19-inch rack
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A 19-inch rack is a standardized (EIA 310-D, IEC 60297 DIN 41494 SC48D) system for mounting various electronic modules in a "stack", or rack, 19 inches (480 mm) wide. Equipment designed to be placed in a rack is typically described as rack-mount, a rack mounted system, a rack mount chassis, subrack, or occasionally, simply shelf.
Because of their origin as mounting systems for telephone switching equipment, they are still sometimes called relay racks, but the 19-inch rack format has remained a constant whilst the technology that is mounted within it has changed beyond all recognition. This standard rack arrangement is widely used throughout the telecommunication, computing, audio and video industries, as well as others.
The rack's mounting fixture consists of two parallel metal strips (also referred to as "rails") standing vertically. The strips are each 0.625 inches (15.9 mm) wide, and are separated by a gap of 17.75 inches (450.9 mm), giving an overall rack width of 19 inches (480 mm). The strips have holes in them at regular intervals, with both strips matching, so that each hole is part of a horizontal pair with a centre-to-centre distance of 18.3 inches.
The holes in the strips are arranged vertically in repeating sets of three, with centre-to-centre separations of 0.5 inch (10 mm), 0.625 inch (15.9 mm), 0.625 inch (15.9 mm). The hole pattern thus repeats every 1.75 inches (44.5 mm). Racks are divided into regions, 1.75 inches in height, within which there are three complete hole pairs in a vertically symmetric pattern, the holes being centred 0.25 inch (6.4 mm), 0.875 inch (22.2 mm), and 1.5 inch (38 mm) from the top or bottom of the region. Such a region is commonly known as a "U", for "unit", and heights within racks are measured by this unit. Rack-mountable equipment is always designed to occupy some integral number of U. For example, an oscilloscope might be 4 U high, and rack-mountable computers are most often 2 U or 1 U high.
Originally the mounting holes were tapped to receive a particular type of threaded bolt. Racks with plain square holes are now common. Square-holed racks allow boltless mounting, and can be adapted for use with bolts by the use of cage nuts. A cage nut consists of a spring steel cage, designed to clip onto a square mounting hole, within which is a captive nut.
Rack-mountable equipment is mounted simply by bolting its front panel to the rack, or with a square-holed rack by clipping or some other variation on the theme. Having all the structural support at one edge of the equipment is a weakness of this system, and so heavier equipment is designed to use a second pair of mounting strips located at the back of the equipment. Various spacings between the front and back strips are used; 800 mm is typical, and equipment is often designed to handle a range of rack depths.
The strength required of the mounting strips means they are invariably not merely flat strips but actually a wider folded strip arranged around the corner of the rack. The strips are usually made of steel of around 1.5 mm thickness, or of slightly thicker aluminium.
Heavy equipment, 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. A light 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, but this is to stop it falling out rather than for structural support. 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.
Because the mounting hole arrangement is vertically symmetric, rack-mountable equipment can be mounted upside-down. Not all equipment is suitable for this, however.