There has so far been known a drive source such as an internal combustion engine, an electric motor and the like drivably connected with wheels through a drive train including a transmission and the like, so that a driving power from the drive source can be transmitted to the wheels through the drive train. This type of a vehicle, however, encounters such a problem that muffled noises or chinking noises are generated by torsional vibrations originated from rotational fluctuations caused for example by torque fluctuations of an internal combustion engine.
Here, the term “chinking noises” is one kind of abnormal noises which sounds “jarajara” caused by collisions of an idling gear pair forming part of a set of transmission gears. The collisions are in general caused by torsional vibrations originated from the rotational fluctuations caused by the torque fluctuations of the internal combustion engine. On the other hand, the term “muffled noises” is also one kind of abnormal noises which is generated in a passenger room due to vibrations caused by the torsional resonance of the drive train originated from the torque fluctuations of the internal combustion engine. The torsional resonance of the drive train is usually generated in the normal area (for example around 2500 rpm of the rotational speed of the internal combustion engine for an FF vehicle) at a low vehicle speed.
For this reason, between the internal combustion engine and the drive train is provided a torsional vibration attenuation apparatus which can absorb the rotational fluctuations of the internal combustion engine, thereby making it possible to concurrently absorb the torsional vibrations of the drive train.
One of the conventional torsional vibration attenuation apparatuses comprises a first rotation member selectively connected with or released from a flywheel, a second rotation member connected with an input shaft extending from a transmission, and coil springs disposed between the first and the second rotation members to have the first and the second rotation members resiliently connected with each other in a rotation direction (for example see Patent Document 1).
The first rotation member is constituted by a clutch disc made of a friction material, and a pair of disc plates connected with the radially inner side of the clutch disc. The second rotation member is, on the other hand, constituted by a hub member which is constituted by a boss splined to the outer peripheral portion of the input shaft extending from the transmission, and a flange radially outwardly extending from the boss.
The coil springs are respectively accommodated in and supported by a plurality of spring accommodation windows formed in the flange, and a plurality of spring accommodation portions formed in the pair of disc plates in opposing relationship with the spring accommodation windows.
The coil springs thus constructed and arranged can be compressed between a pair of input plates, i.e., disc plates and the hub member in the circumferential direction to the input plates when the pair of disc plates and the hub member are relatively rotated. The circumferential torsional vibrations inputted from the pair of disc plates to the hub member can be absorbed by the coil spring, thereby making it possible to suppress the chinking noises from being generated.
On the other hand, between the hub member and the pair of disc plates is provided a hysteresis mechanism constituted by a thrust member which serves to generate a hysteresis torque based on a friction force between the hub member and the pair of disc plates, thereby making it possible to suppress the torsional resonance of the driving train and to reduce the muffled noises remarkably generated in the passenger room at the low speed of the vehicle.
It is known that the rotational fluctuation properties of the internal combustion engine are different between an acceleration time and a deceleration time of the vehicle, the acceleration time being a time when the hub member is relatively rotated in the positive side with respect to the pair of disc plates by the rotational torque of the internal combustion engine transmitted from the pair of disc plates to the hub member, and the deceleration time being a time when the hub member is relatively rotated in the negative side with respect to the pair of disc plates by the rotational torque of the internal combustion engine transmitted from the hub member to the pair of disc plates by the engine brake.
FIG. 10 is a view showing rotational fluctuations at the acceleration time and at the deceleration time. As shown in FIG. 10, the rotational fluctuations at the acceleration time are large in the low rotational speed area of the internal combustion engine, while the rotational fluctuations at the deceleration time are large in the high rotational speed area of the internal combustion engine.
For this reason, it is necessary that the hysteresis torque of a damper mechanism be heightened around the torsional resonance point at the acceleration time to suppress the torsional resonance of the drive train in the low rotational speed area, while the hysteresis torque of the damper mechanism be reduced to a smaller level around the high rotational speed area of the internal combustion engine with the large rotational fluctuations at the deceleration time to suppress the torsional vibrations by increasing the damping force.
The conventional damper mechanism is, however, constructed to have hysteresis torques at the acceleration time and at the deceleration time set at the same level, so that a damping mechanism can attenuate the torsional resonance of the drive train in the low rotational speed area at the acceleration time for the large hysteresis torque, but there is a possibility that the damping mechanism cannot sufficiently attenuate the torsional vibrations at the deceleration time.
For the hysteresis torque reduced to absorb the torsional vibrations at the deceleration time, the torsional vibrations are increased by the torsional resonance of the drive train around the resonance point at the acceleration time (shown by a dashed line in FIG. 11), thereby leading to generating the muffled noises.
In view of the above facts, there has so far been provided another vibration attenuation apparatus as one of conventional apparatuses which is constructed to have the hysteresis torques changed and different from each other at the acceleration time and at the deceleration time. The vibration attenuation apparatus comprises a friction generation mechanism for generating friction between the pair of disc plates and the hub member when the disc plates and the hub member are relatively rotated with respect to each other. The friction generation mechanism comprises a first friction generation portion and a second friction generation portion, the first friction generation portion functioning to generate frictions between the pair of disc plates and the hub member in the torsional property positive and negative sides while the second friction generation portion having a float member to be engaged with the hub member in the torsional property positive side to operate the first friction generation portion, and to be disengaged from the hub member in the torsional property negative side to prevent the first friction generation portion from being operated (for example see Patent Document 2).
The vibration attenuation apparatus thus constructed can suppress the torsional resonance by heightening the hysteresis torque at the acceleration time, while can increase the damping force by reducing the hysteresis torque at the deceleration time.