EP2241763B1 - Hydraulic circuit and pressure compensating valve therefor - Google Patents

Description

The invention relates to a hydraulic control device according to the preamble of patent claim 1.

Hydraulic control devices with a pressure switch are known in various embodiments in practice. In one embodiment (s. Fig. 1 ), the pressure switch between the pressure source and the valve assembly is arranged in a main flow path and formed as a 3/2-way slide valve which connects the pressure source to the valve assembly in one control position and separates the valve assembly from the reservoir, whereas in the other control position, the pressure source shut off and connects the valve assembly with the reservoir. For reasons of the smooth adjustment of the control member, the slide valve needs a sliding fit which causes inaccurate leakage losses to the reservoir in units, for example with a pump with a low flow rate, at a high supply pressure or delivery volume flow. For certain control devices, for example, with a pressure source for high maximum pressure at low flow, leakage at driven pressure source are not tolerable. Among other things, the reason for a pressure switch in such hydraulic control devices is that after switching off the pressure source of the pressure applied to the valve assembly pressure is to be reduced to the reservoir, for example, so that the drive motor acting as a pressure source pump when switching does not have to work immediately against relatively high resistance. For example, a single-phase AC motor can bad start against impending pressure, which requires an oversized design of the drive motor so that it is able to properly perform the start-up phase despite counter-pressure. In order to avoid such start-up difficulties, it is also known to provide the pressure switch with an auxiliary volume in a control pressure channel which contains the control pressure acting on the control member in the first adjustment direction. This auxiliary volume is defined in a chamber in which a piston against spring force yields so that when the drive motor starts and the pressure switch has separated the valve assembly from the reservoir and connected to the pressure source, the pressure source fills the auxiliary volume, whereby the drive motor initially only has to overcome low resistance. This auxiliary volume means additional structural complexity and provides the desired effect at most up to a maximum pressure of, for example, about 300 bar. At higher maximum pressures of eg up to 700 bar, however, the effect is no longer satisfactory.

Out DE 295 18 025 U1 a generic hydraulic control device is known, wherein the consumers are fed from a variable displacement, which is cooled in a stand-by operating situation without switched consumers by a flushing valve in a small flow rate to the reservoir effluent hydraulic medium. The flush valve has the function of a pressure switch with a throttle as an integral part and is arranged in a slide of a pressure compensator. If the variable displacement pump is switched off, then the flushing valve opens the outflow path to the reservoir, so that the pressure line leading to the valve arrangements is depressurized.

Further prior art can be found in DE 94 10 796 U1 ,

The invention has for its object to provide a hydraulic control device of the type mentioned above, in which no loss losses occur when shut off to the reservoir Abströmweg and working pressure source, and in which the starting resistance of the drive motor is minimized.

The stated object is achieved with the features of claim 1.

Since the 2/2-way seat valve in the hydraulic control device works by design with leak-free shut-off, leakage losses are avoided when shut off with driven pressure source Abströmweg to the reservoir, which may be important for small units. The throttle in the permanent main flow connection between the pressure source and the valve assembly due to the start of the drive motor that only a predetermined pressure difference across the throttle or a predetermined volume flow through the throttle adjust before the pressure switch shuts off the Abströmweg leak-free, ie, the outflow to the reservoir is shut off with a delay or remains open after startup. Thus, the switched-pressure source promotes without significant counter-pressure initially in the reservoir, so that the drive motor easily comes to a sufficient speed. Even a small-sized single-phase AC motor can thus be used as a cost-effective drive source for a pump, since the pressure switch in combination with the throttle to the reservoir "soft" and shuts off only with delay. This pressure switch works satisfactorily up to about 700 bar and shut off without leakage, for example as soon as about ¾ of the maximum pump flow is reached. The pressure difference need only be relatively small, so that the pressure switch responds as desired, so that the loss due to the throttle during operation remains negligible, eg about 5 to 10 bar at a maximum pressure of eg 500 bar.

In an expedient embodiment, the valve cone for the two control pressures on differently sized loading surfaces, such that the loading surface for the valve cone of the first adjustment acting on the control pressure is smaller than the loading surface for the valve cone acting in the second adjustment control pressure. By adjusting the loading surfaces in relation to each other, the desired soft response of the directional seat valve can be particularly easy to achieve, and ensures that at small pressure difference in the directional seat valve, a large closing force acts, so that even up to high supply pressures of about 700 bar leakage is guaranteed.

