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, subrack, 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 (less commonly RU). The industry standard rack cabinet is 42U tall.
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 (482.6 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 (25.4 mm) centers, prevails in telecommunications.
19-inch racks are often used to house professional audio and video equipment, including amplifiers, effects units, interfaces, headphone amplifiers, and even small scale audio mixers. They are also widely used for computer server equipment, allowing for dense hardware configurations without occupying excessive floorspace or requiring shelving. 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 inches / 0.787 millimetres) less than the allotted number of Us. Thus, a 1U rackmount computer is not 1.75 inches (44.5 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 (88.9 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.
In 1965, a durable fiber reinforced plastic 19-inch rackmount case was patented by ECS Composites and became widely used in military and commercial applications for electronic deployment and operation. State-of-the-art rackmount cases are now also constructed of thermo stamped composite, carbon fiber and DuPont’s Kevlar for demanding military and commercial uses.
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