Abstract:
The steam generating device ( 1 ) is used to heat and/or froth liquids, especially milk. It includes an electrically driven pump ( 3 ) supplied with water from a water source ( 2 ), which pump delivers water in a controlled fashion to a continuous flow heater ( 6 ) heated by an electrically operated heating ( 22 ) by way of a conduit ( 13 ). The continuous flow heater ( 6 ) heats the water it receives in its steam pipe ( 18 ) to steam that is subsequently supplied to a steam tapping point, preferably a steam nozzle ( 7 ), where the steam exits under pressure. An intermediate tank ( 5 ) with an overflow chamber ( 53 ) which stores a defined quantity of water is provided in the conduit ( 13 ) between the pump ( 3 ) and the steam pipe ( 18 ). When the steam generating device ( 1 ) is switched on, the pump ( 3 ) is switched on simultaneously with the heating ( 22 ), and after the switch-on the water delivered by the pump ( 3 ) flows into the chamber ( 53 ) of the intermediate tank ( 5 ) and is retained there so long and is not delivered further to the continuous flow heater ( 6 ) until the continuous flow heater ( 6 ) has reached a temperature sufficient to evaporate water.

Description:
TECHNICAL FIELD 
     The present invention relates to generating steam for heating and/or frothing liquids, especially milk. 
     BACKGROUND 
     EP 0 781 520 Al discloses a steam generating device of this type. In the Cappuccino position, that is the position where steam for frothing milk is desired, switches S 1  and S 2  adopt the position in which the tube heating element of the continuous flow heater is energized. Simultaneously, the electric control means is actuated by way of the input so that the electric motor drives the piston pump and pumps water into the water pipe of the continuous flow heater, where it evaporates and is conducted via the output line to the steam nozzle. The steam is accelerated in the steam nozzle and will then exit at the end of the steam nozzle (FIGS. 1 and 2 of EP 0 781 520 Al). 
     Thus, the instant when the pump starts to deliver water is the very instant when the heating device begins to heat the water heater. The result is that the water heater is not yet so heated up as to be in a position to entirely evaporate the volume of water it receives. The final result is that hot water or a hot steam/water mixture exits from the steam nozzle. This impairs the quality of the frothing result. 
     Further, it is known from Espresso machines that a waiting time is necessary after switch-on of the Espresso machine until the continuous flow heater has reached its operating temperature that is required for the generation of steam so that only steam will exit from the nozzle. This necessitates a signalling means which indicates when the frothing operation can be started. This arrangement causes prolonged waiting times and disadvantages of use of the appliance. 
     SUMMARY 
     An object of the present invention is to provide a steam generating device for heating and/or frothing liquids, especially milk, for a hot beverage machine which produces steam by especially simple means in a shortest possible time. An additional objective is to provide a method therefor which permits automatically controlling the steam generation after switch-on of the appliance with equally simple means. 
     The provision of an intermediate tank with an overflow chamber (which represents in the system a sort of buffer for the intermediate storage of water or a delay time element) between the pump and the steam pipe of the continuous flow heater permits achieving by especially simple means that water will not exit from the discharge of the intermediate tank and propagate to the continuous flow heater until the continuous flow heater has reached its operating temperature. The rate of delivery of the pump, the filament power of the continuous flow heater and the accumulation volume of the intermediate tank must be so conformed to one another that water will only be conducted into the continuous flow heater when the latter has reached the operating temperature that is required for the generation of steam. Thus, the present invention eliminates the need for sophisticated time delay elements because, according to the present invention, activation of the steam generating device causes simultaneous switch-on of the pump and the heating device of the continuous flow heater, and the intermediate tank will retain the water delivered until the continuous flow heater has reached its operating temperature necessary for steam generation. 
     Instead of an intermediate tank, it is also possible to choose a hose portion of a certain length in which the quantities of water delivered by the pump are received until the continuous flow heater has reached its operating temperature. In this event, too, it would be advantageous that the hose-side end to the continuous flow heater has an elevated position vis-à-vis the intake to prevent a too quick propagation of water into the continuous flow heater. The hose acts as a type of overflow chamber also in this arrangement. 
