Rudimentary Treatise on the Construction of Locks

by A. C. Hobbs; George Dodd

fig. 17. Simple tumbler lock.

One of the most elementary forms of tumbler-lock is shewn in [fig. 17]. In this case the bolt, instead of having two notches in the bottom edge, like those in the back-spring lock, [fig. 6], has two square notches or slots in the upper edge; and as the key acts upon the bolt, these notches must of course share in whatever movements the bolt is subjected to. Behind the bolt is a kind of latch or tumbler (the lower part of which is shewn by dotted lines), with a stump or projecting piece of metal at a; the tumbler moves freely on a pivot at the other end, and is made to rise through a small arc whenever the key acts upon the bolt. When the bolt is wholly shot, the stump falls into one notch and prevents the motion of the bolt; when wholly unshot or withdrawn, the stump falls into the other notch, and equally prevents the motion of the bolt. It is not, therefore, until the key, by elevating the tumbler, has raised the stump out of the notch, that the bolt has freedom of movement. If the shape of the key does not enable its web to effect this elevation to a sufficient degree, the bolt remains immovable; and to this extent a certain additional security is obtained by making the shape of the key significant as well as the wards.

fig. 18.

fig. 19. Old French lock.

The tumbler-principle, as we have said, is difficult to trace to its origin on account of the various aspects which it presents; but the great French treatise proves that the locksmiths of France were familiar with tumbler-locks a century ago. The plates of that work represent the details of numerous locks, on the upper edge of the bolts of which were notches called encoches, as at o k [fig. 18]; into these notches sank a small iron stud or stump called the arrêt du pêne, or bolt-stop, shewn in [fig. 19], attached to the upper portion of the gâchette or tumbler, which, for the sake of economy of metal, is made in the form of a triangular spring in front of the bolt k i; and not until the key, by its circular action, had raised this stud out of one or other of the notches, could the bolt move to the right or left. The stud was generally fixed to a spring which forced it down again into the notch as soon as the action of the key had ceased. Sometimes, however, the stud was fixed to the bolt, and the notches were in a separate tumbler or gâchette (see E E, [fig. 21]); and in other instances, again, the stump was fixed to the case of the lock and caught into notches in the bolt. It will be seen, when we come to treat of tumbler-locks of later date, that there was much in these early locks to point out the way. [Fig. 19], copied from the French work, represents a lock of the box or casket kind. Two staples, fixed into the cover, fall into two cavities or receptacles at C d; and a short bolt in each receptacle catches into each staple, one near g and one near h. The small bolt q is attached to the upper extremity of the lever q r s, [fig. 19], and shewn separately in [fig. 20]; and by the pressure of a spring a ([fig. 19]) upon this lever, the bolt q is kept locked in the staple. The vertical portion of this spring presses at its lower end on another spring p ([fig. 19]) of singular curvature; and attached to the horizontal part of this second spring is the stud, which falls into a notch in the top of the bolt. The action of these parts, then, is as follows: when the key is placed upon the key-pin at Z, and turned round in the direction in which the hands of a watch move, the bitt presses against the tail s of the lever, moves it upon its centre Z, [fig. 19], v, [fig. 20], to the left, and consequently moves the upper part q to the right, drawing it out of the receptacle and liberating the staple within C. Thus it will be seen that the lever q r s, held in one position by the spring a, forms in itself a simple kind of spring catch-lock, and was, in fact, formerly used as such, without any other appendages except the staple in the lever, into which the catch q fitted on shutting down the lid. So also we may regard the other portion, [fig. 18], or k i p h ([fig. 19]), as forming a separate lock; for the key after having passed S comes in contact with the triangular spring, which it raises thereby, lifting the stud out of the bolt, and exerting pressure against the barbs of the bolt n. [Fig. 18] shoots the bolt k, and also the short bolt l, which passes through the staple in the cavity d, [fig. 19].

fig. 20.