Editing 19-inch rack

{{Short description|Standard-width frame for electronic equipment}}
{{redirect|Rack-mounted|text=See also [[Industrial PC]]}}
{{Use American English|date=April 2023}} 

[[Image:Chassis-Plans-Rack.jpg|thumb|160px|A full-height rack cabinet]]

A '''19-inch rack''' is a standardized frame or enclosure for mounting multiple electronic equipment modules. Each module has a front panel that is {{convert|19|in|mm|sigfig=4|abbr=}} wide. The 19&nbsp;inch dimension includes the edges or ''ears'' that protrude from each side of the equipment, allowing the module to be fastened to the rack frame with screws or bolts. Common uses include [[computer servers]], [[telecommunications equipment]] and [[networking hardware]], [[audiovisual]] production gear, [[professional audio]] equipment, and [[scientific equipment]].

== 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 cabinet''', '''rack-mountable''', or occasionally simply '''shelf'''. The height of the electronic modules is also standardized as multiples of {{convert|1.75|in|mm|sigfig=4}} or one [[rack unit]] or U (less commonly RU). The industry-standard rack cabinet is 42U tall;<ref>{{cite book |last1=West |first1=Jill |last2=Dean |first2=Tamara |last3=Andrews |first3=Jean |date=2015 |title=Network+ Guide to Networks |chapter=Structured Cabling and Networking Elements |chapter-url=https://books.google.com/books?id=KjadBAAAQBAJ&pg=PA169 |location=Boston, MA |publisher=Engage Learning |page=169 |isbn=9781305480865 |edition=Seventh |access-date=9 December 2019 |quote=Racks are measured in rack units (RU or U) with the industry standard being 42U tall — about 6 feet.}}</ref> however, many data centers have racks taller than this.<ref>{{cite web | url=https://www.datacenterknowledge.com/servers/data-center-racks-getting-taller-wider-deeper | title=Data Center Racks Getting Taller, Wider, Deeper }}</ref>

The term '''relay rack''' appeared first in the world of [[telephony]].<ref>{{cite journal |last=Lowenthal |first=Max |date=February 2, 1899 |title=The New Exchange of the Central New York Telephone and Telegraph Co. at Syracuse, N.Y. |url=https://books.google.com/books?id=bH5NAAAAYAAJ&pg=RA2-PA144 |journal=The Electrical Engineer |volume=XXVII |issue=561 |pages=142–147 |access-date=9 December 2019 }}</ref>

By 1911, the term was also being used in [[railroad signaling]].<ref>{{cite magazine |author=<!--Staff writer(s); no by-line.--> |title=New Electric Interlocking at Allentown, PA |url=https://books.google.com/books?id=z8w0AQAAMAAJ&pg=PA344 |magazine=The Signal Engineer |location=Chicago |publisher=The Railroad Gazette |date=September 1911 |volume=4 |issue=9 |pages=344–345|access-date=9 December 2019 }}</ref> There is little evidence that the dimensions of these early racks were standardized.

[[file:Rack_mounted_telephone_equipment_(1923).png|thumb|right|upright|Telephone equipment racks (1923)]]
The 19-inch rack format with rack-units of {{convert|1.75|in|mm|2}} was established as a standard by AT&T around 1922 in order to reduce the space required for [[repeater]] and termination equipment in a telephone company [[Central office (telecommunications)|central office]]. The earliest repeaters from 1914 were installed in ad hoc fashion on shelves, in wooden boxes and cabinets. Once serial production started, they were built into custom-made racks, one per repeater. But in light of the rapid growth of the toll network, the engineering department of AT&T undertook a systematic redesign, resulting in a family of modular factory-assembled panels all "designed to mount on vertical supports spaced 19{{frac|1|2}} inches between centers. The height of the different panels will vary,... but... in all cases to be a whole multiple of {{frac|1|3|4}} inches."<ref>{{cite magazine |last=Demarest |first=Charles S. |date=July 1923 |title=Telephone Equipment for Long Cable Circuits |url=https://archive.org/details/bstj2-3-112 |magazine=Bell System Technical Journal |volume=2 |issue=3 |location=New York |publisher=American Telephone and Telegraph Company |pages=112–140 |access-date=9 December 2019 }}</ref>

