Rack and Enclosures: Know Your Racks

A 19-inch rack is a standardized (EIA 310-D, IEC 60297 and DIN
41494 SC48D) system for mounting various electronic modules in a
"stack", or rack, 19 inches (482.6 mm) wide. Equipment designed to be
placed in a rack is typically described as rack-mount, a rack mounted system, a
rack mount chassis, subrack, or occasionally, simply shelf. The slang expression
for a subrack (generally 1U = 1.75 in = 44.45 mm height) is "pizza
box" due to the similarity in size and shape.

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. This standard rack arrangement is
widely used throughout the telecom, computing, audio, entertainment, and other
industries.

The racks mounting fixture consists of two parallel metal
strips (also referred to as "rails" or "panel mount")
standing vertically. The strips are each 0.625 inches (15.875 mm) wide, and are
separated by a gap of 17.75 inches (450.85 mm), giving an overall rack width of
19 inches (482.6 mm). The strips have holes in them at regular intervals, with
both strips matching, so that each hole is part of a horizontal pair with a
center-to-center distance of 18.3 inches (464.82 mm).

The holes in the strips are arranged vertically in repeating
sets of three, with center-to-center separations of 0.5 inch (12.7 mm), 0.625
inch (15.875 mm), 0.625 inch (15.875 mm). The hole pattern thus repeats every
1.75 inches (44.45 mm). Racks are divided into regions, 1.75 inches in height,
within which there are three complete hole pairs in a vertically symmetric
pattern, the holes being centered 0.25 inch (6.35 mm), 0.875 inch (22.225 mm),
and 1.5 inch (38.1 mm) from the top or bottom of the region. Such a region is
commonly known as a "U", for "unit", and this unit measures
heights within racks.

Equipment Mounting

Originally, the mounting holes were tapped to receive a particular type of
threaded bolt. These are no longer typically used for frequently changed server
racks rendering the mounting hole unusable. Tapped-hole racks are still used for
hardware that rarely changes, such as phone or network cabling panels and relay
racks.

The tapped-hole rack was first replaced by round-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 cage nut consists of
a spring steel cage, designed to clip onto the open mounting hole, within which
is a captive nut.

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. 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.

Rack-mountable equipment is mounted simply by bolting its front
panel to the rack, or with a square-holed rack by clipping or some other
variation on the theme. Having all the structural support at one edge of the
equipment is a weakness of this system, and so heavier equipment is designed to
use a second pair of mounting strips located at the back of the equipment.

The strength required of the mounting strips means they are
invariably not merely flat strips but actually a wider folded strip arranged
around the corner of the rack. The strips are usually made of steel of around 2
mm thickness, or of slightly thicker aluminium.

Heavy equipment, 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. 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.

Computer servers designed for rackmounting often include a
number of extra features to make the server easy to use in the rack:

• The sliding rails can lock in the extended position to
  prevent the equipment from moving when extended out from the rack into the
  service position

• The server often has locking pins on the sides that just
  drop into slots on the extended rail assembly. This permits a very easy
  server installation and removal

• The rackmount hardware often includes a folding cable tray
  behind the server, so that the cables are held into a neat and tidy folded
  channel when inside the rack, and unfolds out into a long strip when pulled
  out of the rack, allowing the server to continue to be plugged in and
  operating normally even while fully extended and hanging in midair in front
  of the rack

• Rack-optimized servers often include an indicator light on
  the front and rear of the rack to help identify the problem machine

• A handle may be provided at the rear of the server rails, to
  help pull or push the server without having to pull on the cables

Due to the large number of computers that can be installed into
a single rack, it is impractical for each computer to have its own separate
keyboard, mouse, and monitor. Instead a sharing device known as a KVM switch is
used to share a single keyboard, mouse, and monitor amongst many different
computers in the rack at once.

Since the mounting hole arrangement is vertically symmetric, it
is possible to mount rack-mountable equipment upside-down. However, not all
equipment is suitable for this type of mounting. For instance, most optical disc
players will not work upside-down because the driving motor mechanism will not
grip the disc.

Source: wikipedia.org