Rack rails - standards?

Mr Ian Primus wrote: ANSI/EIA-310-D-1992 or ISO/IEC 60297 (not 60927 as listed in innumerable Sun documents!) are current, but the standard goes back decades. The standard defines the spacing between left and right front rails (17.75"), between the hole centres on the left and right rails (18.25"), and vertically between the holes (see below). The 19" designation you often see is the width of the front panel that bolts onto the rails. The distance from front rail to back rail isn't standardised and in practice that does vary widely between racks, in fact some racks only have rails at the front. We have racks that are 600mm deep, 650mm, 850mm, and 950mm, and with the front rails set back various different distances from the front door (on racks that actually have a front door). American racks often have round holes (I've seen Sun, DEC and SGI racks like this) and use Tinnerman nuts, usually 10-32 UNF thread, which is very similar to European M5 thread. European racks tend to have square holes to put cage nuts into; these are usually M6 thread/size but sometimes M5. On racks with round holes, sometimes a small plate with 3 or 4 tapped holes to match four holes in the rack is used instead of Tinnerman nuts. To see a Tinnerman nut, look at the the picture of a Palnut at http://www.boltproducts.com/tinnerman/nut-bolt-retainers.html (often there's a thin hexagonal nut welded onto the back instead of the simple formed thread shown there) and another common style is shown in the diagram 1/4 of the way down http://techpubs.sgi.com/library/tpl/cgi-bin/getdoc.cgi/hdwr/bks/SGI_EndUser… For a cage nut see http://www.canford.co.uk/commerce/ProductDetails.aspx?ProductID=16-026. However all the racks and frames I've seen, US or European, have the same vertical hole spacing to ANSI/EIA-310-D-1992, not the same spacing between every hole but repeated in "units" of three holes -- hence the "42U" designation you sometimes see for a six-foot rack. 1U is 1.75", and the distance between centres of the middle hole and the upper and lower holes are 5/8", and that means the distance between centres between the lower hole of one U and the upper hole of the next U down, is 1/2". There's sometimes a mark -- a scribed line or a small notch in the rail edge -- between one U and the next. -- Pete Peter Turnbull Network Manager University of York