Ratchets and pawls are mechanical assemblies that are used to transmit intermittent rotary motion, or to permit a shaft to rotate in one direction but not the other. Ratchets and pawls are usually made of steel, stainless steel, cast iron, brass, or other metal materials. Ratchets are sometimes called ratchet wheels because they consist of a rotating gear or rack with angled teeth. Pawls, which are sometimes misidentified as ratchets, are thin protrusions that rest against a ratchet to restrict its motion. When the ratchet is rotated in one direction, the pawl is raised and moves smoothly between the angled teeth. When the ratchet’s rotation stops, the pawl rests between the teeth and makes a clicking noise. Ratchets and pawl that permit rotation in only one direction cause the pawl and teeth to clash if the ratchet is turned the opposite way. Applications for these single-direction devices include turnstiles, spanners, winders and jacks. The action of a ratchet can be either harsh or smooth, depending on the configuration of the ratchet teeth and pawl.
Product specifications for ratchets and pawls include the number of teeth, outside diameter, bore diameter, face width, and pitch. The number of teeth is usually an even number. The outside diameter for English unit ratchets is usually measured in fractional increments such as .500 in, .625 in, or 3.125 in. The outside diameter for metric unit ratchets is usually measured in increments of 10 mm (e.g., 40 mm, 60 mm, and 100 mm). Bore diameter is the size of the hole through the center of the ratchet. Face width measures the part of the ratchet that includes the teeth but excludes the hub, a cylindrical protrusion of the ratchet that is concentric with the outside diameter. Ratchets and pawls differ in terms of pitch, an amount that represents the number of teeth per unit of measure. Suppliers who measure pitch in English units provide the diametrical pitch, an amount calculated by dividing the number of teeth by the outside diameter or the ratchet. Suppliers who measure pitch in metric units provide the circular pitch, an amount calculated by dividing the outside diameter of the ratchet by the number of teeth and multiplying by pie. English unit ratchets and pawls usually have 16, 24, 32, or 48 teeth per inch. Most metric unit ratchets and pawls span 2.09, 3.14, 4.71, 6.28, 9.42, and 12.56 mm per tooth.