Students having trouble with a faculty member should consult the director of graduate studies. The Graduate School has adopted a university-wide grievance procedure, to which you may turn if less formal measures fail. If you want to discuss your situation with someone outside the department, contact the Dean of the Graduate School; to review the university grievance policy on the Institutional Diversity webpage.
If one of your students complains about your teaching and you cannot resolve the problem yourself, you should consult with the director of Graduate Studies or the chair. If the issue cannot be resolved at that point, the department will work with you, perhaps taking the matter to the Graduate School’s grievance process.