I had the same problem with my Sem Cycle and the crank arm nut coming loose.
I would tighten it very tight and it would still loosen within 1/2 mile.
When I replaced the nut I looked at the old one and noticed that the
scallops on the nut surface were worn off. I have not had a problem with
it now for 2 weeks (20 miles or so). This may help if the crank arm has
not been damaged.