By 1934, it was an established standard with holes [[Tap and die|tapped]] for [[Unified Thread Standard|12-24 screws]] with alternating spacings of {{convert|1.25|in|mm|2}} and {{convert|0.5|in|mm|2}} <ref>{{cite magazine |last=Mezger |first=G. Robert |date=November 1934 |title=The Relay Rack in Amateur Construction |magazine=QST |publisher=American Radio Relay League |pages=27–30 }}</ref> The [[List of EIA standards#300–399|EIA standard]] was revised again in 1992 to comply with the [[Omnibus Foreign Trade and Competitiveness Act|1988 public law 100-418]], setting the standard U as {{convert|15.875|mm|in|3|abbr=on}} + {{convert|15.875|mm|in|3|abbr=on}} + {{convert|12.7|mm|in|3|abbr=on}}, making each ''U'' {{convert|44.45|mm|in|2}}.<ref>ANSI/EIA-310-D-1992</ref>

The 19-inch rack format has remained constant while the technology that is mounted within it has changed considerably and the set of fields to which racks are applied has greatly expanded. The {{convert|19|in|mm|1|adj=on}} standard rack arrangement is widely used throughout the [[Telecommunications industry|telecommunications]], [[computer|computing]], [[Sound recording and reproduction|audio]], [[video]], [[entertainment]] and other industries, though the [[Western Electric]] [[23-inch rack|23-inch standard]], with holes on {{convert|1|in|mm|1|adj=on}} centers, is still used in legacy [[ILEC]]/[[Competitive local exchange carrier|CLEC]] facilities.

Nineteen-inch racks in two-post or four-post form hold most equipment in enterprise [[data centers]], [[ISP]] facilities, and professionally designed corporate [[server room]]s, although [[hyperscale computing]] typically use wider racks.<ref>{{Cite web |title=Open Compute Project |url=https://www.opencompute.org/projects/rack-and-power |access-date=2024-03-01 |website=Open Compute Project |language=en}}</ref><ref>{{Cite book |last1=Barroso |first1=Luiz André |url=https://books.google.com/books?id=b951DwAAQBAJ |title=The Datacenter as a Computer: Designing Warehouse-Scale Machines, Third Edition |last2=Hölzle |first2=Urs |last3=Ranganathan |first3=Parthasarathy |date=2018-10-29 |publisher=Morgan & Claypool Publishers |isbn=978-1-68173-434-7 |language=en}}</ref> They allow for dense hardware configurations without occupying excessive floor space or requiring shelving.

[[File:Tektronix oscilloscope 7603 in 19 inch rack version with three plug-ins.jpg|thumb|right|350px|A professional-grade [[oscilloscope]] [[Tektronix analog oscilloscopes|Tektronix 7603]] (1970s) for use in electronics and scientific laboratories. Typically for rack-mounted devices, the width of the front panel exceeds the width of the device itself, which provides the overlap zone (including screw holes) with the left and right rack rails.]]
Nineteen-inch racks are also often used to house professional audio and video equipment, including [[amplifier]]s, [[effects unit]]s, interfaces, [[headphone]] amplifiers, and even small-scale audio mixers. A third common use for rack-mounted equipment is [[industrial control system|industrial power, control, and automation hardware]].

Typically, a piece of equipment being installed has a front panel height {{convert|1/32|in|in mm|sigfig=2}} less than the allotted number of Us.  Thus, a 1U rackmount computer is not {{convert|1.750|in|mm}} tall but is {{convert|1.719|in|mm}} tall.  If ''n'' is number of rack units, the ideal formula for panel height is {{nowrap|1=''h'' = 1.75''n'' − 0.031}} for calculating in inches, and {{nowrap|1=''h'' = 44.45''n'' − 0.794}} for calculating in millimeters. 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.
{{clear}}

==Equipment mounting==
[[Image:Server rack rail dimensions.svg|thumb|upright|A typical section of {{convert|19|in|mm|1|adj=on}} server rack rail, dimensions noted in US inch]]
[[Image:19" Server rack rail dimensions in mm.svg|thumb|upright|A typical section of {{convert|19|in|mm|1|adj=on}} server rack rail, dimensions noted in mm (exact)]]

