A multiplate friction clutch can transmit power by pressing a stack of friction plates, which comprises plural separator plates and plural friction plates arranged alternately side by side, with a piston movable within a clutch casing. The multiplate friction clutch includes an annular holding groove formed in a pressing wall of the piston, a cushioning wave spring received in the holding groove, and an annular protuberance formed on an outer circumferential edge of an opening of the holding groove. The cushioning wave spring can be brought into resilient contact with one of the friction plates, which one friction plate is located adjacent to the piston. The annular protuberance prevents the wave spring from falling off the holding groove.

FIELD OF THE INVENTION

This invention relates to a multiplate friction clutch useful in an automatic transmission of an automotive vehicle.

BACKGROUND OF THE INVENTION

FIG. 1is a cross-sectional view illustrating the basic construction of a multiplate friction clutch10.FIG. 1shows a clutch casing1, a hub2as a counterpart element to which power is to be transmitted, and a piston3movable within the clutch casing1. Designated at numerals4and5are separator plates and friction plates, respectively, which are arranged alternately side by side to make up a stack of friction plates. In the illustrated example, the separator plates4are in fitting engagement at outer circumferential portions thereof with spline grooves11in the clutch casing1, and on the other hand, the friction plates5are in fitting engagement at inner circumferential portions thereof with spline grooves21in the hub2.

When desired to engage the clutch, a hydraulic pressure is introduced into between the clutch casing1and the piston3through an oil hole12. As a result, the piston3is pressed rightwards as viewed in the figure so that the stack of friction plates is pressed toward a backing plate7. As the backing plate7is prevented from moving rightwards by a stopper ring8, the clutch is brought into engagement. When desired to disengage the clutch, on the other hand, the hydraulic pressure is released. As a result, the piston3is allowed to return leftwards owing to the arrangement of a return spring9.

Keeping in step with the recent move toward high-speed and high-power engines, it has become a practice to apply a cushioning resilient member to a piston on a surface thereof where the piston is brought into contact with an associated stack of friction plates. It was firstly contemplated to use a coned disk spring as a resilient member for relatively large loads. Specifically referring toFIG. 1, it was contemplated to form a holding groove31in a pressing wall of the piston3and to insert a coned disk spring in the holding groove31.

In a high-torque-capacity clutch, however, an increase in the number of friction plates in a stack of friction plates leads to a greater overall clearance between the piston3and the backing plate7. As the clutch is operated repeatedly, the wear of friction linings on the friction plates progresses, resulting in a still greater overall clearance. A potential problem, therefore, arises that the coned disk spring may fall off the holding groove when the clutch is not in operation. To avoid such a potential problem, it was proposed to form an annular protuberance on an edge of an opening of the holding groove and further, to form the coned disk spring in an elliptical shape (see JP-A-2004-144197).

When the holding groove is provided at the edge of its opening with the annular protuberance for the prevention of falling-off of the cushioning coned disk spring as mentioned above, the coned disk spring is required to ride over the annular protuberance upon mounting the coned disk spring on the piston. Formation of a cut in a coned disk spring may, however, structurally lead to a potential problem that, when pressed, the resulting coned disk spring may not be able to produce resiliency as much as it would do if the cut were not formed. The formation of such a cut further requires to form the coned disk spring in an elliptical shape and to fit the resulting elliptical coned disk spring in the holding groove while pressing the elliptical coned disk spring inwardly from both sides along a major axis into a shape close to a true circle. Moreover, after the mounting, the coned disk spring is in contact with the protuberance only at parts thereof in the direction of a minor axis.

As there is an ever-increasing demand for clutches of higher performance, it is required to mount a cushioning resilient member even on a clutch of relatively small torque capacity. In such a case, the use of a wave spring is suited. A wave spring allows to form a cut therein so that its handling is easier than a coned disk spring. The resulting wave spring with the cut formed therein is, however, accompanied by an inconvenience that under centrifugal force and vibrations, it has greater tendency of falling off the holding groove from the position of the cut.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the present invention, there is thus provided a multiplate friction clutch for transmitting power by pressing a stack of friction plates, which comprises plural separator plates and plural friction plates arranged alternately side by side, with a piston movable within a clutch casing, comprising:

an annular holding groove formed in a pressing wall of the piston,

a cushioning wave spring received in the holding groove such that the cushioning wave spring can be brought into resilient contact with one of the friction plates, said one friction plate being located adjacent to the piston, and

an annular protuberance formed on an outer circumferential edge of an opening of the holding groove to prevent the wave spring from falling off the holding groove.

There is an outstanding desire for still further improvements in the performance of automatic transmission equipment for automotive vehicles. The arrangement of a spring on a pressing wall of a piston in a multiplate friction clutch makes it possible to reduce shift shocks. As this spring, it is desired to use a coned disk spring when relatively large loads are applied or a wave spring when small loads are applied. Depending on each automatic transmission equipment, either a coned disk spring or a wave spring can be used selectively. A wave spring permits easier mounting in a holding groove than a coned disk spring, as the wave sprig allows to form a cut therein.

