A spring-loaded camming device (also SLCD, cam or friend) is a piece of rock climbing or mountaineering protection equipment. It consists of three or four cams mounted on a common axle or two adjacent axles, so that pulling on the axle forces the cams to spread farther apart. The SLCD is used by pulling on the "trigger" (a small handle) so the cams move together, then inserting it into a crack or pocket in the rock and releasing the trigger to allow the cams to expand. At this point the climbing rope can be attached to a sling and carabiner at the end of the stem. A pull on the rope, such as that generated by a climber falling, will cause a properly placed SLCD to convert the pulling force along the stem of the unit into outwards pressure on the rock, generating massive amounts of friction and preventing the removal of the unit from the rock. Because of the large forces which are exerted on the rock when an SLCD is fallen on, it is very important that SLCDs are only placed in solid, strong rock.
In 1973 Greg Lowe filed for a patent for a spring-loaded version of the Abalakov Cam.
Modern SLCDs were invented by Ray Jardine in 1978 (US patent 4,184,657) and sold under the brand name of "Friends". Ray designed a spring-loaded opposing multiple cam unit with a more stable 13.75 degree camming angle and an innovative triggering mechanism. ("Friend" is now widely used by climbers to refer to SLCDs in general, but properly speaking it refers to the brand now manufactured by Wild Country.) Other popular brands include Black Diamond Camalots, Metolius PowerCams, DMM 4CUs, Trango FlexCams, and CCH Aliens.
The invention of SLCDs revolutionised rock climbing because it meant that climbs with parallel cracks could be protected. Furthermore, unlike pitons, SLCDs can be removed easily without causing damage to the rock, and made clean climbing (climbing without damaging the rock) practical on almost all climbs. Since the invention of the Technical Friend, there has been a great deal of development of the SLCD by a variety of manufacturers, such as the virtual replacement of solid stem units for more flexible and durable wire stemed cams, the adoption of the dual axel design by Black Diamond, the invention of three-lobed camming units to fit smaller cracks, to the more recent invention of the Link Cam by Omega Pacific, an attempt to allow one SLCD to span an even larger range of crack sizes. SLCDs are sold in various sizes to fit a diverse range of cracks from about 6 mm to 300 mm wide, though devices of below about 10mm or above about 100 mm are not often seen.
Modern trad climbers often climb with numerous and variously sized SLCDs to cover a wide range of crack sizes, sometimes with duplicate units of the most used sizes.