The problem in most cases seems to be that the plastic gear on the motor that opens and closes the aperture develops a crack. The gear no longer fits tightly on the axle, which just spins around unable to rotate the gear. A company called MYDC in Taiwan will replace the plastic gear with a brass one. Or they will sell you a brass gear to install yourself. I did the latter on my 30-110mm lens - and can say that repair is NOT for the faint of heart. But after doing it my 30-110mm lens now functions properly. BTW, I did have Nikon Canada do the same type of repair once, and the lens’ aperture mechanism failed again within three years. The repair part they used was not durable enough.