Let's sacrifice one Lego axle to see how it deforms or breaks when high torque is applied.
The results (using PF M-motor):
1:1 (40 mNm) - ok
3:1 (110 mNm) - ok
5:1 (180 mNm) - slight permanent deformation
9:1 (300 mNm) - serious permanent deformation
15:1 (500 mNm) - even more serious permanent deformation
- (? mNm) - breaks in half
