A previously proposed fluid pump has an inner rotor, an outer rotor, a pump housing and a rotatable shaft. The inner rotor includes external teeth, and the outer rotor includes internal teeth for meshing with the external teeth. The pump housing receives the inner rotor and the outer rotor. The rotatable shaft drives the inner rotor to rotate the same. When the inner rotor is rotated by rotating the rotatable shaft, a rotational force of the inner rotor is transmitted from the external teeth to the internal teeth. Thereby, the outer rotor is also rotated. When the inner rotor and the outer rotor are rotated, the volume of the respective pump chambers, which are formed between the external teeth and the internal teeth, changes. In response to increasing of the volume of the pump chamber, the fluid is drawn into the pump chamber. Thereafter, in response to decreasing of the volume of the pump chamber, the fluid is compressed in the pump chamber and is discharged from the pump chamber (see, for example, JP2013-60901A).
In a case where a repulsive force, which is applied from the fluid to the inner rotor, is large, like in a case where viscosity of the fluid is high, a force (tilting force), which is applied from the fluid to the inner rotor in a direction for tilting the inner rotor relative to the rotatable shaft, is increased. Thereby, a slide resistance between a radial bearing, which rotatably and slidably supports the rotatable shaft, and the rotatable shaft is increased to cause an increase in the energy loss or generation of damage at a sliding portion between the radial bearing and the rotatable shaft.
With respect to the above point, the inventors of the present application have studied a structure for coupling the inner rotor to the rotatable shaft through a joint member rather than directly coupling the inner rotor to the rotatable shaft. With this structure, the above-described tilting force can be absorbed through resilient deformation of the joint member, and thereby the slide resistance between the radial bearing and the rotatable shaft can be reduced.
In the above coupling structure, since the inner rotor is not directly coupled to the rotatable shaft, it is necessary to provide a member that rotatably and slidably supports the inner rotor. The inventors of the present application have studied a structure that slidably supports the rotatable shaft through a cylindrical inner peripheral surface of a radial bearing and also slidably supports the inner rotor through a cylindrical outer peripheral surface of the radial bearing.
However, the inventors of the present application have noticed that the above-described bearing structure poses the following new disadvantage. That is, the rotatable shaft is placed to extend over both of a high pressure passage, which conducts the fluid discharged from each corresponding one of pump chambers, and an inside of the pump housing. Thereby, the fluid in the high pressure passage penetrates into an area between the cylindrical inner peripheral surface of the radial bearing and the rotatable shaft to implement lubricating function. In contrast, it is difficult to provide a structure, which enables penetration of high pressure fluid between the cylindrical outer peripheral surface of the radial bearing and the inner rotor, so that the lubricating function of the fluid cannot be expected. Therefore, the slide resistance of the inner rotor cannot be sufficiently reduced in comparison to the slide resistance of the rotatable shaft.
That is, in the case where the above structure is adapted, although the tilting force can be absorbed through the joint member, there is required a structure that slidably supports the inner rotor. In this case, there is the new disadvantage of that the slide resistance of the inner rotor cannot be sufficiently reduced.