I noticed that GearSmall.stl should not be printed with PLA filament because these gears are attached to Steppers and when steppers become hot this gear's hole become larger and it starts slipping on the shaft of stepper and lose grip with shaft. 3D print this file with ABS filament.
So I solved this issue by remixing GearSmall.stl file a little bit. I added two M3 nuts with two M3x6 Grub Screws (also called Set Screw) and also made the diameter of gear's base a little bit large to make it stronger.

UPDATE: I have set ampere value too high on DRV8825 stepper drivers that is why my stepper get hot. Then I changed stepper ampere to optimal value by following the video guide here: https://www.youtube.com/watch?v=89BHS9hfSUk&feature=youtu.be
BTW It is good to use this remixed gear instead of a gear without screws....

Graber i3

No

No

0.2 mm layer height

100%