... what is it?
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.
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.
the SRS range of 19" front and rear panels from
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
- IEC 60297-3-101 Part 3-101: Subracks and associated
- IEC 60297-3-102 Part 3-102: Injector/extractor
- IEC 60297-3-102 Part 3-103: Keying and alignment
- 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
- DIN 41494-8 Components on front panels; mounting
- DIN IEC 60297-3-100 (see above in IEC section)
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
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.
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.
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.
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:
- 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
- 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.
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"