19-inch rack

A 19-inch rack is a standardized frame or enclosure for mounting multiple electronic equipment modules. Each module has a front panel that is 19 inches (482.6 mm) wide. The 19 inch dimension includes the edges or ears that protrude from each side of the equipment, allowing the module to be fastened to the rack frame with screws or bolts. Common uses include computer servers, telecommunications equipment and networking hardware, audiovisual production gear, music production equipment, and scientific equipment. 

Several 19-inch racks in a professional audio application

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 cabinet, rack-mountable, or occasionally simply shelf.enclosures-cabinet The height of the electronic modules is also standardized as multiples of 1.75 inches (44.45 mm) or one rack unit or U (less commonly RU). The industry-standard rack cabinet is 42U tall.

  A full-height rack cabinet

The term relay rack appeared first in the world of telephony.

By 1911, the term was also being used in railroad signaling. There is little evidence that the dimensions of these early racks were standardized.

Telephone equipment racks (1923)

Telephone equipment racks (1923)

The 19-inch rack format with rack-units of 1.75 inches (44.45 mm) was established as a standard by AT&T around 1922 in order to reduce the space required for repeater and termination equipment in a telephone company central office. The earliest repeaters from 1914 were installed in ad hoc fashion on shelves, in wooden boxes and cabinets. Once serial production started, they were built into custom-made racks, one per repeater. But in light of the rapid growth of the toll network, the engineering department of AT&T undertook a systematic redesign, resulting in a family of modular factory-assembled panels all "designed to mount on vertical supports spaced 19¹⁄₂ inches between centers. The height of the different panels will vary,... but... in all cases to be a whole multiple of 1+³⁄₄ inches." cabinet

By 1934, it was an established standard with holes tapped for 12-24 screws with alternating spacings of 1.25 inches (31.75 mm) and 0.5 inches (12.70 mm) The EIA standard was revised again in 1992 to comply with the 1988 public law 100-418, setting the standard U as 15.875 mm (0.625 in) + 15.875 mm (0.625 in) + 12.7 mm (0.500 in), making each U 44.45 millimetres (1.75 in).

The 19-inch rack format has remained constant while the technology that is mounted within it has changed considerably and the set of fields to which racks are applied has greatly expanded. The 19-inch (482.6 mm) standard rack arrangement is widely used throughout the telecommunication, computing, audio, video, entertainment and other industries, though the Western Electric 23-inch standard, with holes on 1-inch (25.4 mm) centers, is still used in legacy ILEC/CLEC facilities.

Nineteen-inch racks in two-post or four-post form hold most equipment in enterprise data centers, ISP facilities, and professionally designed corporate server rooms, although hyperscale computing typically use wider racks. They allow for dense hardware configurations without occupying excessive floor space or requiring shelving.

A professional-grade oscilloscope Tektronix 7603 (1970s) for use in electronics and scientific laboratories. Typically for rack-mounted devices, the width of the front panel exceeds the width of the device itself, which provides the overlap zone (including screw holes) with the left and right rack rails.

Nineteen-inch racks are also often used to house professional audio and video equipment, including amplifiers, effects units, interfaces, headphone amplifiers, and even small-scale audio mixers. 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 in; 0.79 mm) less than the allotted number of Us. Thus, a 1U rackmount computer is not 1.750 inches (44.5 mm) tall but is 1.719 inches (43.7 mm) tall. If n is number of rack units, the ideal formula for panel height is h = 1.75n − 0.031 for calculating in inches, and h = 44.45n − 0.794 for calculating in millimeters. 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.

Equipment mounting

Originally, the mounting holes were tapped with a particular screw thread. When rack rails are too thin to tap, rivet nuts or other threaded inserts can be used, and when the particular class of equipment to be mounted is known in advance, some of the holes can be omitted from the mounting rails.

Threaded mounting holes in racks where the equipment is frequently changed are problematic because the threads can be damaged or the mounting screws can break off; both problems render the mounting hole unusable. Tapping large numbers of holes that may never be used is expensive; nonetheless, tapped-hole racks are still in use, generally for hardware that rarely changes. Examples include telephone exchanges, network cabling panels, broadcast studios and some government and military applications.

The tapped-hole rack was first replaced by clearance-hole (Round Hole, Round Unthreaded Holes, and Versa Rail 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. 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.

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.

A typical section of 19-inch (482.6 mm) server rack rail, dimensions noted in US inch

Structural support

Rack-mountable equipment is traditionally mounted by bolting or clipping its front panel to the rack. Within the IT industry, it is common for network/communications equipment to have multiple mounting positions, including tabletop 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 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 bending stress placed on the mounting brackets of the equipment, and the rack itself. As a result, 4-post racks have become common, 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 backward, e.g. APC SX-range racks), it is 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 is common for such rails to have an adjustable depth), allowing the equipment to be supported by four posts, while also enabling it to be easily installed and removed.

