Bayonet mount

A bayonet mount

A bayonet mount before and after insertion

Early-19th century socket bayonet

Socket of a bayonet

A bayonet mount (mainly as a method of mechanical attachment, as for fitting a lens to a camera) or bayonet connector (for electrical use) is a fastening mechanism consisting of a cylindrical male side with one or more radial pins, and a female receptor with matching L-shaped slot(s) and with spring(s) to keep the two parts locked together. The slots are shaped like a capital letter L with serif (a short upward segment at the end of the horizontal arm); the pin slides into the vertical arm of the L, rotates across the horizontal arm, then is pushed slightly upwards into the short vertical "serif" by the spring; the connector is no longer free to rotate unless pushed down against the spring until the pin is out of the "serif".

To couple the two parts, the pin(s) on the male are aligned with the slot(s) on the female and the two pushed together. Once the pins reach the bottom of the slot, one or both parts are rotated so that the pin slides along the horizontal arm of the L until it reaches the "serif". The spring then pushes the male connector up into the "serif" to keep the pin locked into place. A practised user can connect them quickly and, unlike screw connectors, they are not subject to cross-threading. To disconnect, the two parts are pushed together to move the pin out of the "serif" while twisting in the opposite direction than for connecting, and then pulling apart.

The strength of the joint comes from the strength of the pins and the L slots, and the spring. To disengage unintentionally, the pins must break, the sleeve into which the connector slides must be distorted or torn enough to free the pins, or the spring must fail and allow the connector to be pushed down and rotate due to, say, vibration.

It is often possible to push down the connector and rotate it, but not far enough to engage and lock; it will stay in place temporarily, but accidental disconnection is very likely.

Uses

The first documented use of this type of fitting (without the name "bayonet") may be by Al-Jazari in the 13th century, who used it to mount candles into his candle-clocks. This style of fitting was later used for soldiers who needed to quickly mount bayonets to the ends of their rifles, whence the name.

In cameras with interchangeable lenses, the bayonet-style lens mount likewise allows the photographer to quickly change lenses. The bayonet-style mount additionally allows for the precise alignment of mechanical and electrical connections between modern camera bodies and lenses. Camera lens mounts usually employ stronger flattened tabs rather than pins, though their function is the same.

Bayonet connectors are employed for use in electronics, electrical power and control circuits, and to supply a physical connection between two cables transmitting any kind of impulse, signal, audio or electronic data. The bayonet connectors are derived from the threaded series to provide faster coupling and anti-vibration resistance. The coupling system is usually made of 2 bayonet ramps machined on the external side of the receptacle connector and 2 stainless steel studs mounted inside the plug connector’s coupling nut. Several classes of electrical cable connectors, including audio, video, and data cables use bayonet connectors. Examples include BNC, C, and ST connectors. (The BNC connector is not exactly as described in this article, as the male, not female, connector has the slots and spring.)

Many types of light bulb are fitted with integral bayonet connectors.

Bulb bayonet mounts

Compact fluorescent lamp with double contact B22d bayonet mount

Incandescent light bulb with double contact B22d bayonet mount, and corresponding socket with sprung connectors

Preceding bulb mounted in the socket

LED lamps with GU10 bi-pin twist-lock mount

Incandescent 40 W BA15d bulb

See also: Automotive lamp types

The bayonet light bulb mount is still prevalent in many former members of the British Empire such as India, Australia, Ireland, New Zealand, the United Kingdom as well as regions of the Middle East. Older installations in countries like France also have this type of B base, as well as the Edison screw (E) base used in the United States, Japan and other countries. These bulbs have two pins on opposite sides of the cap; however, some specialized bulbs have three pins (cap designation B22d-3) to prevent use in domestic light fittings. Examples of three pin bulbs are found in mercury street lamps and fireglow bulbs in some older models of electric radiative heater. Bayonet cap bulbs are very common worldwide in applications where vibration may loosen conventional bulbs, such as automotive lighting and other small indicators, and in many flashlights.

Some bulbs may have slightly offset lugs to ensure they can be only inserted in one orientation; this is case in bulbs like the 1157 automobile tail-light which has two different filaments to act as both a running light and a signal light. In the 1157 automotive bulb, each filament has a different brightness and is connected to a separate contact on the bottom of the base, the two contacts being symmetrically positioned about the axis of the base but the pins are offset so proper orientation of the bulb is automatic because it only fits one way. Newer bulbs use a wedge base which can be inserted either way with no issues. Some special-purpose bulbs, such as infrared, have 3 pins spaced 120 degrees apart to prevent them being used in any but the intended socket.

Bayonet bases or caps are often abbreviated to BC, often with a number after. The number refers to the diameter of the base (e.g., BC22 is a 22 mm diameter bayonet cap lamp). BC15, a 15 mm base, can also be referred to as SBC standing for small bayonet cap. The lowercase letter s or d refers to whether there are one (single) or two (double) contacts on the bottom of the bulb.

Type	IEC	DIN	ANSI
B15d	IEC 60061-1 (7004-11)	DIN 49721
BA15d	IEC 7004-11 A	DIN 49720
BA15s	IEC 7004-11 A	DIN 49720
BA20d	IEC 7004-12	DIN 49730
B21s-4
B22d	IEC 60061-1 (7004-10)
BY22d
B24s-3
GU10	IEC 60061-1 (7004-121)
GZ10	IEC 60061-1 (7004-120)
GU24			Pending (Mar 2007)

While G actually indicates bi-pin, those listed above have a twist-lock, but with parallel pins from the end instead of opposing pins on the side.

These are the available sizes in the UK:

Designation	Alternative designation	Contacts	Dimension, etc.
BA5s		1	5 mm
BA7s		1	7 mm
BAX9s		1	9 mm
BA9s	Miniature bayonet cap (MBC)	1	9 mm
BA15d	Small bayonet cap (SBC)	2	15 mm
BAX15s		1	15 mm
BA15s	Single centre contact (SCC)	1	15 mm
BA20s		1	20 mm
BA20d		2	20 mm
BA21d		2	21 mm
B21-4			21 mm 4 pin
BA22d	Bayonet cap (BC)	2	22 mm
BC-3	Bayonet cap (BC)	2	22 mm 3 bayonet pins
B22d-3		2	22 mm double ended (railway)
BX22d		2	22 mm

Of these, only the BC (BA22d) is widely used in homes; the BA20d (sometimes confusingly called a Bosch fitting) was once a common automotive (twin filament) headlamp fitting but has largely been superseded by more modern, higher-rated H-series sockets and is only used for lower powered applications (e.g. combination tail/stop lamps) if at all.

See also

• Edison screw
• Bipin
• Arri bayonet
• Joseph Swan
• Bayonet – a type of knife that attaches to a rifle.

References

• IEC 61184: Bayonet lampholders, International Electrotechnical Commission, 1997. (also: BS EN 61184) – specifies requirements and tests for the B15 and B22 bayonet holders for light bulbs used in some Commonwealth countries

External links

• GU-24 Goes Mainstream
• Line-voltage Socket Design Competition (GU24)
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