Assembly Assistance for the Printrbot Simple - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 45

Figure 3.8 The Simple's base is ready for more parts to be added.
The X-axis motor and the Y-axis motor both have a small piece of clear vinyl
tubing that is inserted over their axles. Don't push the vinyl tubing all the way into
the axle hole, because the tube will rub and will provide resistance to the motor
when it wants to spin. After you've inserted the tubing over the axle, put a zip tie
on it near but not touching the motor. I put mine on with about 1/8” space of tubing
left behind the zip tie, and both motors rotate just fine. Pull that zip tie as tight as
you can get it, too. You'll hear a click as it tightens up, and you can use a pair of
pliers (pulling on the loose end) and a screwdriver (placed against the nub) to get
one or two more clicks of tightness.