I have a rock tumbler or rock polisher where the belt wasstretched out and needed an idler wheel to take up the slack.  There is two here, one for compression andone for tension.  They consist of aframe, an arm and the wheel, which are assembles with 4-40 screws, washers andnuts.  Each needs a spring, onecompression and one tension.  It workedvery well.  Printed these parts separatelywith a slice of 0.29 mm in PLA. Assemble with 4-40 machine screws, washers and nuts.  Enjoy!