In general, a clutch device of this kind forming part of a power transmission apparatus is provided with a cam mechanism that can increase an output torque of an actuator to enhance a frictional engagement force of the clutch device, thereby increasing a torque transmission amount of a power transmission apparatus. The torque increasing function of the cam mechanism as such makes it possible not only to downsize the actuator but also to realize designing a small clutch pack.
The cam mechanism is schematically shown in FIG. 11 to comprise a drive side cam plate 101 driven by the actuator to rotate in a direction shown by an arrow “a”, a driven side cam plate 102, and a spherical member 103. The spherical member 103 is sandwiched between a cam surface 101a formed on the drive side cam plate 101 and a cam surface 102a formed on the driven side cam plate 102 to be capable of being rolled between the cam surface 101a and the cam surface 102a. When the drive side cam plate 101 is rotated around its center axis in the direction shown by the arrow “a” to have the cam surface 101a moved by a length Ls, the spherical member 103 is adapted to be moved in an axial direction shown by an arrow “b”. By the movement of the spherical member 103, the driven side cam plate 102 is moved by a length Lx to press friction engagement elements forming parts of the clutch device.
The cam mechanism is constructed to have a circumferential force Fs acted on the spherical member 103 and an axial force Fx acted on the driven side cam plate 102 when the torque is inputted to the drive side cam plate 101 from the actuator, the circumferential force Fs and the axial force Fx being associated with the shapes of the cam surfaces 101a, 102a. The circumferential movement amount Ls of the drive side cam plate 101 and an axial movement amount Lx of the driven side cam plate 102 are also associated with the shapes of the cam surfaces 101a, 102a. 
In other words, the circumferential force Fs and the axial force Fx are acted on the driven side cam plate 102 through the spherical member 103 when the drive side cam plate 101 is inputted with the torque in the direction shown by the arrow “a” as shown in FIG. 12. If the angle between the cam surface 102a and the surface 102b perpendicular to the axial direction of the driven side cam plate 102 is assumed to be a cam angle α, the force Fx can be represented by the equation Fx=Fs÷tan α. The movement amount Lx can be indicated by the equation Lx=Ls×tan α.
The axial force Fx acted on the driven side cam plate 102 is increased in response to the reduced cam angle α to increase the force pressing the friction engagement elements of the clutch device with the same magnitudes of the circumferential forces Fs as shown in FIG. 13, thereby increasing the frictional engagement force of the clutch device. On the other hand, with the same circumferential movement Ls as shown in FIG. 14, the axial movement amount Lx acted on the driven side cam plate 102 is increased in response to the increased cam angle α, thereby improving the responsiveness of the clutch device.
In the clutch device provided with the cam mechanism thus constructed, increasing the frictional engagement force of the clutch device results in lowering the responsiveness of the clutch device, while enhancing the responsiveness of the clutch device leads to lowering the frictional engagement force of the clutch device. This means that the frictional engagement force of the clutch device and the responsiveness of the clutch device are in contradicting relations with each other, so that there is a requirement for structural improvement of the clutch device, which results in requiring the clutch device to structurally be improved in order to enhance both of the frictional engagement force and the responsiveness of the clutch device.
On the other hand, there have been developed in recent years power transmission apparatuses adapted to cut off the power transmitted from the driven driving wheel forming part of a four-wheel drive vehicle and to stop the rotation of a rotation member in the power transmission path, thereby removing the frictional resistance generated by the rotation of the rotation member for the purpose of improving the fuel consumption of the vehicle. The power transmission apparatuses thus constructed are required to enlarge gaps between the friction engagement elements of the clutch pack in order to reduce the rotational resistance or a drag torque caused in the released state of the clutch device capable of cutting off the power. The power transmission apparatus thus required encounters such a problem that the clutch pack is structurally enlarged. In addition, the enlarged gaps between the friction engagement elements of the clutch pack leads to the strokes of the friction engagement elements of the clutch pack to be brought into engagement with one another, so that the power transmission apparatus encounters such a problem that the responsiveness is lowered.
In this respect, the power transmission apparatus provided with such a clutch device to be operated by the magnetic force of an electromagnet coil is relatively improved in responsiveness. However, the power transmission apparatus has such a drawback to generate the drag torque between the friction engagement elements of the clutch pack prior to an engagement state of the clutch device with the magnetic force of the electromagnet coil. For reliably reducing such a drag torque, there is the clutch device which is operated by the output torque of an actuator such as a motor and the like. The above clutch device is required to reduce the rotational speed of the actuator with a high reduction gear to obtain a predetermined frictional engagement force of the clutch device, so that the power transmission apparatus encounters such a problem that the responsiveness of the clutch device is further lowered in addition to the increased gaps between the friction engagement elements of the clutch device.
As a cam mechanism forming part of the power transmission apparatus, there has so far been known a cam mechanism having two different cam angles (see for example Patent Documents 1, 2).
The cam mechanism disclosed in the Patent Document 1 is constructed to have a plurality of grooves equi-distantly on the circumference of the cam plate. The grooves include a first large cam angle groove and a second small cam angle groove alternatively in the circumference direction of the cam plate. On the other hand, the cam mechanism disclosed in the Patent Document 2 is constructed to have a plurality of grooves equi-distantly on the circumference of the cam plate. The grooves include two different cam angle areas constituted by a non-linear area between a cam angle θ0 and a cam angle θ1 and a linear area from the cam angle θ1 to the cam angle θmax. Each of the disclosed cam mechanisms has a spherical member sandwiched between the grooves to be capable of being rolled in the grooves. Known cam mechanisms are constructed to have the cam angles of the first large cam angle groove and the non-linear area enlarged, so that the axial movement amount is increased with respect to the circumferential movement amount, thereby making it possible to promptly narrow the gaps between the friction engagement elements, and thus to obtain a relatively high responsiveness. On the other hand, the known cam mechanisms are constructed to have the cam angles of the second large cam angle groove and the linear area reduced, so that the circumferential movement amount is increased with respect to the axial movement amount, thereby making it possible to increase the axial force, and thus to obtain a relatively high frictional engagement force of the clutch device.