pistons 42b thru e tends to pressurize system 1 the movement of the pistons 40 and 42 also causes the spider 52 to nutate which rotates the central shaft 48 to move the rotary valves 60 and 62 to a new valving position so that alternately each piston pair is valved to pressure and return smoothly transferring hydraulic energy from one system to the other of course when system 1 is the higher pressure system the unit 20 transfers energy in the opposite direction since the pressure applied to the pistons by means of the valves 60 and 62 automatically will cause the nutator spider 52 to rotate in the opposite direction it should be understood that many different types of valves such as flat plate valves may be substituted for the rotary valves diagramatically shown in fig1 and 2 without materially affecting the invention however it is important that the interconnection means 58 be such that positive positioning of the piston pairs with respect to the spider 52 is maintained at all times so that the phase relationship therebetween and between the rotary valves 60 and 62 is maintained the construction of a preferred interconnection means is shown in detail in fig3 and 5 the interconnection means 58 includes a bushing 68 having a cylindrical bore 70 therethrough which bore 70 permits pivotal and sliding motion of the rod 56 retained therein rod 56a being shown the bushing 68 is mounted in a bore 72 in the interconnection means 38 which is generally at a right angle to the bore 70 in.