Hydraulic unit and method for providing a pressurized hydraulic fluid

Abstract

An hydraulic unit provides a pressurized hydraulic fluid at an outlet. The unit has an electric motor and a pump operated by the motor for generating pressure. A reservoir having a variable compensation volume stores the hydraulic fluid primed by the pump in a gas-free manner. The hydraulic fluid is especially pressurized in order to safely and reliably guarantee that the hydraulic fluid is gas-free. The inventive design allows for the safe and operationally reliable use of the hydraulic unit even at high acceleration values. The hydraulic unit is especially adapted for use in an industrial robot.

Description

The invention relates to a hydraulic unit and a method for providing a pressurized hydraulic fluid.

Hydraulic units are employed in the most diverse possible technical sectors. In particular, the hydraulic unit is also used in a machine tool which has to execute an axial movement with high force during a machining operation. Such a machine tool is, for example, a press or a punching machine, in which a hole is punched out or a punching element punched in with the aid of an axially displaceable ram.

The hydraulic unit serves, in particular, also in the sector of riveting technology, for connection to a setting appliance for setting a rivet, in particular a blind rivet. In the case of a blind rivet, this is introduced, with its rivet sleeve in front, from one side into a bore of two components to be connected, until its setting head comes to lie on the upper component. In the rivet sleeve, a rivet plug is arranged, which is drawn in the axial direction with the aid of the setting appliance. In this case, the rivet sleeve is deformed and forms a closing head, so that the components to be connected are clamped between the closing head and setting head. Where specific tensile force is overshot, the plug tears off, and the operation of setting the blind rivet is concluded. A continuous pressure build-up occurs during the setting operation. In this case, first, under low pressures, comparatively long axial strokes are executed. At the end of the setting operation until the rivet plug is torn off, high deformation forces and consequently high pressures must be provided, along with comparatively low axial strokes. In order to achieve as short cycle rates as possible, therefore, the hydraulic unit must be capable both of executing long axial travels quickly and of applying very high forces.

In process automation, in particular, for example, in the automobile industry, efforts are aimed at a fully automated and monitored blind rivet setting operation with the aid of an industrial robot. In this case, however, there is a problem that hydraulic lines have to be led from the fixed hydraulic unit to the robot and along its robot arms to a rivet setting appliance fastened to the robot hand. The routing of the hydraulic lines is difficult under these circumstances. Particularly in confined workspace situations, for example in body components of a motor vehicle, there is the additional problem that only very little space is available to the robot and there is the risk of chafing of the hydraulic lines on sharp-edged components.

The object on which the invention is based is to specify a compact hydraulic unit which can be employed, in particular, in combination with an industrial robot. The object on which the invention is based is, furthermore, to specify a method for providing a pressurized hydraulic fluid.

The object relating to the hydraulic unit is achieved, according to the invention, by means of the features of patent claim 1. Accordingly, a hydraulic unit for providing a pressurized hydraulic fluid is provided at an outlet of the unit which has an electric motor and at least one pump for pressure generation, operated via the electric motor and designed, in particular, as a piston pump. In this case, for the hydraulic fluid sucked in by the pump, a storage space with a variable compensating volume is provided, in which the hydraulic fluid is stored free of gas.

The invention in this case proceeds from the consideration that, because of the problems of routing the hydraulic lines along the robot, it is advantageous for the hydraulic unit to be arranged directly on the robot, in particular on the robot hand, so that no hydraulic lines are routed via a movable robot axis. This presents the problem, however, that, in a conventional hydraulic unit, air or gas would pass into the hydraulic fluid on account of the acceleration, so that a reliable and defined hydraulic actuation of a machine tool, for example of a blind rivet setting head, would not be possible.

Owing to the arrangement of the storage volume with a variable compensating volume in which the hydraulic fluid is arranged so as to be free of gas, the penetration of air into the hydraulic fluid and the foaming of the latter are avoided. The change in a hydraulic volume of the machine tool, occurring during the operation of the machine tool, leads to a variation in the compensating volume of the storage space. Here, therefore, in contrast to conventional hydraulic units, no air is used for volume compensation in the compensating volume. In the hydraulic unit proposed here, therefore, rapid movements and, in particular, abrupt accelerations, for example direction changes, do not lead to a foaming of the hydraulic fluid.