===Fastening===
Originally, the mounting holes were [[taps and dies|tapped]] with a particular screw thread. When rack rails are too thin to tap, [[rivet nut]]s or other [[threaded insert]]s can be used, and when the particular class of equipment to be mounted is known in advance, some of the holes can be omitted from the mounting rails.<ref>[http://www.cs.uiowa.edu/~jones/pdp8/UI-8/guide.shtml#pdp8rack The Computer Rack] section of [http://www.cs.uiowa.edu/~jones/pdp8/UI-8/guide.shtml The University of Iowa's DEC PDP-8], documents a relay rack made in 1965; Nov. 2011.</ref>

Threaded mounting holes in racks where the equipment is frequently changed are problematic because the threads can be damaged or the mounting screws can break off; both problems render the mounting hole unusable. Tapping large numbers of holes that may never be used is expensive; nonetheless, tapped-hole racks are still in use, generally for hardware that rarely changes. Examples include telephone exchanges, network cabling panels, broadcast studios and some government and military applications.

The tapped-hole rack was first replaced by clearance-hole (Round Hole, Round Unthreaded Holes,<ref>{{cite web|url=http://www.server-racks.com/rack-hole-types.html|title=Define: Rack Hole Types|website=server-racks.com|date=14 October 2007 }}</ref> and Versa Rail<ref>{{cite web|url=http://www.server-racks.com/dell-versa-rail.html|title=What is a Versa Rail|website=server-racks.com|date=15 October 2007|archive-url=https://web.archive.org/web/20221129080420/https://www.server-racks.com/dell-versa-rail.html|archive-date=2022-11-29|url-status=dead}}</ref>) 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 nut]]s. In the event of a nut being stripped out or a bolt breaking, the nut can be easily removed and replaced with a new one. Production of clearance-hole racks is less expensive.

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 traditionally mounted by bolting or clipping its front panel to the rack. Within the IT industry, it is common for network/communications equipment to have multiple mounting positions, including tabletop and wall mounting, so rack-mountable equipment will often feature L-brackets that must be screwed or bolted to the equipment prior to mounting in a 19-inch rack. With the prevalence of [[#Related standards|23-inch racks]] in the Telecoms industry, the same practice is also common, but with equipment having 19-inch and 23-inch brackets available, enabling them to be mounted in existing racks.

A key structural weakness of front-mounted support is the bending stress placed on the mounting brackets of the equipment, and the rack itself. As a result, 4-post racks have become common, featuring a mirrored pair of rear mounting posts. Since the spacing between the front and rear mounting posts may differ between rack vendors and/or the configuration of the rack (some racks may incorporate front and rear rails that may be moved forwards and backward, e.g. APC SX-range racks), it is common for equipment that features 4-post mounting brackets to have an adjustable rear bracket.

Servers and deep pieces of equipment are often mounted using rails that are bolted to the front and rear posts (as above, it is common for such rails to have an adjustable depth), allowing the equipment to be supported by four posts, while also enabling it to be easily installed and removed.

Although there is no standard for the depth of equipment, nor specifying the outer width and depth of the rack enclosure itself (incorporating the structure, doors and panels that contain the mounting rails), there is a tendency for 4-post racks to be {{convert|600|mm|in|2|abbr=on}} or {{convert|800|mm|in|2|abbr=on}} wide, and for them to be {{convert|600|mm|in|2|abbr=on}}, {{convert|800|mm|in|2|abbr=on}} or {{convert|1010|mm|in|2|abbr=on}} deep. This of course varies by manufacturer, the design of the rack and its purpose, but through common constraining factors (such as raised-floor tile dimensions), these dimensions have become quite common. The extra width and depth enables cabling to be routed with ease (also helping to maintain the minimum bend radius for fiber and copper cables) and deeper equipment to be utilized. A common feature in IT racks is mounting positions for ''zero-U'' accessories, such as [[power distribution unit]]s (PDUs) and vertical cable managers and ducts, that utilize the space between the rear rails and the side of the rack enclosure.

The strength required of the mounting posts means they are invariably not merely flat strips but actually a wider folded strip arranged around the corner of the rack. The posts are usually made of [[steel]] of around 2&nbsp;mm thickness (the official standard recommends a minimum of 1.9&nbsp;mm), or of slightly thicker [[aluminum]].

