Ratchet gears only turn in one direction and are useful for tasks that require discrete points. They consist of a gear, pawl, and shaft, and allow for a small amount of rearward motion. The most common manual ratchet is in a socket wrench, which has a switch to change the direction of the ratchet.
A ratchet gear is a specialized type of gear that can only turn in one direction. Most ratchet gears have round teeth on one side and flat on the other. The section of the ratchet that interacts with the gear, called the pawl, will slide on the rounded side, but will catch on the flat side. This one-way rotation makes ratchet gears well suited for tasks that require discrete points, such as clicking a dial. Unlike many one-way gear systems, ratchets allow for a small amount of rearward motion; this makes them better for non-powered applications where slippage will cause no damage.
Ratchets are actually very simple mechanisms. They typically consist of three parts: the ratchet gear, pawl, and shaft. The gear is at the end of the shaft and the shaft is fixed or capable of rotation. The pawl sits next to the gear, often with a small spring that forces the pawl tip into the ratchet teeth. As the gear turns, the pawl moves over the rounded portions of the teeth and snaps into place on the flat side, giving the ratchets their unique clicking sound.
Other one-way gear mechanisms, such as a worm gear, are unable to move backwards. This is not entirely true for a ratchet gear. Between clicks, i.e. after the pawl clicks one tooth but before it clicks the next, the ratchet can turn in either direction. This amount of movement is very small, but can 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 manual ratchet is in a socket wrench. These ratchets have a fixed gear and shaft that connect to a socket. The handle and pawl are connected to each other, but separate from the gear. When the user turns the wrench 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 tightening it.
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 set of ratcheting gears. When the switch moves to the other position so it can take out a bolt, the pawl moves to a second gear with the teeth reversed from first. The internal process works the same way; the wrench instead simply unscrews the bolt.
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