A 19-inch rack is a standardized (EIA 310-D, IEC 60297 and DIN 41494 SC48D) system for mounting various electronic modules in a "stack", or rack, wide. Equipment designed to be placed in a rack is typically described as rack-mount, a rack mounted system, a rack mount chassis, subrack, rack mountable, or occasionally, simply shelf. The slang expression for a subrack (generally 1U = 1.75 in = 44.45 mm height) is "pizza box" due to the similarity in size and shape, see also pizza box form factor. Most racks are sold in the 42U form: that is, a single rack capable of holding 42 1U pizza box servers, or any combination of 1U, 2U, 3U or other height units. See Rack unit.
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. This 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 centers, prevails in telecommunications.
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 tall but is tall. 2U would be instead of 3.5-inches. 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.
The formal standards for a 19-inch rack are available from the following:
A rack's mounting fixture consists of two parallel metal strips (also referred to as "rails" or "panel mount") standing vertically. The strips are each wide, and are separated by a gap of , giving an overall rack width of . 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 center-to-center distance of .
The holes in the strips are arranged vertically in repeating sets of three, with center-to-center separations of , , . The hole pattern thus repeats every . 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 centered , 0.875 inch (22.225 mm), and 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 usually designed to occupy some integral number of U. For example, an oscilloscope might be 4U high, and rack-mountable computers are most often 2U or 1U high. Occasionally, one may see fractional U devices such as a 1.5U server, but these are much less common.
The height of a rack can vary from a few inches such as in a broadcast console to a floor mounted rack whose interior is (45 rack units) high. Many wall mounted industrial equipment enclosures have 19" rack rails to support mounting of equipment.
The tapped-hole rack was first replaced by round-hole 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. A cage nut consists of a spring steel cage, designed to clip onto the open mounting hole, within which is a captive nut. In the event of a nut being stripped out or a bolt breaking, the nut can be simply removed and replaced with a new one.
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.
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 2 mm thickness (the official standard recommends a minimum of 1.9 mm), or of slightly thicker aluminum.
Heavy equipment or equipment which 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.
Slides or rails for computers and other data processing equipment such as for a disk array or router often need to be purchased directly from the equipment manufacturer as many are non-standard in terms of how thick they are (from the side of the rack to the equipment) or how they get mounted to the equipment.
Computer servers designed for rack-mounting often include a number of extra features to make the server easy to use in the rack:
Due to the possibility of installing large number of computers into a single rack, it is impractical for each computer to have its own separate keyboard, mouse, and monitor. Instead a controlling device known as a KVM switch is used to share a single keyboard, mouse, and monitor amongst many different computers in the rack at once.
Since the mounting hole arrangement is vertically symmetric, it is possible to mount rack-mountable equipment upside-down. However, not all equipment is suitable for this type of mounting. For instance, most optical disc players will not work upside-down because the driving motor mechanism does not grip the disc.
In all cases, especially with two-post racks, the rack must be secured to that floor or adjacent building structure so as to not fall over. This is required by code in seismic zones. Seismic racks rated according to Telcordia GR-63-CORE are available, with Zone 4 representing the most demanding environment.