1. Locking and Arresting Mechanisms
Author: John Smith
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.
2. 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.