[QCWA] history of the rack dimensions

Thu Oct 20 17:51:14 EDT 2011

As a former Bell System employee (1937 - 1940 [interrupted by service in US 
Navy] and 1945 - 1948) let me offer this: Bell System was highly resistant 
to accepting anything that was not developed by Bell Labs or within the Bell 
System (The "NIH" Not invented here complex, we called it. Yet the 19" relay 
rack was standard throughout the system. So I believe it is safe to say that 
it was first used by Bell System. Let me add also: Don't rely to heavily on 
anything you find in Wikipedia!
73, W4TG

----- Original Message ----- 
From: "Don Tucker" <w7wll at arrl.net>
To: <qcwa at mailman.qth.net>
Sent: Wednesday, October 19, 2011 10:31 PM
Subject: Re: [QCWA] history of the rack dimensions

> 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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