Hex key

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Hex keys of various sizes.

Hex screws of various sizes.

Part of the series on
Screw drive types
	Slotted
	Phillips ("Crosshead")
	Pozidriv (SupaDriv)
	Torx
	Hex (Allen)
	Robertson
	Tri-Wing
	Torq-Set
	Spanner Head
	Triple Square (XZN)
	Polydrive
Others:
spline drive, double hex
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A hex key, also known as an Allen, Alum, hex-head, or zeta key or wrench, is a tool used to drive screws and bolts that have a hexagonal socket in the head.

Some features of this type of tool are:

The tool is simple, small and light.
The contact surfaces of the screw or bolt are protected from external damage.
The tool can be used with a headless screw.
The screw can be inserted into its hole using the key.
There are six contact surfaces between bolt and driver.
Torque is constrained by the length and thickness of the key.
Very small bolt heads can be accommodated.
The tool can be manufactured very cheaply, so is often included with products requiring end-user assembly.
Either end of the tool can be used to take advantage of reach or torque.

Many manufacturers have adopted hex key bolts and screws for assembling bicycles and for ready-to-assemble furniture.

This tool is also commonly referred to by various trademarks or brand names. "Allen wrench" was originally a trademark of the Allen Manufacturing Company in Hartford, Connecticut, taken out in 1943. In France, it is called "clef Allen". In Germany, the Netherlands, Poland and in Russia, this type of screw is known by the name "Inbus", after the company that patented them in 1936, Bauer & Schaurte Karcher in Beckingen, Saarland, Germany (stands for Innensechskantschraube Bauer und Schaurte). (It is still known as an Inbus-key—often misspelled as Imbus-key—in parts of Europe.) In Italy it is known as brugola, after Egidio Brugola, who invented it in 1926. In Sweden it is called insexnyckel (sex being the Swedish word for six). In the other non-English speaking parts of Europe, mainly in Norway, it is usually known as an "Unbrako key" (often misspelled "Umbrako") ^[1], which is a brand name established in 1911. The brand is owned by SPS Technologies. The name Zeta key refers to the fact that Zeta is sixth letter of the Greek alphabet.

[edit] Hex key standard sizes

Hex keys are measured across-flats (AF), which is the distance between two flat edges of the key, as opposed to across-corners. Standard metric sizes are defined in ISO 2936:2001 "Assembly tools for screws and nuts—Hexagon socket screw keys", also known as DIN 911, and, measured in mm are 0.7, 0.9, 1.0, 1.25, 1.3, 1.5, 2 to 6 in 0.5 mm increments, 7 to 22 in 1 mm increments, followed by 24, 25, 27, 30, 32, 36, 42 and 46 mm.

Metric hex wrench sizes are sometimes referred to using the designation "M" followed by the size in millimeters of the tool or socket, e.g. "M6", although this may be confused with the standard use of "M6" which refers to the threads of a metric screw or bolt.

American sizes are defined in ANSI/ ASME standard B18.3-1998 "Socket Cap, Shoulder, and Set Screws (Inch Series)". Values given here are taken from Machinery's Handbook, 26^th Edition, section "Fasteners", chapter "Cap and Set Screws", table 4 (p. 1601).

Screw size (nominal)	Socket size (inches)	Approximate socket size (mm)
#6	7/64	2.78
#8	9/64	3.57
#10	5/32	3.97
1/4	3/16	4.76
5/16	1/4	6.35
3/8	5/16	7.94
7/16	3/8	9.52
1/2	3/8	9.52
5/8	1/2	12.7
3/4	5/8	15.9
7/8	3/4	19.0
1	3/4	19.0

Note that numerous other sizes are defined; this is an apparently arbitrary subset.

Using a hex wrench on a socket that is too large may result in damage to the fastener or the tool. An example would be using a 5 mm tool in a 5.5 mm socket. Because hex-style hardware and tools are available in both metric and English/Imperial sizes (with English sizes most commonly used in the United States, and metric in other places), it is also possible to select a tool that is too small for the fastener by using an English-unit/Imperial tool on a metric fastener, or the converse.

Some hex keys have a ball on one end, which allows the tool to be used at an angle off-axis to the screw. They were invented in 1964 by the Bondhus Corporation^[2], and are now manufactured by several other companies. While providing access to otherwise inaccessible fasteners, thinning of the tool shaft to create the ball shape weakens it compared to the straight-shaft version, limiting the torque that can be applied. The tool also makes point contact with the fastener as opposed to the line contact seen in the straight style tools. This can increase the chance of stripping the head of the fastener, particularly on smaller fasteners.