A 19-inch rack is a standardized frame or enclosure for mounting multiple electronic equipment modules. Each module has a front panel that is wide. The 19-inch dimension includes the edges or that protrude from each side of the equipment, allowing the module to be fastened to the rack frame with screws or bolts. Common uses include computer servers, telecommunications equipment and networking hardware, audiovisual production gear, professional audio equipment, and scientific equipment.
Overview and history Equipment designed to be placed in a rack is typically described as rack-mount/rack-mountable, a rack-mount instrument, rack-mounted system, or rack-mount chassis. The rack may include subracks or shelves. The height of the mountable equipment is standardized as multiples of or one rack unit or U (less commonly RU). The industry-standard rack cabinet is 42U tall; however, many data centers have racks taller than this.
The term relay rack appeared first in the late 19th century, in the world of telephony.
By 1911, the term was also being used in railroad signaling. There is little evidence that the dimensions of these early racks were standardized.
The 19-inch rack format with rack units of was established as a standard by AT&T around 1922 in order to reduce the space required for repeater and termination equipment in a telephone company central office. The earliest repeaters from 1914 were installed in ad hoc fashion on shelves, in wooden boxes and cabinets. Once serial production started, they were built into custom-made racks, one per repeater. But in light of the rapid growth of the toll network, the engineering department of AT&T undertook a systematic redesign, resulting in a family of modular factory-assembled panels all "designed to mount on vertical supports spaced 19 inches between centers. The height of the different panels will vary,... but... in all cases to be a whole multiple of inches."
By 1934, it was an established standard with holes tapped for 12-24 screws with alternating spacings of and . The EIA standard was revised again in 1992 to comply with the 1988 public law 100-418, setting the standard U as + + , making each U .ANSI/EIA-310-D-1992
The 19-inch rack format has remained constant while the technology that is mounted within it has changed considerably and the set of fields to which racks are applied has greatly expanded. The standard rack arrangement is widely used throughout the telecommunications, computing, audio, video, entertainment and other industries, though the Western Electric 23-inch standard, with holes on centers, is still used in legacy ILEC/CLEC facilities.
Nineteen-inch racks in two-post or four-post form hold most equipment in enterprise data centers, Internet service provider facilities, and professionally designed corporate server rooms, although hyperscale computing typically use wider racks. They allow for dense hardware configurations without occupying excessive floor space or requiring shelving.
Nineteen-inch racks are also often used to house professional audio and video equipment, including amplifiers, effects units, interfaces, headphone amplifiers, and even small-scale audio mixers. A third common use for rack-mounted equipment is industrial power, control, and automation hardware.
Typically, a piece of equipment being installed has a front panel height less than the allotted number of Us. Thus, a 1U rackmount computer is not tall but is tall. If n is the number of rack units, the ideal formula for panel height is for calculating in inches, and for calculating in millimeters. This gap allows a bit of room above and below an installed piece of equipment so it may be removed without binding on the adjacent equipment.
Equipment mounting
Fastening Originally, the mounting holes were tapped with a particular screw thread. When rack rails are too thin to tap, rivet nuts or other threaded inserts can be used, and when the particular class of equipment to be mounted is known in advance, some of the holes can be omitted from the mounting rails.The Computer Rack section of The University of Iowa's DEC PDP-8, documents a relay rack made in 1965; Nov. 2011.
Threaded mounting holes in racks where the equipment is frequently changed are problematic because the threads can be damaged or the mounting screws can break off; both problems render the mounting hole unusable. Tapping large numbers of holes that may never be used is expensive; nonetheless, tapped-hole racks are still in use, generally for hardware that rarely changes. Examples include telephone exchanges, network cabling panels, broadcast studios and some government and military applications.
The tapped-hole rack was first replaced by clearance-hole (Round Hole, Round Unthreaded Holes, and Versa Rail) racks. The holes are large enough to permit a bolt to be freely inserted through without binding, and bolts are fastened in place using cage nuts. In the event of a nut being stripped out or a bolt breaking, the nut can be easily removed and replaced with a new one. Production of clearance-hole racks is less expensive.
The next innovation in rack design has been the square-hole rack. Square-hole racks allow boltless mounting, such that the rack-mounted equipment only needs to insert through and hook down into the lip of the square hole. Installation and removal of hardware in a square-hole rack is very easy and boltless, where the weight of the equipment and small retention clips are all that is necessary to hold the equipment in place. Older equipment meant for round-hole or tapped-hole racks can still be used, with the use of cage nuts made for square-hole racks.
