Rudimentary Treatise on the Construction of Locks

by A. C. Hobbs; George Dodd

We now come to Chubb’s lock, patented 3d February, 1818, which consisted of double-acting tumblers and a peculiar kind of detector. This lock has been made the subject of various patents obtained in the years 1824, 1833, 1846, and 1847. This lock[4] consists of six separate and distinct double-acting tumblers, all of which must be raised to a particular height, neither more nor less, in order that the bolt may pass. It also comprises a detector, by which, should any one of the tumblers be lifted too high in an attempt to pick or open the lock by a false key, it would be immediately detected on the next application of the proper key. The tumblers are flat pieces of iron or steel, with the plane of the surface vertical, and pivoted at one end; and the following is the mode in which the key, the tumblers, and the bolt, are brought into mutual action.

[4] The lock about to be described is the latest and most complete form of Chubb lock up to the date of the Great Exhibition. The various additions and alterations which have been made in the lock since that date will be noticed in a subsequent chapter.

The bolt shoots in and out of the lock in the usual way. It has a square stud or stump riveted on one surface; and it is to furnish obstructions to the passage of this stud that the tumblers are provided. All the six tumblers are pivoted to one pin at the end, giving to each of them a small leverage, each independent of the others. There are six springs which press these tumblers downwards, one to each tumbler. There is a longitudinal slot or gating in each tumbler, large enough to receive the stud of the bolt; and unless all the six slots (supposing there to be six tumblers) coincide in height or position, the stud will not have a clear passage for moving to and fro. Now the slots are purposely made nearer the upper edge in some of the tumblers than in others, all the six being different in this respect; so that if they are all lifted equally, the slots do not coincide, and the bolt and its stud will not pass. The tumblers must then be raised unequally, those to be most raised which have the slot nearest to the lower edge. To effect this, the bit of the key is cut into six steps or inequalities, each to act upon one particular tumbler, and each cut or stepped to the exact depth which will suffice for the proper raising of the tumbler. The key is inserted in the keyhole, and is turned; the six steps raise the six tumblers all to the proper height, to leave a clear passage along the slots; and the extreme end of the key then acts upon the bolt itself, and shoots it. To unlock it again, the same or a duplicate key must be used; for if another key be employed, differing by ever so little from the proper one, some one or more of the tumblers will be lifted either a little too much or not quite enough; and in either case the stud of the bolt will catch above or below the slot, instead of having a clear line of movement along the slot itself. After both locking and unlocking, the springs force the tumblers down as far as they can go, burying the stud in the recesses above the slot; so that the tumblers must be raised by the key both for locking and unlocking.

The doctrine of chances has wide play in determining the relative position of the six tumblers. In Mr. Chubb’s essay this part of the subject is treated in the following way: “The number of changes which may be effected on the keys of a three-inch drawer-lock is 1 × 2 × 3 × 4 × 5 × 6 = 720, the number of different combinations which may be made on the six steps of unequal lengths (on a six-tumbler lock), without altering the length of either step. The height of the shortest step is, however, capable of being reduced 20 times; and each time of being reduced, the 720 combinations may be repeated; therefore 720 × 20 = 14,400 changes. The same process, after reducing the shortest step as much as possible, may be gone through with each of the other five steps; therefore 14,400 × 6 = 86,400, which is the number of changes that can be produced on the six steps. If, however, the seventh step, which throws the bolt, be taken into account, the reduction of it only ten times would give 86,400 × 10 = 864,000, as the number of changes on locks with the keys all of one size (that is, with one key of definite size in all save the lengths of the steps). Moreover, the drill pins of the locks and the pipes of the keys may be easily made of three different sizes; and the number of changes will then be 864,000 × 3 = 2,592,000, as the whole series of changes which may be gone through with this key. In smaller keys, the steps of which are capable of being reduced only ten times, and the bolt-step only five times, the number of combinations will be 720 × 10 × 6 × 5 × 3 = 648,000. On the other hand, in larger keys, the steps of which can be reduced thirty times, and the bolt-step twenty times, the total number of combinations will be 720 × 30 × 6 × 20 × 3 = 7,776,000.”

These enormous numbers have been the cause of much of the wonderment which the six-tumbler locks have excited; and, as we shall see further on, the Bramah lock presents still more of the marvellous in respect to this ringing of the changes.

fig. 31. Chubb lock, with detector and six tumblers.

The construction and action of the Chubb lock may be further illustrated by means of an engraving, [fig. 31], in which b is the bolt of the lock, with a stump riveted to it marked s. The six tumblers are shewn perspectively, the front or anterior one being marked t; they all move on the centre-pin a, but are nevertheless perfectly distinct and separate, to allow of being elevated to different heights. At d is shewn one end of a divided spring, the divisions being equal to the number of tumblers, one to each, and so bent that each spring may press upon its particular tumbler. At e is the detector-spring, so placed that a projecting piece in the hindmost tumbler shall be near it; this tumbler having also fixed into it a stud or pin p. This being the arrangement, especially in relation to the stump s and the tumblers, it follows that all the tumblers must be lifted to exact and regulated heights in order that the stump may pass through the longitudinal slits of the tumblers; unless it can do so, the bolt cannot be withdrawn. As there are gaps or notches in each tumbler both above and below the proper line of passage, and as there are no ordinary means of ascertaining when any one tumbler is lifted too high or not high enough, the safety of the lock is greatly increased by this uncertainty; especially when it is considered that this uncertainty is multiplied sixfold by the different modes in which the six tumblers are slotted. If, through the insertion of a false key, or by any other cause, any one of the tumblers be raised above its proper position, the detector spring e will catch the hindmost tumbler, and retain it so as to prevent the bolt from passing; and thus, upon the next application of the true key, it will be instantly felt that some one of the tumblers has been overlifted, because the true key will not unlock it. To relieve the bolt from this temporary imprisonment, the key must be turned the reverse way, as for locking; all the tumblers will thus be brought to their proper position, and allow the stump to enter the notches n n´; the bevelled part of the bolt will then lift up the detector-spring, and allow the hindmost tumbler to fall down into its proper place; and all this being effected, the lock may be opened and shut in the ordinary way. The pin p is so adjusted that if any one of the tumblers—front, back, or intermediate—be lifted too high, the pin will be lifted with it, and will catch into the detector-spring, thus producing the result just described.

fig. 32.
Key to Chubb’s lock.