Nissan tec. on board?

I have an 04 frontier V6, and figured the altenator was going out, only to discover the crank pully that drives the alt. is loose/spinning, on its hub. Is this by design? I figure no? It does power the alt, but figured it may be slipping when chargeing demand is needed. anybody? This is my christmas present this year, left me stranded friday/X-eve:angry7: