1. Field of the Invention
The present invention relates to a fluid coupling for an engine cooling system and more particularly to a fluid coupling for use in controlling the revolution of the cooling fan of an engine of a vehicle, etc.
2. Background of the Related Art
A conventional fluid coupling is disclosed in U.S. Pat. No. 4,702,360, and is shown in FIG. 6. There, a rotor 71 is fixed to a shaft 72, and has helical grooves 71a on an outer peripheral surface and a hole 71b. A labyrinth 71c of rotor 71 is formed on an annular surface of the rotor 71. A body 73 is supported by the shaft 72 via a bearing 74. A labyrinth 73a of body 73 is formed on an interior surface of the body 73. A working volume 75 is defined by the labyrinth 71c of rotor 71 and the labyrinth 73a of the body 73.
A cover 76 is fixed to the body 73. A fan (not shown) is fixed to the circumference of the body 73 and the cover 76. A dividing plate 77 is fixed to the cover 76, and divides inner portion of the body 73 and the cover 76 into the working volume 75 and a storage volume 78. The dividing plate 77 has a hole 77a, and the hole 77a is opened or closed by a plate 79 with a hole 79a. The plate 79 is rotated by a bimetal 80 fixed to the cover 76.
In the above-mentioned fluid coupling, the plate 79 is rotated by the bimetal 80 which detects the temperature of air drawn through a radiator (not shown) by the fan. At high air temperatures, the hole 79a overlaps with the hole 77a with the result that the viscous fluid which is in the storage volume 78 flows into the working volume 75. Thus, the rotation of the shaft 72 is transmitted to the body 73 and the cover 76, so that the fan is rotated.
However, the quantity of the viscous fluid in the working volume 75 depends on the quantity of the fluid which is transmitted from the storage volume 78 to the working volume 75 due to the action of the bimetal 80. Since the bimetal 80 detects the temperature of air drawn through the radiator by the fan, and since this air temperature will not necessarily vary in correspondence with the water temperature in the engine, it is difficult to obtain a revolution of the fan in accordance with the temperature of the engine.