Ratchets

What Is A Ratchet:

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. 

What Is A Ratchet Made Of:

Figure 1. "Ratchet Drawing"

A ratchet is composed of three main parts: a round gear (or a linear rack), a pawl (also called a "click"), and a base (or mount). 

1. Gear: 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 circumference) 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.
2. Pawl ("Click"): 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. 
3. Mount: Gears or Linear Racks and Pawls are typically mounted in a fixed relationship to one another on a mount.

There may be additional and distinct parts that make up a particular ratchet device. In both types of ratchet describe above, 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.

How Do Ratchets Work:

1        . Ratchet (Gear):

2 . Ratchet (Linear Rack):

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.

What Are Some Common Examples Of Ratchets:

Figure 2. General Gear Ratchet Mechanism

Ratchets can serve as a useful mechanism in many different applications.

• Socket Wrench: 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. 

Figure 3. Ratchet Example: Socket Wrench

• Turnstile: 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. 

Figure 4. Ratchet Example: Turnstile

• Zip Tie: 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.

Figure 5. Ratchet Example: Zip Tie

• Ratchet Straps: 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. When the handle is pushed back to the starting position, one pawl prevents the strap from unwinding.

Figure 6. Ratchet Example: Ratchet Straps

What Are The Limitations To Ratchets:

Ratchets can only stop backward motion at discrete locations (i.e. at each successive gear). As a consequence ratchets can allow a limited amount of backward motion called backlash or "play." Backlash is the maximum amount of distance or the largest angle of rotation that can be lost in one direction (typically the direction opposite that in which the device is designed to move freely) prior to a resistive force being applied to the next part (tooth) in the mechanical sequence. If a design requires one to minimize play in the device, it is possible to make a toothless ratchet utilizing two high friction surfaces (e.g. metal and rubber). Since backlash is principally a function of the surface's compressibility, something like a rubber surface can significantly reduce losses due to backlash.