Although 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 600 mm (23.62 in) or 800 mm (31.50 in) wide, and

for them to be 600 mm (23.62 in), 800 mm (31.50 in) or 1,010 mm (39.76 in) deep. This of course varies by manufacturer, the design of the rack and its 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 the minimum bend radius for fiber and copper cables) and deeper equipment to be utilized. A common feature in IT racks is mounting positions for zero-U accessories, such as power distribution units (PDUs) and vertical cable managers and ducts, that utilize 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 steel of around 2 mm thickness (the official standard recommends a minimum of 1.9 mm), or of slightly thicker 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 seismic zones. According to 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 GR-63, NEBS Requirements: Physical Protection, are available, with Zone 4 representing the most demanding environment. GR-3108, Generic Requirements for Network Equipment in the Outside Plant (OSP), specifies the usable opening of seismic-compliant 19-inch racks.

A typical section of 19-inch (482.6 mm) server rack rail, dimensions noted in mm (exact)

Computer mounting

Example of 19-inch computer rack with servers

Computer servers designed for rack-mounting can include a number of extra features to make the server easy to use in the rack:

• The sliding rails can lock in various extended positions to prevent the equipment from moving when extended out from the rack for service.
• The server itself might have locking pins on the sides that just drop into slots on the extended rail assembly, in a manner similar to a removable kitchen drawer. This permits very easy server installation and removal since there is no need for the server to be held in midair while someone fastens each rail to the sides of the server with screws.
• Some manufacturers of rack-mount hardware include a folding cable tray behind the server, so that the cables are held into a neat and tidy folded channel when inside the rack, but can unfold 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 mid-air in front of the rack. This piece of equipment thus simplifies maintenance but at the cost of providing a restriction to airflow.
• Rack-optimized servers might duplicate indicator lights on the front and rear of the rack to help identify a machine needing attention or provide separate identify LED indicators on both sides of the server (which can be turned on in software or by pushing an associated button). Since some configurations permit over fifty 1U servers in a single rack, this provides a simple method to determine exactly which machine is having a problem when at the rear of the rack.
• 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.

When there is a large number of computers in a single rack, it is impractical for each one to have its own separate keyboard, mouse, and monitor. Instead, a KVM switch or LOM software is used to share a single keyboard/video/mouse set amongst many different computers.

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 does not grip the disc.

Rack types

19-inch server racks can vary in quality. A standard 19-inch server rack cabinet is typically 42u in height, 600 millimetres (24 in) wide, and 36 inches (914.40 mm) deep. Newer server rack cabinets come with adjustable mounting rails allowing the user to place the rails at a shorter depth if needed. There are a multitude of specialty server racks including soundproof server racks, air-conditioned server racks, NEMA-rated, seismic-rated, open frame, narrow, and even miniature 19-inch racks for smaller applications. Cabinets are generally sized to be no wider than the standard 24-inch-wide (610 mm) floor tiles used in most data centers.

Racks carrying telecom equipment like routers and switches often have extra width to accommodate the many cables on the sides.

Four-post cabinet racks

Four-post racks allow for mounting rails to support the equipment at the front and rear. These racks may be open in construction without sides or doors or may be enclosed by front and/or rear doors, side panels, and tops Most data centers use four-post racks.

Two-post relay racks

Two-post racks provide two vertical posts. These posts are typically heavy gauge metal or extruded aluminum. A top bar and wide foot connect the posts and allow the rack to be securely attached to the floor and/or roof for seismic safety. Equipment can be mounted either close to its center of gravity (to minimize load on its front panel), or via the equipment's front panel holes. The Relay Racks name comes from early two-post racks which housed telephone relay and switching equipment. Two-post racks are most often used for telecommunication installations.

ATA road case racks

Rack-mounted audio power amplifiers of a sound reinforcement system.

The racks each have large heavy-duty rollers, a typical setup for open-air concerts and the like, here at the Quadfest music festival.

19-inch equipment that needs to be moved often or protected from harsh treatment can be housed in a road case approved by the Air Transport Association of America (ATA), sometimes also referred to as a flight case. Road cases typically have plywood sides laminated with polyvinyl chloride (PVC), extruded aluminum edges, steel corners, handles, and latches. Larger cases typically have wheels for easy transport. Road case racks come in different heights based on the 1U standard and different depths. Non-isolated cases simply mount 19-inch mounting posts inside the case. To protect equipment from shock and vibration road rack cases use an inner and outer case. These cases can be isolated by thick layers of foam or may use spring-loaded shock mounting. Touring musicians, theatrical productions and sound and light companies use road case racks.

Fiberglass-reinforced plastic case racks

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. Rackmount cases are also constructed of thermo-stamped composite, carbon fiber, and DuPont's Kevlar for military and commercial uses.

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