The hydraulic unit proposed here is therefore arranged, in particular, on machine parts which are accelerated during operation. These are, in particular, the robot hand of an industrial robot, crane or gripper devices, motor vehicles, in particular motor trucks, and, for example, mobile entertainment equipment for amusement parks. When an industrial robot is concerned, accelerations of, for example, 20 times gravitational acceleration and above may in this case occur. The mobile hydraulic unit is capable of executing such high accelerations, without its functioning capacity being impaired.

Expediently, in this case, the hydraulic fluid in the storage space has an overpressure with respect to an ambient pressure. Foaming is thereby reliably avoided. Preferably, this overpressure is in the region of a few 10⁵ Pa, in particular between 3 and 50×10⁵ Pa.

To form the variable compensating volume, according to an expedient development a compensating wall of the storage space is arranged displaceably in the manner of a piston and so as to be sealed off with respect to a stationary housing wall of the storage space. This design having the mechanically particularly rigid compensating wall achieves a very robust construction. Moreover, the preferred configuration as a piston has the advantage of a simple construction. The pressure generation unit is therefore designed in the manner of a piston storage space. Alternatively to the mechanically rigid configuration, the compensating wall is designed, for example, as an elastic diaphragm.

To generate the overpressure in the storage space, in this case, the compensating wall can expediently be acted upon by a counterforce or a counterpressure. For this purpose, advantageously, on the outside facing away from the storage space a pressure space is provided to which a pressure line can be connected. The generation of the counterforce therefore takes place, in particular, pneumatically or else hydraulically. In principle, a mechanical application of the counter-pressure, for example by means of a spring element, is also possible. By contrast, the advantage of pneumatic or hydraulic pressure action is that the magnitude of the counterpressure can be controlled in a simple way. The pressure unit for generating the counterpressure is therefore designed, in the case of pneumatic pressure generation, in the manner of a media converter, that is to say converts pneumatic pressure into hydraulic pressure. Preferably, in this case, the pressure unit is designed in such a way that pressure intensification is achieved.

In order, in particular, to ensure reliable operation, in particular, for example, in the event of an interruption of the pneumatic line for generating the counterpressure, in a preferred version a securing spring is additionally provided for generating the counterpressure. A securing spring is in this context to be understood, in general, to mean an elastic element which exerts a fixed elastic restoring force. Preferably, in this case, a spring element in the actual sense, for example a compression spring, is employed. Alternatively to the additional arrangement of the spring element, this is provided instead of the pressure space.

In order to achieve as compact a construction of the hydraulic unit as possible, the electric motor and the pump are arranged in a housing of the unit, and the inner space surrounded by the housing forms the storage space, that is to say is filled with hydraulic fluid. The electric motor and the pump are therefore arranged in the hydraulic fluid, in particular hydraulic oil. The housing is sealed off, overall, hermetically relative to the outside. By virtue of this configuration, a separate compensating vessel is not required. Furthermore, there is no need for any supply lines from the compensating vessel to a suction-intake side of the pump.

Furthermore, with a view to as compact a construction as possible, the housing is preferably closed by an, in particular, end-face function block in which a plurality of hydraulic components are integrated. Hydraulic components of this type are, for example, hydraulic lines and valves. The function block therefore forms a cover of the housing and consequently of the unit. By the hydraulic components being integrated into the cover, there is no need for a separate space requirement for these components and the unit overall can have a very compact construction.

Expediently, the function block is designed for controlling and routing the hydraulic fluid provided at the outlet. For this purpose, a multiplicity of lines and also hydraulic control elements, such as valves, are arranged in the function block. The function block therefore serves, for example, for shutting off or releasing the pressurized hydraulic fluid generated by the pump.

Preferably, the pressure side of the pump is connected via a line to the function block. All further hydraulic components following the pump on the pressure side are integrated in the function block. By all the hydraulic function elements being arranged within the function block, the construction of the remaining unit is kept comparatively simple and robust.