The ratio of the loading areas may be between about 2: 1 and 4: 1, but is preferably about 3: 1.

In an alternative embodiment, the loading surfaces may be about the same size. For a sufficient closing force can then be used a throttle, which causes a higher pressure difference.

It is advantageous if the valve cone between the smaller loading surface and the valve seat is sealed by at least one ring seal. On the one hand, the ring seal dampens the movement of the valve cone, and on the other hand ensures that no leakage occurs between the possibly high control pressure P1, possibly high application pressure and the reservoir R in the shut-off position, which could falsify the control pressure acting on the smaller or different application area.

Suitably, the throttle has a fixed throttle cross-section, which specifies a suitable for the respective supply pressure flow rate or pressure difference. The throttle may be, for example, a screw-in in a main channel connecting the pressure source and the valve assembly, and can be replaced if necessary against another Einschraubdrossel with another throttle cross-section. Basically, the size of the throttle is selected depending on the flow rate.

Alternatively, the throttle may have a variable throttle cross-section, which can be adapted to the respective operating condition or flow rate.

In a structurally simple embodiment, the valve cone has a first piston defining the larger loading area and a second piston defining the smaller loading area. Between the first and second pistons is a annular seat, preferably provided with at least approximately the size of the smaller loading surface. The two pistons are guided displaceably sealed in corresponding bores and control the movements of the valve cone in dependence on the two pilot pressures and the force of a spring acting in the opening direction.

Conveniently, a concavely rounded cone transition is provided between the seating surface on the valve cone and the first piston. The cone transition ensures a clean flow when opening the seat valve. A constriction may be provided between the seat surface and the second piston, preferably a concavely rounded constriction. This flow channel defined via the constriction serves to guide the flow when opening the directional seat valve.

Suitably, the seat is conical, preferably with a cone angle of about 70 °. The valve seat may also be correspondingly conical, or slightly spherical, to ensure the leak-free shut-off. On the seat can follow in the direction of the cone transition, a cylindrical approach, and then another conical surface. This is due to manufacturing (grinding) and also represents an advantageous Hubhilfe for the opening process.

In an expedient embodiment, the valve cone is arranged in one of the stepped bore contained in the valve seat. The stepped bore has, preferably, two stepped bore portions, with lateral channels leading to the stepped bore. In an expedient embodiment, the stepped bore is contained in a bush, which has a plurality of outer and spaced sealing regions, and can be inserted into a simple inner bore, for example, of a housing.

In a further expedient embodiment, the bush is arranged sealed in the housing in an inner bore having two annular channels. To an annular channel pass channels which are connected to a pressure source port and a valve assembly port of the housing, while the other annular channel is connected via a channel with a reservoir port of the housing. In the inner bore a continuous Buchsicherungs screw can be set, for. B. for ease of installation, while a free inner bore end is closed by a screw plug, on the one hand allows the convenient mounting of the components of the directional seat valve in the housing, and on the other hand with the derived between the pressure source and the throttle Control pressure acted upon control chamber limited. In the control chamber, the control pressure acts on the larger actuation surface of the valve cone via the continuous Buchsensicherungsschraube. The piston of the valve cone defining the larger loading surface may be guided with a sliding fit in the stepped bore, without any special further sealing, since the control pressure in the control chamber is in the shut-off position of the directional seat valve in contrast to the supply pressure. Alternatively, it would also be possible to give the piston which defines the larger loading surface a ring seal in the stepped bore.

The spring which holds the poppet in the open position in the depressurized state of the control device, can advantageously be arranged on the second piston defining the smaller impingement surface and be supported on the base of the inner bore. Thanks to the guidance of the second piston in the stepped bore, the spring is also aligned.

Suitably, the throttle, the spring and the loading surfaces on the valve cone are matched to one another such that upon switching on the pressure source with a start of a drive motor of the pressure source, preferably a single-phase AC motor, facilitating delay first a predetermined pressure difference across the throttle or a predetermined volume flow through the throttle is generated before the outflow to the reservoir is shut off leak-free. As a result, a "soft" response of the pressure switch is achieved without having to arrange further structural measures for this function in the control device. After switching off the drive motor of the pressure source, both the pressure applied by the valve assembly pressure decreases to the reservoir, as well as the pending from the pressure source forth pressure, the latter via the throttle. The pressure applied to the valve assembly may possibly be maintained via a check valve.

