Servo-assistance device for motor vehicle gear shift
transmission

The invention relates to a servo-assistance device of a gear shift device of a motor vehicle transmission of the type defined in detail in the preamble of claim 1.

Present-day utility vehicles such as buses and trucks, with their forward-control construction, have spatial conditions governing the installation of the transmission which necessarily locate the transmission remote from the driver's seat. This remoteness is especially great in vehicles with under-floor or rear engines. Precise gear shifting is often made difficult by the long, sometimes stiff-running shift rod linkages of mechanically shifted gearboxes.

For the driver of a motor vehicle to be able to direct his full attention to the road traffic, he must as far as possible be relieved of distractions and supported in all the activities necessary for driving a motor vehicle.

Pneumatic shifting aids of dispersed construction, which consist of a mechanical-pneumatic control part and a separate pneumatic power part, are known. A shifting aid of dispersed construction is known from Loomann, Zahnradgetriebe, 2nd edition, Springer Verlag, 1988, p. 225. The control part is a mechanically actuated control valve which is operated by the shift rod linkage. The selecting movement when shifting gear is transmitted by mechanical means directly to the gearbox. As the shifting movement is transmitted, the control valve is actuated and simultaneously the manual shifting force is transmitted mechanically to the gearbox via a lever. In this case the manual shifting force is additionally assisted pneumatically by a compressed air cylinder. This compressed air cylinder, as a two-position cylinder with integrated hydraulic damper, forms the pneumatic power part. A direct, proportional replication of the manual shifting force is not achieved in this case. The paths between control part and power part are long and the installation is wasteful of space. Damage to the compressed air lines between control part and power part is unavoidable.

DE 198 39 850 A1 discloses a gear shift device with a servo-assistance device for a transmission of a motor vehicle. A control rod of the servo-assistance device is provided to be axially displaceable inside a piston rod and cooperates with a shift lever via a shift rod linkage. The piston rod of the servo-assistance device cooperates with means for shifting a gear shift transmission and has a latching device with three detents corresponding to two gear-engaged positions and a neutral position located between them, for secure definition of the particular shift position of the transmission.

DE 10 2006 006 652 A1 discloses a gear shift device with a servo-assistance device for a transmission of a vehicle, which includes means for shifting and selecting a gear step of a transmission and a control rod of the servo-assistance device on which acts a manual shifting force which is to be assisted. Spring elements are provided in the gear shift device in order to modify, within the servo-assistance device, the manual shifting force acting on the servo-assistance device before and/or during the generation of the servo-assistance force.

A valve which includes actuating pistons and valve slides arranged on the control rod is present in the servo-assistance device. The valve slides are held axially apart by a valve spring and each rest against a valve seat of the piston rod. The valve slides are actuated by the actuating pistons, whereby they are released from the valve seat of the piston rod and the valve opens.

By means of a spring element arranged on the control rod of the servo-assistance device, the shift lever, which cooperates via the shift rod linkage with the control rod of the servo-assistance device and with a mechanical interface for bridging the gear shift device, is fixed in the vehicle cabin practically without free play. In this case the spring force of the spring element must not be too large, since the spring force, in conjunction with a valve force of the servo- assistance device, determines the triggering force of the servo-assistance device and therefore the start of servo assistance of the shifting force. The valve force is understood here to mean a force consisting of the valve spring force and the pneumatic valve pressure force, the pneumatic valve pressure force acting on the valve slide. A configuration of the mechanical interface for bridging the servo-assistance device is disclosed, for example, in DE 198 40 052 Al, to which reference is made here.

If the servo-assistance device is supplied continuously with reservoir air, a small actuation of the shift lever merely has the effect that the spring element arranged on the control rod, by which the shift lever is fixed, is compressed and the actuating piston comes into abutment against the valve slide of the servo- assistance device. From this abutment point onwards, the valve force acts additionally to the spring force of the spring element in opposition to the manual shifting force, whereby a significantly greater manual shifting force is now required to move the shift lever further.

