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Sunday, December 14, 2008

Everything you need to know about racks

Width, depth, screw size and all the rest

I see questions on the web about racks. "What kind of screws should I buy?" "If I build my own, then is there a uniform depth?"

Here's what I know about racks in the USA. By the way, supposedly standards are international, but when I see that inches are involved I shy away from making such claims. The story goes that the industry standard 19 inch rack width came about as the choice made by George Westinghouse to hold electronic switching devices used on his railroad. Early telephone companies also adopted the same width.

If you want to read about the different international standards then get busy at Google with these standards: 310-D from the EIA (Electronic Industries Alliance); 60297, set by the IEC (International Electrotechnical Commission); and the German Institute for Standardization or DIN (Deutsches Institut für Normung e.V.) standard, 41494 SC48D. If you want an everyday overview for recording equipment racks, then read on.

Let's talk about screws first as this is something you'll need to know when buying racks or rack rails to make your own racks.

The popular screw size in the USA from what I've seen is 10-32. These screws can come with Phillips heads (truss head or pan head), starpost heads, square-drive heads, or thumb-style heads. Thumb-style means you can tighten them with your hands. There are also proprietary systems.

I've also seen racks tapped for 12-24 screws (and sometimes hex bolts at this size) and for metric M6 screws.

I recommend 10-32 truss head screws for recording unless you need some sort of proprietary attachment system for 10-32 such as the rack release system. I got no recommendations for you if you want a rack for computer servers or telecommunications equipment. Whatever screw size you choose, remember the nylon washers to keep from scratching your rack gear as you tighten the screw to the rack.

How do the screws attach to the rack rails? You're probably thinking there's only one way for this to be done. Wrong. Most racks seem to be tapped for the appropriate screw size such as 10-32. However, some racks contain punch-outs (little squares, really, which are usually sized 3/8 inch) to attach cage nuts or upon which to place clip nuts. I've seen cage nuts that apparently fit all three screw sizes. Whew.

I recommend that you choose tapped rack rails or those with the 3/8 inch square punch outs. Just remember that if you choose rack rails with the punch outs that you'll need four cage nuts or clip nuts for ever piece of gear you mount in your rack. I sort of like the cage nuts better than the clip nuts. Installation of some cage nuts can be done by hand. Others require you to pry them into place with a screw driver or other tool. There is actually a special cage nut tool that sort of looks like a giant pair of nail clippers.

I am going to assume for the remainder of this tutorial that you've choosen to buy or make a rack with 10-32 tapped holes.

You need to understand "RU." RU is the unit of measurement for rackmounted devices. One RU is equal to 1 3/4 inches. Two RU equals 3 1/2 inches. Three RU equals 5 1/4 inches. I've not ran across anything bigger than three RU so I'll stop there but you add 1 3/4 inches per RU.

The standard rack width is 19 inches. The width from tapped hole to tapped hole on the corresponding level is 18 3/8 inches. This is the important dimension if you design your own rack. You need to make sure that from the center of one tapped hole to its corresponding hole on the other rail is 18 3/8 inches from center of the hole to center of the other hole.

It's quite easy to make your own racks. You need to determine how many RU you need. I've got one piece of gear that needs one RU on top and one on bottom to breathe. The manufacturer told me so. It's a two RU piece. So, I got to figure in four RU in choosing rack rails.

Here are some things I've learned. First, always get more RU than you need for stationary racks. I'm always getting stuff going in and out of my rack. Second, always get a cheap, battery operated screw driver for screwing and unscrewing things to your rack. Third, if you collaborate with others or use other studios outside your own, then you should probably go with case racks in the Anvil style because you'll be moving your rack(s) around. There are a few vendors who sell parts so that you can fabricate your own traveling rack cases if you so choose.

In designing or buying a rack, one important dimension to consider is depth. I suggest at looking the gear you might possibly acquire and check the depth of that gear. That should be your guide for the appropriate rack depth for you. If you are going to go with mobile cases, then you should probably figure in additional depth for cabling and plugs.

I use only one nylon washer per screw. I place the washer between the screw head and the piece of gear being racked.

I also use rack shelves for certain pieces of gear that are not rackmountable. Some come tapped with holes on the bottom to allow attachment with screws to the rack shelves. I've found these tapped screw holes to be delicate. Be careful. They strip easily sometimes. Some people use Velcro or some other sort of alternative stick 'em solution to attach items to rack shelves. I've got one heavy piece that just sits on a heavy two RU shelf -- no screws, no Velcro, no stick 'em.

--Steven Langer

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