the gatings have to be made wider than the stump, to allow a sufficient amount of play, thus introducing a fatal element of insecurity in the construction of the lock, since the security is of course enhanced in proportion as the gatings fit the stump accurately. In the new lock, on the contrary, the arc T, [fig. 58], in each lever, can be shaped truly to its own proper radius, independent of all the rest of the levers; and as the action of the stump is instantaneous in catching the gatings as soon as they are all brought simultaneously under it, the stump and gatings can be made to fit one another with the most perfect accuracy, and without the slightest play.

On turning the cylinder C further round, the bit N passes from under the levers, which remain held back by the insertion of the stump in the gatings; and just before reaching the position shown in [fig. 59], the slide block R has pushed the bit completely out of the radial slot, and the bit falls down as shown in [fig. 59], and drops through a hole into the inside of the safe that is locked. At this point the back pin of the slide block comes in contact with the lower side of the cam groove O in the stump-bolt, [fig. 54]; and by turning the cylinder C onwards to the position shown in [fig. 60], the withdrawal of the bolt B is completed, bringing the parts into the position shown in [fig. 60]. In these drawings only one lever L is shown; but there are altogether six levers, as shown in the sectional plan, [fig. 56]. The pin P is fixed in the tail D of the main bolt, so as to travel with the bolt; and by this means the springs I are released from strain, as shown in [fig. 60], as soon as the bolt is withdrawn.

fig. 59.

From the nicety with which the various parts of this lock are constructed, it is evident that the levers must be very accurately lifted by the bit of the key in order to withdraw the bolt; and therefore any error in the bit, such as would occur with a false bit, will effectually prevent the lock from being opened. This may be illustrated by supposing the false bit to be so close an imitation as to have five of its steps absolutely correct, and the sixth only slightly wrong: though it is almost impossible that such a near approach to correctness could be attained in practice. The counterfeit bit being inserted in the lock, and the cylinder turned round, all will go on the same as with the true bit, up to the time when the false bit reaches the point T of the levers, as previously shown with the true bit in [fig. 58]. Here a change of action takes place; but what is the nature of the change the operator has no means as yet of ascertaining. In the case supposed, where five of the steps in the bit are right, but the sixth is wrong, the gating of the sixth lever does not precisely coincide with the others, nor with the stump S; and the consequence is that, at the critical moment when the stump ought to spring into the gatings and hold back the levers from falling forwards, it will be prevented from entering the gatings, owing to the entrance being partly blocked up by the one lever, which stands more or less across it.

fig. 60.

The fact, however, that the stump cannot enter the gatings, does not become known to the operator until the cylinder C has been turned further round, so as to bring the slide-block pin in contact with the lower side of the cam groove O in the stump-bolt; and before this point has been reached the false bit has already passed clear of the levers, which, not being retained by the stump, are instantly thrown forwards again by their springs, and locked in their original position by the stump entering the notches. At the same time the false bit has dropped into the inside of the safe in the same manner as the true bit, as shown in [fig. 61].