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A belay device (braking device) is a portable piece of climbing equipment used to control the tension of a rope during recreational climbing, caving, or professional rope work. It is primarily used to belay (protect) another climber in case of a fall. This involves paying out and taking in slack during climbing, locking off the device in case of a fall, and lowering the climber back to the ground.

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Depending on the design, belay devices can be used for other climbing-related purposes such as:
 * Descending a rope (also known as "abseiling" or "rappelling")
 * Roped solo climbing
 * Progress capture (also known as ratcheting) – Allowing rope to pass in only one direction, for example during hoisting of a load

Principles of operation
A belay device works by multiplying the force of the belayer´s hand, through rope bending (distortion) and friction, allowing the user to vary the braking force of a rope passing through the device. A belay device can multiply the force of the braking hand by a factor of 5-10. When the climber is making progress, braking force is minimized and the belayer can give slack by pulling out rope. In case the climber falls, braking force is increased either by placing the braking hand in a position where friction and bending is maximized, or by the action of a mechanism in the device. This allows the belayer to slow down and stop a falling climber with minimum effort.

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A belay device is always used with a locking carabiner, by which it is connected to the belayer´s climbing harness or to a fixed anchor. In many models, the carabiner itself is part of the braking mechanism.

A skilled belayer can fine-tune the braking force (and thus the length of the fall) by adjusting body position, or by allowing a small amount of slippage in the device.

Material
Current commercial designs are commonly manufactured from aluminium, sometimes with steel reinforcement, and some plastic parts where structural strength is not required. Devices weight from around 40 grams to around 250 grams, with even heavier devices available for professional work.

Characteristics of belay devices
The design of a belay device usually balances the following requirements:
 * Sufficient friction to stop the climber´s fall early enough and without causing injury to the belayer.
 * Low enough friction to reduce peak forces on climber, belayer, and rock anchors (bolts, nuts, etc).
 * Low weight
 * Usability – ease of use both for first-time users and for experts.
 * Redundancy - ability to stop a fall even if the belayer is distracted or incapacitated.
 * Robustness – functions in difficult conditions such as cold, wet, or sandy environments.
 * Versatility – can be used in a variety of climbing situations, such as abseiling, using double-rope technique, using varying rope diameter, etc.
 * Commercially viable price and production process

Few devices fulfil all these requirements. Instead, most designs cater to a subset of users by optimizing for only some of them.

History
First appear in the 60s Yates Belay Slave? MSR Brake Bar? Sticht?

Types of belay devices
Over time, designs have evolved from simple plates with holes, to highly engineered contraptions with many moving parts. However, all the early designs are available on the market today.

While commercial requirements have resulted in an abundance of visual designs, the basic engineering can be divided into the following main categories.

Note that in the climbing community, the name of a pioneering or widely used model is often used to signify a whole category of devices. Examples of this are Sticht, ATC and Grigri.

Plates
The simplest and oldest design is the belay plate, sometimes referred to as Sticht plate. It is a flat metal plate with one or two holes. The rope passes through a hole, around a carabiner, and back out through the same hole. By varying the position of the braking hand, the rope is bent differently. The S-configuration generates high friction, while the U-shape generates lower friction. Because the friction depends on the position of the braking hand, the device is very dependent on correct handling. Plate designs exist for double-rope handling, and sometimes have a “guide mode” whereby the device will automatically provide higher friction when under load. Thanks to their simplicity, versatility, and low weight, plates are still in use in modern versions, especially in professional applications. An early commercial design was the Salewa Sticht plate from the 1960´s. A modern version is the Camp Ovo.

Tubes (tubers)
An evolution from the plate, tube designs arrived in XXXX. The function is the same as for plates, with the advantage of greater heat dissipation during long abseils and greater ease of handling. Many tube models can also be oriented in two ways, to provide a high- and a low-friction mode. Modern designs such as the Black Diamond ATC and Petzl Verso are variations of the basic tube design.

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Like plates, some tube designs have a special attachment allowing them to be used in “guide mode” for assisted belay while bringing up a second. Examples are DMM Pivot and Wild Country Pro Guide.

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Figure 8
The figure 8 consists of a larger and a smaller ring. The rope is passed through the larger ring, around the “neck” of the 8, and out through the same hole. The smaller hole is used to attach the device to the harness with a carabiner. A modern version of this is the Black Diamond Super 8. This type of device can be used for belaying as well as abseiling, but provides relatively little friction compared to other devices. Some designs include features to increase friction when needed.

Assisted-braking devices
Assisted-braking devices provide additional safety when belaying lead climbing and top-roping, by blocking the rope in case of a fall, with minimal intervention by the belayer. This can prevent accidents that involve an unfocused, unskilled, or incapacitated belayer. These devices are referred to as "assisted-braking" rather than "automatic-braking" because manufacturers do not guarantee fully automatic operation in all scenarios. The belayer should always keep a hand on the passive side of the rope.

