Cage nut

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A cage nut consists of a square nut in a spring steel cage which wraps around the nut. The cage has two wings that when compressed allow the cage to be inserted into the square holes in equipment racks, and when released hold the nut in position behind the hole. The nut is usually slightly loose in the cage to allow for minor adjustments in alignment.

The most common use for cage nuts is to mount equipment in square-holed 19-inch racks (the most common type nowadays). There are four common sizes: 10/32 and to a lesser extent 12/24 are generally used in the United States; elsewhere M6 (6 mm outside diameter) for light and medium equipment and M8 for heavier equipment such as uninterruptible power supplies (UPSs).

Although some modern rack-mount servers have bolt-free fitting, other components such as shelves, networking hardware, UPSs and KVM switches still tend to be fitted using cage nuts.