Abstract:
In order to minimize an effect of centrifugal force on a release operation of a clutch piston, this invention provides a return apparatus including a spring support rotating at a same speed as the clutch piston, and a return spring abutted at an end against the spring support and exerting an elastic force on the clutch piston, the elastic force increasing as the clutch piston rotating speed increases.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a clutch of an automatic transmission, and more particularly, to a return apparatus of a clutch piston of an automatic transmission. 
     BACKGROUND OF THE INVENTION 
     A multi-plate clutch is structured having a plurality of alternating clutch plates and disks, and is used to transmit power in motorcycles and automatic transmissions of vehicles. The conventional multi-plate clutch assembly, with reference to FIG. 5, includes a plurality of clutch plates  104 , which are mounted substantially in parallel within slots of an inner slotted portion  102  of a clutch retainer  100 , and a plurality of clutch disks  110 , which are mounted between the clutch plates  104  within slots of an outer slotted portion  108  of a hub  106 . That is, the clutch plates  104  and the clutch disks  110  are inserted partially within the slots of the inner slotted portion  102  of the clutch retainer  100  and the outer slotted portion  108  of the hub  106 , respectively. 
     A piston  112  is mounted in an area defined by the clutch retainer  100 , and a hydraulic pressure chamber  114  is formed between the piston  112  and the clutch retainer  100 . A return spring  116 , which is a compression spring, is provided on a side of the piston  112  opposite to that where the hydraulic pressure chamber  114  is formed to provide a constant biasing force to the piston  112  in a direction toward the hydraulic pressure chamber  114 . 
     With the supply of a sufficient hydraulic pressure to the hydraulic pressure chamber  114 , the piston  112  overcomes an elastic force of the return spring  116  such that the piston  112  moves in a direction toward the clutch plates  104  and the clutch disks  110  to compress the same, thereby engaging the multi-plate clutch to allow for the transmission of power. If the hydraulic pressure is then exhausted from the hydraulic pressure chamber  114 , the piston  112  moves in the opposite direction as a result of the biasing force of the return spring  116 . This removes the force applied to the clutch plates  104  and the clutch disks  110  by the piston  112  so that the transmission of power is discontinued. 
     However, the exhaust of hydraulic fluid is often incomplete. That is, although the hydraulic pressure in the path between a hydraulic pressure control apparatus and the hydraulic pressure chamber  114  is fully exhausted, part of the fluid in the hydraulic pressure chamber  114  is not exhausted as a result of a centrifugal force generated by the rotation of the clutch, thereby generating a centrifugal force hydraulic pressure. In the case where the clutch is rotating at a high speed, the centrifugal force hydraulic pressure may maintain engagement of the clutch. 
     FIG. 5 shows two types of clutch return mechanisms according to the prior art, one of which is, as shown in an upper-half part, a balance piston type, and the other of which is, as shown in a lower-half part, a check ball type. 
     According to the balance piston type, a balance piston  118  is mounted inwardly from the piston  112  as shown in the upper part of the example of a conventional multi-clutch assembly of FIG. 5, and the return spring  116  is interposed between the piston  112  and the balance piston  118 . Also, an opposing hydraulic pressure is supplied to a space between the piston  112  and the balance piston  118  such that the opposing hydraulic pressure enables the piston  112  to return. 
     According to the check ball type, with reference to the lower part of the example of a conventional multi-clutch assembly of FIG. 5, a check ball  120  is mounted at an outer extremity portion of the piston  112 , with this area of the piston  112  and the adjacent member being formed in such a manner as to allow operation of the check ball  120 . Various forces act on the check ball  120  including the force of the hydraulic pressure, centrifugal force resulting from the rotation of the multi-plate clutch, and a buoyancy force provided by the fluid used to create hydraulic pressure. If a rotational speed of the clutch is increased, the centrifugal force acting on the check ball  120  overcomes the centrifugal force hydraulic pressure to create the flow of fluid, that is, to enable exhaust of the remaining fluid. 
