A 19-inch rack is a standardized frame or enclosure for mounting multiple electronic equipment modules. Each module has a front panel that is 19 inches (482.6 mm) wide. The 19-inch dimension includes the edges or ears 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 1.75 inches (44.45 mm) 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 1.75 inches (44.45 mm) 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 191⁄2 inches between centers. The height of the different panels will vary,... but... in all cases to be a whole multiple of 1+3⁄4 inches." By 1934, it was an established standard with holes tapped for 12-24 screws with alternating spacings of 1.25 inches (31.75 mm) and 0.5 inches (12.70 mm). The EIA standard was revised again in 1992 to comply with the 1988 public law 100-418, setting the standard U as 15.875 mm (0.625 in) + 15.875 mm (0.625 in) + 12.7 mm (0.500 in), making each U 44.45 millimetres (1.75 in). 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 19-inch (482.6 mm) 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 1-inch (25.4 mm) 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 1⁄32 inch (0.031 in; 0.79 mm) less than the allotted number of Us. Thus, a 1U rackmount computer is not 1.750 inches (44.5 mm) tall but is 1.719 inches (43.7 mm) tall. If n is the number of rack units, the ideal formula for panel height is h = 1.75n − 0.031 for calculating in inches, and h = 44.45n − 0.794 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. 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-mount 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.
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