Locking and Arresting Mechanisms
A ratchet mechanism is used widely as a means of arresting a motion and producing intermittent action of a force so that it develops its action at particular instants. The ratchet bar is provided with teeth with which the pawl B engages. The pawl, which is controlled by a spring, is pivotally mounted in a frame C in which the ratchet bars is also mounted in a sideway.
The pawl is thus able to arrest the motion of the ratchet bar when the latter is thrust to the right and can, for e.g.: periodically releases it, so that the bar moves in successive jerks. In the grip-roller locking device the bar A can move in relation to the cage C. When A moves to the right, the roller (or ball) jams and thus locks the two parts immovably together. When a moves to the left, the roller is released.
In the friction brake, the counterclockwise rotation of the wheel causes friction to build up as a result of the thrust exerted by the brake block on its pivoted angle lever, so that the wheel is broken; no braking action is developed when the wheel rotates in the clockwise direction. An ordinary lock is also essentially a device of this general type. In the catch lock the sliding element A is slid forward in the guide C and is locked by means of the catch B. The latter is released when it is lifted by the key, so that a can then be slid back.
The grip brake that acts as a safety device for elevators in the event of cable fracture is a locking mechanism of this class. When the rope exerts an upward pull, the gripping jaws are released, so that the grip brake as a whole can be moved in relation to the fixed guide rod. If the cable breaks, the powerful spring will push the slide down, causing the pivoted angle lever to thrust the brake blocks with considerable force against the rod.
A ratchet-wheel mechanism is used to produce intermittent motion, as already stated. The mechanism is equipped with two pawls. The upper pawl is operated by an eccentric drive, and its successive thrusts cause the ratchet wheel to rotate counterclockwise in an intermittent motion. The lower pawl prevents clockwise rotation of the ratchet wheel while the driving pawl is performing its return motion.
The star wheel mechanism operates on the same principle. The large driving wheel drives the small driven wheel only as long as the drive pins on the former engage with the teeth of the latter. As there are eight pins and eight teeth, the small wheel will always perform one complete revolution as long as there is engagement. A mechanism of this type is used in most counting devices: e.g., the mileage counter in a motor vehicle.
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