Ahh, the wily cage nut, bane of IT existence. The feisty little cage nut would rather run away and fall into the bottom of your server rack, or allow itself to be mangled into uselessness, than be held captive in a rack post, pierced through by a cruel stainless steel bolt.

IT folks are well served by the computer equipment rack: Like a high-rise apartment building, It lets you pack a lot of equipment into a lot of space, and it also makes you look like you've hit the Big Time.

Racks are essentially four vertical metal posts with horizontal cross-bracing attached. You can accessorize with fancy doors or side panels, and secret compartments for PDUs and the like, but the rack's whole purpose is to hold equipment in place, and that's what the four vertical posts are for.

Most equipment is attached to the rack by bolting rails especially designed for the equipment in question to the posts. Trouble is, there are several different sorts of rails and each type has a different way of attaching itself to the posts. Some rails are designed for extra-heavy equipment such as UPSes and need extra-strong bolts. Others hold lightweight equipment such as KVM switches or 1U servers and simply snap in.

Some rack manufacturers solve the problem by putting large square holes in a metal flange extending from each post. Each type of rail is adapted to fit into the square hole.



|   o   |
|  ___  |
| |   | |
| |___| |
|       |
|   o   |
|  ___  |
| |   | |
| |___| |
|       |
|   o   |
|  ___  |
| |   | |
| |___| |
|       |
|   o   |

The simplest way to attach rails to a rack is with a nut and bolt. But a large square hole is not a threaded round hole. What's worse, you can't hold onto the nut, the post, the screwdriver, and the rail all at the same time -- Too many hands in too small a place, and the rail you're installing frequently removes access to the nut.

   ________________________________________________________
              RAIL                                         |          _
                                            CAN'T     .-.| |  _______| |
                                              REACH   | |: : |///////| |
                                               THIS!  `-'| |         |_|
                                                         |
   _________POST_________________________________________|

To the rescue comes the cage nut, a square nut with a metal strip embedded in the middle, then wrapped around to form a little square "cage":

    __________       _________
   |  _____         |         |
   | |_____|_       |   .-.   |
   | |_____| |      |  ( O )  |
   |_________|      |   `-'   |
                    |_________|

     SIDE               TOP

The flat clip slides over the flange, and the nut goes into the square hole. The metal cage acts as a spring, holding the nut in the hole so that the bolt can be insterted:


   ________________________________________________________
              RAIL                                    ____ |          _
                                                     |.-.|||  _______| |
                                                     || |:|: |///////| |
                                                     |`-'|'|         |_|
                                                         |
   ___________POST_______________________________________|

Cage nuts can be tricky, as the cage usually isn't constructed of metal tempered to be a spring. It can be difficult to pry the clip away from the rest of the nut, and easy to overbend the clip and ruin the nut at the same time. What's worse, the spring is often weak, and if the bolt is even a little bit out of alignment, it will push the nut out of the hole rather than go into the nut's threads. If you're not careful, you'll just twist the nut into uselessness.

So here are some useful tips:

  • Don't use your fingernails pry the clip from the rest of the nut. Use the post flange instead.
  • Only pry the clip away enough to get it on the post. If you push the nut sideways onto the flange while pulling the body of the nut back away from the flange, it will slide on as soon as it there is enough room.
  • As soon as the flange is between the nut and the clip, wiggle the nut towards the hole and it will slide into place.
  • If you accidentally put the nut in the wrong hole, don't panic. Repeat the process by grabbing the body of the cage nut behind the flange, while pulling up or pushing down. with just the right amount of pressure, the nut will slide out of the hole. If the correct hole is very close, don't pull the nut all the way off the flange. Instead, slide the nut up or down the post until it is near the correct hole, then push in in to place.
  • SEE....the cage nut. BE....the cage nut.

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