Known from DE 198 39 854 A1 is a gear shift device with servo-assistance device in which the compressed air supplied can be limited or interrupted by means of a reducing device. For example, the compressed air supply to the servo-assistance device can be interrupted when the transmission is in a gear-engaged position and the clutch is engaged. If the compressed air supply to the servo-assistance device is interrupted, the pneumatic valve pressure force is eliminated and with it a major part of the stop-effect of the valve, since only the valve spring force of the valve force is then effective. This has the result that the shift lever can be moved relatively far beyond its latched gear-engaged position, at most until the mechanical bridging of the servo-assistance device is brought into effect. In conjunction with shift systems which have high friction, a shift lever which is moved beyond the latched gear-engaged position would stay in its position when the driver releases it.

Depending on how far the shift lever is moved by the vehicle driver, therefore, it can remain in a position between the intended latched gear-engaged position and the mechanically bridged position, so that an unequivocal position for the latched gear-engaged position is no longer present. This also applies analogously to the latched neutral position of the shift lever. The latched gear-engaged position and the latched neutral position of the shift lever are understood here to mean the position of the shift lever assigned to it when the gear shift device is in a corresponding latched state for securely defining the particular shift position of the transmission. The latching may be effected, for example, by a latching contour in the piston rod of the servo-assistance device and corresponding ball catches, as disclosed, for example, in DE 198 39 850 A1.

It is the object of the present invention to specify a servo-assistance device of a gear shift device of a motor vehicle transmission with which positioning of the shift lever is improved and with which the disadvantages of the prior art can be minimized or eliminated.