     The overflow chamber in one embodiment may extend substantially horizontally in its longitudinal direction and may be separated by a vertical wall from the discharge so that the excess water flows over the wall and into the discharge of the intermediate tank and, thus, into the steam pipe only when the overflow chamber is filled with water. In this embodiment, the intake and the discharge may be designed at the same level or at different levels. If, however, according to another embodiment, the intermediate tank extends vertically in its longitudinal direction, the discharge is required to be arranged at the chamber at the level which defines the chamber volume that is sufficient to take up so much water that the heating device has reached its operating temperature exactly when overflow begins. 
     It is possible that the intake is arranged above the discharge on the intermediate tank, that means, the actual intermediate tank is positioned below the intake and also below the discharge. In another variation, wherein the intake is designed on the bottom of the intermediate tank, the actual intermediate tank is arranged above the intake. In this event, the discharge is also disposed in the longitudinal direction of the intermediate tank above the intake. 
     The intermediate tank may include a discharge which has a pressure relief valve that opens in the presence of inadmissible excess pressure in the intermediate tank and discharges steam or water to the atmosphere. Inadmissibly high pressures in the intermediate tank which might cause explosion or leakage of the intermediate tank or of parts of the entire water/steam system are this way prevented. 
     In some embodiments, the intermediate tank is succeeded by a pressure chamber in which a piston that is displaceable in opposition to the force of a spring is guided, the said piston shutting off the discharge to the atmosphere in the closed position and opening the pressure chamber with the discharge in the open position. A pressure relief valve of this type is especially easy to manufacture and can be integrated especially simply into the pressure chamber that is directly adjacent to the intermediate tank. Advantageously, the pressure chamber and the intermediate tank form a chamber which is configured as a cylindrical bore. This chamber may be provided especially easily. 
     The piston may have an indentation in the shape of a hollow chamber on its side facing the intermediate tank. This indentation serves as a pressure cushion and, additionally, as a steam accumulator. The purpose of the pressure cushion containing a steam/air mixture is to prevent the steam which flows back into the intermediate tank when the pump and the continuous flow heater are switched off from immediately condensing in the cold water. 
     Another aspect of the invention features a method for steam generation in a steam generating device wherein both the pump and the continuous flow heater switch on after the appliance is switched on. The water delivered by the pump is now initially supplied to the intermediate tank configured as overflow tank and accumulated therein until the continuous flow heater has reached its operating temperature required for the steam generation. The rate of delivery of the pump and the accumulation volume of the intermediate tank are conformed to the filament power of the continuous flow heater so that water will not flow into the steam pipe until the continuous flow heater or the steam pipe has reached its temperature that is required for the generation of steam. Exclusively steam which exits at the steam tapping point, preferably from the steam nozzle, is generated by the method of the present invention in a time-delayed manner after the appliance is switched on. The method of the present invention does not require sophisticated electronic time elements or other electronic sensing means which indicate to a user at what point of time the continuous flow heater has reached its operating temperature so that the appliance activates the pump only then, or manual activation is rendered possible. 
     In some cases the method permits evacuation of the intermediate tank after each process of steam generation in order to make the intermediate tank available again in its function for the next case of use. 
     One embodiment is illustrated in the accompanying drawings and will be explained in detail hereinbelow. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic view of the design of a steam generating device 
     FIG. 2 is a longitudinal cross-section taken through the top part of the intermediate tank, wherein the pressure relief valve has adopted its closed position. 
     FIG. 3 is a longitudinal cross-section taken through the top part of the intermediate tank, as shown in FIG. 2, however, in this arrangement the pressure relief valve has reached its open position, at excess pressure in the pressure chamber. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a steam generating device  1  which is generally comprised of a water tank  2  that is preferably used only for the generation of steam, a pump  3  configured as a piston pump with a gear unit  4 , an intermediate tank  5 , a continuous flow heater  6 , and a steam nozzle  7 . The water tank  2  is connected to the inlet port  9  of the pump  3  by way of a water conduit  8 . The outlet port  10  communicates by way of a first portion  11  of conduit  13 , which connects the pump  3  to the steam pipe  18  of the continuous flow heater  6 , with the intake  15  arranged on the bottom  14  of the intermediate tank  5 . 
