1. Technical Field
The present invention relates to oil pumps suitable for supplying working oil to automatic transmissions in vehicles such as automobiles.
2. The Related Art
An oil pump for an automatic transmission designed to reduce cavitation erosion is disclosed in Japanese Patent Kokai Publication No. 2003-161269. As disclosed therein, the oil pump includes a cast-iron pump body having a circular hollow formed in a face thereof; and a light-alloy pump cover closing the hollow within the pump body, thereby forming a gear compartment (pump chamber) therebetween. A drive gear is supported and driven by a drive shaft journaled in the gear compartment of the pump body, and a driven gear is disposed in the gear compartment so as to be rotatable eccentric to the drive gear and driven by the drive gear meshed with the driven gear; an arc-shaped suction port and an arc-shaped discharge port are located at the bottom of the gear compartment in a suction area and a discharge area, respectively. Working fluid spaces are defined between teeth on the outer circumferential surface of the drive gear and teeth on the inner circumferential surface of the driven gear and between side walls of the chamber bottom and the cover. Thus, the working fluid spaces are arrayed circumferentially around the pump chamber. An arc-shaped suction port is formed in the pump body and an arc-shaped discharge port is formed in the inner side face of the pump cover in the suction area and in the discharge area, respectively, for communication with the rotating working spaces.
With the oil pump disclosed in Japanese Patent Kokai Publication No. 2003-161269 (hereinafter simply referred to as the “prior art”), cavitation erosion is limited to an expected normal or tolerable level when the rotational speed of the drive gear is in a normal range of use (for example, up to 7,000 rpm). However, when the rotational speed of the drive gear is higher than that (for example, 7,500 rpm), the cavitation erosion of the pump cover greatly increases to an unacceptable level. This problem will now be described with reference to FIGS. 6 and 7.
In the prior art oil pump, a notch 5a is formed in the chamber side face of the pump body, i.e. in the “bottom” of the pump chamber (also see pump body 10 and chamber 11 in FIG. 1), and extends circumferentially from the front end of a discharge port 4a formed in the pump body to the rear end of a suction port 3a formed in the pump body in the suction area for the working spaces. In addition, a notch 5b formed in the pump cover 2, shorter than the notch 5a, extends circumferentially from the front end of a discharge port 4b formed in the cover to the rear end of a suction port 3b formed in the cover. When the drive gear 6a and driven gear 6b are rotated in the direction of the arrow during operation of the oil pump, working spaces R formed between the teeth of the gears 6a and 6b first come into communication with the discharge port 4a through the notch 5a. Since the working spaces R were in communication with the suction ports 3a and 3b until immediately before, the working spaces R are filled with low-pressure working oil entraining bubbles of a gas of volatiles from the working oil and air released from the working oil. Because the pressure of the working oil in the discharge ports 4a and 4b is significantly higher than that at the suction ports, when the working spaces R come into communication with the notch 5a, the high-pressure working oil in the discharge port 4a temporarily flows back from the pump body 1 toward the opposing inner side face of the pump cover 2 and into the working spaces R as indicated by an arrow f. Thus, the bubbles in the working spaces R collapse (become smaller), and the impact pressure occurring due to that collapsing causes cavitation erosion at the inner chamber side face of the pump cover in the vicinity where the bubbles collapse.
When the rotational speed of the oil pump is less than or equal to a predetermined limit, a small number of bubbles are present in the working spaces R, the pressure of the working oil in the discharge ports 4a and 4b is not very high, and the rate of inflow into the working spaces R is also low. Therefore, the collapsing of the bubbles mainly occurs adjacent the bottom of the pump body 1, but is not relatively noticeable. Thus, cavitation erosion of the pump body 1 can be prevented by forming the pump of a material, such as cast iron, having a high resistance to cavitation erosion. Accordingly, the prior art technology is effective in preventing cavitation erosion when the rotational speed of the oil pump is less than or equal to the predetermined limit.
However, when the rotational speed of the oil pump exceeds the predetermined limit, the pressure in the working spaces R is reduced, the volume of bubbles is increased, and the bubbles accumulate adjacent the inner circumference due to the increased centrifugal force. Moreover, the pressure of the working oil in the discharge ports 4a and 4b is increased, and the rate of inflow into the working spaces R is also increased. Accordingly, the position where the collapsing of the bubbles occurs is shifted to the inner side face of the pump cover 2, and more bubbles collapse. Since the pump cover 2 is composed of a material, such as aluminum, having low resistance to cavitation erosion, cavitation erosion occurs at the position indicated by symbol E1 at the inner side face of the pump cover 2, as shown in FIG. 7(b). Thus, gaps are formed between the pump gears 6a and 6b, and pump efficiency is reduced due to leaking of the working oil. It is believed that cavitation erosion occurs at the pump cover 2 by the above-described mechanism when the rotational speed of the oil pump exceeds the predetermined limit.
To solve the above-described problem, theoretically it would be possible to form the pump cover 2 of a metallic material having high resistance to cavitation erosion, e.g. aluminum with, for example, T6 heat treatment for increasing the surface strength or high-silicon aluminum alloy. However, such materials do not always solve the problem because of the large volume of bubbles in the working spaces R which are collapsed (crushed), and therefore, a material such as cast iron having high resistance to cavitation erosion is required. In such a case, the weight of the oil pump is disadvantageously increased since both the pump body 1 and the pump cover 2 are composed of cast iron. When such a heavy oil pump is installed in an automotive automatic transmission, the pump body or the pump cover of the oil pump cannot be integrated with the transmission housing which is composed of a light alloy, resulting in a complicated structure.