1. Field of the Invention
The present invention relates to a viscous clutch or coupling assembly adapted for installation in torque transmission systems in a motor vehicle for effecting a drive connection between a pair of rotary members coaxially arranged for relative rotation.
2. Discussion of the Prior Art
Conventional viscous coupling assemblies of this kind are, in general, classified into two types. A viscous coupling assembly of the first type is adapted to automatically effect torque transmission between drive and driven rotary members in response to relative rotation thereof. A viscous coupling assembly of the second type is adapted as a limited-slip differential to restrict relative rotation between drive and driven rotary members, between a pair of drive rotary members or between a pair of driven rotary members. The coupling assembly of the first type is mainly installed in one of torque transmission systems in a four-wheel drive vehicle of the real-time type. The coupling assembly of the second type is mainly installed in a differential unit.
In the other point of view, conventional viscous coupling assemblies of this kind may also be classified into two other types. Viscous coupling assemblies of the first type are disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 61-102330 and 61-191434, wherein transmitted torque is determined to be in response to a difference in rotation between drive and driven rotary members, hereinafter simply called "relative-rotation responsive type". A viscous coupling assembly of the second type is disclosed, for example, in Japanese Patent Laid-Open Publication No. 61-191432, wherein transmitted torque is determined to be in response to a difference in torque between drive and driven rotary members, hereinafter simply called "torque responsive type".
Transmitted torque T in the relative-rotation type is represented by the following equation. EQU T=K.times.N.sup..alpha.
where K and .alpha. are coefficients, respectively, and N is a difference in rotation between drive and driven rotary members. On the other hand, transmitted torque T in the torque responsive type is represented by the following equation: EQU T=I.times..omega.
where I is the moment of inertia of the driven rotary member, and .omega. is the angular acceleration of the driven rotary member. Between the two equations described above, there exists a relationship as represented by the following equation: ##EQU1##
As is understood from the above-described equations, the torque responsive type is capable of instantaneously responding to a torque change in the rotary members caused by an instantaneous change in torque generated by a prime mover of the vehicle or in grip performance of tire during running, while the relative-rotation responsive type is more slowly responsive to a torque change in the rotary members than the torque responsive type because N is first determined after integration of the angular acceleration .omega. by time.
In the case that the viscous coupling assemblies of the two types as described above have been installed in one of torque transmission systems in a four-wheel drive vehicle of the real-time type, characteristics of the two responsive types are compared with each other as follows:
(1) Phenomenon of tight corner braking
Since the radius Rf of a turning circle for the front road wheels in cornering is larger than that Rr for the rear road wheels, the number of rotation per unit time Nf for the front road wheels becomes larger than that Nr for the rear road wheels to cause a difference N=(Nf-Nr) in rotation therebetween. In this connection, the relative-rotation responsive type has disadvantages such as an increase in steering effort and an increase in the radius of the turning circle due to torque transmission to the respective front and rear road wheels. For this reason, in a motor vehicle provided with the relative-rotation responsive type, the characteristic of torque transmission is determined to be in a range where phenomenon of tight corner braking does not make a driver feel uncomfortable. On the contrary, in a motor vehicle provided with the torque responsive type, any angular acceleration (.omega.) does not occur unless torque generated by the prime mover is suddenly increased. Thus, no phenomenon of tight corner braking takes place during cornering.
(2) Running performance
With its slower response to a torque change in the rotary members, the relative-rotation responsive type is inferior to the torque responsive type, for example in getting out of slippery roads.
(3) Running feel
With the relative-rotation responsive type, rough acceleration work, for example, causes no any sudden change in torque transmission between front and rear drive wheels to keep running of a motor vehicle stable. With the torque responsive type, however, rough acceleration work causes a sudden change in torque transmitted to one pair of front and rear road wheels to make running of the vehicle unstable, resulting in deterioration of running feel of the driver.
(4) Recognition of slip during running
With the relative-rotation responsive type, slip of the drive wheels is easily recognized by the driver during running of the vehicle, if any. With the torque responsive type, however, slip of the drive wheels causes transmitted torque to immediately distribute to front and rear drive wheels to always maintain the motor vehicle at the upper limit of running performance thereof, so that the drive does not recognize the slip.