     As can be seen in FIGS. 1 to  3 , a discharge  16  projects alongside the intake  15  from the bottom  14  in a downward direction, discharge  16  being connected to the steam inlet  19  of the steam pipe  18  of the continuous flow heater  6  by way of the second portion  17  of conduit  13 . The steam outlet  20  is connected via steam conduit  21  to the steam nozzle  7  which is detachably fastened and sealed at the end of the steam conduit  21 . Steam nozzle  7  will not be referred to herein in detail because its operation and design is widely known in patent literature, such as, for example, from the description of DE 43 27 085.9-16. 
     Because the steam generating device  1  illustrated in FIG. 1 shows only the necessary component parts, it is not shown how it can be installed into a housing of a machine for the preparation of hot beverages (not shown). Other than shown herein, the continuous flow heater  6  will not be installed vertically but horizontally in a beverage preparing machine in order that the water is prevented from immediately flowing to the middle of the steam pipe  18 , but instead flows slowly into the steam inlet  19  so that it will heat already at the beginning in the steam pipe  18 . It should be mentioned herein that, according to FIG. 1, the heating rod  22  of the continuous flow heater  6  is provided below the steam pipe  18  and soldered or welded to it, or attached to it in a properly heat-conducting manner in any other way, in order that the heat emanating from the heating rod  22  flows as evenly as possible from downwards into the steam pipe  18 . 
     According to FIG. 1, the heating rod  22  includes contact lugs  23 ,  24  at its ends which are connected to electric lines  25 ,  26 . The electric line  26  is connected to a temperature switch  27  which is secured to a transverse web  28  arranged on the continuous flow heater  6 . Because the continuous flow heater has a generally U-shaped design (other configurations such as a thermoblock or a vessel-shaped structure are also possible), the transverse web  28  interconnects the two legs  29 ,  30  of the continuous flow heater  6  and forms a mounting support for the continuous flow heater  6  itself. The transverse web  28  is preferably soldered or welded to the two legs  29 ,  30 . 
     The temperature switch  27 , by way of an electric inlet line  31 , is connected to an electric switch  32  which switches the steam generating device  1  on and off according to FIG.  1 . The contact lug  23  of the heating rod  22  is connected to the negative pole of the electric switch  32  by way of the electric line  25 . In parallel to the lines  25 ,  31 , the electric motor  35  is connected to the electric switch  32  by way of the electric lines  33 ,  34  so that, when switch  32  is switched on, both the electric motor  35  and the heating  22  of the continuous flow heater  6  will be energized. 
     According to FIG. 1, the drive shaft  36  of the electric motor  35  is unrotatably connected to a first toothed wheel  37  which drives a second toothed wheel  38  of larger diameter. Attached in the center of the second toothed wheel  38  in an unrotatable fashion is, in turn, a third toothed wheel  39  of smaller diameter which, on its part, drives a fourth toothed wheel  40  of larger diameter on which a point of support  41  is eccentrically arranged. The point of support  41  causes a driving rod  42  to execute a stroke movement which carries a piston on its end not illustrated in the drawing. The piston is guided in a bore (not shown) of the piston housing  43 . Above the piston housing  43 , a valve housing  44  follows in which, as is conventional practice in piston pumps, a suction and a pressure valve (not shown) are designed. The piston pump  3  with gear unit  4  will not be dealt with more closely at this point because assemblies of this type are already known from the above-mentioned Espresso machine sold by Braun. 
     In FIG. 1, a two-way valve  45  which is only hinted at schematically is arranged in the first portion  11  of conduit  13 . The two-way valve  45  is connected to the switch  32  in a preferably mechanical manner, as is indicated by the dotted line  46  in FIG.  1 . The two-way valve  45  subdivides the first portion  11  of conduit  13  into two conduit portions  11   a ,  11   b , and in the off-position of the two-way valve  45  the conduit portion  11   b  is connected by way of a relief conduit  47  whose end  48  opens into the feed opening  49  of the water tank  2 . The water tank  2  includes at its top area proximate the feed opening  49  a handle  50  which is used to remove the water tank  2  from the steam generating device  1  for the replenishment of water. However, the outlet (not shown) of the water tank  2  must be closed by a valve assembly (not shown) in the outlet when the outlet is isolated from the water conduit  8 . A window  51  is provided in the outside wall so that the filling level of the water tank  2  can be better seen. 
