Shifting element with three shift positions

#This application claims priority from German patent application serial no. 10 2012 204 839.1 filed Mar. 27, 2012.

#The invention concerns a method for actuating a shifting element with three shift positions and a device according to 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.

#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

#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 SA, SB, SC along the control path. At the first shift position A a certain actuating pressure p1 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 p2 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.