How do you tighten the nut on the fan shaft tight enough to hold the eccentric in place and the appropriate tension on the fan belt without causing the fan belt pulley to bind up and make the fan belt slip?
Fred,
Sounds like you have a problem with the shoulder on your fan shaft or spacer washer or seal. It should not bind when tightening.
See the drawing at:
http://www.mtfca.com/discus/messages/179374/209786.html?1304904881
Jim