A socket wrench is a type of wrench or spanner that has a socket attached at one end, usually used to turn a fastener.
The most prevalent form is the ratcheting socket wrench, often improperly called a ratchet. A ratcheting socket wrench is the device within a hand tool in which a metal handle is attached to a ratcheting mechanism, which attaches to a socket, which in turn fits onto a type of bolt or nut. Pulled or pushed in one direction, the ratchet loosens or tightens the bolt or nut attached to the socket. Turned the other direction, the ratchet does not turn the socket but allows the ratchet handle to be re-positioned for another turn while staying attached to the bolt or nut. This ratcheting action allows the fastener to be rapidly tightened or loosened in small increments without disconnecting the tool from the fastener. A switch is built into the ratchet head that allows the user to apply the ratcheting action in either direction, as needed, to tighten or loosen a fastener. Other common methods of driving socket wrenches include pneumatic impact wrenches, hydraulic torque wrenches, torque multipliers and breaker bars. Some lesser known hybrid drivers include striking wrench tools with square drive, and hydraulic impact wrenches (typically powered by on site hydraulic power such as present with military tanks, and many rail car applications).
Socket wrenches are most commonly hexagonal, or more commonly referred to as “6-point” in lay terms. Sockets are driven by being attached to the driving tool via a male/female square connection fitting (called the square drive). Standard sizes of square drives around the world include 1/4″, 3/8″, 1/2″, 3/4″, 1″, 1-1/2″, 2-1/2″ and 3-1/2″ square drive sizes (a de facto international standard with no metric equivalents) along with some lesser used drivers such as 5/8″ square drive, and both #4 and #5 spline drives specified in ANSI B107 specifications. This wide range of square drive sizes provides for a wide variety of socket types and sizes to suit small to very large nuts and bolts. Some square drivers have a through hole to attach the socket to the driver (using a retaining ring with o-ring and pin type, or single piece molded retaining rings), a locking pin, or friction ball. Some common hand ratchets employ a quick release button on their top for quick socket release of smaller sockets.The tool chosen to drive the socket wrench ultimately supplies the mechanical advantage needed by the user to provide the torque needed to loosen or tighten the fastener as may be required. Larger drivers are typically used with higher torque, while smaller drivers are used for convenience in smaller low torque applications. Given the limits of human strength and fatigue, torque above 600 ft-lbs of torque will generally involve some kind of power assist, instead of the user simply pushing on the handle of a wrench. Very large sockets and drivers are typically powered by hydraulics to achieve torque.
A socket is typically a cylinder which has a female six– or twelve-point recessed opening sized to fit over the common male hexagonal head of a bolt or nut fastener. The opposite end of the socket wrench has a standardized (ANSI B107, ISO, or other consensus standard) square recess to accept the socket wrench’s drive size. Male drivers are also produced for use with socket head cap screws, and are often called Allen drivers (trademark) or the generic term male bit drivers
The principal advantage of interchangeable sockets is that, instead of a separate wrench for each of the many different fastener sizes and types, only separate sockets are needed for each size and type. Because of their versatility, nearly all screw and bolt types now have sockets of different types made to fit their bolts or nuts. Sockets often come as a “socket set” with many different sizes or types of sockets to fit the heads of different-sized fasteners. A ratchet of the “set size” is often included with the socket set. Sockets are commonly available in fractional inch and metric sizes, and in short (shallow) and longer (deep) varieties.