Bayonet mount

Fastening mechanism

A bayonet mount (mainly as a method of mechanical attachment, such as fitting a lens to a camera using a matching lens mount) 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". The bayonet mount is the standard light bulb fitting in the United Kingdom and in many countries that were members of the British Empire including Australia, Hong Kong, Fiji, India, Pakistan, Sri Lanka, Ireland and New Zealand, parts of the Middle East and Africa and, historically, in France and Greece.

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—for example due to vibration. It is 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. Bayonet electrical connectors are used in the same applications where other connectors are used, to transmit either power or signals. Bayonet connections can be made faster than screw connections, and more securely than push fit connections; they are more resistant to vibration than both these types. They may be used to connect two cables, or to connect a cable to a connector on the panel of a piece of equipment. The coupling system is usually made of two 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.) The GU 10 light fittings in common use for both halogen and LED miniature spotlight lamps have a similar means of connection but the retaining pins are fitted to the end of the lamp and also double as the electrical contacts. The pins are cylindrical but the ends have a larger diameter, resembling a T when viewed from the side. The receptacle has two slots resembling curved keyholes which have holes at one end sized to accept the pin ends. The lamp is inserted into the receptacle by placing the pins in the holes and rotating in a clockwise direction. Note that, unlike the traditional bayonet fitting, the retaining springs act laterally on the pins so no inward pressure is required to lock the lamp in the fitting. GU 10 fittings are available in heat resistant form for use with halogen lamps which generate heat.

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 type of fitting was later used for soldiers who needed to quickly mount bayonets to the ends of their rifles, hence the name.

Many cameras with interchangeable lenses use a bayonet lens mount to allow lenses to be changed rapidly and locked accurately in position. Camera lens mounts usually employ stronger flattened tabs rather than pins, though their function is the same. A bayonet mount is often used to mate a cylinder with a base in cylindrical packaging such as that for CD spindles.