According to a particularly preferred embodiment, at least two pumps are provided for the provision, on the one hand, of a low-pressure part stream and, on the other hand, of a high-pressure part stream of the hydraulic fluid. A two-stage hydraulic unit is thus provided. The advantage of this is that different pressure stages are provided at a low energy outlay as a function of the respective application. Different pressure requirements are therefore served simply and in an energy-saving way. Particularly in a blind rivet setting operation, there is no need for high pressure to be provided at the commencement of the setting operation.

Expediently, the at least two different pumps are actuated jointly by the electric motor. A plurality of hydraulic part streams of different pressures and/or of different feed quantities are therefore generated via one and the same electric motor, so that the most diverse possible pressure requirements can be fulfilled by means of only one electric motor and therefore in a highly space-saving way. Particularly in two-stage or multistage machining operations in which different pressure requirements are demanded within one operation, this embodiment is of particular advantage. For example, long axial strokes must be executed at only low pressure and short axial strokes at high pressure, as, for example, in a blind rivet setting operation.

Expediently, in this case, the pumps are actuated jointly via an eccentric shaft of the electric motor and are therefore arranged approximately annularly around the eccentric shaft. The pumps are therefore actuated directly by the electric motor, without a gear being interposed. In the provision of two hydraulic part streams, in this case, expediently a plurality of pumps are provided for generating the low pressure part stream and a plurality of pumps are provided for generating the high-pressure part stream, a pump for the high-pressure part stream and a pump for the low-pressure part stream preferably being alternately adjacent to one another.

Preferably, furthermore, a valve arrangement for controlling the at least two part streams is provided, which is designed in such a way that in each case only one part stream is provided at the outlet of the hydraulic unit. There is therefore no need for any external control valves outside the unit for changing over from one part stream to the other part stream, so that, overall, a compact construction is achieved. The valve arrangement is in this case designed, in particular, in such a way that an automatic changeover between the part streams takes place as a function of the current pressure requirement.

This valve arrangement is in this case integrated, in particular, in the function block. Preferably, the valve arrangement has a pressure switching valve which automatically switches off the low-pressure part stream when a predeterminable pressure of the hydraulic fluid provided at the outlet is overshot.

In order to keep energy necessary for generating the pressure as low as possible, furthermore, the valve arrangement is preferably designed in such a way that in each case one of the part streams can be switched to pressureless. There is therefore provision, in particular, for in each case one of the part streams to be pressureless during operation. The electric motor therefore needs to build up pressure in only one part stream and can therefore have a lower-power and compact design.

According to an expedient development, the electric motor is controllable and, in particular, regulatable. The electric motor is in this case started only as required, that is to say when there is a pressure requirement. The pressure is therefore generated, only as required, in an energy-saving way without a pressure accumulator.

Preferably, in this case, the electric motor is regulated to a constant rotational speed. A constant stream of hydraulic fluid is thereby provided. In particular, in addition to this, the electric motor is regulated to a constant torque, so that a specific pressure, for example a limited maximum pressure, is generated and maintained. Torque regulation is advantageous particularly in the case of a 0-travel stroke, that it is to say, for example, when, during the operation of setting a blind rivet, a setting or forming force has to be maintained without or virtually without a movement of the blind rivet.

The advantage of controlling the pressure via the electric motor is to be seen, in particular, in that no pressurized hydraulic fluid has to be stored. There is therefore no pressure storage volume provided between the pump for generating the pressure in the hydraulic fluid and the outlet. The generation of pressure therefore takes place instantaneously, that is to say without a buffer or the like, by the electric motor being started up and controlled. The pressure is provided very quickly at the outlet via the regulation of the electric motor.

The electric motor is expediently designed, in particular, as a servomotor.

Furthermore, the object is achieved, according to the invention, by means of a method for providing a pressurized hydraulic fluid according to patent claim 20. The advantages and preferred embodiments listed with regard to the hydraulic unit are also to be applied accordingly to the method.

