may way para ma tighten ang bearing, the old fashion way is to tighten it until you cannot turn the hub, then loosen it until it turns freely. this is the same as seating your bearing. of course nowadays a torque wrench is use to tighten it. your lock nut getting loose could be due to the following, the lock cap is not bend enough and the lock pin was not spread enough to rest on one of the groove of the nut. also the two screw of the lock nut in the case of a axle with a locking hub (4x4) could come loose if nut impact tighten properly.