Demystifying Rack Units: How to Get More from Electronics Enclosures and Cabinets

Rackmount units may not seem glamorous, but they physically and functionally support electronic components that drive industry. While engineers innovate servers, test equipment, connectivity hubs, and more, standardized rackmount units like those from Hammond Manufacturing reliably house electronic components and use limited space efficiently. A quick primer into the internal logic of rack units gives engineers the skills they need to choose the rack units and accessories needed for their project.

Rack arithmetic

Engineers measuring their components for installation in a rack system need to learn one more unit of measurement: the rack unit. Abbreviated as U and equal to 1.75" or 44.45 mm, this unit of measurement replaces inches and millimeters in descriptions of rack dimensions.

Rack units are advertised by the maximum usable mounting space on the internal rails. For example, Hammond’s RCK series of rack cabinets (Figure 1) is 24.00" wide, comes in a 25.00" or 32.00" depth, and is available in six heights ranging from 26.38" to 78.00". Their usable dimensions are 19.00" wide, 22.56" or 29.56" deep, and 21.00" to 73.50" tall. Using the above definition of the U dimension, the cabinets are approximately 15U to 44U in height with 12U to 42U of usable mounting space, the figure of merit to consider when specifying a rack system.

Figure 1: The RCK series rack cabinets are made from 16-gauge, powder-coated steel. Mounting rails streamline the use of the 12U to 42U of available space. (Image source: Hammond Manufacturing)

Rack rails, such as those supplied with RCK series cabinets, have #10-32 threaded holes spaced every 0.625" to accommodate rack components sized in 1U increments, in compliance with Electronic Components Industry Association (ECIA) standard EIA-310D. Cabinets in the RCK series are made from 16-gauge, powder-coated steel, with a self-squaring design that ensures components fit as designed, allowing cabinets to be ganged together.

In addition to being available in six heights and two depths, RCK series rack cabinets have many additional ordering options. Users can specify solid, vented, or tinted-window doors; choose the placement of vents, cable-management punch-outs, and built-in fans; and add feet, casters, cable tie-offs, and electrical grounds.

Enclosing electronics

When designing a rackmount system, users start with the reason a rack system is needed in the first place: the enclosure or enclosures holding the primary electronic components. Electronics enclosures like those in Hammond Manufacturing’s RM series (Figure 2) are designed to contain and protect delicate electronics components using strong, lightweight aluminum.

Figure 2: Electronics enclosures in the RM series are 1U to 5U in height and made of user-assembled, powder-coated aluminum panels. (Image source: Hammond Manufacturing)

The enclosures’ vertical panels cover the side, front, and rear of the enclosure with 0.125" thick powder-coated aluminum. Larger top and bottom panels measure 0.064" thick. The enclosures come in standard sizes from 1U to 5U tall, 8.30" and 16.60" wide, and from 4.25" to 28.00" deep.

Enclosures ship unassembled as a kit of flat aluminum panels, adhesive rubber feet, and assembly hardware. For deeper models, pull handles and hardware for mounting on a rear rack are sold separately. Full-width versions also come with mounting hardware designed to interface with standard rack rails.

For components that don’t fit within an enclosure or are not configured for use in a rackmount cabinet, users turn to universal rack shelves like those in Hammond Manufacturing’s RAS series (Figure 3). The black powder-coated shelves are 19" wide, come in 14- or 16-gauge steel, and can hold 42 lbs to 200 lbs, depending on their height from 1U to 4U.

Figure 3: Rack shelves in the RAS series are 1U to 4U in height and can hold up to 200 lb of components that are not designed for rack-mount installation. (Image source: Hammond Manufacturing)

Once a user knows the required enclosure or shelf size, it is easy to calculate the remaining space in the rack. For instance, the smallest enclosure in the RCK series has 12U of available space. Using a 4U RM enclosure leaves an additional 8U of space for accessories.

Adding accessories

Common accessories in rackmount systems include power distribution units (PDUs), cable-management systems, ventilation fans, and rack shelves. Each of these is available in the standard 19" width and heights that are in 1U increments.

In the example above of a 4U RM enclosure mounted in a 12U RCK cabinet, a cable manager like one in Hammond Manufacturing’s RB-HRM series (Figure 4) will most likely be needed to keep the installation space-efficient and tidy. These 1U accessories, designed to be horizontally mounted in pairs arranged back-to-back within the cabinet, include four pass-through cutouts  and rings that extend 3.69" from the panel front to corral cables.

Figure 4: RB-HRM cable-management panels that use rings and pass-throughs to keep cables organized are made of 16-gauge steel and fit a 1U x 19" rack space. (Image source: Hammond Manufacturing)

Another reason for wanting to corral cables is to ensure adequate airflow to keep delicate electronics from overheating. Rack cabinets have vent options to promote airflow, but many applications also deploy one or more exhaust fans. Two-fan units like those in Hammond Manufacturing’s PPG series (Figure 5) fit in a 3U panel and work with 115 V or 230 V service to create 210 ft³/min (CFM) of airflow.

Figure 5: PPG series fans move up to 210 CFM of air to exhaust heat from rack cabinets. Wire and punch-out coverings keep users from interacting with fan blades. (Image source: Hammond Manufacturing)

Providing reliable, surge-free power to both the electronics in the enclosure and the accessory fans is another top consideration. Power distribution units (PDUs), like those in Hammond Manufacturing’s 1582e series (Figure 6), fit horizontally in a 1U x 19" rackmount installation.

Figure 6: PDUs in the 1582e series are 1U high and have eight outlets, a green on/off indicator, and a 10 A circuit breaker. They handle up to 240 V_AC at 50 Hz or 60 Hz. (Image source: Hammond Manufacturing)

PDUs in the 1582e series are rated for 100 V_AC to 240 V_AC at 50 Hz or 120 V_AC to 240 V_AC at 60 Hz, and include a 10 A circuit breaker. They comply with the standards of multiple worldwide and regional bodies including UL, CAN/CSA, CE, UKCA, and TÜV SÜD.

If the example discussed above requires two 1U PDUs in addition to a 4U enclosure, a 3U fan, and a 1U cable manager, 2U of space in the smallest RCK cabinet will be unfilled. Many users choose to cover extra rack units with blanking panels such as those in Hammond Manufacturing’s PBPA series (Figure 7).

Figure 7: PBPA aluminum blanking panels are 1U to 10U in height and 0.125" thick. They are finished to match the cabinet on which they will be mounted. (Image source: Hammond Manufacturing)

The 0.125" thick 5052-H32 aluminum panels mount securely to the rack via four attachment points. In addition to providing a clean aesthetic for the front of the rack, they improve air flow patterns within the cabinet.

Conclusion

Standardized rackmount systems streamline the process of creating space-efficient housings for electronic components. Accessories designed for the rackmount ecosystem offer plug-and-play functionality for power supplies, thermal management, cable organization, and more, giving engineers the flexibility they need for electronics housing.