The object of the invention is achieved by a generic servo-assistance device also having the characterizing features of the main claim.
A servo-assistance device of a gear shift device of a motor vehicle transmission includes a control rod on which acts a manual shifting force to be assisted. In addition, the servo-assistance device includes a piston rod which cooperates with means for shifting a gear shift transmission and at least one spring element which is arranged on the control rod and radially inside the piston rod. According to the invention, the spring element is arranged on the control rod of the servo-assistance device in such a manner that play between the spring element and the piston rod is present in one or both operating directions of the control rod, whereby the control rod can be moved by the extent of the play in one or both operating directions without the spring element being compressed. The spring element may be in the form, for example, of a spiral spring or a disk spring.
In this case the spring element arranged on the control rod of the servo-assistance device may have a high spring force, since the spring element has no effect on the triggering force of the servo-assistance device. The start of assistance of the shifting force is therefore no longer dependent on the spring element, whereby a reduction in the triggering force of the servo-assistance device is effected. The spring element may be arranged on the control rod in a pretensioned state. The play between the spring element and the piston rod is configured sufficiently large, in one or both operating directions of the control rod, for a valve of the servo-assistance device to be actuated before the play between the spring element and the piston rod has been fully traversed. This has the effect that, after a short operating travel of the control rod, the valve of the servo-assistance device is opened and, with further increasing manual shifting force, the control rod bears against the piston rod of the servo-assistance device via the spring element arranged on the control rod, bringing about a stop effect, since the spring has to be compressed for further actuation of the control rod.
In a preferred embodiment of the servo-assistance device according to the invention, the spring element is limited in the axial direction by stop elements and the play between the spring element and the piston rod, in one or both operating directions of the control rod, is defined by the stop elements and stops of the piston rod. The stop elements may be fixed, for example, by the spring element and by stops on the control rod, and the stops of the piston rod may be formed, for example, integrally with the piston rod or implemented by means of circlips.
Through the servo-assistance device according to the invention, the problem of the non-unequivocal positioning of the shift lever can be solved and the triggering force of the servo-assistance device reduced. Unequivocal positioning of the shift lever can be achieved even with shift systems having high friction. For this reason the inventive servo- assistance device can be used advantageously both in shift systems in which the shift lever acts on the gear shift device via a shift rod linkage and also in shift systems in which the shift lever acts via a cable linkage.
The servo-assistance device according to the invention may preferably be used in conjunction with a gear shift device of a motor vehicle transmission. The gear shift device includes means for selecting and shifting gear steps of the transmission, a manual shifting force to be assisted acting on a control rod of the inventive servo-assistance device. Through the gear shift device which includes the servo-assistance device proposed according to the invention, therefore, precise positioning of a shift lever of the gear shift device can be achieved. Through the reduction in the triggering force of the servo-assistance device, comfort when operating the gear shift device is enhanced. The manual shifting force to be assisted may- act via a shift rod linkage or a cable linkage, or may act hydraulically on the control rod of the servo- assistance device.
In an advantageous embodiment of the gear shift device with servo-assistance device, elastic elements are provided in the gear shift device in order to modify or limit the manual shifting force acting on the servo- assistance device before and/or during its application to the servo-assistance device. An arrangement and functioning of the elastic elements is known, for example, from DE 10 2006 006 651 Al.
The gear shift device with the servo-assistance device according to the invention may preferably be used in a motor vehicle transmission. The motor vehicle transmission having the gear shift device with the inventive servo-assistance device may be, for example, a conventional manual transmission having shift elements internal to the gearbox which are connected to the gear shift device and therefore to the inventive servo-assistance device in order to shift a gear step of the transmission.
The basic principle of the invention, which admits a plurality of embodiments, is explained in more detail below with reference to a drawing, in which:
Fig. 1 shows a shift system according to the prior art;
Fig. 2 shows a part of an embodiment of a servo- assistance device in a sectional representation, and
Fig. 3 is a characteristic-curve diagram.
Fig. 1 is a sketch of the shift system 2 of a motor vehicle according to the prior art. A shift rod 6 leads from a shift lever 4 via a lever deflection 8 to a gear shift device 11 with a pneumatic servo-assistance device 10. The pneumatic servo-assistance device 10 includes a connecting line 12 which leads to a reservoir 14 from which the pneumatic servo-assistance device 10 is supplied with compressed air. The lever deflection 8 has a first lever 16 which is connected, preferably articulated, to the shift rod 6. The lever deflection 8 has a second lever 18 which in turn engages in a control rod 20 which is arranged in the pneumatic servo-assistance device 10. Also provided in the pneumatic servo-assistance device 10 is a piston rod 22 in which engages a lever 24 which is connected via a rotatable selector shaft 26 to a lever 28 in the vehicle transmission 30. The lever 28 engages in a selector rail 32 by means of which gear ratios of the transmission can be shifted in known fashion. A movement of the lever 24 is converted via the selector shaft 2 6 into a movement of the lever 28, so that the lever 28 can cause the selector rail 32 to execute an axial movement. In this movement the selector rail 32 preferably adopts three positions, two axial end positions which each correspond to a selected gear ratio, and a middle position located between the end positions, which corresponds to a neutral position of the transmission.