Racks, especially two-post racks, are often secured to the floor or adjacent building structure so as not to fall over. This is usually required by local building codes in [[Earthquake|seismic zones]]. According to [[Telcordia Technologies]] Generic Requirements document GR-63-CORE, during an earthquake, telecommunications equipment is subjected to motions that can over-stress equipment framework, circuit boards, and connectors. The amount of motion and resulting stress depends on the structural characteristics of the building and framework in which the equipment is contained and the severity of the earthquake. Seismic racks rated according to  [https://telecom-info.njdepot.ericsson.net/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-63& GR-63], ''NEBS Requirements: Physical Protection,'' are available,<ref>[https://telecom-info.njdepot.ericsson.net/ido/AUX/GR_63_TOC.i03.pdf Telcordia GR-63-CORE, NEBS Requirements: Physical Protection]</ref> with Zone 4 representing the most demanding environment.<ref>{{cite web|title=Telcordia GR-1502-CORE, Central Office/Network Environment Detail Engineering Generic Requirements |url=https://telecom-info.njdepot.ericsson.net/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-1502& |archive-url=https://web.archive.org/web/20110716204652/https://telecom-info.njdepot.ericsson.net/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-1502& |archive-date=2011-07-16 |url-status=live |access-date=2009-07-27 }}</ref><ref>{{Cite web |url=http://www.hoffmanonline.com/stream_document.aspx?rRID=16186&pRID=16185 |title=Seismic Enclosures Provide an Extra Measure of Protection |access-date=2007-10-29 |archive-date=2009-01-13 |archive-url=https://web.archive.org/web/20090113154540/http://www.hoffmanonline.com/stream_document.aspx?rRID=16186&pRID=16185 |url-status=dead }}</ref>

[https://telecom-info.njdepot.ericsson.net/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-3108& GR-3108], ''Generic Requirements for Network Equipment in the Outside Plant (OSP),'' specifies the usable opening of seismic-compliant 19-inch racks.

===Rails (slides)===
Heavy equipment or equipment that 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.<ref>William B. George, ''Chassis Slide Mechanism'', {{US patent|3092429}}, granted June 4, 1963.</ref> Some rack slides even include a tilt mechanism allowing easy access to the top or bottom of rack-mounted equipment when it is fully extended from the rack.<ref>Scott F. Herbert, ''Electronic Assembly Chassis Supporting Track'', {{US patent|2809085}}, granted Oct. 8, 1957.</ref>

Slides or rails for computers and other data processing equipment such as [[disk array]]s or [[router (computing)|router]]s often need to be purchased directly from the equipment manufacturer, as there is no standardization on such equipment's thickness (measurement from the side of the rack to the equipment) or means for mounting to the rail.

A rails kit may include a cable management arm (CMA), which folds the cables attached to the server and allows them to expand neatly when the server is slid out, without being disconnected.

===Computer mounting===
[[Image:My Opera Server.jpg|thumb|Example of 19-inch computer rack with servers]]

Computer servers designed for rack-mounting can include a number of extra features to make the server easy to use in the rack:
* The sliding rails can lock in various extended positions to prevent the equipment from moving when extended out from the rack for service.
* The server itself might have locking pins on the sides that just drop into slots on the extended rail assembly, in a manner similar to a removable kitchen drawer. This permits 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.
* Some manufacturers of rack-mount hardware include a folding cable tray behind the server, so that the cables are held into a neat and tidy folded channel when inside the rack, but can unfold 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 mid-air in front of the rack. This piece of equipment thus simplifies maintenance but at the cost of providing a restriction to airflow.
* Rack-optimized servers might duplicate indicator lights on the front and rear of the rack to help identify a machine needing attention or provide separate ''identify'' [[LED]] indicators on both sides of the server (which can be turned on in software or by pushing an associated button). Since some configurations permit over fifty 1U servers in a single rack, this provides a simple method to determine exactly which machine 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.

When there is a large number of computers in a single rack, it is impractical for each one to have its own separate keyboard, mouse, and monitor. Instead, a [[KVM switch]] or [[Out-of-band management|LOM]] software is used to share a single keyboard/video/mouse set amongst many different computers.

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 [[electric motor|motor]] mechanism does not grip the disc.

