fig. 35. Exterior of a Bramah lock.
fig. 36. Details of the Bramah lock.
This is the principle which Mr. Bramah adopted; and we have now to trace it, step by step, by means of illustrative details. [Fig. 35] represents the exterior of a box or desk lock, one among many varieties which the Bramah lock presents. A A shews the bolt, formed something like two hooks rising out of a bar of metal, which bar has a backward and forward motion upon the plate B B. The upper edge of this plate is turned over at right angles, forming a small horizontal surface through which two openings are cut to receive the two hooked portions of the bolt. The movements of the bolt are otherwise guided by the edges of square holes through which it works; the holes being made in the edge-pieces of the lock, riveted to the main plate. The bolt is further prevented from rising out of its place by means of a plate of metal C, which is secured to the edge-pieces by two screws 1, 1, and by two steadying pieces. This plate has on its surface a cylindrical projection D, which contains in effect all the working mechanism of the lock. The pins 4 4 are employed for securing a plate, which we shall have to describe presently. When such a lock is fixed upon a desk or box, the portion D projects to a small distance through a hole in the wood-work, forming in itself a very neat escutcheon, with a key-hole in the centre.
fig. 37. The slides.
So much for the exterior. We must now proceed to examine the interior of the lock, especially the part contained within the cylinder. In [fig. 36], for convenience of arrangement, the several parts are exhibited separately, and as if the plane of the lock were horizontal, with the key acting vertically. The essential part of the mechanism is a barrel or cylinder E, pierced or bored with a cylindrical hole down its centre. The inside of the bore has six narrow grooves, cut parallel with the axis, and in the direction of radii; the grooves are not cut through the thickness of the cylinder, but leave sufficient substance of metal for strength. In every groove is fitted a steel slide of peculiar form, such as is shewn at a´ a´ in [fig. 37]. Each slide is split in its thickness (seen in section), so that it may move up and down in its groove with a slight friction, and thereby not fall simply by its own weight. Each slide has three small notches (3, 2, 3´), the use of which will presently appear. Reverting to [fig. 36], the lower part of the opening through the cylinder E is closed by a circular plate of metal, fixed to it by two screws; this plate is represented at F, in the lower part of the figure. This plate has a vertical pin rising from its centre (also seen at b, [fig. 39]), and serving as a key-pin on which the pipe of the key may work or slide; and it has also a short circular stud c projecting from its under side, and fitted to enter into a curved opening in the bolt presently to be described.
The point to be now borne in mind is this, that if the cylinder E turns round, the plate F will also turn round, and with it the stud c; and as this stud works into the peculiarly formed cavity d in a portion of the bolt ([fig. 38]), it causes the bolt to be shot backwards or forwards. Now, in order to prevent this rotating of the cylinder unless the proper key be employed, the following mechanism is introduced: the cylinder has a groove cut round its circumference at e e, extending sufficiently near to the internal bore to produce the desired effect without too much weakening the metal. Into this notch is introduced the thin circular plate of metal f f, it being divided into two halves for this purpose; and when so placed, it occupies the position shewn by the dotted portion e e. When this plate is screwed to the case of the lock by the screws 4, 4, it cannot of course turn round; but the cylinder itself will or will not turn round according to the position of the slides. The plate f f has six notches, 5, 5, 5, &c. in the inner edge or circle; so adjusted that, when the plate is in its place, the slides a a can move up and down. The cylinder cannot move round in a circle without carrying the slides with it; and these cannot so move unless they are all depressed to such exact distances in their respective grooves, that the deep notch of each slider (shewn at 2 in [fig. 37]) shall come into the plane of the circular plate: when all are so brought, the cylinder can be turned. If any one of the slides be pressed down either too low or not low enough, this turning of the cylinder cannot be effected, because the slides will be intersected by the edges of the notches 5, 5; and it is the office of the key, therefore, to press all the six slides down to the exact distances required. When the slides are not pressed upon by the key, they are forced upwards to the top of the cylinder by a spiral spring 6, coiled loosely round the pin b; this pressure forces up a small collet, 7, on which the upper part of the slides rest by a sort of step.