Fig. 2 shows a section of the servo-assistance device 10 in a configuration according to the invention. The servo-assistance device 10 comprises a control rod 20, a piston rod 22 and a spring element 4 0 which surrounds the control rod 20 radially and is arranged radially inside the piston rod 22. The control rod 20 of the servo-assistance device 10 is displaceable axially inside the piston rod 22 and cooperates with a shift lever via a shift rod linkage (see Fig. 1) . The piston rod 22 cooperates with means for shifting the gear shift transmission (see Fig. 1) . A spring element 6 0 surrounds the control rod 20 radially and cooperates in an axial direction with a stop 48 and a stop element 62. The stop 48 is fastened to the control rod 20. The stop element 62 rests against a stop 64 on the control rod 20. Here, a prolongation 74 is provided on the control rod 20. Arranged on the prolongation 74 is a further spring element 66 which is enclosed between a stop element 68 and a stop 72 securely arranged on the prolongation 74. The stop element 68 rests axially against a further stop 70 fastened securely to the prolongation 74. A valve 54 includes actuating pistons 38, 58 and valve slides 36, 56. The actuating pistons 38, 58 and the valve slides 36, 56 are displaceable axially on the control rod 20. The valve slides 36, 56 are held apart axially by a spring element 34 and each rest against a valve seat of the piston rod 22. According to the invention the spring element 4 0 is arranged on the control rod 20 in such a manner that a play 50, 52 is present in both operating directions of the control rod between the spring element 4 0 and the piston rod 22. The spring elements 34, 40, 60, 66 are here in the form of spiral springs and the stops 46, 48, 64, 70, 72 in the form of circlips. The spiral spring 4 0 is limited in the axial direction by stop elements 42, 44. The spiral springs 40, 60 may have different spring rates or elasticities. It is likewise possible that the spiral springs 40, 60 are pretensioned together or individually. The play 50, 52 between the spiral spring 4 0 and the piston rod 22 is defined in both operating directions of the control rod 20 by the stop elements 42, 44 and stops of the piston rod 22. The stop elements 42, 44 are here fixed by the spring element 40 and by stops 46, 48 on the control rod 20, and the stops of the piston rod 22 are here formed integrally with the piston rod 22.
If the control rod 20 is moved to the right in the drawing plane by a manual shifting force, the actuating piston 38 arranged on the control rod 2 0 is also moved to the right. The valve slide 36 is actuated by the actuating piston 3 8 before the stop element 44 abuts the stop of the piston rod 22. The valve slide 36 is released from the valve seat of the piston rod 22 and the valve 54 is opened. By means of an existing reservoir pressure the open valve 54 controls a servo pressure according to the prevailing control rod force or manual shifting force. If the control rod 20 is now moved further to the right by an increase in the manual shifting force, the actuating piston 38 and the valve slide 3 6 remain in their open positions previously reached because of the equilibrium of forces which has been attained, while the control rod 2 0 continues to move relative to these two components, and the spiral spring 60 between stop 48 and stop element 62 is compressed further until the stop element 44 comes into abutment with the stop of the piston rod 22. If the stop element 44 rests against the stop of the piston rod, a still greater manual shifting force is required to move the control rod 20 further to the right, since the spring force of the spring element 40 must now also be overcome.
The spiral spring 4 0 is now compressed by increasing of the manual shifting force. If the compressed air supply to the servo-assistance device is then interrupted and the shift lever 4 (see Fig. 1) is moved beyond its latched gear-engaged position, the shift lever 4 moved beyond its latched gear-engaged position is repositioned in the direction of its latched gear- engaged position by the spring force of the spring element 40, even in shift systems with high friction, so that the shift lever 4 is now spaced from the gear- engaged position only by the lateral play 50, 52. The spring element can be designed according to the friction of the shift system.
The case is analogous for actuation of the control rod 20 to the left in the drawing plane.
Through the servo-assistance device 10 according to the invention a reduction in the triggering force can be effected, since the valve 54 has already been opened before the spring element 4 0 is compressed. This is also advantageous in shift systems with high friction. The triggering force is understood here to mean the manual shifting force which must be exerted in order to open the valve 54 of the servo-assistance device 10.
Fig. 3 shows a characteristic-curve diagram in which the servo-assistance force FS is represented as a function of the manual shifting force Fh . In the servo- assistance device 10 according to the invention, the triggering force FH2 is smaller than the triggering force FHi of a servo-assistance device known from the prior art. The characteristic curve 78 which can be achieved by the servo-assistance device 10 is therefore displaced parallel to the characteristic curve 76 known from the prior art, whereby a higher servo-assistance force FS is achieved with the same manual shifting force Fh in the initial region of the characteristic curve 7 8.
The characteristic curve 78 has a region between two deflection points 80, 82 which is disposed practically horizontally. The deflection point 80 is produced when the stop element 44 comes into abutment against the stop of the piston rod 22. In the region between the two deflection points 80, 82, the spring force of the spring element 40 must first be overcome before the valve 54 can open further, whereby only a small servo- assistance force FS is generated here in relation to the manual shifting force FH. From the deflection point 8 2 onwards both spring elements 40, 6 0 are compressed, so that the characteristic curve 78 has a constantly- rising gradient up to the maximum servo-assistance force FSmax- Through a corresponding design of the spring element 40, for example by means of the spring rate or by a pretensioned arrangement of the spring element 40, the region between the two deflection points 80, 82 can be varied correspondingly. The manual shifting force FH at which the first deflection point 8 0 is produced is therefore dependent on the spring force of the spring element 60, while the second deflection point 82 is produced as a function of the spring force of the spring element 40.
Even with a small manual shifting force FH, therefore, a given servo-assistance force FS can be achieved, a practically constant servo-assistance force FS then being achievable for a certain range from a given level of manual shifting force FH onwards, and then, with a still greater manual shifting force FH, a rising servo- assistance force FS can again be achieved. Here, the spring force of the spring element 40 is selected such that the region between the two deflection points 80, 82 is disposed practically horizontally and the characteristic curve 78 is displaced so far to the right that, in the region between the deflection point 82 and the maximum servo-assistance force FSMAX, less servo-assistance force FS is generated in comparison to the characteristic curve 76 for an equal manual shifting force FH.
References
2 Shift system
4 Shift lever
6 Shift rod
8 Lever deflection
10 Servo-assistance device
11 Gear shift device
12 Connecting line 14 Reservoir
16 Lever
18 Lever
2 0 Control rod 22 Piston rod 24 Lever
26 Selector shaft
28 Lever
3 0 Vehicle transmission 32 Selector rail
34 Spring element
3 6 Valve slide
3 8 Actuating piston
4 0 Spring element 4 2 Stop element
4 4 Stop element
46 Stop
4 8 Stop
5 0 Play 52 Play 54 Valve
56 Valve slide
58 Actuating piston
6 0 Spring element 62 Stop element 64 Stop
66 Spring element
68 Stop element
70 Stop
72 Stop
74 Prolongation
7 6 Characteristic curve according to the prior art
78 Characteristic curve
80 Deflection point
82 Deflection point
Fh Manual shifting force
FS Servo-assistance force
FHI Triggering force according to the prior art
FH2 Triggering force
Fsmax Maximum servo-assistance force

