A multistage centrifugal pump for pumping fluid, the pump comprising: - a pump housing having a fluid inlet and a fluid outlet and a flow channel between the inlet and the outlet in the housing, the pump housing is comprising a number of pumping stages, - a shaft mounted by bearings for rotation about its center axis (A-A), - a number of impellers, the number of impellers is corresponding to the number of pumping stages, the impellers are rotatable by the shaft and arranged to be rotated about the axis (A-A) in the flow channel, and each impeller comprises a hub having an axial opening configured to be fitted on the shaft and defining the length of the impeller in the axial direction, - when in use, power is transmitted though the rotating shaft to the impellers, causing torque, radial forces and axial forces, the shaft comprises a shaft collar for receiving the axial forces, - over the length of the shaft where the impellers are configured to be assembled, the shaft has a constant diameter that is smaller than the inner diameter of the hub and adaptor rings are arranged into the axial opening of the hub to transmit the radial forces between the impeller and the shaft, - the hub is configured to be tighten in radial direction to the shaft by two adaptor rings, each of the adaptor rings extending from an axial end of the hub towards an axial centre of the hub, characterized in that, - the hub is provided with circumferential cavities for the adaptor rings to fit in, the length of said cavities is longer than the width of the adaptor ring so that the adaptor ring does not protrude out from the cavity to prevent the axial load to be delivered through the adaptor rings. - a pump housing having a fluid inlet and a fluid outlet and a flow channel between the inlet and the outlet in the housing, the pump housing is comprising a number of pumping stages, - a shaft mounted by bearings for rotation about its center axis (A-A), - a number of impellers, the number of impellers is corresponding to the number of pumping stages, the impellers are rotatable by the shaft and arranged to be rotated about the axis (A-A) in the flow channel, and each impeller comprises a hub having an axial opening configured to be fitted on the shaft and defining the length of the impeller in the axial direction, - when in use, power is transmitted though the rotating shaft to the impellers, causing torque, radial forces and axial forces, the shaft comprises a shaft collar for receiving the axial forces, - over the length of the shaft where the impellers are configured to be assembled, the shaft has a constant diameter that is smaller than the inner diameter of the hub and adaptor rings are arranged into the axial - the hub is configured to be tighten in radial direction to the shaft by two adaptor rings, each of the adaptor rings extending from an axial end of the hub towards an axial centre of the hub, characterized in that, - the hub is provided with circumferential cavities for the adaptor rings to fit in, the length of said cavities is longer than the width of the adaptor ring so that the adaptor ring does not protrude out from the cavity to prevent the axial load to be delivered through the adaptor rings.