Abstract:
A method of actuating a shifting element with three shift positions, having a simply controlled shifting cylinder as the actuating element, a shifting element for shifting to the three shift positions and a pressure regulator, such that the shifting cylinder is designed as a cylinder with one working line and the shifting element is pushed by the spring force of at least one spring element to an end position and moved to the other shift positions in opposition to the spring force. The central position is recognized in that, when the correct shifting element position is reached, the force for moving the shifting element in opposition to the spring force abruptly increases and, because of this, a specific pressure can be set by a pressure regulator for the central position.

Description:
This application claims priority from German patent application serial no. 10 2012 204 839.1 filed Mar. 27, 2012. 
     FIELD OF THE INVENTION 
     The invention concerns a method for actuating a shifting element with three shift positions and a device according to the invention. 
     BACKGROUND OF THE INVENTION 
     Shifting elements for torque transmission, for example in the form of shifting clutches or claws, have long been known in geared mechanical transmissions such as countershaft transmissions. Countershaft transmissions are usually made with gearwheels, so-termed loose wheels, mounted rotatably, on a shaft, which mesh with gearwheels connected in a rotationally fixed manner to other shafts so that they form so-termed gear pairs. By alternately connecting the gearwheels mounted rotatably on the shaft to the shaft in a rotationally fixed manner, the gear pairs can be connected into the force flow of a countershaft transmission by means of appropriate shifting elements such as synchronizers, claws or frictional elements in order to obtain various gear steps. As a rule this requires an actuating element in the form of a manual actuator, an electric actuator or a piston/shifting-cylinder arrangement that can be actuated hydraulically or pneumatically. In such piston/shifting-cylinder arrangements a piston is arranged and able to move axially in a shifting cylinder. The piston can move actively to at least one shift position and in its thrust direction is directly or indirectly connected fixed to the shifting element. In this case the actuating element can actuate the shifting elements mechanically, hydraulically, pneumatically or magnetically in relation to the shaft on which the rotatably mounted gearwheels are arranged. 
     A shifting element usually belongs to a shifting group of a geared mechanical transmission. In such a case a shifting group is preferably formed of two rotatably mounted gearwheels arranged next to one another in the transmission, which can alternately be engaged in the torque transmission by means of a shifting element. As is known, for example from the document DE 197 56 639 A1 by the present applicant, in such cases for each shifting group of a geared transmission, one shifting element with a cylinder that acts on both sides, having a piston which is moved by a fluid, can be used. 
     Depending on the design, it can also be necessary to maintain a central position in a shifting group, this as a rule being the neutral position. In the central or neutral position it is important that no part of the shifting element engages with any loose wheel. Thus, for example the central position constitutes the passive position of the actuating element. In that case the actuating element is held in the central position for example by springs, and has to be pushed actively to the other two shifts positions. For this, in pneumatic or hydraulic control systems two working lines and two piston faces are needed. Without such holding in the central position by spring action there is a risk that the central position will not be able to be approached, set and maintained exactly so that parts of a shifting element are at least partially engaged with a loose wheel. 
     The problem of a safe approach to the central position can also be solved by mechanical systems without the central position being held by springs. One solution is described, for example, in the document DE 40 38 170 A1. A shifting cylinder is considered, which has a two-sided controllable piston coupled to a shifting element, the piston being acted upon with a pressure medium by way of control elements such as displacement valves. The piston consists of two double pistons that act in opposition, which can move within a common cylinder housing, such that in the cylinder housing a respective pressure space with a pressure medium connection is associated with each of the two double pistons. The first piston of each double piston is rigidly connected to move with a shifting element and the second piston of each double piston is connected to a first piston so that it can move freely. When the pressure is equalized on both sides, the two double pistons are held in the neutral position with their freely movable, second pistons against a stop in the cylinder housing, and in each case when acted upon alternatively by pressure on one side or the other, they are pushed out of the neutral position to the shifting position, in such manner that the unpressurized double piston with its two pistons has its shifting displacement imparted to it by the pressurized double piston. 
     However, for example due to lack of fitting space or even by virtue of a component identity approach or platform concept, it can be in part not possible, or only so with difficulty, to realize a shifting cylinder that can be actuated to both sides with two working lines and with a corresponding control system. 
     SUMMARY OF THE INVENTION 
     Accordingly, with a hydraulically or pneumatically controlled actuating element in the form of a shifting cylinder with one working line, the purpose of the invention is to ensure reliable movement to three shift positions of a shifting element for the shifting of a shifting packet. In this, the central position in particular should be secure so that in this position there can be no contact between parts of the loose wheels and the shifting element. 