The arrangement of the annular protuberance on the edge of the opening of the holding groove has made it possible to prevent the wave spring from falling off under centrifugal force and vibrations. The inclusion of the cut in the wave spring, however, involves a potential inconvenience that the wave spring may fall off from the position of the cut. However, the formation of the wave spring into a modified quadrilateral, cross-sectional shape corresponding to a cross-sectional shape of the annular protuberance can bring about a further advantage that the wave spring is rendered more resistant to falling-off.

DETAILED DESCRIPTION OF THE INVENTION

The wave-spring-holding groove formed in the pressing wall of the piston is provided on the outer circumferential edge of its opening with the annular protuberance having an inclined surface. Preferably, the wave spring can be formed in a modified quadrilateral, cross-sectional shape such that the wave spring is provided on an outer circumferential wall thereof with an inclined surface corresponding to the inclined surface of the annular protuberance. Owing to a contact between the inclined surface of the annular protuberance formed on the outer circumferential edge of the opening of the holding groove and the inclined surface of the outer circumferential wall of the wave spring, the wave spring is prevented from falling off the holding groove under centrifugal force and vibrations.

FIGS. 2 to 4show the wave spring6, a cut61formed in the wave spring6, a wave valley62, and a cross-section63of the wave spring6.FIGS. 6 to 8illustrate on an enlarged scale an encircled part A ofFIG. 5, where the wave spring6is received in the holding groove31.

FIG. 6illustrates the conventional wave spring6received in the holding groove31and having a rectangular cross-section as the cross-section63. It is to be noted that the conventional wave spring6has been chamfered at and along all the corner edges thereof. When the clutch is in a non-operated state, the wave spring6is pressed under centrifugal force against an upper wall, that is, an outer circumferential wall of the holding groove31, and assumes the position shown inFIG. 7. Under vibrations, the wave spring6then tends to gradually fall off the holding groove31as depicted inFIG. 8. After the wave spring6has fallen off the holding groove31, the wave spring6is caused to fit into between the piston and the adjacent separator plate so that the clutch is brought into a continuously-engaged state. The automotive vehicle is hence rendered incapable of being shifted, thereby developing an inconvenience that friction linings on friction plates burn out.

It may be contemplated to make the peak of each wave higher for preventing the wave spring from falling off. This approach, however, leads to an increase in the depth of the holding groove31. As a consequence, a piston having a greater thickness is required. The resulting clutch, therefore, has a greater overall axial length. This is certainly contrary to the requirement for a more compact clutch.

As the wave spring includes the cut, the wave spring is more susceptible to falling-off from the position of the cut.

FIGS. 9 to 12are enlarged cross-sectional views similar toFIGS. 6 to 8, and show a holding groove41of a piston33in a multiplate friction clutch according to an embodiment of the present invention. The holding groove41formed in the piston33is provided on an outer circumferential edge of an opening thereof with an annular protuberance42having an inclined surface43.

FIG. 10shows the cross-sectional shape of a wave spring46in the multiplate friction clutch according to the embodiment of the present invention. This wave spring46has a modified quadrilateral cross-section47. Described specifically, the cross-section of the wave spring46is not a simple rectangular cross-section unlike the cross-section63of the conventional wave spring depicted inFIG. 4. The wave spring46is provided on the outer circumferential wall thereof with the inclined surface48which corresponds to the inclined surface43of the protuberance42on the outer circumferential edge of the opening of the holding groove41. It is to be noted that the wave spring46has been chamfered at and along all the corner edges thereof.

FIG. 11illustrates the position of the wave spring46received in the holding groove41of the piston33when the clutch casing1(seeFIG. 1) of the multiplate friction clutch according to the embodiment has not rotated yet.

FIG. 12shows the position of the wave spring46when the clutch casing1is rotating but the separator plates4and the friction plates5have not engaged yet (seeFIG. 1). The wave spring46is pressed under centrifugal force against the outer circumferential wall of the holding groove41, and force is applied to the wave spring46in such a direction that the wave spring46may fall off the holding groove41under vibrations.

In the multiplate friction clutch according to this embodiment, the holding groove41is provided on the outer circumferential edge of its opening with the annular protuberance42having the inclined surface43, and the wave spring46has the modified quadrilateral cross-section and is provided on the outer circumferential wall thereof with the inclined surface48corresponding to the inclined surface43of the annular protuberance42. Even when the wave spring46becomes about to fall off under centrifugal force and vibrations, the contact of the inclined surface48with the inclined surface43can surely prevent the falling-off of the wave spring46.

This application claims the priority of Japanese Patent Application 2006-112195 filed Apr. 14, 2006, which is incorporated herein by reference.