The Collaborative International Dictionary of English v.0.48:

Rock shaft \Rock" shaft`\ [Cf. Rock, v. i.] (Mach.)
   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]