A ratchet is a device which is designed to only allow movement in one direction. The design of a ratchet is extremely simple, and ratchets can be found in use in a wide variety of applications, from clocks to socket wrenches. Many hardware stores carry tools which utilize a ratchet mechanism, and ratchets can also be seen on surgical instruments and in a number of mechanical moving parts in everything from cars to watches.
The design of a ratchet includes a toothed wheel with teeth which are angled. A finger, known as a pawl, articulates with the teeth. The wheel can move freely in one direction, but the angles prevent the wheel's motion from reversing. A small amount of play in the reverse direction is allowed by the distance between the teeth in the wheel; precision ratchets are sometimes designed to limit this motion. Ratchets can be made from metal, plastic, wood, and composite materials, depending on what they are being used for and the cost of the item.
One way in which a ratchet can be used is to lock something in place. The ratchet can be moved to the desired position, and then it will hold itself in place with friction. This technique is often used with surgical clamps. To open the clamp to remove it, the pawl is disengaged from the wheel. Another example of this kind of ratchet can be seen in some styles of blind, in which the pawl locks the blind in place at a desired height.
In tools such as socket wrenches, ratchets can be tremendously useful. In many tasks, it is not possible to complete a full revolution of a wrench. This would require the user to make a partial revolution, remove the wrench, move it back into position, make another partial revolution, and so forth. With a ratchet, the partial revolution can be completed before the pawl is disengaged to slide the handle of the wrench back into position without moving the bolt backward. This makes tasks easier, and reduces the risk of stripping or damaging bolts.
The socket wrench is probably one of the most well known examples of this device. Socket wrenches usually come in a set, allowing people to fit different attachments into the head of the ratchet to deal with bolts of different sizes and shapes. Ratchet sets can include heads meant to match with a variety of bolts and screws, and they are a tremendously useful thing to keep around the workshop or tool box.