From DT Online
A Ratchet mechanism allows movement in one direction only.
Features and Applications
In the illustration, the Ratchet Wheel lifts the Pawl as it rotates anti-clockwise but clockwise rotation is stopped by it. Such a system is used in Winches to allow loads to be lifted up without slipping down again. The Ratchet Wheel could be fixed to the Winch shaft via a Key as shown. Alternatively, use of a square or hexagonal winch shaft would ensure it rotates with the Ratchet.
A similar idea can be used to create a Linear Ratchet which allows motion in one direction but not the other. A Pawl type mechanism allows each squeeze of a trigger to tighten a Bar Clamp for example. They are found also as Quick Release Mechanisms on some Wood VIces and are the basis of the humble Cable Tie,
|Ratchet Wheel - ratchet wheel can rotate only anticlockwise because reverse rotation is prevented by flat spring.
|Ratchet Mechanism being used to convert the continuous rotary motion of a drive shaft into the intermittent rotary motion of a driven shaft such that by flicking over the blue pawl the motion direction of the driven shaft can be changed without changing the input motion direction
|Pendulum Clock Escapement - the gravity pendulum performs a harmonic angular oscillation and the ratchet wheel tends to rotate clockwise due to the weight which is suspended by a cord wound around its shaft. The anchor rocks with the pendulum and allows the wheel to rotate only two teeth during each oscillation - the characteristic ‘Tick-tock’ sound is caused when the anchor collides the wheel teeth. The mechanism is used in pendulum clocks where the wheel motion is transmitted to hands through a gear train to show time. Note that the weight driven wheel also transfers energy to the pendulum (timekeeper) to replace the energy lost to friction during its cycle and keeps the pendulum oscillating.
|Bicycle Free-Wheel. - sprocket receives motion from the pedaling bicyclist but the hub rotates only when the sprocket rotates clockwise and clockwise rotation of the hub has no influence to the sprocket. The pawl is always pressed toward the sprocket's teeth by a spring. In practice two pawls are used.