Understanding Why a “u” so Important to Servers and 19” Racks?

Have you ever been tasked to find storage for servers? Well, if you have you will know how frustrating it can be, especially with all the different terminology.

When you are looking for server storage, you get a mixture of conflicting information, the racks are called 19” rack mount racks, the width and depth is in millimetres and the mounting post’s are in U’s.

A 19 inch rack is a standardised system for mounting various electronic equipment in a rack 19” wide, the rack take’s its’ name from the dimensions between the mounting rails at the front and rear of the racks.

The racks can come in different depths: 600 mm deep, 800 mm deep, 1,000 mm deep and different widths: 600 mm wide and 800 mm wide. This totally depends upon what equipment is being stored in the racks.

The internal storage heights are called “U’s”, this is a measurement that equals 44.45 mm or 1.75“, for example a rack mount server comes in 1U, 2U 4U, so for example a 42U rack will theoretically accommodate 10 x 4U servers. However you do have to allow for air flow, as you do not want the cabinet to over heat, so for every 4U of space used a gap of 1U is left blank, this can be closed off using blanking plates – this is the Industry standard. Some say the letter U is used as a standard for the word Unit. The standard the racks were created by the Electronic Industries Alliance and is referred to as EIA-310-D.

When we call the servers and selves 1U, they are not exactly 1.75 inches (1U), they are 1/32 inch smaller (normally 1.719 inches instead of 1.75 inches) and this is allowing a small tolerance throughout the cabinet, so the equipment will not catch on other equipment in the rack.

Where are these racks used? They are used in telecommunications applications for storing all the data networks and telephone systems, ISP’s to host all the websites, the audio industry to accommodate the recording equipment and the railways to store signal relays.

A rack’s mounting fixtures are called mounting rails and run vertically, top to bottom in the rack, each rails is 5/8” wide with a gap between them of 17.75 inches, giving the overall internal rack size of 19 inch.

You mount the equipment to the mounting rails; some racks have pre-drilled holes that are tapped to take the bolt that is supplied with the server, other racks have cage nuts, these are nuts that are help captive in steel wrap that fits snugly into the square mounting holes in the rails.

If you were to store a piece of electronic equipment for example that is 450mm deep in a 600mm deep rack, (this depth would be dictated by the deepest piece of equipment to be mounted), that is heavy, you would mount support rails, these run from the front rails to the rear rails in the rack and distribute the load evenly.

Cooling is essential, if you plan on filling a rack to its capacity, there are several ways to cool a rack. Firstly, if you are storing only servers, you will require front to rear cooling, this is achieved in data centres with air conditioned rooms and vented front and rear doors, these doors have either fine slots or holes in them for the air to be drawn into the cabinet and cool each server.

The second option is to have air blown into the racks from the void in the floor and exhausted from the room, this is more expensive.

The final option is to have a fan unit or air conditioning unit fitted into the roof recess, these come in different configurations to suit the application.

Some companies like to bay (join together - this is achieved with a baying kit and comprises of 4 long bolts) 2 racks, 1 storing the active equipment (servers) and 1 storing the passive equipment (hubs, routers, etc) if this is the case the racks need to be earth bonded, so that if there is a short circuit no one gets an electrical shock.