1984 Ford f150 belt slips when steering?

Every time i turn left or right my powersteering belt loosens causing it to slip and sometimes killing the engine. I have put a new belt on and I have it on as tight as possible. What could be causing this to happen?

Check the power steering pump pulley is it hard to turn. Check any idler pulleys if they run on this belt. Check everything for a bad bearing. Then look at other parts in the steering that could put an abnormal load on the pump.
Such as a connection hit by a rock or road debris,bad hoses or bad power steering unit. Bad front end parts can also cause problems. Pump bearings also go bad.