idler shaft

noun

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 & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012