Structural support Rack-mountable equipment is traditionally mounted by bolting or clipping its front panel to the rack. Within the IT industry, it is common for network/communications equipment to have multiple mounting positions, including tabletop and wall mounting, so rack-mountable equipment will often feature L-brackets that must be screwed or bolted to the equipment prior to mounting in a 19-inch rack. With the prevalence of 23-inch racks in the Telecoms industry, the same practice is also common, but with equipment having 19-inch and 23-inch brackets available, enabling them to be mounted in existing racks.
A key structural weakness of front-mounted support is the bending stress placed on the mounting brackets of the equipment and the rack itself. As a result, 4-post racks have become common, featuring a mirrored pair of rear mounting posts. Since the spacing between the front and rear mounting posts may differ between rack vendors and/or the configuration of the rack (some racks may incorporate front and rear rails that may be moved forward and backward, e.g., APC SX-range racks), it is common for equipment that features 4-post mounting brackets to have an adjustable rear bracket.
Servers and deep pieces of equipment are often mounted using rails that are bolted to the front and rear posts (as above, it is common for such rails to have an adjustable depth), allowing the equipment to be supported by four posts, while also enabling it to be easily installed and removed.
Although there is no standard for the depth of equipment, nor specifying the outer width and depth of the rack enclosure itself (incorporating the structure, doors and panels that contain the mounting rails), there is a tendency for 4-post racks to be or wide, and for them to be , or deep. This, of course, varies by manufacturer, the design of the rack and its purpose, but through common constraining factors (such as raised-floor tile dimensions), these dimensions have become quite common. The extra width and depth enable cabling to be routed with ease (also helping to maintain the minimum bend radius for fiber and copper cables) and deeper equipment to be utilized. A common feature in IT racks is mounting positions for zero-U accessories, such as power distribution units (PDUs) and vertical cable managers and ducts, that utilize the space between the rear rails and the side of the rack enclosure.
The strength required of the mounting posts means they are invariably not merely flat strips but actually a wider folded strip arranged around the corner of the rack. The posts are usually made of steel of around 2 mm thickness (the official standard recommends a minimum of 1.9 mm), or of slightly thicker aluminum.
Racks, especially two-post racks, are often secured to the floor or adjacent building structure so as not to fall over. This is usually required by local building codes in seismic zones. According to Telcordia Technologies Generic Requirements document GR-63-CORE, during an earthquake, telecommunications equipment is subjected to motions that can over-stress the equipment framework, circuit boards, and connectors. The amount of motion and resulting stress depends on the structural characteristics of the building and framework in which the equipment is contained and the severity of the earthquake. Seismic racks rated according to GR-63, NEBS Requirements: Physical Protection, are available,Telcordia GR-63-CORE, NEBS Requirements: Physical Protection with Zone 4 representing the most demanding environment.
GR-3108, Generic Requirements for Network Equipment in the Outside Plant (OSP), specifies the usable opening of seismic-compliant 19-inch racks.
Rails (slides) Heavy equipment or equipment that is commonly accessed for servicing, for which attaching or detaching at all four corners simultaneously would pose a problem, is often not mounted directly onto the rack but instead is mounted via rails (or slides). A pair of rails is mounted directly onto the rack, and the equipment then slides into the rack along the rails, which support it. When in place, the equipment may also then be bolted to the rack. The rails may also be able to fully support the equipment in a position where it has been slid clear of the rack; this is useful for inspection or maintenance of equipment, which will then be slid back into the rack.William B. George, Chassis Slide Mechanism, , granted June 4, 1963. Some rack slides even include a tilt mechanism allowing easy access to the top or bottom of rack-mounted equipment when it is fully extended from the rack.Scott F. Herbert, Electronic Assembly Chassis Supporting Track, , granted Oct. 8, 1957.
Slides or rails for computers and other data processing equipment, such as disk arrays or routers, often need to be purchased directly from the equipment manufacturer, as there is no standardization on such equipment's thickness (measurement from the side of the rack to the equipment) or means for mounting to the rail.
A rails kit may include a cable management arm (CMA), which folds the cables attached to the server and allows them to expand neatly when the server is slid out, without being disconnected.
Computer mounting
Computer servers designed for rack-mounting can include a number of extra features to make the server easy to use in the rack: The sliding rails can lock in various extended positions to prevent the equipment from moving when extended out from the rack for service. The server itself might have locking pins on the sides that just drop into slots on the extended rail assembly, in a manner similar to a removable kitchen drawer. This permits very easy server installation and removal since there is no need for the server to be held in midair while someone fastens each rail to the sides of the server with screws.