ratchet has 4 distinct synonyms spread across 2 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
mechanical device consisting of a toothed wheel or rack engaged with a pawl that permits it to move in only one direction
move by degrees in one direction only
“a ratcheting lopping tool”
Synonyms, antonyms and the broader and narrower relations for ratchet come from Princeton University’s WordNet. Full sources and licences.