For smaller units with low flow and high pressure, the throttle generates only a negligible back pressure or a small pressure difference when starting the drive motor. Only at approx. ¾ of the pump delivery flow is the Δp large enough (approx. 5 to 10 bar) and the directional seat valve moves into the shut-off position. Now promotes the pressure source through the throttle to the valve assembly, with only a negligible loss of a few percent is to be accepted.

Fig. 1

ein Blockschaltbild einer hydraulischen Steuervorrichtung gemäß des Standes der Technik,

Fig. 2

ein Blockschaltbild einer erfindungsgemäßen hydraulischen Steuervor- richtung,

Fig. 3 und Fig. 4

zwei einander zugeordnete Vertikalschnitte einer konkreten Ausfüh- rungsform einer Druckweiche für die hydraulische Steuervorrichtung von Fig. 2,

Fig. 5

einen Aufriss eines Ventilkegels der Druckweiche, und

Fig. 6

einen Vertikalschnitt einer Buchse der Druckweiche der Fig. 3 und 4.

Embodiments of the subject invention will be explained with reference to the drawings. Show it:

Fig. 1

FIG. 2 is a block diagram of a hydraulic control device according to the prior art. FIG.

Fig. 2

a block diagram of a hydraulic control device according to the invention,

FIG. 3 and FIG. 4

two mutually associated vertical sections of a concrete embodiment of a pressure switch for the hydraulic control device of Fig. 2 .

Fig. 5

an elevation of a valve cone of the pressure switch, and

Fig. 6

a vertical section of a socket of the pressure switch of 3 and 4 ,

A hydraulic control device H according to the prior art has in Fig. 1 a 3/2-way spool valve 1 between a pressure source P and a valve assembly, not shown (from the valve assembly pending pressure P1) on. The control member of the 3/2-way spool valve 1 connects in the pressureless state of the hydraulic control device H P1 with a reservoir R, while P is shut off. This control position is supported by a spring. The control member of the 3/2-way shift valve 1 is acted upon from a pilot control line 2 with a control pressure derived from P in the direction of the second control position, and acted upon by a derived from P1 via a control line 3 control pressure parallel to the spring to the first control position. Due to the sliding fit required for the ease of movement of the slider formed as a slider, leakage can not be avoided in any control position, especially in the shut-off position to the reservoir R. The pressure source P is for example a pump and is driven by a not shown on and off electric motor. From the valve arrangement, not shown, at least one hydraulic consumer is controlled.

In the first control position shown, the pressure P1 to R is reduced. The connection from P to P1 is blocked. When starting the drive motor initially acts only to build up the control pressure in the control line 2, the counterforce of a spring until the 3/2-way spool valve switches to the second control position. There is then immediately significant back pressure from P1. This back pressure must overcome the drive motor, which can lead to problems, for example, with a single-phase AC motor. To facilitate the startup of the drive motor, the pressure switch W can therefore be assigned an auxiliary volume 4. In a chamber in the control line 3 is against the force of a spring 6, a piston 5 slidably. Once the 3/2-way spool valve 1 switches over after overcoming the spring in the second control position, the auxiliary volume 4 is filled via the control line 3, wherein the piston 5 is displaced against the spring 6, so that the drive motor can start more easily. Purpose of the pressure switch W is also to reduce the pressure applied by the valve assembly P1 then to the reservoir R when the drive motor is turned off.

Fig. 2 shows a hydraulic control device H according to the invention with a pressure switch W, which has a "soft" response without additional structural measures to allow a drive motor M of the pressure source P, regardless of the respective maximum pressure, eg to about 700 bar, initially without significant back pressure start , And only then build the desired supply pressure P1 to the valve assembly V when the drive motor M has reached a certain speed and is powerful enough or reaches the flow rate about ¾ of the respective maximum.

Between the pressure source P and the valve arrangement V, a main channel 10, 12 is provided in a housing 21 of the pressure switch W. At a node 11 of the main channel 10, 12, a discharge path 13 branches off to a reservoir line 20 and a reservoir R. In Abströmweg 13 a 2/2-way seat valve 14 is arranged, the pressure-dependent between a first control position (passage position) as in Fig. 2 shown, and a second control position (leak-free shut-off position, not shown) is switchable, and a control member 16 includes. The second control position (leak-free shut-off position) is leak-tight in this case in both directions of flow.

