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A socket wrench (often called a ratchet) most commonly refers to a hand tool in which a metal handle is attached to a ratcheting mechanism which attaches to a socket, which in turn fits on 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 socket plus socket wrench 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. The sockets are attached to the ratchet through a square fitting (called the "drive") that contains a spring-loaded ball detent mechanism to keep the sockets or extensions in place. The drive on the ratchet, which comes in standard sizes of 1/4", 3/8", 1/2", 3/4", and 1" (a de facto international standard with "no" metric equivalents), allows a wide variety of socket types and sizes to attach to a given ratchet. Some ratchets have quick release buttons on their top for quick socket release. The ratchet handle supplies the mechanical advantage needed by the user to provide the torque needed to loosen or tighten the fastener. For cases of where low torque is required palm ratchets or ratchet spinners may be used. For greater torque, a breaker bar or torque wrench can be used instead of the socket wrench.
To see how socket wrenches/ratchets are made see ratchet manufacturing video.
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 fastener and a standardized square recess on the other end called the drive, to accept the ratchet handle's standardized drive size. Sockets are made to be interchangeable (indexable) for a given socket wrench size and a wide range of fastener sizes. Because of their versatility nearly all screw and bolt types now have sockets of different types made to fit their bolts or nuts. Other common male sockets include Allen hex wrenches, Torx (T-3 thru T-50) spline wrenches, 4-point (square), 8-point (double square) and 12-point (triple square) male sockets and a wide variety of other screw/bolt types. The socket's female drive recess snaps onto the handle's male drive. 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. One advantage of a socket wrench 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, with only one to three ratchet sizes needed for use on a wide variety of socket/fastener sizes and types—saving cost, storage space, time, and effort.
There are also a power tool versions of "air" ratchets which use compressed air power to drive air powered socket wrenches which tighten or loosen nuts or bolts. A second major variety of compressed air powered tools are impact wrenches which are used for common tasks such as lug nuts on wheels. Electric powered impact wrenches for the same tasks are not uncommon. Small cordless 12 Volt and 18 Volt impact drivers are often used today as powered "ratchets to remove and install nuts and bolts. Hydraulic motor ratchets with their characteristic higher torque characteristics are rare outside of heavy industry. The sockets used for impact duty (called "impact sockets") are made with thicker walls and tempered to a lower hardness so as not to shatter under the impacts of the impact tool. They are typically finished in black oxide rather than the chrome plating typical of the hand-tool variety. Standard sockets (i.e. non-impact sockets) should never be used with impact wrenches as they may shatter.
Nut drivers also use a female socket to envelop and drive a male fastener head. They are typically built like a screwdriver with a handle built for turning and a built in socket head of either metric or fractional inch sizes and may be of different lengths. Typically nut drivers are not called socket wrenches.
The socket wrench typically is of the ratchet type. The ratcheting mechanism allows the nut to be tightened or loosened with a reciprocating motion, without requiring that the wrench be removed and refitted after each turn. Typically, a small lever on the ratchet head switches the wrench between tightening and loosening mode. These drive fittings come in four common sizes: 1/4 inch, 3/8 inch, 1/2 inch, and 3/4 inch (referred to as "drives", as in "3/8 drive"). Despite being denominated in inches, these are international standards and no metric counterparts exist. Larger drive sizes such as 1 inch and above are usually only encountered on fasteners of larger industrial equipment, such as tractor-trailers (articulated lorries), large cargo aircraft and passenger airliners, and marine work (merchant fleets, navies, shipyards). The sockets themselves come in a full range of inch and metric sizes. ("SAE" is often used as a blanket term for the nonmetric sizes, despite the technical inaccuracy of that usage.) The advantages of the system of a ratchet wrench with indexable sockets are speed of wrenching (it is much faster than a conventional wrench, especially in repetitive bolt-on or bolt-off usage) and efficiency of tooling cost and portability (it is much more efficient than a set of nonratcheting wrenches, with every size head having its own handle).
Different types of sockets for different fasteners types: hex heads, Allen heads, star heads, Torx heads, square heads, etc. are easy to buy and use with a ratchet in both inch and metric sizes.
Wrenches in the form of sockets, that is, a female driver to envelop the male head of a fastener, have existed for centuries. Early examples include the keys used to wind clocks since the Middle Ages. The heads and sockets were typically square; hex heads eventually became more common starting in the 20th century. The ratcheting socket wrench, with interchangeable (indexable) sockets, was invented by an American, J.J. Richardson, of Woodstock, Vermont, USA. The tool was patented through the Scientific American Patent Agency on June 18, 1863. The first illustration of the tool appears on pg. 248 of the April 16, 1864 issue of Scientific American. In current English usage the term "socket wrench" connotes indexable sockets so strongly that most English speakers would resist calling a large non-indexable, non-racheting, socket-head wrench a "socket wrench"; instead it might be called a "nut driver", although that term usually connotes smaller sizes.
