Previously, an intake air control device, which opens and closes an intake air passage formed in a valve body by rotating a throttle valve received in the intake air passage, is widely used.
In a device, which is disclosed in the patent literature 1 as one such intake air control device, a valve shaft is fixed to the throttle valve at two axial locations by a pair of fixing members. With this fixing structure, rotatable shaft portions of the valve shaft, which respectively project from two opposite axial sides of the throttle valve, are rotatably supported by two separate bearings, respectively, which are held by the valve body. Among these two bearings, a first bearing, which rotatably supports the rotatable shaft portion located on one side, includes an inner race, which is fixed to the rotatable shaft portion located on the one side, and an outer race, which is fixed to the valve body. Furthermore, among the two bearings, a second bearing, which rotatably supports the rotatable shaft portion located on the other side, includes an inner race, which is axially slidably fitted to the rotatable shaft portion located on the other side, and an outer race, which is fixed to the valve body.
In the intake air control device disclosed in the patent literature 1, an aluminum-based material is used as a material of the valve body and the throttle valve, and an iron-based material is used as a material of the valve shaft. In this case, generally, a linear expansion coefficient of the throttle valve and a linear expansion coefficient of the valve body are larger than a linear expansion coefficient of the valve shaft in the axial direction. Therefore, the valve shaft, which is axially slidable at the second bearing side that is opposite from the first bearing having the inner race fixed to the valve shaft, will have relative displacement relative to the valve body due to a thermal expansion coefficient difference. Therefore, a recess for relieving the relative displacement around at least the shaft portion of the valve shaft located on the second bearing side is formed at an axial rim section of the throttle valve. In this way, even when the relative displacement of the valve shaft is generated, interference between the throttle valve and the valve body is limited, and thereby characteristics as of the intake air control device can be ensured.
However, when the recess is formed at the throttle valve like in the disclosure of the patent literature 1, the intake air leaks through the recess even in the state where the throttle valve is fully closed. Therefore, at this time, the intake air flows through the intake air passage while the flow amount of this intake air in the intake air passage exceeds an inevitable flow amount of the intake air that inevitably flows through a full closing state clearance that is a clearance formed between the throttle valve in the full closing state and the valve body (hereinafter also simply referred to as a full closing state clearance). Thus, it is difficult to ensure the intended characteristics as of the intake air control device.