Windshield wiper repair

The front windshield wipers in my 1998 Olds Silhouette suddenly stopped working, and when I looked under the hood, the main part that turns the wiper arms was completely loose. I tightened the one bolt in the center that held the part in place, and the wipers worked okay, but only temporarily. Today the same thing happened, and the same bolt was loose again. What is wrong? Can something be stripped? I haven't the money for an expensive repair and am hoping there is a simple, inexpensive solution. Thanks.