Direct-acting (?), a. (Mach.) Acting directly, as one part upon another, without the intervention of other working parts.
[1913 Webster]

Direct-acting steam engine, one in which motion is transmitted to the crank without the intervention of a beam or lever; -- also called direct-action steam engine. -- Direct-acting steam pump, one in which the steam piston rod is directly connected with the pump rod; -- also called direct-action steam pump.
[1913 Webster]