The hydraulic unit described here is distinguished, on the one hand, by its mobility, that is to say the hydraulic unit can be moved and accelerated very quickly, without its functioning capacity being impaired, and is therefore functionable, in particular, even independently of position. Owing to this property, the hydraulic unit is suitable, in particular, for arrangement on an industrial robot and there is, in particular, part of an exchangeable robot hand. Owing to arrangement directly on the robot hand, the hydraulic lines to the machine tool, for example a setting tool, are reduced to the necessary minimum amount, so that damage to these on account of the movements of the robot arms is not to be feared.

Furthermore, by virtue of the compact configuration, even confined workspaces are accessible.

The hydraulic unit described here is distinguished, on the other hand, by its highly compact construction, at the same time with the generation of very high pressures. Expediently, the hydraulic unit has an approximately cylindrical housing which has a length of only about 30-40 cm, with a diameter of about 12 cm. At the same time, the hydraulic assembly is provided for the provision, in particular, of the two pressure part streams, the low-pressure part stream being provided, for example, for about 200×10⁵ Pa and the high-pressure part stream preferably being provided for 500×10⁵ Pa. Even with an overall construction space of 3000 to 10 000 ccm, therefore, a mobile hydraulic unit is afforded which makes it possible to have two hydraulic part streams with 100 to 300 bar and 300 to 700 bar pressure. The overall volume of the hydraulic fluid within the hydraulic unit in this case preferably amounts to only about 500 ml. The hydraulic unit is therefore distinguished by a high power density along with the use of low energy. Since no pressure limiting valves of any kind are provided and the hydraulic unit is operated in switch-off mode, that is to say only when there is actually a pressure requirement, only low energy losses occur and the necessary use of energy is low. This makes it possible to use a comparatively low-power and compact electric motor.

An exemplary embodiment of the invention is explained in more detail below with reference to the drawings, in which, in each case in diagrammatic illustrations:

FIG. 1 shows a longitudinal section through a hydraulic unit,

FIG. 2 shows a view of the front end face, designed as a function block, of the hydraulic unit,

FIG. 3 shows a view of the rear end face of the hydraulic unit, and

FIG. 4 shows a hydraulic diagram of the hydraulic unit.

Identically acting parts are given the same reference symbols in the figures.

The hydraulic unit 2 illustrated in FIGS. 1 to 3 has, overall, an essentially cylindrical housing 4, the inner space of which forms a storage space 5 for the hydraulic fluid and is sealed off hermetically. The housing 4 is closed on its left end face by means of a control or function block 6 designed in the manner of a housing cover. On its right end face lying opposite the function block 6, the hydraulic unit 2 has a compensating block 8 which closes the housing 4 on the rear end face. A pressure generation block 10 is arranged between these two blocks 6, 8. As is evident from the figure, the individual housing components of the hydraulic unit 2 are fastened to one another by means of screw connections. At the parting planes or parting points of two components, which are in each case in the form of metallic components, in each case sealing elements 12 are provided, so that a hermetic sealing off of the overall inner space 5 with respect to the surroundings is achieved.

The pressure generation block 10 is formed essentially by a suboil electric motor 14, designed as an alternating current servomotor, and by a plurality of pumps 16 designed as piston pumps. The electric motor 14 has a stator 14A with a stator winding and a rotor 14B with a permanent magnet. Provided on the rotor 14B, on its end face, is an eccentric shaft 18, the axis of which is arranged so as to be offset radially with respect to the rotor axis 20. The pumps 16 arranged annularly around the eccentric shaft 18 are actuated alternately via the eccentric shaft 18. The piston of the respective piston pump 16 is actuated via the eccentric shaft 18 for the suction intake and expulsion of the hydraulic oil. A bearing 22 is arranged between the eccentric shaft 18 rotating during operation and the stationary pumps 16.

Each of the pumps 16 is followed on the pressure side by a pressure line 24 which leads to the function block 6. The pressure line 24 is in this case formed by a duct worked into the housing wall. The suction side of the pumps 16 is connected in each case to the inner space 5 in which the hydraulic oil is located.

In the exemplary embodiment, six pumps 16 overall are arranged annularly around the eccentric shaft 18, alternately adjacent pumps 16 being provided for generating two different pressures, to be precise a low pressure of the magnitude of about 200×10⁻⁵ Pa and a high pressure of the magnitude of about 500×10⁻⁵ Pa.

