Hydraulic system

Abstract

A hydraulic system (1) for electro-hydraulically pressing and/or moving a first (8) and a second (9) set of disks of a CVT transmission (10) is provided. The hydraulic system (1) includes two electrically driven hydraulic pumps (4, 5). In order to reduce energy consumption during operation of the hydraulic system, the hydraulic pumps (4, 5) are in the form of self-inhibiting hydraulic pumps (4, 5) which, when a hydraulic pressure is applied and the associated electric drive is switched off, substantially maintain the applied hydraulic pressure as a result of a static friction and a residual moment of the associated electric drive.

Claims

1. A hydraulic system for electrohydraulically pressing or adjusting a first pulley set and a second pulley set of a CVT transmission, the hydraulic system comprising: two electromotively driven hydraulic pumps configured as self-locking hydraulic pumps, the two electromotively driven hydraulic pumps configured such that when a hydraulic pressure is applied and a respective associated electromotive drive is switched off the two electromotively driven hydraulic pumps substantially maintain the applied hydraulic pressure as a result of a static friction and a residual torque of the respective associated electromotive drive.

2. The hydraulic system as claimed in claim 1, wherein the two electromotively driven hydraulic pumps are configured as reversing pumps.

3. The hydraulic system as claimed in claim 1, further comprising a hydraulic pressure accumulator connected in parallel to the two electromotively driven hydraulic pumps.

4. The hydraulic system as claimed in claim 3, wherein the hydraulic pressure accumulator is connected to a branching arranged between the two electromotively driven hydraulic pumps, the branching being connected to the second pulley set of the CVT transmission.

5. The hydraulic system as claimed in claim 4, wherein a first of the two electromotively driven hydraulic pumps is configured as a support pump connected between the first pulley set of the CVT transmission and the branching.

6. The hydraulic system as claimed in claim 5, wherein a second of the two electromotively driven hydraulic pumps is configured as a pressing pump connected between a hydraulic medium reservoir and the branching.

7. The hydraulic system as claimed in claim 4, further comprising a hydraulic resistor configured for acting only in a charging direction of the hydraulic pressure accumulator, the hydraulic resistor being arranged between the branching and the hydraulic pressure accumulator.

8. The hydraulic system as claimed in claim 7, further comprising a blocking valve configured for blocking flow from the branching toward the hydraulic pressure accumulator, the blocking valve being arranged between the branching and the hydraulic pressure accumulator parallel to the hydraulic resistor.

9. A method for actuating the two electromotively driven hydraulic pumps of the hydraulic system as claimed in claim 1, the method comprising: switching off at least one of the two electromotively driven hydraulic pumps as soon as the applied hydraulic pressure reaches an upper pressure limit value.

10. The method as claimed in claim 9, wherein the at least one of the two electromotively driven hydraulic pumps is switched on again as soon as the applied hydraulic pressure reaches a lower pressure limit value.

11. A method of constructing a hydraulic system for electrohydraulically pressing or adjusting a first pulley set and a second pulley set of a CVT transmission, the method comprising: providing a first electromotively driven self-locking hydraulic pump to deliver a contact pressure from a hydraulic medium reservoir into a pressure chamber of the second pulley set; providing a second electromotively driven self-locking hydraulic pump to displace pressurized hydraulic medium between a pressure chamber of the first pulley set and the pressure chamber of the second pulley set and connecting a hydraulic pressure accumulator to the first and second electromotively driven self-locking hydraulic pumps via a branching between the first and second electromotively driven self-locking hydraulic pumps, and providing a hydraulic resister and a blocking valve arranged in parallel with each other between the hydraulic pressure accumulator and the branching.

12. The method as recited in claim 11 wherein the first and second electromotively driven self-locking hydraulic pumps are configured such that when a hydraulic pressure is applied and a respective associated electromotive drive is switched off the two electromotively driven hydraulic pumps substantially maintain the applied hydraulic pressure as a result of a static friction and a residual torque of the respective associated electromotive drive.