=== Rack types ===
19-inch server racks can vary in quality. A standard 19-inch server rack cabinet is typically 42u in height, {{convert|600|mm|in}} wide, and {{convert|36|in|mm|2}} deep.<ref>{{Cite web |url=https://www.apc.com/shop/th/en/products/NetShelter-SX-42U-600mm-Wide-x-1070mm-Deep-Enclosure-with-Sides-Black/P-AR3100 
|title=APC Netshelter SX, Server Rack Enclosure, 42U, Black, 1991H x 600W x 1070D mm - APC Thailand}}</ref> This comprises a volume of 974 L, or just under a cubic meter. Newer server rack cabinets come with adjustable mounting rails allowing the user to place the rails at a shorter depth if needed. There are a multitude of specialty server racks including soundproof server racks, air-conditioned server racks, NEMA-rated, seismic-rated, open frame, narrow, and even miniature 19-inch racks for smaller applications. Cabinets are generally sized to be no wider than the standard {{convert|24|in|mm|adj=mid|-wide}} floor tiles used in most data centers.

Racks carrying telecom equipment like routers and switches often have extra width to accommodate the many cables on the sides.

==== Four-post cabinet racks ====
''Four-post racks'' allow for mounting rails to support the equipment at the front and rear. These racks may be open in construction without sides or doors or may be enclosed by front and/or rear doors, side panels, and tops.<ref>{{cite web|title=Telcordia GR-3108-CORE, NEBS Requirements for Telecommunications Data Center Equipment and Spaces |url=https://telecom-info.njdepot.ericsson.net/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-3108& |archive-url=https://web.archive.org/web/20090828052023/https://telecom-info.njdepot.ericsson.net/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-3108& |archive-date=2009-08-28 |url-status=live |access-date=2009-07-24 }}</ref> Most data centers use four-post racks.

==== Two-post relay racks ====
''Two-post racks'' provide two vertical posts. These posts are typically heavy gauge metal or extruded aluminum. A top bar and wide foot connect the posts and allow the rack to be securely attached to the floor and/or roof for seismic safety. Equipment can be mounted either close to its center of gravity (to minimize load on its front panel), or via the equipment's front panel holes.<ref name="RelayRack">{{cite web | url= http://www.budind.com/pdf/hb1272.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.budind.com/pdf/hb1272.pdf |archive-date=2022-10-09 |url-status=live
| title= Aluminum Relay Rack
| publisher= Bud Industries
| access-date= 2017-12-26}}</ref> The ''Relay Racks'' name comes from early two-post racks which housed telephone relay and switching equipment. Two-post racks are most often used for telecommunication installations.

==== ATA road case racks ====
[[File:Amps (QuadFest) (edited).jpg|thumb|Rack-mounted audio [[power amplifier]]s of a [[sound reinforcement system]]. The racks each have large heavy-duty rollers. This is typical for relatively impromptu venues such as open-air concerts.]]

19-inch equipment that needs to be moved often or protected from harsh treatment can be housed in a ''[[road case]]'' approved by the [[Air Transport Association of America]] (ATA), sometimes also referred to as a ''flight case''. Road cases typically have plywood sides laminated with [[polyvinyl chloride]] (PVC), extruded aluminum edges, steel corners, handles, and latches. Larger cases typically have wheels for easy transport. Road case racks come in different heights based on the 1U standard and different depths. Non-isolated cases simply mount 19-inch mounting posts inside the case. To protect equipment from shock and vibration road rack cases use an inner and outer case. These cases can be isolated by thick layers of foam or may use spring-loaded shock mounting.  Touring musicians, theatrical productions and sound and light companies use road case racks.<ref name="ATARoadCase">{{cite web
| url= http://www.anvilcase.com/applications/racks/
| title= Standard Rack Mount Cases
| publisher= Anvil Case
| access-date= 2017-12-26
| archive-url= https://web.archive.org/web/20180226172125/http://www.anvilcase.com/applications/racks/
| archive-date= 2018-02-26
| url-status= dead
}}</ref>

==== Fiberglass-reinforced plastic case racks ====
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.{{citation needed|date=March 2023}} Rackmount cases are also constructed of thermo-stamped composite, [[carbon fiber]], and [[DuPont]]'s [[Kevlar]] for military and commercial uses.{{citation needed|date=March 2023}}

==== Polyethylene molded case racks ====
Portable rack cases using a rotary-molded [[polyethylene]] outer shell are a lower-cost alternative to the more durable ATA-approved case. These cases are marketed to musicians and entertainers for equipment not subject to frequent transportation and rough handling. The polyethylene shell is not fiberglass reinforced and is not rigid. The shape of small cases is maintained by the rack rails and the cover seal extrusions alone. Larger cases are further reinforced with additional plywood or sheet metal. The outer shell is frequently embossed in a self-mating pattern to combat the tendency for stacked cases to deform slightly creating a slope that encourages the upper case to slide off. The cases typically use extruded aluminum bands at the ends of the body with tongue-and-groove mating to like bands for the covers. End covers are typically secured with either a simple draw latch or a rotary cam ''butterfly'' latch, named for the shape of the twist handle.