The function block 6, designed as a solid metal cover, has a thickness d which amounts, for example, to about 10% of the overall length 1 of the hydraulic unit 2. A multiplicity of ducts for forming hydraulic lines 28 and bores 30 for arranging hydraulic valves are introduced into the function block 6, so that the function block forms a valve block.

The arrangement of the individual hydraulic lines 28 and of the bores 30 or the valves is also apparent, in particular, from the end view according to FIG. 2, in which the hydraulic lines 28 and the bores 30 are illustrated by broken lines. As can be seen from this, a multiplicity of bores 30 and consequently hydraulic valves are provided. Of these, a directional seat valve 32A and a pressure switching valve 32B can be seen in FIG. 1. Furthermore, on the top side, a filling or top-up valve 32C is provided, via which the storage space 5 can be filled. Moreover, a venting valve 32D is arranged on the end face. A further orifice 34, which is open only during filling with the hydraulic fluid, serves for pressure compensation during filling.

All the hydraulic control elements are therefore integrated in the function block 6. Via the function block 6, the hydraulic oil provided at an outlet 36A, B (cf. FIG. 4) is controlled, that is to say, via the function block 6, the hydraulic pressure at the outlet 36A, B is controlled. Hydraulic control elements are no longer required thereafter. Instead, a hydraulic line can be linked directly to the outlet 36 and be connected to a corresponding hydraulic inlet on a example a blind rivet setting tool 38 functioning of the function block 6 and the significance of the individual valves are also apparent, in particular, from the description of the hydraulic diagram according to FIG. 4.

The compensating block 8 comprises an annular or cylindrical housing wall which is formed by the housing 4 and which forms a cylinder 40 open to the inner space. A piston 42 forming a compensating wall is arranged with an exact fit in this cylinder 40. The piston 42 is sealed off with respect to the inner wall of the cylinder 40 by means of sealing elements 12 and is arranged so as to be displaceable in the longitudinal direction in relation to the cylinder 40. The piston 42 is designed as a hollow piston which, like the cylinder 40, widens in a step-shaped manner, as seen in cross section. The cavity of the piston 42 forms a pressure space 44 which can be acted upon with a predeterminable pressure via a pneumatic connection 46 (cf. FIG. 3). The pressure space 54 is delimited on the rear side by a fixed end wall 48 of the housing. A securing spring 50 designed as a compression spring is supported on the end wall 48 and exerts a pressure force on the piston 42. The configuration illustrated affords a pressure intensifier and media converter.

To operate the hydraulic unit 2, the inner space 5 is filled completely with a hydraulic fluid, in particular hydraulic oil, so that the electric motor 14 and, with it, the pumps 16 are mounted in hydraulic oil. Complete venting takes place via the venting valve 32D, so that the overall inner space 5 is free of gas and of air. In order to maintain this reliably, a counterpressure of about 5-15×10⁻⁵ Pa is generated in the pressure space 44 via the pressure compensating block 10 by the application of a corresponding pneumatic pressure. The overall housing inner space 5 is therefore under an overpressure. Furthermore, filling level monitoring, not described in any more detail here, is provided, so that an automatic check of the hydraulic oil quantity is carried out.

To provide the hydraulic fluid at the outlets 36A, B under high pressure, the electric motor 14 is started, as required. That is to say, the hydraulic pressure is generated only when there is actually a requirement for this, that is to say when the blind rivet is already introduced into the blind rivet hole and the setting operation commences by drawing on the rivet plug. There is no pressure vessel provided. The eccentric shaft 18 is set in rotational movement via the electric motor 14, so that the individual pumps 16 are actuated alternately and in rotation and in each case convey a predefined quantity of hydraulic fluid into the pressure line 24 and consequently to the function block 6.

Since the hydraulic quantities in the inner space 5 vary during operation, the volume of the inner space 5 can be varied in order to avoid the occurrence of gas bubbles in the hydraulic fluid. The volume of the inner space 5 therefore forms a compensating volume and the inner space 5 forms a storage space. To vary the volume, the piston 42 moves automatically within the cylinder 40 according to the respective current requirements.

