A ratchet gear is a specialized gear type that may only turn in one direction. Most ratchet gears have teeth that are round on one side and flat on the other. The section of the ratchet that interacts with the gear, called a pawl, will slide over the rounded side, but will get caught on the flat side. This one-directional spin makes ratchet gears well-suited for tasks that require discrete points, such as clicks on a dial. Unlike many one-directional gear systems, ratchets do allow a small amount of backward motion; this makes them better for non-powered applications where slippage won’t cause damage.
Ratchets are actually very simple mechanisms. They typically consist of three parts: the ratchet gear, the pawl and the shaft. The gear sits on the end of the shaft, and the shaft is either fixed or is able to spin. The pawl sits next to the gear, often with a small spring forcing the pawl’s point into the ratchet gear’s teeth. As the gear turns, the pawl moves over the rounded parts of the teeth and clicks into place on the flat side, giving ratchets their unique clicking sound.
Other one-directional gear mechanisms, like a screw gear, are incapable of moving backward. This isn’t completely true for a ratchet gear. Between clicks—that is, after the pawl clicks off one tooth, but before it clicks off the next—the ratchet may turn both ways. This amount of motion is very small, but may be significant depending on the purpose of the gear. As a result, ratchets are more common on human-powered tools and machines.
The most common hand-powered ratchet is in a socket wrench. These ratchets have a fixed gear and shaft that connect to a socket. The handle and the pawl are connected to each other, but separate from the gear. As the user turns the wrench in one way, the resistance of the bolt head in the socket causes the ratchet to move freely. When turned the other way, the pawl pushes against the gear, moving the power generated into the bolt, and screws it in.
Most socket wrenches have a switch that changes the direction of the ratchet. While there are several ways to design a socket wrench, the most common method uses a second ratchet gear set. When the switch moves to the other position so it may take a bolt out, the pawl moves to a second gear that has teeth reversed from the first. The internal process works the same way; the wrench just unscrews the bolt instead.