     However, in the case of the first example above in which the balance piston  118  is used, to effectively offset the centrifugal force hydraulic pressure, an inner diameter of the balance piston  118  must be smaller than an inner diameter of the piston  112 . As a result, the size of the transmission is increased. 
     Further, with respect to the application of the check ball  120 , it is difficult to design the check ball  120  and maintain the quality of the same to meet the precise design parameters. It is also difficult to fabricate the check ball  120  to the precise design parameters, as the surfaces of the check ball  120  and its contacting surface must be fabricated with exactitude. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a return apparatus for a clutch piston of an automatic transmission which includes a return spring, the elastic force of which increases as the rotational speed of the clutch piston increases such that the clutch piston is prevented from engaging when hydraulic pressure is released. The return apparatus according to the invention further includes a spring support rotating at a same speed as the clutch piston, with the return spring abutted at an end against the spring support and exerting an elastic force on the clutch piston, the elastic force increasing as the clutch piston rotational speed increases. 
     The return spring of the invention preferably includes a plane spring plate curved convexly at its central portion, in a radial direction with respect to the rotation axis of the clutch piston, and a mass unit disposed at the curved central portion, where the return spring is abutted against the spring support at an end of the plane spring plate. The opposite end of the plane spring plate is connected to the clutch piston. The mass unit is, preferably, cylindrically shaped. 
     The return spring may also comprise a central portion having a predetermined length and mass, and a pair of elastic fins extending in a radial direction with respect to the rotation axis of the clutch piston, at a predetermined angle on opposite sides of the central portion. Also a distal end of one of the elastic fins is preferably abutted against the spring support and a distal end of the other elastic fin is abutted against the clutch piston. 
     In a further aspect of the invention a return spring for a clutch piston of an automatic transmission comprises a v-shaped body with a force generating mass disposed at the apex of the body. The v-shaped body has two resilient arms configured to bear in opposition against a spring support and the clutch piston. The force generating mass disposed at the apex is configured and dimensioned to exert a spreading force on the resilient arms in response to rotation of the clutch piston sufficient to overcome any centrifugal force hydraulic pressure acting on the piston. In a preferred embodiment, the force generating mass is formed integrally with the v-shaped body. 
     In another alternative embodiment of the invention, a clutch piston system for an automatic transmission includes a clutch piston acting on at least one clutch plate in response to applied hydraulic pressure, and centrifugal force means for returning the clutch piston to a non-acting position in response to discontinued hydraulic pressure. In a preferred embodiment, the centrifugal force means comprises support means rotating with the clutch piston and centrifugal force biasing means bearing against the support means. The centrifugal force biasing means acts on the clutch piston with increasing force proportional to rotational speed of the clutch piston. In one preferred embodiment, the centrifugal force biasing means comprises a v-shaped body and a force generating mass. The v-shaped body defines an apex and two resilient arms. The force generating mass is disposed at the apex and is configured and dimensioned to exert a spreading force on the resilient arms in response to rotation of the clutch piston sufficient to overcome centrifugal force hydraulic pressure acting on said piston. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: 
     FIG. 1 is a schematic diagram of a return apparatus for a clutch piston of an automatic transmission according to a preferred embodiment of the present invention; 
     FIG. 2 is a perspective view of a return spring according to a preferred embodiment of the present invention; 
     FIG. 3 is a side view of the return spring of FIG. 2; 
     FIG. 4 is a side view of the return spring of FIG. 2 used to describe an operation of the return spring; and 
     FIG. 5 is sectional views of two types of conventional return apparatus for a clutch piston of an automatic transmission. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     Referring to FIG. 1, multi-plate clutch  2  includes a plurality of clutch plates  8 , which are mounted substantially in parallel within slots of an inner slotted portion  6  of a clutch retainer  4 , and a plurality of clutch disks  14 , which are mounted between the clutch plates  8  within slots of an outer slotted portion  12  of a hub  10 . That is, the clutch plates  8  and the clutch disks  14  are inserted partially within the slots of the inner slotted portion  6  of the clutch retainer  4  and the outer slotted portion  12  of the hub  10 , respectively. 
