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19-Inch Rack

A 19-inch rack is a standardized frame or enclosure for mounting multiple equipment modules. Each module has a front panel that is 19 inches (482.6 mm) wide, including edges or ears that protrude on each side which allows the module to be fastened to the rack frame with screws.

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 shelf. The height of the electronic modules is also standardized in 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.

Rack-mounts are still sometimes called relay racks because they originated in the railroad industry. 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 rack standard arrangement is widely used throughout the telecommunication, computing, audio, entertainment industries, among others. However, the Western Electric 23-inch standard, with holes on 1-inch (25.4 mm) centers, prevails in telecommunications.

Usage

19-inch racks are often used to house professional audio and video equipment, including amplifiers, effects units, interfaces, headphone amplifiers, and small-scale audio mixers. They are also widely used for computer server equipment. This is because they allow for dense hardware configurations without occupying excessive floorspace or requiring shelving. Another common use for rack-mounted equipment is industrial power, control, and automation hardware.

Physical Dimensions

A piece of equipment being installed has a front panel height 1⁄32 inch less than the allotted number of Us. Because of this, a 1U rackmount computer is not 1.75 inches tall. It is 1.719 inches  tall. A 2U rackmount computer would be 3.469 inches tall, instead of 3.5 inches. 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. It 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.

19-Inch Rack Equipment Mounting

19-Inch Rack Fastening

Originally, the mounting holes were tapped to receive a particular type of threaded bolt. This method is still frequently used in some government and military applications in conjunction with slide rails for ease of maintenance. However, it is no longer typical to use this method for frequently changed server racks. This is because of the possibility that the threads will become damaged or a bolt will 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.

Clearance-Hole Racks

The tapped-hole rack was first replaced by clearance-hole racks. They are round hole or round unthreaded holes and Versa Rail racks. The holes are large enough to permit a bolt to be freely inserted through without binding. 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 inside of which is a captive nut. The nut can be easily removed and replaced with a new one if it becomes stripped out or a bolt breaks. Production of clearance-hole racks is less expensive because the process of tapping holes is eliminated and replaced with fewer, less expensive, cage nuts.

Square-Hole Racks

The next innovation in rack design is the square-hole rack. Square-hole racks allow boltless mounting. That means 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. The weight of the equipment paired with 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. There are cage hole nuts designed for this purpose.

Structural Support for 19-Inch Rack Mounts

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. Because of this, 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, with equipment having 19-inch and 23-inch brackets available. This enables them to be mounted in existing racks.

Structural Weakness

A key structural weakness of front-mounted support is the 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 the configuration of the rack, 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. Just like above, it's common for such rails to have an adjustable depth which allows the equipment to be supported by 4-posts. This also enables it to be easily installed and removed.

Depth

There is no standard for the depth of equipment or for specifying the outer width and depth of the rack enclosure itself. There is a tendency for 4-post racks to be 600 mm or 800 mm wide, and for them to be 600 mm, 800 mm or 1010 mm deep. This varies by manufacturer, the design of the rack, and its purpose. These dimensions have become common due to similar constraining factors, such as raised floor tile dimensions. The extra width and depth enable cabling to be routed with ease, and helps to maintain bend radius for fiber and copper cables. It also allows deeper equipment to be utilized. 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 utilize the space between the rear rails and the side of the rack enclosure.

The strength required of the mounting posts means they are not merely flat strips but actually a wider folded strip arranged around the corner of the rack. The posts are usually made of steel close to 2 mm thickness (the official standard recommends a minimum of 1.9 mm), or of slightly thicker aluminum.

Securing Racks for Strength and Support

Racks, especially two-post racks, are often secured to the floor or to 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.

Rails (slides)

Heavy equipment or equipment which is commonly accessed for servicing is often not mounted directly onto the rack but instead is mounted via rails (or slides). This makes servicing easier. A pair of rails is mounted directly onto the rack, and the equipment then slides into the rack along the rails to support it. When in place, the equipment may also be bolted to the rack. The rails may also 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 can then be slid back into the rack.

Slides or rails for computers and other data processing equipment, such as disk arrays or routers, often need to be purchased directly from the equipment manufacturer. There is no standardization for such equipment's thickness or means for mounting to the rail.

Computer mounting

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

• 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 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. This is because 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. This is to hold the cables in a neat and tidy folded channel when inside the rack. It can unfold out into a long strip when pulled out of the rack, allowing the server to continue to be plugged in and continue operating, while fully extended in front of the rack. This piece of equipment simplifies maintenance; however, it is 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. This 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. This helps pull or push the server in or out without having to pull on the cables.

KVM or LOM Software

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.

19-Inch Rack Types

Racks are available with either four or two vertical posts. Four-post racks allow for mounting rails to support the equipment at the front and rear. These racks may be open in construction. (similar to the traditional open-style two-post racks) Or they may be enclosed by front and/or rear doors, side panels, or tops. Two-post racks provide two vertical posts. A piece of equipment can be mounted either via its front panel holes or close to its center of gravity, in order to minimize load on its front pane. This depends on the design of the rack. Two-post racks are most often used for telecommunication installations.

Specifications for 19-Inch Racks

A rack's mounting fixture consists of two parallel metal strips, called posts or panel mounts, standing vertically. The posts are each 0.625 inches (15.88 mm) wide and are separated by a gap of 17.75 inches (450.85 mm), giving an overall rack width of 19 inches (482.60 mm). The posts have holes in them at regular intervals, with both posts matching, so that each hole is part of a horizontal pair with a center-to-center distance of 18.312 inches (465.12 mm).

The holes in the posts are arranged vertically in repeating sets of three, with center-to-center separations of 0.5 inches (12.70 mm), 0.625 inches (15.88 mm), 0.625 inches (15.88 mm). The hole pattern thus repeats every 1.75 inches (44.45 mm).

These holes can either be tapped (usually 1⁄4-inch UNC thread, more seldom metric 5 or 6 mm) or square. The square holes are meant for cage nuts. Tapped holes are more common in USA whereas square holes for cage nuts are common in Europe, especially in German cabinetry.

Regions

Racks are divided into regions, 1.75 inches (44.45 mm) in height, within which there are three complete hole pairs in a vertically symmetric pattern, the holes being centered 0.25 inches (6.35 mm), 0.875 inches (22.23 mm), and 1.5 inches (38.10 mm) from the top or bottom of the region. Such a region is commonly known as a "U", for "unit", or in German "HE" (for Höheneinheit) and heights within racks are measured by this unit. Rack-mountable equipment is usually designed to occupy some integer number of U. For example, an oscilloscope might be 4U high, and rack-mountable computers are most often 1U or 2U high. A blade server enclosure might require 10U. Occasionally, one may see fractional U devices such as a 1.5U server, but these are much less common.

The height of a rack can vary from a few inches, such as in a broadcast console, to a floor-mounted rack whose interior is 45 rack units (78.75 inches / 200 centimeters) high. Many wall-mounted industrial equipment enclosures have 19-inch rack rails to support mounting of equipment.

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