What is a 19" Subrack?

19" Rack
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, 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, 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.

What is a "U"

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 1.719 inches (43.7 mm) tall. 2U would be 3.469 inches (88.1 mm) 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. See the SRS range of 19" subracks and enclosures

Specifications
The formal standards for a 19-inch rack are available from the following:

• Electronic Industries Alliance EIA-310-D, Cabinets, Racks, Panels, and Associated Equipment, dated September, 1992. (Latest Standard Now REV E 1996)
• International Electrotechnical Commission Multiple documents in available in French and English versions.
  • IEC 60297 Mechanical structures for electronic equipment - Dimensions of mechanical structures of the 482,6 mm (19 in) series
  • IEC 60297-1 Replaced by IEC 60297-3-100
  • IEC 60297-2 Replaced by IEC 60297-3-100
  • IEC 60297-3-100 Part 3-100: Basic dimensions of front panels, subracks, chassis, racks and cabinets
  • IEC 60297-3-101 Part 3-101: Subracks and associated plug-in units
  • IEC 60297-3-102 Part 3-102: Injector/extractor handle
  • IEC 60297-3-102 Part 3-103: Keying and alignment pin
  • IEC 60297-3-104 Part 3-104: Connector dependent interface dimensions of subracks and plug-in units
  • IEC 60297-3-105 Part 3-105: Dimensions and design aspects for 1U chassis
  • IEC 60297-4 Replaced by IEC 60297-3-102
  • IEC 60297-5 Multiple documents, -100, 101, 102, ... 107, replaced by IEC 60297-3-101
• Deutsches Institut für Normung DIN 41494 - Multiple documents in German but some documents are available in English.
  • DIN 41494 Equipment practices for electronic equipment; mechanical structures of the 482,6 mm (19 inch) series
  • DIN 41494-7 Dimensions of cabinets and suites of racks.
  • DIN 41494-8 Components on front panels; mounting conditions, dimensions
  • DIN IEC 60297-3-100 (see above in IEC section)

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 0.625 inches (15.9 mm) wide, and are separated by a gap of 17.75 inches (451 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 center-to-center distance of 18.3 inches (460 mm). The holes in the strips are arranged vertically in repeating sets of three, with center-to-center separations of 0.5 inches (13 mm), 0.625 inches (15.9 mm), 0.625 inches (15.9 mm). The hole pattern thus repeats every 1.75 inches (44 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 centered 0.25 inches (6.4 mm), 0.875 inch (22.225 mm), and 1.5 inches (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 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 78.75 inches (2,000 mm) (45 rack units) high. Many wall mounted industrial equipment enclosures have 19" rack rails to support mounting of equipment.

Equipment Mounting: Fastening Originally, the mounting holes were tapped to receive a particular type of threaded bolt. This is still frequently used in government and military applications, often in conjunction with slide rails for ease of maintenance. However, it is no longer typical for frequently changed server racks, due to the possibility for the threads to become damaged or for a bolt to bind and break off, rendering the mounting hole unusable. Tapped-hole racks are still used for hardware that rarely changes, such as phone or network cabling panels and relay racks. 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.

Structural Support
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; 31.5 inches (800 mm) is typical, and equipment is often designed to handle a range of rack depths. Depth of 39.4 inches (1,000 mm) is getting increasingly common, more depth allows for more space for routing cables at the back.

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.

Rails (Slides)
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 Mounting
Computer servers designed for rack-mounting often include a number of extra features to make the server easy to use in the rack:

• The sliding rails can lock in the extended position to prevent the equipment from moving when extended out from the rack into the service position.
• The server often has locking pins on the sides that just drop into slots on the extended rail assembly. This permits a very easy server installation and removal since there is no need for the server to be held in midair while someone fastens each rail to the sides of the server with screws.
• The rack-mount hardware often includes a folding cable tray behind the server, so that the cables are held into a neat and tidy folded channel when inside the rack, and unfolds out into a long strip when pulled out of the rack, allowing the server to continue to be plugged in and operating normally even while fully extended and hanging in midair in front of the rack. This cable tray also helps prevent an often huge cable tangle from forming at the rear of the rack, as unbound cables from upper equipment drape down onto equipment below.
• Rack-optimized servers often include an indicator light on the front and rear of the rack to help identify the problem machine. Since there can be up to 45 1U servers in a single rack, it can be difficult to determine exactly which machine seen from the front is having a problem when at the rear of the rack.
• A handle may be provided at the rear of the server rails, to help pull or push the server without having to pull on the cables.

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.

See the SRS range of 19" subracks and enclosures

This article is licensed under the GNU Free Documentation License.
It uses material from the Wikipedia article "19" Rack".