Square Taper Spindle size & shape

On the left is a Shimano BB-UN55. On the right a Sempu Torque Sensing Bottom Bracket from Grin Tech. The left crank arm worked loose for the 2nd time today. I didn't use a torque wrench the first time & installed it, but after that, I did & used 30 ft lb. (360 in lb. 40.7 Nm) which felt pretty stout & certainly more than 1st time. I used https://www.parktool.com/blog/repair-help/torque-specifications-and-concepts

Looking at Sheldon's page

https://www.sheldonbrown.com/bbtaper.html

it looks like his picture of an ISO crank on a J.I.S. spindle- only about 1/2 of the arm's taper is engaged, but the crankset (FSA Dyna-Drive triple 9 speed) is supposed to be JIS also. My concern isn't so much the depth, but the small size of the flats on the Sempu. How does the TDCM compare?

I weigh 230+ lb. and I try to spin, but helping the motor starting from a stop, I do mash pretty hard.

Have you removed your crank arms a bunch? That is a fault with the old square taper system. The more you remove the arms, the more they get worn down. Make sure you put some locktite on the crank arm bolts.

square taper is press fit and crank arms must be tightened to manufacturer torque specs and checked often after installation .. if there is any deformation of the aluminum crank arms from incorrect assembly/ tightening torque then they will likely never stay properly tight again and may need crank arm replacement..

If it breaks it will probably totally shear off not mash the flats down. I've seen them break before from jumping.

If the thing is properly attached you will not break it off by spinning with muscle power alone at 230 lbs.

I don't know if you have two different/incompatible tapers there or not, but if you do it likely won't stay done up no matter what you do.

If the tapers are compatible then I've overcome this same problem before by degreasing and lightly abrading the spindle and crank arm. They stop coming loose after that. No loctite.