A socket wrench is a type of wrench, or tightening tool, that uses separate, removable sockets to fit many different sizes of nuts. It generally includes a ratcheting mechanism that allows the nut to be tightened or loosened with a continuous motion, rather than requiring that the wrench be removed and refitted after each turn. Typically, a lever behind the socket switches the wrench between tightening and loosening modes. The sockets attach to the ratchet through a square fitting that locks onto them and come in four standard sizes: 1/4 inch, 3/8 inch, 1/2 inch, and 3/4 inch. (Despite being denominated in inches, these are international standards and no "metric" counterparts exist.) The largest fitting is usually reserved for use on fasteners used on larger industrial vehicles.
A socket wrench, some extension bars, and some sockets
The modern socket wrench, with interchangeable sockets, was invented by an American tool manufacturing employee named Joe Johnson in Milwaukee, Wisconsin. Previously, sockets were attached to the handle, so different-sized nuts required different wrenches. He and a co-worker, William Seidemann, formed the Snap-on Wrench Company, which is now Snap-on Inc. of Kenosha, Wisconsin, a large tool manufacturer.
Sockets come in a variety of types:
Modern nuts and bolt heads are made with hexagonal gripping surfaces and as such limit the number of positions a wrench can adopt when placed over them. When working in a confined area with limited turning space, 12-point sockets double the number of starting postions. If the nut or bolt is damaged then 6-point sockets are preferred since they offer a better grip with less chance of slipping.
Deep sockets are useful for turning nuts onto bolts when the bolt extends upwards into the socket.
High strength fasteners in demanding situations often require the use of an impact or air wrench to deliver the amount of torque required to tighten or loosen them. Standard sockets are made of a strong, but brittle steel. When used with an impact wrench they can shatter explosively if they break. Specially made impact sockets are made of a weaker, but more malleable steel that will deform and split instead of shattering when they break. Impact sockets are not chrome-plated, as chrome will often rapidly chip when used with impact wrenches. Instead, they are made with a coating of black oxide or a plastic coating to prevent corrosion.
Sockets are available in different lengths so that a user can access nuts in narrow spaces or that have too much length of the bolt protruding behind them. Extender arms allow access to nuts that are difficult to reach.