idler shaft
n (General Engineering) a shaft carrying one or more gearwheels that idles between a driver shaft and a driven shaft, usually to reverse the direction of rotation or provide different spacing of gearwheels, esp in a gearbox
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014