Ratchets are mechanisms that serve to limit either rotary or linear motion to only one direction. Ratchet designs vary but the basic composition of parts is fairly universal. A ratchet is composed of three main parts: a round gear, a pawl, and a base.
There may be additional and distinct parts that make up a particular ratchet device. A force is often applied to the pawl in order to maintain contact with the gear. Springs or a lever system are usually used to accomplish this. Lever systems involve turning the pawl into a first order lever (see our page on levers for more on the topic). In this capacity the pawl rotates into a position where it engages the gear. This type of design can be used to create a mechanism where the direction of the restricted motion can be changed.
The geometry of the gear or rack is usually designed with a ramp feature on one side of the tooth leading to a sharp drop off which restricts motion of the pawl when the linear or rotational direction is reversed. Most ratchet mechanisms are not very large as only a small vertical wall is needed to prevent motion in one direction. However, in applications that feature substantial forces, consideration needs to be given to material selection, thickness, and overall design in order to sufficiently support those forces.
Ratchets composed from gears are typically round and are composed of uniform but asymmetric teeth designed to limit motion to a single direction. The edges on one side of the gear's teeth have a steep slope (oftentimes nearly perpendicular to the tangent of the gear's circumferance) while the other edges of the gear's teeth have a moderate or gradual slope. Linear Rack: Some ratchet designs utilize a linear rack in place of a round gear. The tooth design on a linear rack is exactly the same as it is with a round gear.
The pawl is the part that makes contact with the gear or linear rack. When the gear (linear rack) is rotated (linearly moved) in one direction (counter-clockwise in the image above), the pawl will slide over the teeth without restricting the natural motion of the device. When the direction of motion is reversed the pawl will come into contact with the steep slope on the gear tooth and will impede motion.
Gears or Linear Racks and Pawls are typically mounted in a fixed relationship to one another on a mount.
Another common ratchet mechanism is the turnstile. The turnstile allows for rotation in one direction, but locks in the reverse direction in order to allow for the one-way flow of human traffic in places like the subway.
Possibly the most common application of a ratchet mechanism is the zip tie. The design of the ratchet mechanism allows for the zip tie to be tightened, but locks when a force is applied in an attempt to loosen the tie.
A set of pawls can be used to rotate the gear one or more teeth at a time, while also preventing movement in the other direction. One application of this can be seen in the tightening mechanism of ratchet straps . As the handle of the ratchet strap is lifted, the gear is advanced one or more teeth.
Many tools feature a ratchet mechanism that allows for fasteners or threaded components to be tightened or loosened without the need for continuous rotation or resetting the position of a tool. The socket wrench is a common example.