     The basic concept of the invention is that to move a piston in a cylinder against a force, a certain fluid pressure is needed as the displacing force. If, from a particular point along the control path, the opposing force increases abruptly from one shift position to the next, that point in the control path can be recognized and maintained by not increasing the fluid pressure and hence the displacing force any further. To move the piston farther, the fluid pressure in the pressure space of the cylinder has to be increased until a displacing force is built up which is larger than the opposing force acting on the piston. 
     The invention solves the problem in that a shifting element is pushed by a spring element to a first end position. To bring about a shift, the shifting element must be actuated hydraulically or pneumatically to move it along the control path against the force of the spring element. To recognize the central, or neutral position precisely, when the central position is reached the lever arm is made shorter by virtue of a special contour on the area contacted by the spring element. The area contacted by the spring element is understood to be the elements that are in contact with the spring element, such as pressure elements for transmitting the force to the spring element, buttress elements or supports that hold the spring element. This produces an abrupt increase of the displacement force required in order to move the shifting element farther and reach the third shift position at the second end position of the piston. In a fluid-actuated system this abrupt force increase enables a specific pressure to be set. For this, cup springs or membrane springs of various designs are used as spring elements. The cup spring can be designed such that it exerts a spring force that remains almost constant over the control path. To move farther along the control path no pressure increase of the actuating element is needed, since the spring element presses the shifting element back with its spring force. 
     An abrupt increase of the displacement force as required by the invention can also be produced by a combination of at least two spring elements. In this case a first spring element exerts a first force on the shifting element in opposition to its movement direction and, beyond a particular position along the control path, this first opposing force is supplemented by a second opposing force produced by a second spring element. This also produces an abrupt increase of the displacement force required. In this variant according to the invention torsion springs are used as the spring elements. Of course, the different spring forces can also be produced by a plurality of spring elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and its embodiment variants will now be described in greater detail with reference to the following drawings, which show: 
         FIG. 1 : Diagram of the hydraulic actuation of a claw (as in the prior art) 
         FIGS. 2 and 2A : Diagram of a lever arm shortened according to the invention, and associated characteristic curves 
         FIG. 3 : Diagram of the actuation of a pressure element with a combination of two springs 
         FIG. 4 : Method according to the invention, with a contour on the buttress element 
         FIG. 5 : Method analogous to  FIG. 4  with a contour on the pressure element and on the buttress element 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1  (showing the prior art) the three shift positions A, B, C will be explained briefly. The figure shows three shifting groups  2 ,  4 ,  6  each with an actuating element  8 ,  12 ,  16  respectively, in the form of a controlled cylinders with a piston, and a shifting element  10 ,  14 ,  18  respectively which is connected directly to the piston. Each of the shifting groups  2 ,  4 ,  6  is pictured in one of the three shift positions A, B, C. The first shifting group  2  is in the shift position A in which the piston of the actuating element  8  is in its first end position. In that shifting position the shifting element  10  is engaged with the gearwheel closest to the actuating element  8 . The second shifting group  4  is shown in the second shifting position B, namely the central or neutral position. The piston of the actuating element  12  of the shifting group  4  is in the central position and the shifting element  14  is then not engaged with either of the gearwheels of the shifting group  4 . The third shifting group  6  is shown with the piston of the actuating element  16  and thus also the shifting element  18  in the third shifting position C. In this third shifting position C, the shifting element  18  is engaged with the gearwheel of the shifting group  6  which is farthest away from the actuating element  16 , and the piston has reached its second end position. 
       FIGS. 2 and 2A  show the diagram of a lever shortened according to the invention and the associated characteristic curves in the shift positions A, B, C already described.  FIG. 2A  shows a device according to the invention with a pressure element  20 , which is connected to an actuating element (not shown here) and which is in contact with a cup spring  22  on one side. The pressure element  20  is part of a shifting element. Radially on the inside and on its side facing away from the pressure element  20  the cup spring  22  is in contrast with a mounting support  24 . The pressure element  20  serves to transmit the actuating pressure, starting from the actuating element, by way of the pressure element  20 , to the cup spring  22 , whereas the cup spring  22  with its spring force is already prestressed in shift position A and presses against the pressure element  20 . The pressure element  20  does not rest against the cup spring over a flat area, but is provided with a special contour. In this example according to the invention this takes the form of two contours elements  26 ,  28  shaped as pointed elevations on the side of the pressure element  20  that faces toward the cup spring  22 . The contour element  26  is radially farther away from the axis of the cup spring  22  than is the contour element  28 . In the first shift position A the pressure element  20  contacts the cup spring  22  only at the contour element  26 . In order to reach the central position, i.e. shift position B, the actuating element has to move the pressure element  20  and the cup spring  22  against the force of the cup spring  22 . During this the pressure transmission takes place via the contour element  26  until the second shift position B is reached and the cup spring  22  adopts a position in which both contour elements  26  and  28  are in contact with the cup spring  22 . From then on, with further actuation of the shifting element toward the third shift position C, the pressure transmission is taken over by the contour element  28 . The distance of the contour element  28  away from the point where the cup spring  22  is in contact with the mounting support  24  is smaller than that of the contour element  26 . Thus, the lever for transmitting the force of the actuating element is shorter, and a larger force must therefore be applied in order to move the pressure element  20  farther in the direction toward shift position C. As a result different pressures are needed in order to move to shift position C, starting from shift position A, so that for each shift position A, B, C specific actuating pressures can be defined. Hence the actuating pressure can be adjusted in such manner that all the shift positions A, B, C can reliably be achieved. 