==Cooling==
{{main|Computer cooling#Servers}}
There is no standard for airflow and cooling of rack-mounted equipment. A variety of airflow patterns can be found, including front intakes and rear exhausts, as well as side intakes and exhausts. Low-wattage devices may not employ active cooling, but use only passive thermal radiation and [[convection]] to dissipate heat.

For rack-mounted computer servers, devices generally intake air on the front and exhaust on the rear. This prevents circular airflows where hot exhaust air is recirculated through an adjacent device and causes overheating.

Although open-frame racks are the least expensive, they also expose air-cooled equipment to dust, lint, and other environmental contamination. An enclosed sealed cabinet with forced air fans permits air filtration to protect equipment from dust.

Large server rooms will often group rack cabinets together so that racks on both sides of an aisle are either front-facing or rear-facing, which simplifies cooling by supplying cool air to the front of the racks and collecting hot air from the rear of the racks. These aisles may themselves be enclosed into a cold air containment tunnel so that cooling air does not travel to other parts of the building where it is not needed or mixes with hot air, making it less efficient. [[Raised floor|Raised or false floor]] cooling in server rooms can serve a similar purpose; they permit cooling airflow to equipment through the underfloor space to the underside of enclosed rack cabinets.<ref>{{Cite web|url=https://schroff.pentair.com/en/schroff/cooling|title=Data Center Cooling {{!}} Pentair|website=schroff.pentair.com|language=en|access-date=2017-08-27|archive-date=2017-08-27|archive-url=https://web.archive.org/web/20170827210722/https://schroff.pentair.com/en/schroff/cooling|url-status=dead}}</ref>

A difficulty with forced air fan cooling in rack equipment is that fans can fail due to age or dust. The fans themselves can be difficult to replace. In the case of network equipment, it may be necessary to unplug 50 or more cables from the device, remove the device from the rack, and then disassemble the device chassis to replace the fans.

However, some rack equipment has been designed to make fan replacement easy, using quick-change fan trays that can be accessed without removing the cabling or the device from the rack, and in some cases without turning off the device so that operation is uninterrupted during replacement.

==Specifications==
[[Image:Chassis-Plans-Rack-Detail.jpg|thumb|Computer keyboard and monitor mounted on a sliding tray in a rack]]
The formal standards for a {{convert|19|in|mm|1|adj=on}} 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)
* [[Consumer Electronics Association]] CEA-310-E design requirements for Cabinets, Panels, Racks and Subracks., dated December 14, 2005
* [[International Electrotechnical Commission]] - Multiple documents are available in French and English versions.
** IEC 60297 ''Mechanical structures for electronic equipment - Dimensions of mechanical structures of the 482,6&nbsp;mm (19&nbsp;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 60297-3-103 ''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
* {{lang|de|[[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&nbsp;mm (19&nbsp;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 ''posts'' or ''panel mounts'') standing vertically. The posts are each {{convert|0.625|in|mm|2}} wide, and are separated by a gap of {{convert|17.75|in|mm|2}}, giving an overall rack width of {{convert|19|in|mm|2}}. The posts have holes in them at regular intervals, with both posts matching, so that each hole is part of a horizontal pair with a center-to-center distance of {{convert|18.312|in|mm|2}}.<ref>{{cite web|url=http://www.server-racks.com/eia-310.html|title=The Server Rack FAQ - Define EIA-310|website=server-racks.com|date=12 October 2007 }}</ref>

The holes in the posts are arranged vertically in repeating sets of three, with center-to-center separations of {{convert|0.5|in|mm|2}}, {{convert|0.625|in|mm|2}}, {{convert|0.625|in|mm|2}}. The hole pattern thus repeats every {{convert|1.75|in|mm|2}}.

Holes so arranged can either be tapped (usually 10-32 [[Unified Thread Standard|UNF]] thread, or, less often, 6mm [[Metric thread|metric]]) or have square holes for [[cage nut]]s.

