there may be times that a radial force ftr may be less a little than a radial fluid force fr during one cycle when a receding angle u03b8 is 12 u00b0 so the 12 u00b0 of a receding angle u03b8 is the lower limit value fig8 shows the simulation result of a radial force ftr and a radial fluid force fr when a receding angle u03b8 is set to 14 u00b0 the 14 u00b0 of a receding angle u03b8 is an optimal value since a radial force ftr always exceeds a radial fluid force fr during one cycle fig9 is the simulation result of a radial force ftr and radial fluid force fr when a receding angle u03b8 is set to 20 u00b0 the orbiting wraps 3 a can be sufficiently adhered on the fixed wraps 2 a since the average value of the radial forces ftr is the twice of the radial fluid force fr during one cycle on the other hand it is not desirable to exert the radial force ftr more than this because the orbiting wraps 3 a are pressed to the fixed wraps 2 a by the greater force than the radial fluid force fr and the contact pressure becomes excessive resulting in the friction loss or the increase in wear therefore 20 u00b0 of a receding angle u03b8 is the upper limit based on the above evaluation it is desired to set a receding angle u03b8 in a range of 12 u00b0 to 20 u00b0 the pressures inside of pumping chambers 5 are equal to the discharge pressure the cross section area in the plane direction of pumping chambers 5 is the largest at the start of an intake and is the minimum at the end of the.