戶樞不蠹

lit. a door hinge never becomes worm-eaten / constant activity prevents decay (idiom)

a household / door / family

hinge / pivot

no; not so / (bound form) not; un-

insect that eats into books, clothing etc / moth-eaten / worm-eaten

lit. flowing water does not rot, nor a door-hinge rust (idiom) / fig. constant activity prevents decay