[SBE] Rack standards

[Bruce Doerle]

Apparently, we are not the only ones who have questioned the origin of the 19" rack width.  Here is a post from a thread from another list server, Classic Computing.

http://www.classiccmp.org/pipermail/cctech/2006-May/020764.html

On May 1, 2006, at 11:51 AM, Lyle Bickley wrote:


> Here's an interesting link on the subject. The short of it the  

> origin of 19"

> racks were as Telco switching "relay racks".

>

> http://encyclopedia.thefreedictionary.com/19%20inch%20rack


And the rest of the story (so to speak) as to _why_ 19":

I asked a fellow "middle aged" phart who (also) worked with genuine
Bell System auld pharts to verify my memory of where this came from --
it was all a matter of convenience (and cost).

The engineering department (Bell Labs) planned equipment layouts  
based on
a 24" grid system. To build the relay racks, channel iron 2.5" deep  
(measured
on the outside) was used as it was very common and relatively cheap.
24 - (2.5*2) = 19 inches for the shelf space.

The above was designed for the first relay-based telephone switches.

Mike


How much weight you put to this is your call.

Bruce
bcd at wucf.org


-----Original Message-----
From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org] On Behalf Of Chriss Scherer
Sent: Friday, October 28, 2011 1:10 AM
To: sbe member discussion mail list
Subject: Re: [SBE] Rack standards

I was just going to say that you got lots of answers, just not the answer to the actual question.





On 10/27/11, Dan Rapak  <DanRapak at verizon.net> wrote:
Okay. but again, why 19" and not 18" or 20" or 19 1/2"?
 
 
From: Edwin Bukont
Well, railroads widths are derived from an english 'coach' standard, that is based on the width of chariot wheels, or more correctly the width of the ruts left by such wheels, which are spaced upon the width that a horse's behind occupies between the wheels. the chariot was imported to england from rome.
 
don't forget we have not only differing widths of 23" and 19" but also differing screws, that being 12-24 (telecom) and 10-32 (eia)
 
the 12-24 is thicker, but with fewer TPI, it actually has less strength than the 'smaller' 10-32 that has a higher TPI.  This is the same dynamic as pertains to speaker hanging, where 'fine' threaded rod is used, rather than coarse, as it withstands vibration better.

 


From: wgbw at lsol.net
this stuff goes back to the age of the Trains, and Western Union / Western Electric, and patents.

long, long, long, before broadcast was catching on....

24" was one standard,   19" allowed for 18" of clearance for equipment.

back then, they actually enforced patents....  you had to change enough of the original invention to make is something different....thus the two sizes.

there was a time, in the United States, that railroad tracks had different spacing, too... it changed in 1865 or thereabouts.







--------------------------------------------------------------------------------
From: "Dan Rapak" <DanRapak at verizon.net> ? 
The article explains the origin, but I still have no idea why 19", etc. were chosen as the numbers. Anyone have any knowledge of that part of the equation?
 
Dan

 
From: Jonstv
At first I was going to comment "wikipedia.... Must be true ;)". But that is actually a good article!



On Oct 25, 2011, at 3:05 PM, "Smith, Jeff" <JEFFSMITH at clearchannel.com> wrote:
Right from Wikipedia...
http://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 rack unit or U (less commonly RU).

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. The 19-inch (482.6 mm) 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 (25.4 mm) centers, prevails in telecommunications.

19-inch racks are often used to house professional audio and video equipment, including amplifiers, effects units, interfaces, headphone amplifiers, and even small scale audio mixers. They are also widely used for 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 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, carbon fiber and DuPont's Kevlar for demanding military and commercial uses


From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org] On Behalf Of Dan Rapak


I just received a question from a colleague that made me scratch my head. Where did the standards for equipment racks (rack units, 19" width, hole spacing) come from and why were they set at these values?
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