Word Study
Rock shaft
CIDE DICTIONARY
Rock shaft, [Cf. Rock, v. i.].
A shaft that oscillates on its journals, instead of revolving, -- usually carrying levers by means of which it receives and communicates reciprocating motion, as in the valve gear of some steam engines; -- called also rocker , rocking shaft , and way shaft . [1913 Webster]
For further exploring for "Rock shaft" in Webster Dictionary Online