Often when I go to school on my bicycle the left crank arm starts to loosen up after one or two really gentle jumps. When this happens I just tighten it with the tool I have in my backpack. It went on like this for almost two weeks after which after only one kilometer of riding it starts to creak very loudly and it also loosens up.
When I go to the repair shop, the worker just tightens it up and says it will work but after only one minute of riding it, it loosens up again. Please tell me how to fix this problem.
It is a square taper crankset where the crank is held on by a nut.