In the present paper the loads, usually modeled as a force and a moment, on the rotors of a three screw pump have been analyzed. These loads, due to the pressure of the working fluid, are periodical, so their actions on the rotors vary. The calculations of the loads are possible thanks to the exact mathematical model of the sectors of surface of the rotors where the pressure works. The computation of the loads is determined as a function of the rotation angle and it is useful for the comprehension of some phenomena, such as the vibration induced and the noise generation, and for the design of a possible balancing system of the rotor loads. On the basis of the results of the calculations described, we can obtain the different types of actions on the rotors. The central screw presents loads different from those of the two satellite screws. In particular we show how the loads depend on the design parameters, especially on the pitch. The criteria for the optimum design of this parameter are indicated in detail.

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