Square heads and sockets were the easiest to make in the era when hand filing was the typical method of manufacture. With the proliferation of modern manufacturing methods, such as milling, shaping, broaching, and advanced die forging, it became just as easy to make hex heads and sockets as square ones. The hex form allows easier wrenching in confined surroundings (where nearby obstacles obstruct the swing of the wrench), because fewer degrees of arc are needed on each swing before it is possible to reposition the wrench onto the next set of flats. Ratchet wrenches further reduce this problem, as the wrench need only swing as many degrees as it takes for the ratchet pawl to catch the next tooth.
Socket Wrench Types 
- Flex ratchets are socket wrenches in which the drive head pivots or swivels back and forth on the handle at a pivot to the rear of the ratchet head.
- Swivel head ratchets swivel the whole ratchet head with handle attachments on the side of the ratcheting head rather than the rear of the ratcheting head.
- Shorter and longer handle ratchets allow the ratchet plus socket to be used in a variety of open or restricted spaces. Some ratchets are built with adjustable length handles or pre-bent handles.
- Fine toothed ratchets have finer teeth on the ratcheting components and may have to only rotate about one degree per "click". These may be useful for very tight locations. Double pawl ratchets allow smaller rotations between turns.
- Palm ratchets have a knurled palm sized circular ratchet handle with reversible socket attachment useful for rapidly loosening or tightening a bolt or nut. They come in a variety of sizes.
- Rotator ratchets (made by Stanley etc.) allow the socket to be twisted by twisting the ratchet handle around the handle axis. Requires less than 1 degree arc swing to rotate socket—ideal for very tight spaces.
- Dual or combination ratchets are made with two sizes of socket attachments—typically 1/4" and 3/8" on opposite sides of the ratchet head.
- Specialized ratchets with hammer heads or other specialized uses are built by some manufacturers.
Socket Types 
|Part of a series on|
|Screw drive types|
Male head 6 & 12 point socket
T & TX T-3 to T-50
Female head uses Allen keys
||Security hex socket
Common sockets are colloquially referred to by counting the number of "points" (pointed corners of walls) present in the shape of the socket nut opening. For example, the hexagon shape is commonly called a "6 point" socket because the hexagon forms 6 "points" where walls intersect to create six sharp pointed corners. The vast majority of larger common nut and bolt heads are produced with the 6-point hexagonal gripping shape. Square 4-point, octagonal 8-point, and 12-point bolt heads are used less frequently, typically for special applications or particular industries such as aircraft or German cars. With rail cars, valve adjustment screws and pipe plugs the 4-point square shaped driver can still be found in wide use. Nuts and bolt heads are also produced in 12-point shapes and various types of splines more common to aircraft and aerospace applications. Most manufacturers of sockets for larger bolts produce them in 6-point (hexagonal) and limited sizes of 12-point (double-hexagonal) configurations. Other female sockets such as 4-point (square), 6-point (hexagonal), 8-point (double square) and 12-point (triple square) sockets are also made.
The other main type of sockets are male sockets which fit inside the female recess in the many types of smaller bolts or fastener heads—commonly called screws. Common male sockets of this type include Allen hex wrenches (in both metric and fractional inch sizes), Torx (T-3 thru T-50) spline wrenches, 4-point (square), 8-point (double square) and 12-point (triple square) male sockets. Some of these socket heads have different types or sizes of male attachment heads that fit inside a "standard" sized six or 12 point socket allowing one socket, with attachments, to be used on a wide array of fittings. Other specialized screw heads that are often installed or removed with screwdriver type handles and appropriate type tip have socket varieties that fit the various screw head types and a can be attached to a socket wrench. Conversely, for low torque situations, a "socket spinner" screwdriver handle with a socket wrench type fitting on one end can be attached to many different types of sockets and extensions.
A socket wrench, with the correct type of socket, can be used to loosen or tighten a large variety of fasteners with lower cost, greater ease and often more torque than can be applied with a screwdriver handle. A common use of these different "screw" head type sockets is to attach to power versions of the socket wrenches which allows the fasteners to be loosened or tightened much faster than can be done by hand.
When working with common 6-point hexagonal fasteners the 12-point shaped socket offers double the amount of starting points or possible positions by which to put the socket on the nut. This is of large importance on a box wrench which is only adjustable in 30 degree increments on a twelve point wrench but only 60 degrees on a six point wrench. This versatility of fitting position is normally of small importance on a socket wrench which can be easily rotated in small increments regardless of socket type. Twelve point sockets have more than adequate strength for all except high impact or high torque situations and are slightly easier to position on the bolt or nut. This versatility is the main purpose of the 12-point shape found in common sockets, in addition to matching up perfectly with 12-point bolt and nut heads found in some applications. Socket strength is mainly a function of the alloys and heat treatments used to make the socket and the wall thickness of the thin parts of the socket not whether they have six or twelve points.
