[SBE] Rack standards

[Rod Simon]

I wish I could remember exactly what my neighbor explained to me 30+ years
ago. He had worked for the railroad for years as a signal technician. I was
just starting my career designing rack mounted equipment.  I do remember it
was a tradeoff of the size and weight of the relays and the size of the
shack they were mounted in.  What I recall was it had to do with using
standard steal channels available at the time and fitting 2 racks side by
side in a four foot area.

 

A 19” rack will not hold equipment that is 18” wide.  I once worked on a
project where we widened the unit to 17.8”or 17.85”  due to using thicker
material and some manufacturing processes.  While it fit in all the racks in
our facility, it did not fit all in the field. The maximum standard for
equipment width is 17.72. 

 

I wish I still had the IEC/EIA standards book I had back in the 70’s it was
some 50-60 pages defining everything you wanted to know about racks and the
equipment mounted in them.  I just read on one website that the EIA was
dissolved in Feb. of this year, CEA, TIA, and other equipment associations
are now the standards for their different industries.  The IEC
(International Electrotechnical Commission) and ANSI are still the major
standards groups for manufacturing.

 

The question I never understood was why the 5/8  -  5/8  -  ½ spacing of the
holes and the 1.75RU minus the  1/32 clearance, creating a maximum equipment
height of  1.718 or can you just round that to 1.72.  Then the question was
do you center the equipment on the 5/8  - 5/8  hole pattern or do you stack
it from starting at the bottom and leave the 1/32 clearance at the top.  I
worked for different mechanical design engineers who used different methods
and both could prove they were correct, depending on how they interpreted
the many different standards and tolerances used in the 70’s. Whatever you
did it made for some really bazar dimensions when manufacturing the front
panels.  

 

In addition does anyone remember there were two different hole spacing
standards prior to the 50’s or 60’s does anyone remember what they were? 

 

 

Rod Simon

 

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Okay. but again, why 19" and not 18" or 20" or 19 1/2"?

 

 

----- Original Message ----- 

From: Edwin Bukont <mailto:ebukont at msn.com>  

To: sbe <mailto:sbe at sbe.org>  

Sent: Thursday, October 27, 2011 9:31 PM

Subject: Re: [SBE] Rack standards

 

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.


Edwin Bukont CSRE, DRB, CBNT 
Nashville, Baltimore, Wherever
V- 240.417.2475; F- 240.368.1265
ebukont at msn.com

 




  _____  


From: wgbw at lsol.net
To: sbe at sbe.org
Date: Thu, 27 Oct 2011 20:53:14 -0400
Subject: Re: [SBE] Rack standards

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.




 

 

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?

 

 

 

Dan Rapak

 

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