The levers or tumblers, the terms being used synonymously, are flat pieces of iron, steel or brass, usually fitted with a spring (Fig. 369), which are so arranged in the lock that the bolt cannot be shot without lifting the levers, which can only be done by a key of the proper size and shape.
There are from one to five levers in an ordinary lock and they are. usually placed one over the other and pivoted over the guide post, as shown in Fig. 367. The bolt post is so placed as to fit in the cut A, Fig. 369, when the bolt is thrown back, and in B when thrown open. The connecting gatings C are cut at different heights so that the levers must be lifted unequally in order to permit the bolt to move. When the key is turned in the lock the cuts in the bit of the key, which are made to match the levers, bear against the bellies D, lifting the levers simultaneously until the gatings are exactly on a line with each other. The key then catches in the notch in the bottom of the bolt, the bolt post passes through the gatings and the levers drop as the key turns, catching behind the bolt post and effectually preventing the bolt from being forced back.
There are many different arrangements of the levers and sometimes more than one set is used, but the general principle is the same in all lever locks. It is obvious from the above description that the more levers a lock has the greater will be the security afforded, and, in fact, the only real security afforded by the common tumbler lock is in the levers. A one-lever lock offers little security, while a three-lever lock offers ten times the security of a one-lever lock.
By transposing the levers and changing the height of the gatings a great many changes can be made, no two of which can be operated by the same key.
The latch is in reality a spring bolt with a beveled face intended to keep the door closed when shut into the jamb, and is operated by the knobs. There are three distinct kinds of latches in common use, the simple spring latch, the anti-friction latch and the stop or front door latch.
Nearly all of the later patterns of locks have an "easy spring" action for the latch bolt, which, although somewhat different in different makes, usually consists of an arrangement of two springs, only one of which is brought into action when the door closes, while both resist the turning of the knob. This permits the latch bolt to be easily pushed back, and at the same time holds the knob firmly.
This is more clearly shown by Fig. 370. When the latch bolt is thrown back by striking the plate on the door jamb it is resisted only by the light spiral spring around the shank of the bolt, but when the hub is turned it moves forward the carriage C, and also the plate D on the end of the latch, thus bringing into play the stronger spring in the carriage and also the lighter one on the bolt.
In Fig. 371 (which represents the knob action of the Yale "Vulcan " locks) the spring A alone opposes the latch when pushed back, but when drawn back by the hub both of the springs A and B are acted on.
In the Sargent lock (Fig. 367) but one spring is used, the "easy" action being obtained by means of the long lever A, which offers but slight resistance to the latch bolt, while the turning of the hub, which draws back the carriage C, is directly resisted by the strong spiral spring. Most lever locks are now made with reversible latches, the latch shank being of such shape that it may be turned over so as to be used for either a right or a left-hand door,
Anti-Friction Strike. - The ordinary form of latch is made with a V-shaped bevel, the long side of the bevel striking against a plate on the door jamb. If the spring on the latch is at all stiff it requires considerable force to push the latch back, besides causing much wear on the bevel of the latch. To overcome this the anti-friction strike was invented. Fig. 372 shows a form of anti-friction strike used by several manufacturers. The strike is about 3/16 of an inch thick and placed at the bottom of the latch (in some makes it is in the middle of the latch). The strike is pivoted as shown and a peg on the strike works in a slot in the latch, which carries it back without friction on the sides. The anti-friction strike is not required where there is an easy spring action, although it is a desirable feature on front door locks and those for heavy doors.
Fig. 370. - Lock and Latch.
Fig. 371.Latch of Yale "Vulcan" Lock.
The hub is a solid piece of metal (bronze in the better grade of locks) which receives the spindle and turns with it. Two arms or cams are usually cast on the hub, which draw back the carriage as the knob is turned. The hub of the Yale " Vulcan " locks is made of two pieces of forged steel, accurately fitting the flanged bearings of the case. This hub has an oblong opening for the spindle, as shown in Fig. 371, with the larger dimension horizontal, to allow for shrinking or swelling in. the door, which is often the cause of the binding of knobs and spindles of the ordinary locks.
Key. - The general shape of the key for ordinary tumbler locks is that shown in Fig. 373, the best keys being made of steel and nickel plated. The portion of key marked A is called the bow, B is the shank and C the bit. The notches on the edge of the bit at E are made to fit the levers, while notches at F show that the keyhole is protected by wards. For locks with a projection on the edge of the keyhole, the key has a groove in one side of the bit to fit the projection, as shown in Fig. 370.