I love Fischertechnik! But something bothered me since I started to play with it in the 1980s: You can't lock the cogwheels on the metal axis fasten enough to build a transmission because the cogwheels always will slip if the trosion force gets to big.
Now that I can print Fischertechnik parts myself I designed a small tube that goes between two cogwheels like shown in the pictures. Both cogwheels are locked together no matter how tight or loose you fastened the locknut on them.
Note: Make sure that you put the screw part of the cogwheel mounting parts towards the force closure on both sides like shown in the image with the exploded parts!