Abstract:
A buckling prevention device including a first rotational element, a second rotational element, a leaf spring arranged for transferring torque between the first and second rotational elements while enabling relative axial movement between the first rotational element and the second rotational element, a connection member fixedly connecting the leaf spring to the second rotational element, wherein the connection member extends axially through a hole in the first rotational element, wherein a gap is formed between the connection member and an edge of the hole when the leaf spring is not experiencing an overly high compression force, and wherein the connection member is operatively arranged to close the gap and engage with the first member when the spring is experiencing an overly high compression force for preventing the leaf spring from buckling and at least partially transferring the torque directly between the first and second rotational elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/423,722 filed Dec. 16, 2010, which application is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention broadly relates to torque converters, more specifically to leaf springs for setting the position of a piston in a torque converter, and even more particularly to a device for preventing the buckling of leaf springs in a torque converter. 
     BACKGROUND OF THE INVENTION 
     Torque converters are well known in the art. Torque converters often include a piston that is axially moveable for engaging a clutch. Leaf springs may be included for transferring torque (directly or indirectly) from the cover of the torque converter to the piston. The leaf springs enable the transfer of torque through the leaf springs to the piston while also being able to flex to enable the piston to move axially with respect to the torque converter cover or a drive plate for the leaf springs. For example of one type of leaf spring arrangement, see United States Patent Publication No. 2008/0190723 (Heck et al.) which Patent Publication is hereby incorporated by reference in its entirety. Typically, the leaf springs are arranged so that in a normal drive mode of operation of the torque converter, the leaf springs are subjected to only tensile forces. Since the springs are, for example, thin plate-like members, they have good tensile strength. 
     While coasting in an automobile, however, the forces are reversed so that the leaf springs are subjected to compression forces. By coasting, it is meant generally that the engine is idling, but the vehicle is moving, including the components of the torque converter. The compression forces on the leaf springs are a result of resistance of the engine that is coupled to the torque converter. The compression forces create a risk that the leaf springs will buckle, and become permanently bent or deformed. The engine resistance is sometimes referred to as providing “engine braking” Basically, the torque converter and other elements are still rotating when the automobile is coasting, but the engine is not (or only to some marginal degree while idling), so the torque converter components generally act to rotate the engine while the engine is idling, instead of the other way around. The engines of many automobiles, such as typical passenger cars, do not usually exhibit engine resistance large enough to make buckling of the leaf springs a substantial risk. However, the engines of some automobiles, such as semitrailers, are arranged to strongly resist rotation while coasting, and provide a large amount of engine braking, which is exerted as compression forces on opposite ends of the leaf springs. As a result, in these vehicles there is a very real risk that the leaf springs will buckle when coasting. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention broadly comprises a buckling prevention device including a first rotational element, a second rotational element, a leaf spring arranged for transferring torque between the first and second rotational elements while enabling relative axial movement between the first rotational element and the second rotational element, a connection member fixedly connecting the leaf spring to the second rotational element, wherein the connection member extends axially through a hole in the first rotational element, wherein a gap is formed between the connection member and an edge of the hole when the leaf spring is not experiencing an overly high compression force, and wherein the connection member is operatively arranged to close the gap and engage with the first member when the spring is experiencing an overly high compression force for preventing the leaf spring from buckling and at least partially transferring the torque directly between the first and second rotational elements. In one embodiment, the first rotational element is a drive plate for the leaf spring. In one embodiment, the second rotational element is a piston for engaging a clutch. In one embodiment, the connection member is a retainer rivet having a head for engaging against the first rotational element for limiting axial movement between the first rotational element and the second rotational element in one axial direction. 
     The current invention also broadly comprises a torque converter including the buckling prevention device described above. In one embodiment, the second rotational element is a piston for engaging a clutch. In one embodiment, the leaf spring is operatively arranged to hold the piston in an open position with respect to the clutch. In one embodiment, the first rotational element is at least coupled mechanically to a torsional input to the torque converter. In one embodiment, the first rotational element is a drive plate for the leaf spring, and the drive plate is connected to a cover for the torque converter, and wherein the cover is connected to the torsional input. In one embodiment, the overly high compression force is a result of resistance in the torsional input, while the torsional input is idling, opposing a rotation of the cover of the torque converter. In one embodiment, the clutch is a lock-up clutch for mechanically coupling a damper of the torque converter to a cover of the torque converter. 
