I am building a robot with 2 motor driven wheels using 2 carseat motors. These motors have shafts which have square threads, so am having trouble coupling them to the wheel shaft as the coupling starts loosening almost causing damage. Attaching pics. Any suggestions appreciated.

 

Drill and tap the couplings for set screws and grind a flat on each motor shaft to lock the set screw on.

 

Thanks, my coupling is of Al so threads may be loose. Any suggestions for a good readily available coupling?

 

You would be lucky to find a coupling with the exact dimensions you need. I have used alumimium couplings without any problem. Use some Locktite threadlocker blue 242 adhesive and the screws will stay in place.