Specialized "wobble" or swivel sockets are made that have a separate socket head and socket wrench connection that allows the socket to wobble or swivel over a limited range independent of the socket wrench or tool position when it is attached to the bolt or nut. This may be a distinct advantage in some applications with restricted access. These "wobble/swivel" sockets, with their typical ball and socket joint swivel attachments, are made in a wide variety of types and sizes including deep, shallow, impact, triple square etc. They are often found in impact sets since they can be built sturdier than a common universal joint socket.
Many socket types and sizes are available with a universal joint between the socket and the ratchet attachment for tightening or loosening fasteners which are not easy to access with the socket wrench plus standard socket arrangement. Also available are short universal joint extensions that allow any socket to be attached to a ratchet at with a universal joint between the two. Wobble extensions can serve the same purpose and may be easier to use since they often give a more solid socket attachment without all the flexing common to universal joint sockets.
Some specialized sockets are made with a specialized "6 flute" etc. socket that attaches to damaged bolts of both metric and factional inch sizes for removal. Some specialized sockets are made to fit specific specialized applications and are designed and sized for that specific application. Spark plug sockets, oxygen sensor sockets, ball joint sockets, axle nut sockets, etc. fit in this category.
Due to the twisting or cam action of the socket on a bolt or nut head, nearly all of the torque is applied at or near the corners of the nut or bolt whether using either a twelve or six point socket the same stresses are applied in the same locations. Under load, the nearly all of the flats of the nut are not touching the socket at all, it is only a small amount of the surfaces near the fastener corners which bears the torque loads. In nearly all applications over-stressing a bolt or nut will result in stripping the bolt or nut threads or breaking the bolt long before the socket will break. Tougher, thicker impact sockets are made for impact gun use. Occasionally when the corners of a bolt head or nut begin to become worn or damaged, the material the nut is made from can eventually smear or tear under load and create a situation where the corners are effectively "rounded off". Under these fortunately rare conditions a six point socket may be slightly better since as the bolt head distorts it brings more of the surface of the socket into contact with fastener head. Many socket users prefer to minimize the probability of rounding the bolt head off by using six point sockets. The other problem may be that a distorted or corroded fastener head may not fit a six point socket but a twelve point socket can be made to fit. When a fastener head is "rounded off" the nut or bolt can often no longer be turned without special repair or extraction tools. Due to the cost and hassle that "rounded" heads can have which prevent "things" to be assembled or disassembled, many efforts have been made to avert this problem. Over the years nearly all manufacturers of quality sockets have employed use of convex walls and other design improvements to the socket geometry in order to direct the torque farther away from the corners of fasteners, and towards the thicker, stronger sections of the nut or bolt head; thus reducing the likelihood of rounding off corners of fasteners under high loads. These designs typically cost little to implement and have been met with commercial success, especially in the automotive maintenance and repair sector.
To see how sockets and extensions are made see socket manufacturing video.
Impact Sockets 
Many applications use air driven or electric impact wrench or impact driver to deliver the amount of shock and torque required to tighten or loosen the different bolts or nuts in use. Inside an impact wrench there is a rotating hammer, usually driven by electricity or air pressure, that is used to incrementally rotate their output shaft. The impact wrench is designed to deliver high torque output with minimal exertion by the user, by storing energy in a rotating mass, then delivering it suddenly to the output shaft. Some impact wrenches can deliver over 600 ft-lb (815 nm) of torque which is enough to break many common fasteners. Even if the socket, attached to the impact wrench, does not turn it is hit with successive hammer blows by the impact wrench's internal hammer. These successive blows are often enough to loosen or tighten a tight or corroded nut or bolt or in some cases break or strip the fastener. Impact wrenches are hard on the sockets since the sockets in use also get hammered. Regular chrome plated "hand sockets" like those commonly included with hand ratchet wrench tool sets are not suitable for this kind of high load impact application. Hand sockets, when used with an impact wrench, can break or shatter explosively if they are used with an impact wrench. Impact sockets are specifically designed and manufactured for impact gun use, and are nearly always made from a thicker, tougher and more ductile alloy steel that is then heat treated for extra durability. Most impact sockets made for "standard" hexagonal fasteners have a six point design. For more flexibility many impact sockets are made with swivel type socket—socket wrench attachment geometry. These impact sockets are rarely, if ever, chrome-plated, as chrome will chip off under impact use, and the process of chrome plating causes some hydrogen embrittlement which slightly weakens "standard" chrome plated sockets. Instead, impact sockets are most commonly finished with a black oxide surface or other coating to provide corrosion and rust protection and improved appearance even after hard impact wrench use. Although ordinary hand sockets should never be used with impact wrenches, impact sockets can be used with hand socket wrenches and other hand drivers. In some breaker bar or high torque applications an impact socket is stronger and a better choice.