     The performance graphs  30  show the actuating element pressures at each shift position A, B, C, at the respectively associated points S A , S B , S C  along the control path. At the first shift position A a certain actuating pressure p 1  is required in order for the contour element  26  to move the cup spring  22  in the direction toward shift position B. At shift position B there is an abrupt increase of the actuating pressure and a pressure p 2  is needed to move the cup spring farther to the third shift position C. In the initial shift position A no pressure has to be applied, since there the actuating element is in its end position. 
     The performance graphs  32  show the functions of the contour elements  26 ,  28  along the control path S A , S B , S C . As far as shift position B the pressure is transmitted by way of the contour element  26 . From the time, in the second shift position B, when both contour elements  26 ,  28  are in contact, during any further movement in the direction toward the third shift position C, the force is transmitted by the contour element  28 . 
     The performance graphs  34  show the overlap or engagement of the shifting element with the gearwheels of the shifting group along the control path S A , S B , S C . In the first shift position A the shifting element is engaged with the claws of a first gearwheel Z 1  of the shifting group. In the third shift position C the shifting element is engaged with the claws of a second gearwheel Z 2  of the shifting group. In the second shift position B, the neutral position, according to the stated objective no part of the shifting element must be in contact with a gearwheel Z 1 , Z 2 . The diagram shows that before reaching the second shift position B the connection of the shifting element with the first gearwheel Z 1  is broken, and only on the way from the second shift position B to the third shift position C does the shifting element engage with the second gearwheel Z 2 . 
     The performance graph  36  shows the variation of the spring force over the control path S A , S B , S C . As already described, a cup spring  22  can be used so that the actuating force remains almost constant over a control distance. In this example the cup spring  22  has been prestressed until it exerts approximately the force F 1 . This corresponds to the prestressing force with which the piston of the actuating element is loaded in its initial position A. To move it from the first shift position A to the second shift position B, an actuating pressure must be applied which is greater than the spring force F 1 . From the second shift position B onward the spring force curve rises. To move from the second shift position B to the third shift position C, namely to the second end position of the piston of the actuating element, it is thus necessary for the actuating element to apply a force at least larger than F 2 . The force increase of the cup spring  22  from the second shift position B onward, indicated by the spring characteristic curve, can additionally support the abrupt pressure increase for the recognition of the central position. 
     As already described, the abrupt pressure increase can be produced not only by means of a defined contour on the contact area of a cup spring, but also by actuating a pressure element in combination with spiral springs. This is illustrated in  FIG. 3 . In this case the pressure element  20  is not provided with a special contour, but is connected to one end of a first spiral spring  38 . At its opposite end in the direction of the shifting path S, the spring is attached to a spatially fixed buttress element  40 . In the initial position the first spiral spring  38  is already acted upon with pressure by the pressure element  20 . The pressure element  20  is part of the shifting element and is connected to an actuating element (not shown). On the buttress element  40 , a second spiral spring  42  is attached to the same surface to which the first spiral spring  38  is attached. The second spiral spring  42  is inserted under compression between the buttress element  40  and a first inner side of a U-shaped holding element  44 . The second inner side of the U-shaped holding element  44  is in contact with the side of the buttress element  40  facing away from the pressure element  20 . The first spiral spring  38  acts upon the pressure element  20  with a force F 3  in the direction of the control path. The control path is the distance to be covered by the actuating element or by the pressure element  20  in order to move from the first shift position A to the second shift position B and then to the third shift position C. In  FIG. 3  the shifting element is shown in all three shift positions. In the initial shift position A, the spring force F 3  of the first spiral spring  38  pushes against the pressure element  20 . To move to the second shift position B, the spring force F 3  of the first spiral spring  38  must therefore be overcome. When the pressure element  20  reaches the outside of the U-shaped holding element  44 , shift position B has been reached. At this point an abrupt pressure increase is produced since, to move farther to the third shift position C, in addition to the spring force F 3  of the first spiral spring  38 , the spring force F 4  of the second spiral spring  42  is also now being applied. In this case the U-shaped holding element  44  serves to ensure exact positioning of the second shift position B and of the abrupt pressure increase produced by virtue of the second spiral spring  42 . To be able to move to the third shift position C, from the second shift position B onward both of the spring forces F 3 ,and F 4  have to be overcome. If now the pressure element moves farther to the third shift position C, not only are the two spiral springs  38 ,  42  compressed but also the U-shaped holding element  44  is pushed in the direction along the control path. 
