The Prism printer has a belt-driven, single-stepper Z axis. A flaw in the original design of the printer was that when the belt was tight, there was no allowance for inconsistent pulleys. If you have non-circular, erratic pulleys, you would get z wobble as the z axis turned. the belt would squeeze everything it was connected to, and what would usually give was the Z bearing block that is attached to the aluminum extrusion, and so the Z leadscrew would pivot back and forth in the x direction. This piece allows some give but still allows proper tension on the belt. use this as an idler instead of a 624 attached directly to the extrusion.