A split nut is a nut that consists of two pieces which can be clamped together to engage the thread of the nut with the bolt, or released, to allow the nut to move along the bolt without turning the bolt. The two pieces have chamfered ends (60° to axis). The nut is split lengthwise so that it may be opened for quick adjustment. A split nut assembly is often used in positioning systems, for example in the lead screw of a lathe. This type of nut can be used only for trapezoidal threads.[1]
Although you can't specifically see it, this educational device utilises a hidden split-nut to allow a threaded shaft to move left and right when the motor turns. http://www.mindsetsonline.co.uk/product_info.php?cPath=16_13_438&products_id=937