The operation of the setting tool 38 via the hydraulic unit 2 may be gathered from the hydraulic diagram according to FIG. 4. This illustration illustrates, on the right half of the figure, the compensating block 8, following this the pressure generation block 10 and, again following this, the function block 6. The setting tool 38 is acted upon by the pressurized hydraulic oil via two outlets 36A, B and supply lines 52A, B.

The pumps 16 are arranged in the pressure generation block 10, in this case three of the pumps 16 being combined to form a high-pressure part stream 54 and three further pumps 16 being combined to form a low-pressure part stream 56. A plurality of nonreturn valves 58 which in each case permit the throughflow of the hydraulic oil in the direction of the arrow only may be gathered from the hydraulic diagram. Furthermore, the pressure switching valve 32B, already mentioned with regard to FIG. 1, two controllable directional seat valves 33A, B and two safety valves 60A, B are arranged.

To start the riveting operation, the electric motor 14 is switched on, so that a hydraulic pressure is provided both in the high-pressure part stream 54 and in the low-pressure part stream 56. The high-pressure part stream 54 is routed via the safety valves 60A and via the directional seat valves 33A which are illustrated on the right half of the figure. The directional seat valve 33A is in this case activated in such a way that the throughflow for the high-pressure part stream 54 is opened as long as a predeterminable pressure is not overshot at the outlets 36A, B. With the directional seat valve 33A open, the high-pressure part stream 54 issues immediately into the housing inner space 5, so that no pressure build-up can occur on the high-pressure side and the part stream 54 is switched to pressureless. The low-pressure part stream 56 is fed via the nonreturn valve 58 and via the supply line 52A to the setting tool 38. The latter has an axially displaceable piston element 62 which, by being acted upon by the low-pressure part stream 54, moves to the right at the commencement of the setting operation. In this first phase of the setting operation, comparatively long travels are covered at only low pressures. An alignment of the blind rivet in the blind rivet hole and a first forming take place in this phase.

To operation of forming the blind operation, the directional seat valve 33A is activated and closed, so that the high pressure at the outlet 36A to the supply line 52A builds up in succession in the MS range. The pressure switching valve 32B is in this case designed in such a way that it switches automatically at a predetermined pressure, for example at a pressure of 80 bar, so that the low-pressure part stream 54 is switched free to the inner space 5 and is consequently switched to pressureless. The pressure supply in this case takes place via the high-pressure part stream 54. Via the nonreturn valve 58 in the high-pressure part stream 54, the latter is provided at the outlet 36 to the supply line 52A.

During the setting operation, the further directional seat valve 33B, which is connected to the second supply line 52B via the second outlet 36B, is in the state shown in FIG. 4. That is to say, the supply line 52B is connected via the directional seat valve 33B to the inner space 5 in the manner of a return line.

After the end of the setting operation, that is to say after the rivet plug has been torn off, the high-pressure part stream 54 is switched to pressureless again due to the switching of the directional seat valve 33A. Owing to the pressure drop caused thereby, the pressure switching valve 32B switches on the low-pressure part stream 56 automatically again.

For the restoring movement required after the setting operation, the directional seat valve 33B is switched so that, as illustrated, the part streams 54, 56 provided by the pumps 16, in particular the low-pressure part stream 54, in this case prevail both on the right side of the piston element 62 and on the left side in exactly the same way. Owing to the selected larger cross-sectional area on the right side of the piston element 62, the latter is pushed to the left back into the initial position again.

Moreover, for protective purposes, the high-pressure part stream 54 is connected to the housing inner space 5 via the safety valve 60A. This safety valve 60A switches, for example, when a pressure of 400×10⁻⁵ Pa is overshot. The safety valve 60B, illustrated on the left, is provided for protecting the pressure prevailing in the inner space 5. This safety valve 60B switches, for example, when a pressure of 25×10⁵ Pa is overshot.

