[QCWA] history of the rack dimensions

Wed Oct 19 22:31:00 EDT 2011

Yup, been through it all, even books at the library, Bob. I even queried a 
lot of the engineers I worked with who were involved in designing products 
that would be rack mounted. They didn't know either.  All of the available 
information does not answer the question of who designed or engineered this 
set of dimensions or for whom and when. I used to work on several 
electronics trade association committees and even though the first standards 
for racks were issued by EIA and the military, no one could explain its 
origins except in very general terms, like in Wikipedia.

There must have been some 'reason' for the odd dimensions. Why 19 inches vs 
20 inches. And why the strange panel heights and vertical mounting spacing. 
Just noting it originated with railroad signaling relays, thus relay rack, 
doesn't convey much information.

It seems like a standard that suddenly just was there without questions.

Don W7WLL
----- Original Message ----- 
From: "Bob Cumming" <w2bzy at cfl.rr.com>
To: <qcwa at mailman.qth.net>
Sent: Wednesday, October 19, 2011 2:41 PM
Subject: [QCWA] history of the rack dimensions

Per:  en.wikipedia.org/wiki/19-inch_rack

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 mountable, or occasionally simply shelf.
The height of the electronic modules is also standardized as
multiples of 1.75 inches (44.5 mm) or one
<http://en.wikipedia.org/wiki/Rack_unit>rack unit or U (less commonly RU).

Because of their origin as mounting systems for
<http://en.wikipedia.org/wiki/Railroad_signaling>railroad signaling
<http://en.wikipedia.org/wiki/Relay>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. The 19-inch (482.6 mm) standard rack
arrangement is widely used throughout the
<http://en.wikipedia.org/wiki/Telecommunication>telecommunication,
<http://en.wikipedia.org/wiki/Computer>computing,
<http://en.wikipedia.org/wiki/Sound_recording_and_reproduction>audio,
<http://en.wikipedia.org/wiki/Entertainment>entertainment and other
industries, though the
<http://en.wikipedia.org/wiki/Western_Electric>Western Electric
<http://en.wikipedia.org/wiki/23-inch_rack>23-inch standard, with
holes on 1-inch (25.4 mm) centers, prevails in telecommunications.

19-inch racks are often used to house professional audio and video
equipment, including
<http://en.wikipedia.org/wiki/Amplifier>amplifiers,
<http://en.wikipedia.org/wiki/Effects_unit>effects units, interfaces,
<http://en.wikipedia.org/wiki/Headphone>headphone amplifiers, and
even small scale audio mixers. They are also widely used for
<http://en.wikipedia.org/wiki/Computer_server>computer server
equipment, allowing for dense hardware configurations without
occupying excessive floorspace or requiring shelving. A third common
use for rack-mounted equipment is industrial power, control, and
automation hardware.

Typically, a piece of equipment being installed has a front panel
height 1/32-inch (0.031 inches (0.787 mm)) less than the allotted
number of Us. Thus, a 1U rackmount computer is not 1.75 inches (44.5
mm) tall but is 1.719 inches (43.7 mm) tall. 2U would be 3.469 inches
(88.1 mm) instead of 3.5 inches (88.9 mm). 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.

In 1965 a durable
<http://en.wikipedia.org/wiki/Fiber_reinforced_plastic>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.
State-of-the-art rackmount cases are now also constructed of thermo
stamped composite, <http://en.wikipedia.org/wiki/Carbon_fiber>carbon
fiber and <http://en.wikipedia.org/wiki/DuPont>DuPont's
<http://en.wikipedia.org/wiki/Kevlar>Kevlar for demanding military
and commercial uses.

Fastening

Originally, the mounting holes were
<http://en.wikipedia.org/wiki/Taps_and_dies>tapped to receive a
particular type of threaded <http://en.wikipedia.org/wiki/Screw>bolt.
This is still frequently used in some 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, network cabling panels, TV
broadcasting facilities, studios and relay racks.

The tapped-hole rack was first replaced by clearance-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
<http://en.wikipedia.org/wiki/Cage_nut>cage nut consists of a
<http://en.wikipedia.org/wiki/Spring_steel>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 easily removed and replaced with a new one.
Production of clearance-hole racks is less expensive because tapping
the holes is eliminated and replaced with fewer, less expensive, cage nuts.

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's
common for network/communications equipment to have multiple mounting
positions, including table-top 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 <http://en.wikipedia.org/wiki/23-inch_rack>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
<http://en.wikipedia.org/wiki/Shear_stress>shear stress placed on the
mounting rails and the leading edge of the equipment. As a result,
4-post racks have become common, with such racks 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 backwards, i.e. APC SX-range
racks), it's 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's common
for such rails to have an adjustable depth), allowing the equipment
to be supported by 4-posts, whilst also enabling it to be easily
installed and removed.

While 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 600mm or 800mm wide, and
for them to be 600mm, 800mm or 1010mm deep. This of course varies by
manufacturer, the design of the rack and it's 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
bend-radius for fibre and copper cables) and deeper equipment to be
utilised. A common feature in IT racks, are mounting positions for
"Zero-U" accessories, such as PDU (power distribution units) and
vertical cable managers/ducts, that utilise 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
<http://en.wikipedia.org/wiki/Steel>steel of around 2 mm thickness
(the official standard recommends a minimum of 1.9 mm), or of
slightly thicker <http://en.wikipedia.org/wiki/Aluminum>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
<http://en.wikipedia.org/wiki/Earthquake>seismic zones. According to
<http://en.wikipedia.org/wiki/Telcordia_Technologies>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 Telcordia GR-63-CORE are
available,<http://en.wikipedia.org/wiki/19-inch_rack#cite_note-0>[1]
with Zone 4 representing the most demanding
environment.<http://en.wikipedia.org/wiki/19-inch_rack#cite_note-1>[2]<http://en.wikipedia.org/wiki/19-inch_rack#cite_note-2>[3]
Telcordia GR-3108-CORE specifies the usable opening of
seismic-compliant 19-inch racks.

3U rackmount system

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 <http://en.wikipedia.org/wiki/Disk_array>disk arrays or
<http://en.wikipedia.org/wiki/Router_%28computing%29>routers 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.

See also:  www.wisegeek.com/what-is-a-relay-rack.htm and
www.facebook.com/pages/Relay-rack/109310605767033?sk=info

www.novexcomm.com/   discusses how Ham Radio is rediscovering the rack 
mount.

If you Google - 19" relay rack history  you will get many. many pages
of info to wade through.

VRY 73

Bob Cumming
W2BZY
QRV 160M-3CM
from el98hr

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