centrifugal force f 1 toward the left side therefore the rotor 40 is pushed against the driving shaft 13 in the rightward direction in fig3 this means that the rotor 40 is pushed against the arc portion of the half circular cross section of the driving shaft 13 in this manner the rotor 40 is tightly coupled to the driving shaft 13 although the small gap is provided between the driving shaft 13 and the center hole 42 of the rotor 40 in this particular embodiment the depressions 45 are formed at the arc portion side of the half circular cross section therefore the arc portion of the center hole 42 closely contacts the arc portion of the driving shaft 13 the rotor 40 and the driving shaft 13 are more closely coupled to each other than they are coupled by making a contact between the chord portions it is not required to machine the center hole 42 and the driving shaft 13 with a high precision because the rotor 40 and the driving shaft 13 are tightly coupled to each other with a help of the imbalance of the centrifugal force operation of the vane pump 10 will be briefly described fluid is sucked into the pump chamber 22 through the inlet port 23 and compressed in the pump chamber 22 and then the compressed fluid is pushed out through the outlet passage 24 the pump chamber between a pair of the neighboring vanes is the largest at the position of the inlet port 23 the pump chamber 22 becomes.