=== Rack unit ===
{{Main|Rack unit}}
Racks are vertically divided into regions, {{convert|44.45|mm|in|2|abbr=}} in height.  Each region has three complete hole pairs on each side. The holes are centered at {{convert|6.35|mm|in|2}}, {{convert|22.25|mm|in|2}}, and {{convert|38.1|mm|in|2}} from the top or bottom of the region. Such a region is commonly known as a '''U''', for ''unit'', '''RU''' for ''rack unit'' or, in German, '''HE''', for ''[[:de:Höheneinheit|Höheneinheit]]''. Heights within racks are measured by this unit. Rack-mountable equipment is usually designed to occupy some integer number of U. For example, an [[oscilloscope]] might be 4U high. Rack-mountable computers and servers are mostly between 1U and 4U high. A [[blade server]] enclosure might require 10U.

Occasionally, one may see fractional U devices such as a 1.5U server or devices that are just 22.5 or 15&nbsp;cm in width, allowing for 2 or 3 such devices to be installed side by side, 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 ({{convert|200.2|cm|in|2|disp=or}}) high. 42U is a common configuration. Many wall-mounted enclosures for industrial equipment use 19-inch racks.

==Related standards==
=== 10-inch rack ===
[[File:19 inch vs 10 inch correct rack dimensions.svg|thumb|Comparison between 10-inch and 19-inch rack dimensions]]
Some telecommunications and networking equipment is available in a narrower 10-inch format with the same unit height as a standard 19-inch rack.
This standard is also gaining in popularity with hobbyists with limited space at their disposal.<ref>{{cite web |title=Project MINI RACK |url=https://mini-rack.jeffgeerling.com/#community |website=Project MINI RACK |publisher=Jeff Geerling}}</ref>

=== 11-foot frame ===

Frames for holding rotary-dial telephone equipment such as step-by-step telephone switches were generally {{convert|11|ft|6|in}} high. A series of studies led to the adoption of frames {{convert|7|ft}} high, with modular widths in multiples of {{convert|1|ft|1|in|}}—most often {{convert|2|ft|2|in}} wide.<ref name="keller">{{cite journal|last1=Keller|first1=A. C.|title=Recent Developments in Bell System Relays — Particularly Sealed Contact and Miniature Relays|journal=Bell Labs Technical Journal|date=January 1964|volume=43|pages=15–44|doi=10.1002/j.1538-7305.1964.tb04057.x}}</ref>

===ETSI rack===
[[File:Dimensions 19-inch ETSI rack.png|thumb|Comparison between 19-inch and ETSI rack dimensions]]
The '''ETSI rack''' is defined by the [[European Telecommunications Standards Institute]] (ETS 300 119). The distance of the right edge of the right mounting rail to the left edge of the left mounting rail is {{convert|535|mm}}. As 535&nbsp;mm is very close to 21 inches, these racks are sometimes called ''21-inch racks''. The gap between the posts is {{convert|500|mm|in|2}}. As 19-inch equipment has a maximum width of {{convert|17+1/4|in|mm|2}}, they can easily be mounted in an ETSI rack by means of an ETSI bracket or adapter plate.

In contrast to the 19-inch standards, ETSI also defined the size of the rack enclosure: the four allowed widths are {{convert|150|,|300|,|600|,|900|mm|in}} and two allowed depths are {{convert|300|and|600|mm|in}}. Hole spacing is {{convert|25|mm|in}}.

===23-inch rack===
[[File:SSEM Manchester museum close up.jpg|left|thumb|200px|The 1948 original [[Manchester Baby]] and the 1998 working replica (pictured) were mounted on 23-inch racking.<ref>{{cite AV media
 | year = 2013
 | title = The Manchester Baby : The First Stored Program Computer
  | publisher = Google
}}</ref>]]

A {{convert|23|in|mm|adj=on}} rack is used for housing [[telephone]] (primarily), [[computer]], [[sound|audio]] and other equipment – although it is less common than the 19-inch rack. The size denotes the width of the faceplate for the installed equipment. The [[rack unit]] is a measure of vertical spacing and is common to both the 19 and 23-inch racks.

