The 19-inch rack format, originally used as mounting systems for railroad signaling relays, has remained a constant while the technology that is mounted within it has changed to completely different fields. Equipment designed to be placed in a rack is typically described as rack-mount, rack mountable or subrack.
RU (rack unit) or U is the unit of measure used to describe the height of equipment intended for mounting in a 19-inch rack. One U is 1.75 inches (44.45 mm) high.
HP (horizontal pitch) is the unit of length defined by the Eurocard standard used to measure the horizontal width of rack mounted electronic equipment. One HP is 0.2 inches (5.08 mm) wide. A standard 19-inch rack is 84 HP wide.
A rack's mounting fixture consists of two parallel vertical metal "posts" or "panel mounts". The posts are each 0.625 inches (15.88 mm) wide, and separated by a gap of 17.75 inches (450.85 mm), giving an overall rack width of 19 inches (482.60 mm). The posts have matching holes in them at regular intervals, with a center-to-center distance of 18.312 inches (465.12 mm). Vertically the holes in the posts are arranged in repeating sets of three, with center-to-center separations of 0.5 inches (12.70 mm), 0.625 inches (15.88 mm), 0.625 inches (15.88 mm). The hole pattern thus repeats every 1.75 inches (44.45 mm).
Tapped-hole racks are used for hardware that rarely changes, such as phone, network cabling panels, TV broadcasting facilities, studios and relay racks. Clearance-hole racks have holes large enough to permit a bolt to be freely inserted through without binding, and held in place by cage nuts, (a spring steel clip within which is a captive nut). Square-hole racks allow boltless mounting, such that the rack-mount equipment only needs to insert through and hook down into the lip of the square hole.
- 19-inch rack article on Wikipedia
- Horizontal pitch article on Wikipedia
- Rack unit article on Wikipedia