The control member 16 of the 2/2-way seat valve 14 is acted upon in the direction of the first control position by a spring 17 and parallel thereto from a control line 15 by a control pressure. The control line 15 branches off from the Abströmweg 13. In the direction of the second control position, however, the control member 16 is acted upon by the pilot pressure in a control line 18 which branches off at a node 19 of the portion 10 of the main channel 10, 12, between the pressure source P and a throttle D, which is between the node 19th and 11 is arranged. The throttle D is used to produce a predetermined pressure drop .DELTA.p, even while the pressure source P still promotes the reservoir line 20, and is built up in the control line 18 sufficient control pressure against adjusts the control pressure in the control line 15 and against the force of the spring 17, the second control position (leak-free shut-off). Only then is the supply pressure P1 to the valve assembly V built up in full and the maximum delivery rate is reached. This facilitates the drive motor M starting from a standstill, since the pressure build-up in the main channel 10, 12 takes place with a predetermined delay, ie, after a predetermined pressure difference Ap (eg, about 5-10 bar) or a certain volume flow with delay through the throttle D was built.

The throttle D may have a fixed throttle cross section 30, or, as indicated at 30 'by dashed lines, an adjustable throttle cross section, and is e.g. depending on the maximum flow rate selected.

The Fig. 3 to 6 illustrate a concrete embodiment of the pressure switch W, for example, in the in Fig. 2 indicated block-shaped housing 21 is housed. The housing 21 has an inner bore 22 which terminates blind, and in which a socket 23 is fixed sealed. In the sleeve 23, a valve plug 24 is guided displaceably and sealed, which cooperates with a formed in the socket 23 valve seat 25. The bushing 23 is positioned in the stepped bore 22, for example, by a continuous bolt lock screw 26. The free end of the inner bore 22 is closed by a sealing screw plug 27, delimiting a control chamber to which the control line 18 leads from the node 19. The control line 15, however, leads to the lower blind end of the inner bore 22. In this area, the spring 17 is arranged. The inner bore 22 has, for example, two ring channels. The upper ring channel in Fig. 3 is connected to the sections 10, 12 of the main channel, while the lower annular channel is connected to the reservoir line 20. The socket 23 ( Fig. 6 ) has in alignment with the annular channels in the housing 21 corresponding lateral passages.

In Fig. 4 are in a parallel opposite Fig. 3 offset vertical section in another sectional plane two passages 28 visible, which serve for securing the housing 21, for example, to the pressure source and / or to the valve assembly. In a partially threaded bore 31, which forms parts of the main channel 10, 12, the throttle D in the form of a throttle screw 29 with the fixed throttle section 30 (eg, 0.8 mm diameter) screwed, for example, adjacent to the node 11 at which branches off the Abströmweg 13.

The poppet 24 has a first piston 32 defining a larger engagement surface A1 (diameter d1) and a second piston 50 axially spaced therefrom defining a smaller engagement surface A2 (pressure gauge d2). Between the piston 32, 50, a seat 34 is formed, for example, a seat with a conical shape and a cone angle α of about 70 °. In the direction of the first piston 32, a circular-cylindrical projection 35 adjoins the seat surface 34, followed by another short, conical surface 36. The conical surface 36 is converted into a concave rounded cone transition 37, which eventually enters the first piston 32 with increasing diameter. Between the seat 34 and the second piston 50, a constriction 38 is provided, which, preferably, is concavely rounded. At the lower end of the second piston 50, a projection 33 is formed, which is used for positioning and supporting the spring 17 (FIG. Fig. 3 ) serves.

The poppet 24 is in the in Fig. 6 shown socket 23 (see Fig. 3 ) mounted displaceably. The bushing 23 has a plurality of sealing grooves 40 on the outside, and optionally also an undercut 48 at the lower end, each for positioning a ring seal (not shown) to the different pressure ranges of the sleeve 23 in the inner bore 22 in the housing 21 (FIGS. Fig. 3 ) against each other. In the sleeve 23, a stepped bore 39 is formed, which has an overhead stepped bore portion 41 for the piston 32, an intermediate portion 42 to the valve seat 25, and a smaller diameter stepped bore portion 44 for guiding the second piston 50. In the stepped bore portion 44, an annular groove 46 is formed for at least one annular seal 47, which seals the second piston 50 on the outer periphery and pressure tightness between side inlets 45 in the stepped bore portion 44 and the lower end of the sleeve 23 ensures. In the intermediate section 42 open lateral inlets 43rd

The lateral passages 43 and 45 respectively open into an annular channel in the housing 21 (FIG. Fig. 3 ) wherein a ring channel 49 for the lateral passages 45 dashed in Fig. 6 is indicated.

