Unlocking the Secrets of a 19-Inch Server Rack Design

I. Introduction:

1- What is a 19″ server rack?

A 19″ server rack generally refers to equipment installed in a 19″ cabinet or rack. The difference between a cabinet and a rack is that a cabinet has a protective shell and top and bottom covers, while a rack does not.

Racks, distinct from cabinets, are often referred to as “frames.” Both have a rectangular shape. The 19″ server chassis is designed to fit inside a cabinet, taking on a rectangular shape and adhering to specific height limitations so that chassis produced by different manufacturers can fit into cabinets designed to the same specifications.

2- Origins of 19″ server racks and cabinets

Originally, 19″ server racks and cabinets were a standard for US military electronic control instruments. The purpose of this standard was to unify the dimensions of these devices and facilitate their quick assembly, replacement, and maintenance. Eventually, this standard was adopted by the civilian sector, becoming widely used in the business world.

3- Types of 19″ server rack products

3-1- The advantages of cabinets include providing protection for server racks and facilitating expansion. They were initially used for industrial control systems, but in recent years, due to the development of networks, they have been increasingly used for network communication equipment.

Therefore, their product types can be broadly divided into two main categories: office-use and industrial-use products.

3-2- Office-use products

  • 3-2-1- Servers
  • 3-2-2- Telecom products
  • 3-2-3- Internet/Intranet products
  • 3-2-4- Other computer peripheral-related products

3-3- Industrial-use products

  • 3-3-1- IPC (Industrial PC)
  • 3-3-2- Control Panels
  • 3-3-3- Other machine-assisted related products

II. 19″ Server Rack External Dimensions and Cabinet Specifications

1- Cabinet Specifications

Cabinets are not merely boxes as their name suggests; they come in various shapes, as shown in the three figures below. These cabinets are all HP products, but their appearance and dimensions vary depending on the operating environment.

All cabinets are manufactured according to relevant specifications, with defined widths, heights, and depths covered in the IEC-60297 and EIA-310 series. The two specifications define the height unit as U (1U = 1.75″ = 44.45mm), the width as 19″ (other width specifications exist, but 19″ is the most common), and the depth has some variation.

1-1- Definition of height unit U: 1U = 1.75 inches = 44.45mm. Cabinets use U as the basic unit, so a 35U cabinet can accommodate a total height of 35U worth of server racks, but the cabinet’s overall height is not 35U.

1-2- The definition of the 19-inch (482.6mm) width refers to the maximum visible width of the server rack when viewed from the front, which is usually the width of the mounting flanges on either side of the server rack, not the width of the cabinet itself. The width of 19″ cabinets is limited by the server rack dimensions, almost always exceeding 500mm (since 19″ server racks are around 483mm wide). Current market cabinet widths range from 500mm to 800mm, with some special specifications exceeding 1000mm, but the most common is a 600mm wide cabinet.

1-3- Although there are regulations for cabinet depth, they are rarely mentioned, as they do not limit server rack designs like height and width. Even short server racks can be used in long cabinets, as long as their design specifications are fully compliant. Cabinet manufacturers produce various depth specifications, with 600mm, 700mm, and 800mm deep cabinets being the most commonly used (depth refers to the cabinet’s external depth, excluding handles).

2- Relationship between server rack external appearance and cabinet during design

2-1- External dimensions: Server rack design is constrained by cabinet size, so designers must consider the following points:

2-1-1- In terms of height: Since height is measured in U, with 1U equal to 1.75″ or 44.45mm, a 1U server rack cannot exceed 44.45mm in height. To facilitate stacking and assembly, the height is typically designed to be less than 44mm. Similarly, the actual height of server racks with different U values will be slightly less than their U value, allowing a gap between each server rack in the cabinet to facilitate assembly and disassembly.

2-1-2- In terms of width: 19″ cabinets have sheet metal (or aluminum rectangular) columns at each corner to support the cabinet and secure the server rack, so the internal width available for server racks is only around 450mm (approximately 17.7″), not the full 19″ width. To account for assembly, server rack width cannot be set to 450mm and usually does not exceed 440mm, leaving a 5mm gap on either side. If slide rails are considered for easy assembly, maintenance, and disassembly, the server rack width must be reduced further. Common slide rail thicknesses range from 10 to 20mm, and both sides need rails, so the server rack width becomes even narrower after accounting for double the thickness.

2-1-3- In terms of depth: Depth considerations include the need for server racks within cabinets to have space for heat dissipation, cable management, and other component arrangements. Since the space on either side is limited, designers must utilize the front and rear spaces. Typically, there is at least 75mm of space at both the front and rear (depending on the mounting bracket location). Therefore, for a 600mm cabinet, it is best to design server racks with a depth of less than 450mm.

2-2- Cabinet Mounting Holes: Server racks must be secured on both sides when placed in a cabinet, with mounting holes usually located on the mounting flanges. The dimensions and positions of these holes are shown in the figure below (data source: EIA-310-D). The figure shows three dimensions for width, each with three numbers. The top layer of dimensions is the standard size for 19-inch cabinets (i.e., 450, 465, and 483.4). The three dimensions represent the following:

450mm – Maximum width a server rack can occupy within the cabinet. 465mm – Distance between the mounting holes on the server rack and cabinet. 483.4mm – Maximum allowable width between the mounting flanges on the server rack.

31.8mm – Distance between the two mounting holes within a 1U height. If there are three mounting holes within a 1U height, the distance between the holes is half of 31.8mm, which is 15.9mm.

12.7mm – Closest distance between mounting holes of two adjacent U units, specifically, the distance between the bottom hole of the upper U and the top hole of the lower U. The junction between 1U and 1U is located in the middle of the 12.7mm distance.

6.75mm – Distance between the top (or bottom) mounting hole of the server rack and the inside top (or bottom) of the cabinet.

2-3- Front Handles: Server racks are pushed in and pulled out from the front, so handles are often designed on the front sides of the server rack for ease of operation. When designing handles, care must be taken to avoid interfering with the screw mounting holes for assembling the server rack and cabinet, as these holes are also located on the sides of the server rack.

2-4- Foot Pads: Some server racks are designed to operate independently outside the cabinet, so their bases are often equipped with foot pads. This prevents damage to surfaces when placed on a table or desk. However, when installing the server rack in a cabinet, the foot pads may interfere with the server rack below. As a result, it is preferable to design foot pads that can be easily removed and reinstalled.

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