     These and other objects and advantages of the present invention will be readily appreciable from the following description of preferred embodiments of the invention and from the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature and mode of operation of the present invention will now be more fully described in the following detailed description of the invention taken with the accompanying drawing figures, in which: 
         FIG. 1  is a cross-sectional view of a torque converter; 
         FIG. 2  is a front view of a leaf spring arrangement for a piston; 
         FIG. 3  is a cross-sectional view of the leaf spring arrangement taken generally along line  3 - 3  in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of the leaf spring arrangement taken generally along line  4 - 4  in  FIG. 2 ; and, 
         FIG. 5  is an enlarged view of a buckling stop device for the leaf spring arrangement of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the invention. While the present invention is described with respect to what is presently considered to be the preferred aspects, it is to be understood that the invention as claimed is not limited to the disclosed aspects. 
     Furthermore, it is understood that this invention is not limited to the particular methodology, materials and modifications described and as such may, of course, vary. It is also understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present invention, which is limited only by the appended claims. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs. Although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the preferred methods, devices, and materials are now described. 
     Referring now to the figures,  FIG. 1  shows torque converter  10 . Torque converter  10  includes cover  12 , impeller  14 , turbine  16 , stator  18 , and vibration damper  20  for transferring torque from an engine (not shown) to an input shaft for a transmission (not shown). 
     Cover  12 , impeller  14 , turbine  16 , stator  18 , and vibration damper  20  could generally take any form known in the art, and the illustrated embodiment is for explanation purposes only. 
     In the embodiment shown throughout  FIGS. 1-5 , torque is transferred from an engine into drive plate  22 , which is then transferred to cover  12  via rivets  24 . The torque is then transferred to leaf spring drive plate  26  via rivets  28 . Retainer rivet  30  is provided between leaf spring drive plate  26  and piston  32 , but it is not arranged for transferring torque directly between plate  26  and piston  32  under normal operating conditions, as there is some play or looseness in the leaf spring drive plate around the retainer rivet. Thus, retainer rivet  30  and plate  26  are moveable with respect to each other to some degree. The retainer rivet is limited with respect to plate  26  on one side due to enlarged head  31  on rivet  30  for setting an axial limit for piston  32  while the piston transitions into an open position, as will be described in more detail below. 
     Torque is transferred from plate  26  to piston  32  via leaf springs  36 . Specifically, as shown in  FIGS. 4 and 5 , the leaf spring is fixed to plate  26  via rivet  38  and to piston  32  via retainer rivet  30 . Thus, torque is not transferred from drive plate  26  to piston  32  directly via retainer rivet  30 , but instead torque is transferred from plate  26  to rivet  38  to leaf springs  36  to retainer rivet  30  and finally to piston  32 . Typically, a plurality of leaf springs is provided about the outer perimeter of the leaf spring drive plate. Piston  32  has two axial positions, namely, an open position and a closed position, for engaging or enabling disengagement of clutch  34 , respectively. That is, surface  35  of piston  32  is brought into engagement with clutch  34  for locking damper  20  to cover  12 , or surface  35  is brought away from clutch  34  to enable the clutch to disengage. 
     In  FIG. 1 , piston  32  is shown in the open position. The piston can be moved into the closed position, for example, by pressurizing and/or depressurizing the various chambers of the torque converter. When the pressure is released or equalized on both axial sides of the piston, the piston returns to the open position, for example, due to the pressure forces and/or slightly urged by the leaf springs, as generally shown in  FIG. 1 . During this transition from the closed position to the open position, the piston in some prior art torque converters will tend to collide with other components of the torque converter, such the damper, for example, due to flexing or shifting of the components. In order to prevent piston  32  from axially moving too far away from clutch  34  and colliding with damper  20 , for example, head  31  is provided on retainer rivet  30  for providing a retaining function for piston  32 . That is, rivet  30  is rigidly secured to piston  32  at one end, with the other end moveable with respect to drive plate  26 . However movement between rivet  30  and plate  26  is limited by head  31 , which acts as a stop to limit how far rivet  30 , and therefore piston  32 , can axially move in the direction toward damper  20 . In this way, rivet  30  can be provided as a positive stop for setting a limit for the open position of piston  32 . It should be appreciated that the ability of the piston to engage clutch  34  is not compromised because rivet  30  is not secured to plate  26 , and head  31  of retainer rivet  30  does not limit the axial movement of the piston in both axial directions. 
