1. Field of the Invention
The present invention relates to a fluid coupling device for use in a vehicle, and more particularly to a temperature-responsive viscous fluid coupling for driving an engine cooling fan assembly, wherein a viscous fluid such as silicone oil transmits the torque from a rotatable input member to a rotatable output member.
2. Description of the Prior Art
The fluid coupling device has been provided for controlling fan rotation in accordance with engine temperature. More specifically, it is desirable to rotate the engine cooling fan at high speed when the engine temperature is above a predetermined value, while at low speed when the engine temperature is below a predetermined value. The fluid coupling device of the present invention is capable of satisfying these requirements.
The problem with conventional devices resided in that the residual fluid functions to transmit torque from the input member to the output member even when the viscous fluid should not work, i.e., the engine temperature is below the above-mentioned predetermined value.
In order to overcome such difficulty, improvements have been presented in the prior art. One such improvement is disclosed in U.S. Pat. No. 3,272,293 wherein a fluid coupling includes notch means provided on the outer periphery of the input member thereby achieving an extremely low idle speed for the output member when the engine is operating at idle speed.
Such improvment has been very effective to obviate the before-mentioned difficulty in conventional devices, and fan rotation according to such improvement has been found to be less than half of the fan rotation of conventional devices with regard to the input member, i.e., engine rotation being below 3,000 r.p.m. - 4,000 r.p.m. Therefore, this improvement seems to be quite satisfactory when the maximum rotation of the vehicle engine is set around 3,000 r.p.m. to 4,000 r.p.m.
A problem arises, however, when the fluid coupling device is to be associated with a vehicle engine having a maximum rotation which is set above 3,000 r.p.m. - 4,000 r.p.m. According to the above-mentioned improvement of U.S. Pat. No. 3,272,293, it has been recognized that the fan rotation is increased as the engine rotation is increased more than around 3,000 r.p.m. and that the fan rotation is finally the same as that of the before-mentioned conventional device when the engine rotation is around 6,000 r.p.m.