     A piston  16  is mounted in an area defined by the clutch retainer  4 , and a hydraulic pressure chamber  18  is formed between the piston  16  and the clutch retainer  4 . A return spring  20  is provided on a side of the piston  16  opposite to that where the hydraulic pressure chamber  18  is formed to constantly provide a biasing force to the piston  16  in a direction toward the hydraulic pressure chamber  18 . 
     With the supply of a sufficient hydraulic pressure to the hydraulic pressure chamber  18 , the piston  16  overcomes an elastic force of the return spring  20  such that the piston  16  moves in a direction toward the clutch plates  8  and the clutch disks  14  to compress the same, thereby engaging the multi-plate clutch to allow for the transmission of power. If the hydraulic pressure is then exhausted from the hydraulic pressure chamber  18 , the piston  16  moves in the opposite direction as a result of the biasing force of the return spring  20 . This removes the force applied to the clutch plates  8  and the clutch disks  14  by the piston  16  so that the transmission of power is discontinued. 
     The return spring  20 , shown in greater detail in FIGS. 2 and 3, includes a central portion  22  that is cylindrically shaped and has a predetermined mass, and a pair of elastic fins  24  and  26  that are integrally formed with the central portion  22  and extend at a predetermined angle on opposite sides of the central portion  22 . 
     The central portion  22  is not limited to a cylindrical shape and may be formed in various other configurations. Also, it is possible for the elastic fins  24  and  26  to be connected to the central portion  22 , for example by a welding process, and they need not necessarily be integrally formed to the central portion  22  as described. 
     The pair of elastic fins  24  and  26  can be formed having a plane spring plate curved convexly at its central portion in a radial direction to a rotation axis of the piston  16 , and a mass unit can be disposed at the curved central portion in order to have the central portion have the predetermined mass. 
     The return spring  20 , referring back to FIG. 1, is mounted in the multi-plate clutch  2  as follows. With a long axis of the central portion  22  placed in a direction perpendicular to the plane created by cutting the multi-plate clutch  2  for the sectional view, a distal end of the elastic fin  24  is abutted against an undersurface of a protrusion  28  of the piston  16 , and a distal end of the elastic fin  26  is abutted against a spring support  39  formed in an area defined by the hub  10  and the piston  16 . The spring support  39  is formed such that the elastic fin  26  does not slip from its position where it makes contact with the spring support  39 . 
     The elastic fins  24  and  26  of the return spring  20  constantly exert a biasing force on the piston  16  in the direction of the hydraulic pressure chamber  18 . Further, with reference also to FIG. 4, if the piston  16  is moved toward the clutch plates and disks  8  and  14  by supplying hydraulic pressure to the hydraulic pressure chamber  18 , the elastic fins  24  and  26  are deformed to decrease a distance therebetween. This increases the elastic force exerted by the elastic fins  24  and  26  on the piston  16 . 
     If the multi-plate clutch  2  is rotated at a high speed, the resulting centrifugal force acts as an upward force (in the drawing) on the central portion  22  of the return spring  20 , and accordingly, the upward force of the central portion acts to compel the elastic fins  24  and  26  to be opened wider, which means that the biasing force of the return spring  20  is increased. 
     Because the biasing force of the return spring  20  increases as the centrifugal force increases, the inadvertent movement of the piston  16  toward the clutch plates  8  and clutch disks  14  is prevented and the piston  16  is sure to draw back when the hydraulic pressure is released from the hydraulic pressure chamber  18 . That is, the force exerted by the elastic fins  24  and  26  during high-speed rotation of the multi-plate clutch  2  is greater than a centrifugal force hydraulic pressure resulting from fluid remaining in the hydraulic pressure chamber  18 , thereby preventing movement of the piston  16  by the centrifugal force hydraulic pressure. Such a configuration of the present invention is simpler and less expensive than conventional structures using balance pistons and check balls. 
     Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.