Referring to the Fig. 3, 4 . 5 and 6 (Shut-off position in Fig. 3 ), the side passages 43 are connected to the pressure source P and the pressure P present at the valve assembly V, while in the annular chamber 49 and the lateral passages 45, the connection to the reservoir R is established. The two pressures P and P1 are in the shut-off position in the stepped bore section 41 or intermediate section 42 and the constriction 37 at. The larger Beaufschlagungsfläche A1 is from the control line 18 with the control pressure acted upon, while the second piston 50 can be acted upon on the smaller loading surface A2 with the control pressure from the control line 15. The spring 17 acts via the projection 33 on the second piston 50 in the opening direction of the valve cone 24.

In the described shut-off position of the pressure switch W promotes the pressure source to the valve assembly V, while the outflow 13 is shut off leak-free.

After switching off the drive motor M eliminates the predetermined Δp at the throttle 30 and the control pressures 15 and 18 are equalized. Finally, the spring force of the spring 17 lifts the valve cone 24 with the seat 34 from the valve seat 25. The Abströmweg 13 is now open to the reservoir line 20, so that the remaining pressure P1, P is completely degraded, possibly to a low reservoir pressure.

If the drive motor M is switched on again, a pressure gradient builds up via the throttle D, the outflow path 13 initially still being open. As soon as the pressure gradient reaches a predetermined level (a certain volume flow), the control pressure in the control line 18 pushes the valve cone 24 with the seat surface 34 onto the valve seat 25 (leak-free shut-off position). This is done until the construction of the predetermined pressure gradient with a delay, which facilitates the startup of the drive motor M. Since only then the Abströmweg 13 is shut off, the drive motor M is already running at a speed at which he is powerful enough to build the desired supply pressure P, P1. As a result, the pressure source P feeds the valve assembly V via the throttle D.

If A2 or d2 approximately corresponds to the seat cross section d3, the closing force generated depends on the pressure difference across the throttle 30. The closing force could also be selected as a function of pressure, if d2 is chosen smaller than the seat cross-section. Then, the force of the spring 17 (for opening) can be set higher.

Alternatively, A1 and A2 (d1 and d2) could be made approximately equal in size, and / or the seat cross-section (d3) could be made larger than A1 (d2). The relative dimensions are e.g. However, depending on the application and / or start-up behavior of the drive motor, all are included in the invention.

Claims (14)

