under spinning-jenny, OED says that the reason for this use of the personal name is uncertain, but under jenny itself, "A female personal name, pet-form or familiar equivalent of Janet (or, by confusion with Jinny or Jeanie, of Jane), and so serving as a feminine of Jack. Hence, like Jack, used as a feminine prefix, and as the name of machines."

also, jenny was a locomotive crane, used for moving heavy weights back and forth.