Hole spacing is either on {{convert|1|in|mm|adj=on}} centers (Western Electric standard), or the same as for {{convert|19|in|mm|adj=on|1|abbr=}} racks ({{convert|0.625|in|abbr=on|disp=or}} spacing).
{{clear}}

===Open Rack===
{{Main|Open Rack}}

Open Rack is a mounting system designed by [[Facebook]]'s [[Open Compute Project]] that has the same outside dimensions as typical 19-inch racks (e.g. 600&nbsp;mm width), but supports wider equipment modules of {{convert|547|mm|in|}}.<ref>{{cite web|url=http://www.opencompute.org/blog/open-rack-1-0-specification-available-now/|title=Open Rack 1.0 Specification Available Now|work=Open Compute}}</ref>

===OpenVPX===
{{Main|VPX}}
OpenVPX is an industry standard for modules that can be installed in chassis in defense applications. The modules can provide a variety of functions, similarly to equipment in a rack.<ref>{{cite web | url=https://www.techbriefs.com/component/content/article/8407-28057-450 | title=OpenVPX — from Concept to Specification | date=September 2010 }}</ref> 

===Eurorack===
{{Main|Eurorack}}
Eurorack is a standard generally used in electronic music production to install components of a [[modular synthesizer]], an electronic music instrument. This is achieved with a rack mounting system specific to the standard.<ref>{{cite web | url=https://www.musicradar.com/news/9-things-every-eurorack-beginner-should-know-1 | title=9 things every Eurorack beginner should know | date=9 February 2022 }}</ref>

===Eurocard===
{{Main|Eurocard (printed circuit board)}}
Eurocard is an [[IEEE]] standard format for [[printed circuit board]] (PCB) cards that can be plugged together into a standard chassis which, in turn, can be mounted in a 19-inch rack. The chassis consists of a series of slotted card guides on the top and bottom, into which the cards are slid so they stand on end, like books on a shelf. At the spine of each card is one or more connectors which plug into mating connectors on a backplane that closes the rear of the chassis.<ref>{{cite journal | url=https://ieeexplore.ieee.org/document/5186728 | doi=10.1049/ep.1983.0192 | title=Eurocard bus standards | journal=Electronics and Power | date=1983 | volume=29 | issue=6 | page=453 }}</ref><ref>{{cite web | url=https://standards.ieee.org/ieee/1101.2/1646/ | title=IEEE Standards Association }}</ref><ref>{{cite book | url=https://books.google.com/books?id=-vQCEAAAQBAJ&dq=eurocard+ieee&pg=PA160 | title=Foundations of Computer Technology | isbn=978-1-000-15371-2 | last1=Anderson | first1=Alexander John | date=25 October 2020 | publisher=CRC Press }}</ref> Several standards such as [[VMEbus]] (Versa Module Eurocard<ref>{{cite web |url=https://www.vita.com/VMEbus-FAQ |title= VMEbus FAQa |access-date=2023-01-17}}</ref> bus), [[STEbus]],<ref>{{Cite journal |last1=Barratt |first1=P. |last2=Lloyd |first2=R. D. |date=1998 |title=Upgrade of The Isis Pre-Injector EHT Generator |s2cid=53471257 }}</ref> [[PCI eXtensions for Instrumentation]], openGear<ref>{{cite web | url=https://www.tvtechnology.com/equipment/opengear-celebrates-15-years-of-success | title=OpenGear Celebrates 15 Years of Success | date=23 December 2021 }}</ref> and others were developed, which share goals with Eurocard.

== Gallery ==

<gallery mode=packed heights="140px" style="text-align:left">
DL380sREAR.jpg|A server rack seen from the rear
Wikimedia Foundation Servers-8055 08.jpg|[[Wikimedia Foundation]] servers as seen from the front
Wikimedia Foundation Servers-8055 23.jpg|Wikimedia Foundation servers as seen from the rear
Wikimedia Foundation Servers-8055 02.jpg|Wikimedia Foundation servers as seen from the rear
Fuji TV (5712870769).jpg|19-inch racks with video equipment
</gallery>

==See also==
* [[Data center]]
* [[Horizontal pitch]]
* [[Rack unit]]
* [[Transit case]], for rack cases
* [[Open Compute Project]], wider, deeper rack design

==References==
{{Reflist}}

== External links ==
* {{Commons category-inline}}
{{Computer sizes}}
{{List of IEC standards}}

[[Category:Computer enclosure]]
[[Category:Mechanical standards]]
[[Category:Server hardware]]
[[Category:American inventions]]
[[Category:1922 introductions]]