Source : Webster's Revised Unabridged Dictionary (1913)

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}.