     Advantageously, leaf springs  36  can flex to enable axial movement of the piston with respect to plate  26  while also enabling the transfer of torque through the springs. Typically, when the engine is being driven, the engine is transferring torque through the leaf spring such that rivets  30  and  38  are pulling on the leaf spring in opposite directions, resulting in tensile forces in the leaf spring. For example, as illustrated in  FIG. 5 , rivet  30  results in tensile force component F t1  on the leaf spring, while rivet  38  results in tensile force component F t2  on the leaf spring. Since the leaf spring is essentially a thin plate-like member, it has good tensile strength and forces F t1  and F t2  will not easily damage the leaf spring. 
     However, when the engine is idling and the vehicle is moving, the torque converter is still rotating in the drive direction, but the engine will not. Instead, as mentioned supra, the engine will resist the rotation, so that cover  12  actually transfers torque back to the engine. That is, while coasting, the engine is idling and instead of driving the torque converter cover, the engine is rotated by the torque converter cover. Again, frictional forces in the engine oppose this rotation while the engine is idling. As a result, rivets  30  and  38  will exert forces on the leaf spring towards each other. That is, rivet  30  will exert force F c1  on the leaf spring toward rivet  38 , while rivet  38  exerts opposing force F c2  on the leaf spring in the direction of rivet  30 . If the torque, and therefore force, is high enough, then these compressive forces can exceed a critical level, resulting in buckling of the leaf springs, which would generally cause the leaf spring to bend, bringing rivets  38  and  30  closer together. Springs are used primarily because they demonstrate good elastic properties, namely, they return to their original shape after an applied force is removed. Under too much compressive force, however, the buckling will cause the springs to yield and become plastically deformed. 
     Retainer rivet  30  is also arranged with leaf spring drive plate  26  to prevent buckling of leaf springs  36 . As mentioned previously, there is some play or looseness between rivet  30  and drive plate  26 . Specifically, as can be seen in  FIG. 4  and the enlarged view of  FIG. 5 , gap  40  is provided between the body of rivet  30  and drive plate  26 . Specifically, rivet  30  extends axially through hole  42  in drive plate  26 . Gap  40  could be formed, for example, by making hole  42  in drive plate  26  greater in diameter than rivet  30 , or by making hole  42  ellipsoidal or some other shape. Gap  40  is positioned such that if leaf spring  36  begins to buckle, rivet  30  will shift toward drive plate  26  in the direction of force F c1  and close the gap. With enough deformation of leaf spring  36 , gap  40  will close completely, and rivet  30  will engage against plate  26  at the edge of hole  42 . Once the gap is closed and rivet  30  is pressed firmly against drive plate  26 , rivet  30  will prevent further buckling of the leaf spring by acting as a stop against the edge of the hole in drive plate  26 . Additionally, torque from piston  32  to drive plate  26  will be at least partially transferred directly through rivet  30 . That is, drive plate  26  and rivet  30  act to provide a hard stop for limiting the compression or buckling experienced by the leaf springs. Accordingly, the leaf springs are only able to buckle a distance approximately equal to gap  40 . 
     The leaf springs are provided so that there is not a hard mechanical link to the piston, such as by a rivet, because such a link may cause a rattling or other performance issues. By creating this hard mechanical link only when necessary, that is, to prevent buckling of the leaf springs, performance is not decreased and the longevity of the system, the leaf springs in particular, is greatly improved. In this way, the size of gap  40  and/or hole  42  can be altered to achieve a balance between the amount of buckling allowed in the leaf springs and how often the rivet will engage with the drive plate. 
     It should be appreciated that while rivets are disclosed as the preferred connection means for connecting the various components of the torque converter together, other connecting members could be used in lieu of rivets. For example, bolts could be used in a similar fashion as any of the rivets, and including a head for a retaining function similar to head  31 . 
     Thus, it is seen that the objects of the present invention are efficiently obtained, although modifications and changes to the invention should be readily apparent to those having ordinary skill in the art, which modifications are intended to be within the spirit and scope of the invention as claimed. It also is understood that the foregoing description is illustrative of the present invention and should not be considered as limiting. Therefore, other embodiments of the present invention are possible without departing from the spirit and scope of the present invention.