This is is a two piece tensioner for the Printrbot Simple that uses a string for the drive belt. It's a compact design and prints quickly. See instruction tab for more info.

Print the pieces
Remove top bed. (The lower bed can stay assembled)
Align the spring holder as shown in the picture
Drill a hole and fasten the holder (3mm x 12mm screw for all)
Attach extension spring (see pic for hardware from Home Depot)
Align the corner piece and fasten
Wrap the string around the motor and tie the string to spring. To get the proper tension, tie the string before placing it over the corner piece. Pull the string over the corner piece and you will have good tension.