     As can be taken from FIGS. 1 to  3 , the outlet  16  of the intermediate tank  5  extends through a pipe  52  into the interior of chamber  53 . The free end, that is the chamber outlet  54 , of the pipe  52  is arranged at a level H relative to the intake  15  at the bottom  14  of the intermediate tank  5 , the said level corresponding to the volume of water which must be received in the chamber  53  until water overflow via the chamber outlet  54  occurs. This volume of water is so dimensioned that, in the event of both the pump  3  and the heating  22  of the continuous flow heater  6  being switched on, the chamber  53  will overflow exactly when the temperature of the continuous flow heater  6  is approximately so high that the water overflowing from the intermediate tank  5  can be evaporated in a sufficient quality in the steam pipe  18 . 
     In FIGS. 2 and 3, the intermediate tank  5  is comprised of a cylindrical housing  55  having a bore  56  of circular cross-section above the chamber outlet  54  in which a valve piston  57  is guided so as to be slidable in a longitudinal direction. A hollow chamber  58  configured as a cylindrical indentation is arranged at the end close to chamber  53 . Longitudinal grooves  60  are arranged on the peripheral surface  59  of the valve piston  57 , spread over said&#39;s circumference, and, starting from chamber  53 , grooves  60  pass over into a first annular groove  61  designed in the mid-area of the peripheral surface  59  of the valve piston  57 . A first ring seal  62  that is preferably configured as an O-ring is mounted into the first annular groove  61  and seals the bore  56  relative to the first annular groove  61  and, thus, in relation to the valve piston  57 . This way, the chamber  53  is formed relative to the chamber  90  connected to the atmosphere. Succeeding the first annular groove  61  in FIGS. 2 and 3 upwards is a second annular groove  63  which has a width that is smaller than the width of the first annular groove  61  though. A second ring seal  64  configured as an O-ring is also mounted into the second annular groove  63  and, in turn, seals the bore  56  relative to the second annular groove  63 . The second ring seal  64  seals the annular chamber  91  towards chamber  90 . 
     According to FIGS. 2 and 3, a blind-end bore  65  is arranged at the end of the valve piston  57  opposite to the hollow chamber  58 , the said bore  65  housing a spiral spring  66  configured as a compression spring which, on the other side, is supported on a cover  67  inserted into bore  56 . Cover  67  at its inside  68  includes a centrally arranged pin  69  which is embraced at its top end by the spiral spring  66  so that the latter is thereby retained in a central position. Diametrally opposite longitudinal grooves  72  which are penetrated by retaining arms  73 ,  74  designed on the cover  67  are provided on the upper free end  70  of the cylindrical housing  55  according to FIGS. 1 and 2. The retaining arms  73 ,  74  are deflected in a downward direction at the outside surface of the cylindrical housing  55  and backgrip detents  75 ,  76  provided on the peripheral surface  92  of the cylindrical housing  55 , as is shown in FIG. 1 only. This arrangement retains the cover  67  tightly on the cylindrical housing  55  and closes the bore  56  towards the top. According to FIG. 1, further longitudinal grooves  77 ,  78  are arranged at the free end  70 , offset by 90° relative to the longitudinal grooves  72 , and engaged by radially outwardly protruding projections  79 ,  80 . The cover  67  is this way,retained unrotatably and without canting in the bore  56  and may thereby support the force of the spiral spring  66  on the housing  55 . 
     Extending transversely to the longitudinal axis  81  of the intermediate tank  5  in FIGS. 1 to  3  is an outlet  82  designed as a bore in the housing  55 , whose discharge opening  83  along with the first ring seal  62  form the actual pressure relief valve  84 . In the position of the pressure relief valve  84  shown in FIG. 2, both the first ring seal  62  and the second ring seal  64  bear pressure-tightly in bore  56  below the discharge opening  83  of the outlet  82  so that, should pressure develop in chamber  53  and hollow chamber  58 , this pressure will be hindered to discharge via the longitudinal grooves  60  and the ring seals  62 ,  64  towards the outlet  82  as long as the valve piston  57  has adopted its closing position as illustrated in FIG.  2 . When the pressure in chamber  53  and hollow chamber  58  rises, the valve piston  57 , after having overcome the friction force between the bore  56  and the ring seals  62 ,  64  and after having overcome the biassing force of the spiral spring  66 , will be moved upwards in the direction X according to FIG. 3 until the ring seal  64  has overridden the outlet  82  in an upward direction and the ring seal  62  has overridden the discharge opening  83  in part or in full in an upward direction. 
