"
Crank_(person)...""Crank"" is a pejorative term for a person who 
holds some belief which the vast majority of his contemporaries would consider false, 
is eccentric, especially one who is unduly zealous,
is bad-tempered.
The term implies that 
a ""cranky"" belief is so wildly at variance with some commonly accepted truth as to be ludicrous,
and arguing with the crank is useless, because he will invariably dismiss all evidence or arguments which contradict his cranky belief.

Common synonyms for ""crank"" include kook and crackpot.  The word quack is usually reserved for someone who promotes a medical remedy or practice that they know to be ineffective. The crank differs from the fanatic in that the subject of the fanatic's obsession is not necessarily widely regarded as wrong, or a ""fringe"" belief....   "