Geometrical (passive) assisted-braking devices
Geometrical assisted-braking devices function much like tube devices, but with a high-friction mode which engages in a leader fall. Depending on rope diameter and the carabiner used, this may fully block the rope so as to provide redundancy in case the belayer is distracted or incapacitated, or may only create extra braking force. Geometrical designs usually have none or few moving parts. The extra braking force is achieved through the shape of the device, guiding the carabiner into a new position when loaded, while still allowing the belayer to pay out rope when unloaded. Geometrical assisted-braking devices are available for bot single and double ropes, making them useful for traditional multipitch climbing.

the European Union´s standards, categorize belay devices of this type as a manual devices. However, functionality is largely similar to mechanical and centrifugal brakes.

The Mammut Smart, CT Click-up, and Edelrid Mega Jul are examples of geometrical assisted-braking devices. , this family of devices is under rapid development.

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Mechanical (active) assisted-braking devices
Mechanical assisted-braking devices have moving parts, allowing the device to fully lock the rope when engaged. The locking mechanism is usually some form of camming action that engages when the device is loaded. A mechanism such as a lever allows the belayer to defeat the locking mechanism for lowering, abseiling, or feeding out rope to a leader. These devices are usually designed for single ropes only, and are therefore common in sports climbing. Examples of such devices are the Petzl Gri-gri and Madrock Life Guard.

Centrifugal (active) assisted-braking devices
Strictly speaking this is a type of mechanical/active assisted-braking device. Centrifugal brakes function much like a seat belt. When the rope moves slowly (a few meters per second) through the device, it functions like a tube and the wheel rotates freely. At higher speeds, a mechanism engages, stopping the wheel and providing a locking mode. An example is the Wild Country Revo.

Solo (self) belay devices
Self-belay devices or Self-locking devices are designed to allow roped solo climbing, where the climber wears the belay device, or secures it to a fixed object on the ground. These devices automatically lock without any intervention when the rope passing through reaches a sufficient velocity (during a fall), but allows the rope to move relatively freely while climbing. Solo devices are usually highly specialized and not suitable for other applications. Examples of devices are the Rock Exotica Soloist and Silent partner.

Autobelays
Autobelays are designed to be permanently mounted at the top of a route, usually at an indoor climbing gym, and allows climbers to top-rope without a belayer. The device takes up slack, and slowly lowers the climber if he or she falls or lets go. Autobelays, as opposed to assisted-braking devices, are designed to be fully automatic, requiring no hands-on operation.

Body belay
Probably the oldest method of belaying, the body belay (also known as hip or waist belay) involves passing the rope around the belayer´s body to increase friction. This method is rarely used today but remains an important skill for survivalists, alpinists and other climbers in need of a backup method. The Dulfersitz is a method for abseiling with a body belay.

Terrain belay
By using features of the environment, such as trees or rock formations, the belayer can increase friction sufficiently to be able to catch a fall, without the use of a belay device. Because it is simple and fast to set up, this method is still in use in alpinism, especially in low-angle terrain.

Friction knots
The HMS or Munter hitch, in combination with a large locking carabiner, provides similar functionality to a plate or tube. Many climbers use it as their primary method when belaying from above.

Improvised belay devices
If a climber loses the belay device, it is possible to achieve similar function using, for example, carabiners, or any piece of equipment that may function as a plate.

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Safety and failure modes
Devices breaking or malfunctioning is highly unusual. However, accidents involving incorrect handling of belay devices are common. Avoiding these types of failures involves knowledge about the specific device, supervised training, and the use of backup techniques such as friction knots when abseiling.

The following are some examples of failure modes:
 * Dropping the passive side of the rope and hence losing control.
 * Improper setup, for example threading the rope incorrectly
 * Accidentally triggering a release mechanism.
 * Changed friction due to water, ice, etc.
 * Hair or clothing getting caught in the mechanism.

Each model requires correct setting up and handling, and it can only be relied upon to provide safety if used as described by the manufacturer.

Standards and certification
In the European Union, belay devices are governed by two EN standards.
 * The EN15151:1 covers "manually assisted locking" devices (corresponding to "Mechanical assisted-braking devices" and "Centrifugal assisted-braking devices" above). These devices must be approved according to the standard to be brought to market in the EU, and are considered Personal Protective Equipment (PPE).
 * The EN15151:2 covers manual belay devices (corresponding to "plates", "tubes", "eights" and "geometrical assisted-braking devices" above), and is not harmonizing. These devices are not considered Personal Protective Equipment (PPE). The reason that "geometrical assisted-braking devices" are in this group is that they are dependent on the position of the braking hand.

The UIAA also provides a standard for belay devices.