Standard Length vs Deep Length Sockets 
Sockets are available in various depths or lengths, often divided by most manufacturers into two categories of "standard" and "deep" according to the ANSI or DIN standard they are made to and the tolerances allowed by those specifications for each length of socket. Because the standards allow for some flexibility in tolerances, it is common to see two manufacturers make Deep sockets of the same size but with slightly different depths even though both meet the same specification. Standard length, otherwise known as "shallow" sockets, have a lower profile and allow a user to access nuts in narrow spaces. Deep sockets are useful for turning nuts onto bolts when the bolt extends upwards into the socket (as in the case of many bolted joints), a very typical example being exhaust clamp bolts on an automobile.
Although most manufacturers offer only those sizes and depths described within the common ANSI or DIN specifications, some exceptions do exist. Specialty manufacturers such as IMPERIAL-Newton Corp offer an expanded range of "extra deep" sockets for special industrial applications; and popular brands like Snap-on or Mac Tools offer what are called "semi-deep" or "mid-length" sockets, which provide much of a deep socket's depth, while fitting in tighter locations.
Pass Through Sockets and Ratchets 
Some sockets are designed to have the same outside diameter and shape within a given set size. Each pass-Through socket, within a given socket set, is designed to be used with a "special" ratchet that fits on the outside of the socket and not to the middle of the socket. By fitting the outside of the socket they allow the bolt or stud to extend through the socket, eliminating the need for a deep socket in some applications. By attaching to the outside of the socket they also allow the socket to be built up to 50% shorter and with 20% less width which is an advantage in some situations. Pass-Through sockets and ratchets are built for 1/4", 3/8" and 1/2" sets in both SAE and metric sizes. By using a fine tooth ratchet and socket system that allow a pass-Through ratchet to be used as a conventional ratchet handle with interchangeable 1/4-inch and 3/8-inch drives some ratchets can be used with standard sockets. They are built by a number of manufacturers with a variety of trade names. Craftsman tools calls theirs--Max Axess, and also sells GearWrench's Pass-Thru brands. Lowes calls their socket system Xtreme Access, etc.. Ratcheting box end wrenches can often be employed in the same application but will nearly always be significantly wider.
These are some of the common accessories that are used with 1/4", 3/8", 1/2", etc. socket wrenches:
- Extensions, sometimes called "extender arms", attach to a socket on one end and a ratchet on the other end of the extension. These "extend" the length of the socket and allow access to nuts or bolts that are difficult to reach. Extensions are typically 1/2" to 20" in length in roughly 1" to 3" increments. They are sometimes attached together to get a needed length extension.
- Wobble extensions have their socket attachment ends ground to allow the socket-extension interface to bend up to about 15 degrees. This additional flexibility often makes using a socket + extension in a cramped location easier. A 1" to 1-1/2" long wobble extension added to the end of any extension will convert it to a slightly longer "wobble" extension.
- Extension Grip Collars are collars with indents that fit on the back of most extensions preventing it from easily rolling away and allow one to easily grip extension and finger tighten or loosen nuts and bolts by turning extension + socket with or without ratchet.
- Ratchet spinners are short (about 1-1/2") extensions that have a knurled attachments on them for easy hand tightening or loosening without the ratchet handle.
- Size adapters allow sockets of one drive size to be used with ratchets of another drive size. They consist of a male drive fitting of one size attached to a female drive fitting of another size. They are typically about 1" long. For example, a 1/4 in. to 3/8 in. adapter allows sockets with 1/4 in. drive holes to attach to a 3/8 in. ratchet, etc..
- Universal joints are two articulated socket joints (about 1" long) combined at a right angle, that allow a bend in the turning axis of the wrench + socket. They are used with extensions and ratchets for turning a bolt or nut at a difficult to access location. Wobble extensions may be substituted for some universal joint applications and have the advantage of not wobbling so much.
- A breaker bar is a strengthened extended-length handle for sockets typically made with a swivel head that adds extra torque for "breaking" loose strongly tightened or "frozen" fasteners. These may be necessary for bolts or nuts that are installed with thread locking compounds or have become corroded in place.
- Torque wrenches are socket attachment handles that allow the torque to be measured when tightening bolts or nuts. This is important to prevent breaking the bolt or nut by torquing it over its strength limits or to put even pressure on a tire rim or cylinder head etc. held on with multiple bolts or nuts to minimize bending or distortion.