A 19-inch rack is a standardized frame or enclosure for mounting multiple equipment modules. Each module has a front panel that is 19 inches (480 mm) wide, including edges or ears that protrude on each side which allow the module to be fastened to the rack frame with screws.
Overview and History
Equipment designed to be placed in a rack is typically described as rack-mount, rack-mount instrument, a rack mounted system, a rack mount chassis, sub-rack, rack mountable, or occasionally simply shelf. The height of the electronic modules is also standardized as multiples of 1.75 inches (44.45 mm) or one rack unit or “U”. Most racks are sold in the 42U form: that is, a single rack capable of holding forty-two 1U devices, or any combination of 1U, 2U, 3U or other rack unit heights that add up to 42 or less. Because of their origin as mounting systems for railroad signaling relays, they are still sometimes called relay racks, but the 19-inch rack format has remained a constant while the technology that is mounted within it has changed to completely different fields. The 19-inch (480 mm) standard rack arrangement is widely used throughout the telecommunication, computing, audio, entertainment and other industries, though the Western Electric 23-inch standard, with holes on 1-inch (25 mm) centers, prevails in telecommunications.
19-inch racks are widely used for computer server equipment, allowing for dense hardware configurations without occupying excessive floor space or requiring shelving.
Typically, a piece of equipment being installed has a front panel height 1/32-inch (.031″) less than the allotted number of Us. Thus, a 1U rackmount computer is not 1.75 inches (44 mm) tall but is 1.719 inches (43.7 mm) tall. 2U would be 3.469 inches (88.1 mm) instead of 3.5 inches (89 mm). 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.
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