An assembly includes at least one piston, a rotating member, a transition arm, and a mechanism. The transition arm is configured to translate between rotational movement of the rotating member and a first linear motion of the piston. The mechanism is configured to superimpose a second linear motion of the piston onto the first linear motion of the piston. The assembly can be a hydraulic (or air) pump or hydraulic (or air) motor, with the second linear motion resulting in reduced output ripple or smooth torque vs. angular rotation, respectively. Also, the assembly can be an alternator or electric motor, with the additional linear motion resulting in an ac waveform that conforms substantially to a sinewave or a back emf that conforms to the input voltage or electric waveform. In addition, the assembly can be an internal combustion engine in which the second linear motion results in the pistons lingering about top dead center during the combustion process.