Abstract: A hydraulic pump includes a case having a plurality of fluid paths and a cylinder-shaped groove with a shaft-bearing hole. A shaft is inserted into the shaft-bearing hole of the groove such that the shaft is rotated. A cylinder block is disposed in the groove of the case with a plurality of cylinders such that the cylinder block is rotated upon receipt of a rotational power of the shaft. The cylinder block has a barrel-shaped bottom portion surrounding the shaft. A piston is inserted into each of the cylinders such that the piston reciprocates. A swash plate is combined with the case such that the swash plate covers an opening portion of the groove. The swash plate has an inclined side facing the cylinder block. A shoe has a top side, a bottom side and a stepped middle portion. The top side of the shoe is connected to the piston via a connecting member. The bottom side of the shoe contacts the inclined side of the swash plate.

Abstract: A rotary motion/constant velocity linear reciprocating motion conversion device capable of converting rotary motion into a constant velocity linear motion or vice versa, and a hydraulic system which can minimize a ripple by using the above-mentioned conversion device, comprises a rotary motion part having a pair of first power transmission elements rotating at the same speed mounted to a common shaft, each element having alternatively positioned teeth capable of transmitting power. A linear motion conversion part comprises a second pair of power transmission elements each positioned adjacent to and engaging the teeth of the first elements. The second elements are each slidably mounted to a helical spline and comprise projections that engage helix grooves arranged in opposite directions along the shaft. The interaction of first and second power elements causes the linear motion of the helix shaft. The helical spline comprises a piston at each end that reciprocates within a respective cylinder.