1. Hydraulic control device (H), comprising a discharge path (13) and a restrictor (D), wherein the hydraulic control device (H) is arranged in use for supplying at least one consumer with pressure from a pressure source (P) which can be switched on and switched off, via at least one valve assembly (V) arranged between the pressure source (P) and a reservoir (R) at one side and the valve assembly (V) at the other side, and comprising a pressure switch (W) in the discharge path (13) extending at least from the valve assembly (V) to the reservoir (R), which pressure switch (W) connects the valve assembly (V) via the discharge path (13) with the reservoir (R), when the pressure source (P) is switched off, but blocks the discharge path (13) to the reservoir (R) when the pressure source (P) is switched on and builds up a supply pressure, wherein the pressure switch (W) contains an adjustable control member (16) which is actuable by a spring (17) and a first pilot pressure originating from the pressure acting at the valve assembly (V) in a first switching direction to a control position for opening the discharge path (13) to the reservoir (R), and is actuable by a second pilot pressure originating from the supply pressure to a control position for blocking the discharge path (13), wherein the pressure switch (W) is a 2/2 multi-way seat valve (14) operating with leakage-free blocking position and containing a valve cone (24) forming the control member (16), and a valve seat (25) arranged in the discharge path (13), the pilot pressure actuating the valve cone (24) in the second adjustment direction to the valve seat (25) being derived from the supply pressure between the pressure source (P) and the restrictor (D), characterised in that the pressure source (P) is permanently connected via the restrictor (D) with the valve assembly (V), that the discharge path (13) containing the pressure switch (W) branches off to the reservoir (R) between the restrictor (D) and the valve assembly (V), and that the pilot pressure actuating the valve cone (24) of the pressure switch (W) in the first adjustment direction is derived from a pressure acting between the restrictor (D) and the valve assembly (V).
2. Hydraulic control device according to claim 1, characterised in that the valve cone (24) comprises pressure receiving areas (A1, A2) of different sizes for both pilot pressures, such that the pressure receiving area (A2) for the pilot pressure actuating the valve cone (24) in the first adjustment direction is smaller than the pressure receiving area (A1) for the pilot pressure actuating the valve cone (24) in the second adjustment direction, wherein, preferably, the ratio between the pressure receiving areas A1:A2 amounts to between 2:1 and 4:1, preferably to about 3:1.
3. Hydraulic control device according to claim 1, characterised in that the valve cone (24) has pressure receiving areas (A1, A2) of at least substantially equal sizes for both pilot pressures.
4. Hydraulic control device according to claim 2, characterised in that the valve cone (24) is sealed by at least one ring sealing (47) between the smaller pressure receiving area (A2) and the valve seat (25).
5. Hydraulic control device according to claim 1, characterised in that the restrictor (D) has a fixed restricting cross-section (30), and preferably, is arranged in a main channel connecting the pressure source (P) and the valve assembly (V).
6. Hydraulic control device according to claim 1, characterised in that the restrictor (D) has a variable restricting cross-section (30').
7. Hydraulic control device according to at least one of the preceding claims, characterised in that the valve cone (24) comprises a first piston (32) defining the larger pressure receiving area (A1, d1) and a second piston (50) defining the smaller pressure receiving area (A2, d2), and that an annular seat surface (34, d3) is provided between both pistons (32, 50), preferably with a size of the size of the smaller pressure receiving area (A2) or larger as the same.
8. Hydraulic control device according to claim 7, characterised in that a concavely rounded cone transition (37) is provided between the seat surface (34) and the first piston (32) and that a narrowed region (38), preferably a concavely rounded narrowed region, is provided between the seat surface (34) and the second piston (50).
9. Hydraulic control device according to claim 7, characterised in that the seat surface (34) is conical, preferably having a cone angle (α) of about 70°, and that a cylindrical projection (35) and a further conical surface (36) continue the seat surface (34) in a direction towards the conical transition (37).
10. Hydraulic control device according to at least one of the preceding claims, characterised in that the valve cone (24) is contained in a stepped bore (39) accommodating the valve seat (35), preferably in a sleeve (23) comprising the two stepped bore sections (41, 44) and sideward channels (43, 45) extending to the stepped bore sections (42, 44), which sleeve (23), preferably, has exterior and axially spaced apart ring sealing portions (40, 48).
11. Hydraulic control device according to claim 10, characterised in that the sleeve (23) is seated in an inner bore (22) of a housing (21), the inner bore comprising two ring channels, that channels leading to a pressure source connection (P) and a valve assembly port (P1) are connected with the one ring channel, and that a channel leading to a reservoir connection (R) is connected with the other ring channel, that an axially open sleeve securing screw (26) is fixed in the inner bore (22), and that a free end of the inner bore is closed by a closing screw (27) for bounding a control chamber (4) for the piston (32) defining the larger pressure receiving area (A1), the control chamber being supplied with pilot pressure derived between the pressure source (P) and the restrictor (D).
12. Hydraulic control device according to at least one of the preceding claims, characterised in that the restrictor (D) is formed as a screw-in restrictor and is threaded into the channel (31) in the housing (21) which channel is connected with the pressure source port (P).
13. Hydraulic control device according to claim 1, characterised in that the spring (17) is arranged at the second piston (50) which is actuated with the pilot pressure derived from the pressure (P1) acting at the valve assembly (V).
14. Hydraulic control device according to at least one of the preceding claims, characterised in that the restrictor (D), the spring (17), and the pressure receiving areas (A1, A2) at the valve cone (24) are adapted such in relation to each other that after switching on the pressure source (P) first a predetermined pressure drop (ΔP) is generated across the restrictor (D) or a predetermined volume flow rate through the restrictor (D) before the discharge path (13) to the reservoir (R) is blocked without leakage, the build up of the predetermined pressure drop or the predetermined volume flow rate resulting in a delay facilitating the start up of a drive motor (M) of the pressure source, preferably, of a single-phase AC electric motor.

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