LIST OF REFERENCE SYMBOLS

• 2 Hydraulic unit
• 4 Housing
• 5 Inner space
• 6 Function block
• 8 Compensating block
• 10 Pressure generation block
• 12 Sealing element
• 14 Electric motor
• 14A Stator
• 14B Rotor
• 16 Pump
• 18 Eccentric shaft
• 20 Rotor axis
• 22 Ball bearing
• 24 Pressure line
• 26 Suction line
• 28 Hydraulic line
• 30 Bore
• 33A, B Directional seat valve
• 32B Pressure switching valve
• 32C Top-up valve
• 32D Venting valve
• 34 Orifice
• 36, 36A, B Outlet
• 38 Setting tool
• 40 Cylinder
• 42 Piston
• 44 Pressure space
• 46 Pneumatic connection
• 48 End wall
• 50 Securing spring
• 52A Supply line
• 52B Supply line
• 54 High-pressure part stream
• 56 Low-pressure part stream
• 58 Nonreturn valve
• 60A, B Safety valve
• 62 Piston element
• d Thickness
• 1 Overall length

Claims

1-20. (canceled)

21. An hydraulic unit for providing a pressurized hydraulic fluid at an outlet, comprising:

an electric motor;

at least one pump for pressure generation driven by said electric motor;

a storage space configured with a variable compensating volume; and

an amount of the hydraulic fluid enclosed free of gas in said storage space and fluidically connected to a suction side of said at least one pump.

22. The hydraulic unit according to claim 21, wherein the hydraulic fluid is disposed in said storage space with an overpressure.

23. The hydraulic unit according to claim 22, wherein the overpressure is approximately in a region of several 105 Pa.

24. The hydraulic unit according to claim 23, wherein the overpressure lies between 3 and 50×105 Pa.

25. The hydraulic unit according to claim 21, wherein said storage space includes a compensating wall displaceably mounted in form of a piston and sealing against a stationary housing wall of said storage space.

26. The hydraulic unit according to claim 25, wherein said compensating wall is disposed to be subject to a counterforce.

27. The hydraulic unit according to claim 26, wherein said compensating wall bounds a pressure space with a connection for a pressure medium on an outside facing away from said storage space.

28. The hydraulic unit according to claim 26, which comprises a spring element disposed to generate the counterforce.

29. The hydraulic unit according to claim 28, wherein said spring element is a securing spring.

30. The hydraulic unit according to claim 21, which comprises a housing forming an inner housing space, and wherein said electric motor and said pump are disposed in said inner housing space, and said inner housing space forms said storage space filled with the hydraulic fluid.

31. The hydraulic unit according to claim 30, which comprises a function block disposed to close off said housing and formed with the outlet, and wherein a plurality of hydraulic components are integrated in said function block.

32. The hydraulic unit according to claim 31, wherein said function block is configured for controlling and routing the hydraulic fluid provided at the outlet.

33. The hydraulic unit according to claim 31, which comprises a line connecting said pump to said function block, and wherein all further hydraulic components following said pump are integrated in said function block.

34. The hydraulic unit according to claim 21, wherein said pump is one of at least two pumps for providing a low-pressure part stream and a high-pressure part stream, respectively.

35. The hydraulic unit according to claim 34, wherein said pumps are jointly driven by said electric motor.

36. The hydraulic unit according to claim 34, which comprises a valve assembly configured to provide in each case only one part stream at the outlet.

37. The hydraulic unit according to claim 36, wherein said valve assembly is configured to enable switching one of the part streams to pressureless in each case.

38. The hydraulic unit according to claim 21, wherein said electric motor is a controlled motor for setting a predetermined profile of the pressure of the hydraulic fluid at the outlet.

39. The hydraulic unit according to claim 21, wherein said electric motor is a servomotor.

40. The hydraulic unit according to claim 21, which comprises a connector for connection to a machine tool.

41. The hydraulic unit according to claim 40 connected, via said connector, to a rivet setting appliance.

42. In combination with a machine part that is subject to accelerations during operation, the hydraulic unit according to claim 21 disposed on the machine part.

43. A method of providing a pressurized hydraulic fluid, which comprises:

providing an hydraulic unit and driving a pump of the hydraulic unit for pressure generation with an electric motor;

supplying the pump with hydraulic fluid from a storage space having a variable compensating volume, wherein the hydraulic fluid is stored in the compensating volume free of gas.