The present invention relates to liquid-cooled disc brakes incorporated in wheels of automobiles, particularly wheels of large-sized vehicles such as motor trucks and buses, and the invention is used for stably braking such large-sized vehicles running at high speeds.
With the spread of a network of highways, disc brakes are more commonly used as brakes of automobiles because, as compared with drum brakes, the disc brakes can offer high heat radiation efficiency and also obtain stable braking force. The diffusion of disc brakes is observed in the field of not only passenger cars but also large-sized vehicles such as buses and motor trucks, particularly sightseeing buses and long-distance motor trucks, which often utilize superhighways. However, disc brakes for use in such large-sized vehicles are to meet brake requirements more severe than those intended for passenger cars, and any disc brake for use in a passenger car, provided its size only is increased, may have heat insufficiently radiated depending on braking conditions, thus lowering the braking force.
For that reason, there has been proposed, as disclosed in Japanese Patent Application (OPI) No. 107132/80, a disc brake designed to give off, by means of liquid, heat generated by the friction between rotors and stators constituting the disc brake when the brake force is applied to the brake.
FIG. 4 shows the construction of a conventional liquid-cooled disc brake.
As shown in FIG. 4, a hub 4 is fitted via a spline mating portion 3 to an end of an axle shaft 2 passing through an axle housing 1 fixed to a vehicle body. The hub 4, together with the axle shaft 2, is rotatable with respect to the axle housing 1.
A plurality of rotors 8 set apart from each other are unrotatably fitted to the outer peripheral surface of an annular braking part 7 anchored with a stud 5 and a nut 6, the rotors 8 being movable with respect to the axle shaft 2.
A caliper 9 surrounding the whole perimeter of the rotors 8 is fixed in the proximity of the braking part 7. The caliper 9, substantially U-shaped in section with its inner peripheral side being open, is provided with an annular support wall 11 secured by a bolt 10 and a nut 12 to the outer peripheral face of the housing, and with an annular cubic body L-shaped in section and fixed close to the outer periphery of the support wall 11. Each support wall 11 of the cubic body 13 thus formed is used to hold the rotors 8 in between first and second annular walls 14, 15.
The outer edges of annular stators 19 set apart from each other are respectively fitted to the inner peripheral surface of a short cylindrical outer peripheral wall 18 for communicating the outer peripheral edges of both the annular walls 14, 15 with each other. The stators 19 are respectively inserted in between the rotors, the end of an annular piston 17 as described later and the rotor 8, or between the rotor 8 and a spacer 47 fitted to the inner surface of the second annular wall 15.
An annular cylinder 16 is fitted to the first annular wall 14 formed with a portion close to the outer periphery of the support wall 11 so that the cylinder 16 opens to the inner face of the first annular wall 14. The annular piston 17 is fitted into the cylinder 16. The piston 17 projects toward the rotors 8 when hydraulic oil is supplied to the cylinder 16.
In a middle portion of the housing 1 matching with the inner periphery of a space 20 enclosed by three walls consisting of the first and second annular walls 14, 15 and the outer peripheral wall 18, there is provided an oil supply channel 21 communicating the inside of the housing 1 with the space 20, whereas an oil discharge channel 22 is formed in the outer peripheral wall 18 to partition the outer periphery of the space 20 and to communicate the inside with the outside of the space 20. An oil supply pipe 23 communicates the oil discharge channel 22 with the housing 1.
The inner peripheral edges of the rotors 8 respectively engage with the outer peripheral surface of the braking part 7, whereas the outer peripheral edges of the stators 19 respectively engage with the inner peripheral surface of the outer peripheral wall 18. In addition, grooves 24, 25 for distributing cooling oil are formed so that the cooling oil reaching the inner peripheral portions of the stators 19 via through-holes 26 formed in the braking part 7 flows through the stators 19 and the rotors 8 and reaches the oil discharge channel 22.
When the conventional liquid-cooled disc brake thus constructed is used to brake an automobile, hydraulic oil is applied to the cylinder 16 installed in the support wall 11 to urge the piston 17 fitted into the cylinder 16 toward the rotors 8. The piston 17 is pushed to narrow the space between the end face of the piston 17 and the inner face of the second annular wall 15 and causes the rotors 8 rotating together with the wheel and the stators 19 to rub against one another. The brake operation for stopping the rotation of wheel is thus performed.
In the case of the conventional liquid-cooled disc brake thus constructed and functioning, however, cooling oil always flows in between the rotors 8 rotated together with the wheels and the unrotatable stators 19, so that the braking parts composed of the rotors 8 and the stators 19 function as a sort of fluid coupling. Consequently, the resistance resulting from the rotation of the axle shaft 2 even during the time the brake force is not applied causes a bad influence on not only the power performance but also fuel consumption rate of the vehicle.