The Volvo wiper system was designed by kindergarten students in Gotenburg. Very poor. I keep a 13mm wrench in the drivers door pocket for just the situation. Klaus has a good Idea with the plastic nut. If the splines are wore from using the wipers with loose nuts you will need to replace the wiper transmission, which is a 172.00 volvo part and a couple frustrating hours. Keep rain x on the windshield and don't use the wipers to move built up snow. I'm thinking of drilling and tapping a hole in the side of the arm and screwing a small pointed screw into the spline, but I will try klauses fix first.