       FIG. 4  illustrates a method according to the invention in which there is a contour on the buttress element. The contour for changing the effective lever length does not necessarily have to be formed on a pressure element or on the side of the actuating element. The contour can also be formed on a separate buttress element positioned adjacent to the cup spring  22  on the side facing away from the pressure element  20 , or as illustrated in this case, it can be provided by the design of the holder  24  itself, in which the contour is formed directly. Again, there are two contour elements  46  and  48 . The first contour element  46  is directly at the contact point of the cup spring  22  on the holder  24 . Again, the shifting element is shown in all three shift positions A, B, C. In the first shift position A, namely the first end position of the piston of the actuating cylinder, a force is exerted by the pressure element  20  on the cup spring  22  so that the cup spring  22  is pre-stressed. The cup spring  22  is in contact only with the contour element  46  on the holder  24 . If now the actuating element is moved along the control path in the direction toward the third shift position C, then when the second shift position B is reached the second contour element  48  too comes in contact with the cup spring  22 . The effective lever of the pressure element  20  on the cup spring  22  is thereby shortened and an abrupt pressure increase takes place. In order to move farther to the third shift position C, a larger force or a higher pressure must be exerted on the pressure element  20  and the cup spring  22 . Thus, the second shift position B can be clearly distinguished. 
     A more pronounced pressure increase can also be produced if a corresponding contour can be formed on different elements of the shifting group. In  FIG. 5 , for example, contours are formed on the pressure element  20  and on the holder  24 .  FIG. 5  also shows that the contour for lever shortening can be designed differently. In this case there are no sharply projecting individual contour elements, but instead contour surfaces  50  and  52 . Other contour designs that have the same effect are possible. To illustrate the shift sequence the shifting element is shown in all three shift positions A, B, C. In the initial position A the pressure element  20 , which is part of the shifting element, presses against the pre-stressed cup spring  22 . The cup spring  22  rests against the holder  24  radially inside at the corner of the contour surface  52 . The pressure element  20  contacts the cup spring  22  only with the radially outer edge of the contour surface  50  located toward the edge of the cup spring  22 . If now the pressure element  20  is pushed in the control path direction to the second shift position B, the actuating element must overcome at least the force of the cup spring  22  to be able to move. When the second shift position B is reached the surfaces  50  and  52  are both in contact with the cup spring  22 . During subsequent movement from the second shift position B to the third shift position C, according to the invention a lever shortening takes place. On moving in the control path direction the cup spring  22  is in contact with the edge of the surface  50  closest to the holder  24  and with the edge of the surface  52  next to the pressure element  20 . The lever length of the cup spring  22  is accordingly shorter and correspondingly a larger force has to be applied in order to shift from the second shift position B to the third shift position C. The abrupt pressure increase required according to the invention is therefore brought about. 
     In present-day hybrid transmissions, already present proportional pressure regulators can be used to produce the pressure jump. In a proportional pressure regulator a specific pressure corresponds to a specific current. No further regulators have to be built in. 
     INDEXES 
     
         
           2  Shifting group 
           4  Shifting group 
           6  Shifting group 
           8  Actuating element 
           10  Shifting element 
           12  Actuating element 
           14  Shifting element 
           16  Actuating element 
           18  Shifting element 
           20  Pressure element 
           22  Cup spring 
           24  Holder 
           26  Contour element 
           28  Contour element 
           30  Actuating pressure performance graph 
           32  Contour element performance graph 
           34  Gearwheel engagement performance graph 
           36  Spring performance graph 
           38  First spiral spring 
           40  Buttress element 
           42  Second spiral spring 
           44  U-shaped holder 
           46  Contour element 
           48  Contour element 
           50  Contour surface on the pressure element 
           52  Contour surface on the holder 
         A First shift position, first end position 
         B second shift position, neutral position 
         C Third shift position, second end position 
         S A  Position along the control path at shift position A 
         S B  Position along the control path at shift position B 
         S C  Position along the control path at shift position C 
         p 1 , p 2 , p 3  Actuating pressure 
         Z 1  First gearwheel 
         Z 2  Second gearwheel 
         F 1 , F 2 , F 3 , F 4  Spring force