[QCWA] history of the rack dimensions

[Bob Cumming]

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