13. A hydraulic system for electrohydraulically pressing or adjusting a first pulley set and a second pulley set of a CVT transmission, the hydraulic system comprising: a first electromotively driven self-locking hydraulic pump arranged to deliver a contact pressure from a hydraulic medium reservoir into a pressure chamber of the second pulley set; and a second electromotively driven self-locking hydraulic pump arranged to displace pressurized hydraulic medium between a pressure chamber of the first pulley set and the pressure chamber of the second pulley set, wherein the first electromotively driven self-locking hydraulic pump is configured to maintain pressure in the pressure chamber of the first pulley set after being switched off and the second electromotively driven self-locking hydraulic pump is configured to maintain pressure in the pressure chamber of the second pulley set after being switched off.

14. The hydraulic system as recited in claim 13 further comprising a hydraulic pressure accumulator connected to the first and second electromotively driven self-locking hydraulic pumps via a branching between the first and second electromotively driven self-locking hydraulic pumps.

15. The hydraulic system as recited in claim 14 further comprising a hydraulic resister and a blocking valve arranged in parallel with each other between the hydraulic pressure accumulator and the branching.

16. The hydraulic system as recited in claim 15 wherein the blocking valve is configured as a nonreturn valve which blocks in a first direction from the branching toward the hydraulic pressure accumulator and opens in a second direction opposite the first direction, the hydraulic resistor producing no effect in the second direction.

17. The hydraulic system as recited in claim 14 further comprising a first hydraulic line extending from the branching to the first electromotively driven self-locking hydraulic pump, a second hydraulic line extending from the branching to second first electromotively driven self-locking hydraulic pump, a third hydraulic line extending from the branching to the pressure chamber of the second pulley set, and a fourth hydraulic line extending from the branching to the hydraulic pressure accumulator.

Description

BRIEF SUMMARY OF THE DRAWINGS

(1) Further advantages, features and details of the present disclosure will emerge from the following description in which different exemplary embodiments are described in detail with reference to the drawing, in which:

(2) FIG. 1 shows a hydraulic system for electrohydraulically pressing and/or adjusting pulley sets of a CVT transmission, having two electromotively driven hydraulic pumps; and

(3) FIG. 2 shows a Cartesian coordinate diagram in which the profile of a pressure applied in the hydraulic system of FIG. 1 is plotted over time.

DETAILED DESCRIPTION

(4) FIG. 1 illustrates a hydraulic system 1 having a hydraulic medium reservoir 3 and two hydraulic pumps 4, 5 in the form of a hydraulic circuit diagram. A hydraulic medium, such as oil, is contained in the hydraulic medium reservoir 3. The hydraulic pump 4 is configured as a reversing pump and, as is indicated by arrow heads, can deliver in opposite delivery directions.

(5) The hydraulic pump 4 is connected between the hydraulic medium reservoir 3 and a branching 6. The branching 6 is a hydraulic branching or a hydraulic node point in which a total of four hydraulic lines open or are connected. Two hydraulic lines connect the two hydraulic pumps 4 and 5 to one another in the branching or the node point 6.

(6) The hydraulic pump 5 is arranged between the branching 6 and a first pulley set 8 of a CVT transmission 10. Extending from the branching 6 is a third hydraulic line to a second pulley set 9 of the CVT transmission 10. Extending from the branching or the node point 6 is a fourth hydraulic line to a hydraulic pressure accumulator 12.

(7) Two further branchings or node points 13, 14 are arranged in the fourth hydraulic line between the branching 6 and the hydraulic pressure accumulator 12. A hydraulic resistor 15 configured as an orifice plate is arranged between the branchings 13 and 14.

(8) The hydraulic resistor 15 configured as an orifice plate acts only in a charging direction of the hydraulic pressure accumulator 12, that is to say from the branching 6 toward the hydraulic pressure accumulator 12. In the reverse direction, the hydraulic resistor 15 configured as an orifice plate has no effect.

(9) A blocking valve 16 is connected parallel to the hydraulic resistor 15 between the branchings 13 and 14. The blocking valve 16 is configured as a nonreturn valve which blocks from the branching 6 toward the hydraulic pressure accumulator 12. The blocking valve 16 configured as a nonreturn valve opens in the opposite direction, with the result that the hydraulic resistor 15 produces no effect in this direction, as is described above.

(10) The two pulley sets 8, 9 represent hydraulic consumers in the hydraulic system 1. These are, for example, hydraulic cylinders each having a piston which is assigned to a variator of the continuously variable conical-pulley wraparound transmission (not shown in further detail).

