I can't believe I'm writing about one of the simplest, most prosaic items in every data center: the cage nut. Everyone who's ever populated a server rack has had some contact with the cage nut, but it's rare I meet someone who groks the cage nut to fullness.
Having mastered the toenail clipper of death and MacGyvered a cage nut insertion tool from a PCI slot cover, I thought I was up on my cage-nut lore. But it was only when I researched for this blog post that I discovered just how much there is to know about the lowly cage nut.
In the beginning, relay and telecom racks were tapped; to mount a router or patch panel, you just put the right-size screw in the holes that line up. Here, children, is where the problem begins. Did the ancient telecommunications industry settle on a single screw size back when that was a possibility? No, it didn't. So while telecom racks are tapped, you can never predict whether any given rack is tapped in 10/32 or the slightly thicker 12/24.
In recent years, data centers have standardized on server cabinets (enclosed racks with mounting rails front and rear) with 3/8" square holes, rather than the pre-tapped holes of the past. Square holes have enabled server and storage vendors to use rails that latch into the rack, which are both stronger and easier to install than bolt-in rails. Since the square holes can just be punched into the rack rails, the manufacturers can save the cost of tapping all those holes.
Much as square-hole racks make it easier to mount rails designed specifically for them, a lot of smaller gear like network switches and even SuperMicro mini servers don't have dedicated rails -- just ears that bolt to the front rack rail for support. To accommodate this gear, we clip nuts into the square holes.
Cage nuts vs. clip nuts
Because having just one standard size or type of nut to clip in would make too much sense, we get to choose between clip nuts that clip around the rail and the more common cage nuts, which clip in from the back side of the rail.
In general, clip nuts are somewhat easier to install without tools. The lightweight types slide on pretty easily, but I've seen them shift under the weight of heavy gear so that the bolt rests on the bottom of the cabinet's square hole. The heavyweight types might be the best for heavy kit, since the actual nut is held in place within the square hole of the rack rail by the spring and the tension of the bolt itself in the hole.
The problem with the heavyweight clip nuts is that it's damned near impossible to remove one without destroying it. You have to push the nut back out through the rail's hole, slide a knife under the little tab on the front side that holds it in place, and then slide it off. Since the lab is a dynamic place, I might need to install some tool-less rails in a slot where I last used nuts. Once I figured out the tricks to installing and removing cage nuts, I banned clip nuts from the lab.
Next page: All about boltsHoward Marks is founder and chief scientist at Deepstorage LLC, a storage consultancy and independent test lab based in Santa Fe, N.M. and concentrating on storage and data center networking. In more than 25 years of consulting, Marks has designed and implemented storage ... View Full Bio