Patent claims

1. A servo-assistance device (10) of a gear shift device (11) of a motor vehicle transmission, including a control rod (20) on which acts a manual shifting force to be assisted, a piston rod (22) and at least one spring element (40) which is arranged on the control rod (20) and radially inside the piston rod (22) , characterized in that the spring element (40) is arranged on the control rod (20) in such a manner that a play (50, 52) is present between the spring element (40) and the piston rod (22) in one or both operating directions of the control rod (20).

2. The servo-assistance device (10) as claimed in claim 1, characterized in that the play (50, 52) between the spring element (40) and the piston rod (22) is configured sufficiently large, in one or both operating directions of the control rod (20) , for a valve (54) of the servo-assistance device (10) to be actuated before the play (50, 52) has been fully traversed.

3. The servo-assistance device (10) as claimed in claim 1 or 2, characterized in that the spring element (40) is limited in the axial direction by stop elements (42, 44), and in that the play (50, 52) is defined in one or both operating directions of the control rod (20) by the stop elements (42, 44) and stops of the piston rod (22).

4. The servo-assistance device (10) as claimed in claim 3, characterized in that the stop elements (42, 44) are fixed by the spring element (40) and by stops (46, 48) on the control rod (20).

5. The servo-assistance device (10) as claimed in at least one of claims 1 to 4, characterized in that the spring element (4 0) is in the form of a spiral spring or a disk spring.

6. A gear shift device (11) of a motor vehicle transmission with a servo-assistance device (10) as claimed in at least one of the preceding claims.

7. A motor vehicle transmission with a gear shift device (11) as claimed in claim 6.