(11) In order to adjust the variator, the pistons in the hydraulic cylinders are adjusted by supplying or removing a hydraulic volumetric flow. If no adjustment is desired, for example in a constant driving operation, a hydraulic volumetric flow is then also not required. In these states, it is sufficient if a desired hydraulic pressure is maintained in the hydraulic system 1.

(12) If a transmission ratio and a pressing of the variator are constant, no volumetric flow is required meanwhile. Nevertheless, the hydraulic pressure set, that is to say the desired pressing, and/or the corresponding moving pulley position, that is to say the desired transmission ratio, are/is maintained.

(13) According to the present disclosure, the hydraulic pumps are configured as self-locking hydraulic pumps 4, 5. The self-locking hydraulic pumps 4, 5 are used for pressing and/or adjusting the CVT transmission 10.

(14) In order to reduce undesired pressure fluctuations, the hydraulic pressure accumulator 12 is connected in parallel. To ensure that the hydraulic pressure accumulator 12 does not inadmissibly slow down a quick pressure buildup in the hydraulic system, the orifice plate 15 acting only in the charging direction of the hydraulic pressure accumulator 12 is interposed.

(15) The two electromotively driven hydraulic pumps 4, 5, which are also referred to as electric pump actuators, advantageously bring about the pressing and the adjustment of the two pulley sets 8, 9 of the CVT transmission 10. The electromotively driven hydraulic pump 4 is configured as a support pump and advantageously serves for supporting and adjusting the variator. The likewise electromotively driven hydraulic pump 5 is configured as a pressing pump and advantageously serves for pressing the variator.

(16) During operation of the hydraulic system 1, the two hydraulic pumps 4, 5 are used to charge the pressure chambers 18, 19 of the pulley sets 8, 9 with pressurized hydraulic medium. For this purpose, the hydraulic pump 4 configured as a pressing pump delivers a contact pressure to the second pulley set 9 in that said pump presses hydraulic medium from the hydraulic medium reservoir 3 into the pressure chamber 19 of the pulley set 9.

(17) The hydraulic pump 5 configured as a variable displacement pump or support pump changes the variator transmission ratio of the CVT transmission 10 by displacing pressurized hydraulic medium between the pressure chambers 18 and 19 of the pulley sets 8, 9.

(18) The two electromotively driven hydraulic pumps 4, 5 are designed in such a way that, under prevailing or applied pressure, when the electric motor used for driving is switched off, they stand still in a simple manner as a result of their static friction and of the residual torque of the electric motor. The applied or prevailing hydraulic pressure is thus maintained even without electrical energy. The electric motor comprises magnets, which lead to the fact that a rotor of the electric motor locks in at certain angles of rotation and thus generates a certain retaining torque even in a deenergized state.

(19) However, unavoidable leakages slowly reduce the pressure, with the result that, after reaching a minimum pressure, the respective hydraulic pump 4, 5 has to be at least briefly started again in order to come again to a higher pressure level.

(20) FIG. 2 illustrates a pressure profile 20 in a Cartesian coordinate diagram. A time t in a suitable time unit is plotted on an x axis 21. A pressure p in a suitable pressure unit is plotted on a y axis 22.

(21) Ascending lines 24 indicate that the pressure increases when the respective hydraulic pump 4, 5 is switched on. Descending lines 25 indicate that the pressure drops when the respective hydraulic pump 4, 5 is switched off. An upper pressure limit value is indicated by a point 28. A lower pressure limit value is indicated by a point 29. A setpoint value for the pressure p is indicated by a dashed line.

(22) It is expedient to keep the interval between the upper pressure limit value 28 and the lower pressure limit value 29 small, since an increased pressure using the example of pressing of increases the variator loss and additionally demands a higher drive torque from the respective hydraulic pump. For a given basic leakage, the difference between the upper limit value 28 and the lower limit value 29 gives the time after which the corresponding hydraulic pump 4, 5 has to be started again.

LIST OF REFERENCE SIGNS

(23) 1 Hydraulic system 2 Hydraulic medium reservoir 3 Hydraulic pump 4 Hydraulic pump 5 Branching 8 First pulley set 9 Second pulley set 10 CVT transmission 12 Hydraulic pressure accumulator 14 Branching 15 Hydraulic resistor 16 Blocking valve 18 Pressure chamber 19 Pressure chamber 20 Pressure profile 21 x axis 22 y axis 24 Ascending line 25 Descending line 28 Point 29 Point