I've had my bike for several years and it's been fine with moderate use, but this summer, I ended up using it a lot (I rode almost 200km in less than three weeks).
It started squeaking pretty badly, so I sprayed some lubricant around all of the joints (pedal, crank-arm, etc.) then took it out for a spin. On that very first ride after the lube, the left crank-arm nearly fell off; I though the pedal broke, but when I stopped and checked, the nut had come loose. I managed to use my keys to tighten the nut and repeat every block or two until I got home.
At home, I used a 14mm socket wrench to tighten the nut, but it keeps coming loose. I've resorted to carrying the socket-wrench with me when I ride and stopping to tighten it about ever kilometer. 😕
I've read some stuff about the holes in the crank-arm wearing out and worn nuts and how Loctite isn't a solution and such, but none of them mentioned lubrication. My issue started immediately after I lubricated the area around the nut, so logic would dictate that it allowed the nut to slip. I've included a photo (fig.1) of the crank-arm hole and nut. To my eye, the hole doesn't look worn (I assume the bulging around the edges is intentional), but the catch-teeth on the nut do seem worn/flat. 🤔
Anyway, I'm wondering if lubrication is indeed the problem and if cleaning the lubrication off (and maybe applying a mild adhesive like Loctite or just hairspray) would suffice.
Figure 1: Crank-arm hole and nut