     In the open position of the pressure relief valve  84  shown in FIG. 3, water will preferably flow over the longitudinal grooves  60  past the ring seal  62  to the outlet  82 , as indicated by arrows  85  in FIG.  3 . The outlet  82  can be in connection to the water tank  2  by way of a return conduit (not shown in the drawings) so that condensate is prevented from discharging sideways at the appliance in an uncontrolled fashion. In FIG. 3, the maximum open position of the pressure relief valve  84  is shown because here the end surface  86  provided on the valve piston  57  abuts on the inside  68  of the cover  67 . 
     As is also shown in FIG. 1, two connecting links  87 ,  88  which extend in a longitudinal direction  81  and are arranged opposite each other are provided at the free end  70  of the wall  71  of the intermediate tank  5 , the said connecting links engaging mating grooves of the housing (not shown) of a steam generating device  1  or a hot beverage preparing machine in order to thereby retain the intermediate tank  5  stationarily in the housing of the appliance. Corresponding retaining arrangements are also provided on the water tank  2 , the pump  3 , the continuous flow heater  6 , the two-way valve  45 , and the switch  32 . These arrangements are not shown in the drawings, they correspond with similar retaining arrangements in the appliance. The arrangement of the parts illustrated in FIG. 1 in a household applicance, preferably in an Espresso or coffee machine, is not necessarily such as is shown in FIG.  1 . Instead, these parts may be positioned farther away from or closer to each other, and they may also be arranged in a different installation position, with the exception of the water tank  2  and the intermediate tank.  5 . The important point is that the water tank  2  and the intermediate tank  5  are arranged vertically corresponding to FIG. 1 so that water will not flow out. 
     The operation of the steam generating device  1  is as follows: 
     Initially, the water tank  2  is isolated from the water conduit  8  by way of the handle  50  and filled with drinking water at a water tapping point. Subsequently, the water tank  2  with its outlet (not shown) is connected to the water conduit  8 , a valve (not shown) designed in the outlet being opened, and water flows to the inlet port  9  of the pump  3 . When now the electric switch  32  is actuated to adopt its switch-on position, simultaneously, pump  3  is energized by way of lines  31 ,  25 ,  33  and the heating  22  of the continuous flow heater  6  is energized by way of lines  31 ,  26 , and  25 . This applies to the heating  22  only if the thermoswitch  27  is closed, that means, the continuous flow heater  6  is cold, so that the line  31  is connected to the line  26 . 
     The electric motor  35  will start to work, causing rotation of the drive shaft  36  and, thus, the first toothed wheel  37 . By way of the toothed wheels  38 ,  39 , and  40 , the point of support  41  is set to rotate so that the connected driving rod  42  and the piston (not shown) in the piston housing  43  will move up and down. The suction valve (not shown) provided in the inlet port  9  will open during the downward movement of the piston of the pump  3  so that liquid is admitted in the chamber for the valve housing  44 . 
     During the upward movement of the piston of pump  3 , the suction valve will close, and simultaneously the pressure valve (not shown) at the outlet port  10  will open. Upon repeated strokes, water is this way delivered into valve  45  via the first conduit portion  11 , from where it propagates via the second conduit portion  11   b  and the intake  15  into the chamber  53  of the intermediate tank  5 . The more strokes the pump  3  performs, the more water flows into chamber  53 . 
     Simultaneously with the activation of switch  32 , which is mechanically connected to the two-way valve  45 , heater  22  will start to heat the steam pipe  18  according to FIG.  1 . In the moment when the water delivered by pump  3  has risen so far in chamber  53  that it reached the level H of the chamber outlet  54 , the excess water will flow through the pipe  52  and the second portion  17  of conduit  13  into the steam inlet  19  of the steam pipe  18  where it now evaporates. The instantaneous evaporation is possible only because the time which was required to replenish the chamber  53  until the filling level H was just about sufficient to heat the continuous flow heater  6  to such a degree as to enable it to instantaneously evaporate inflowing water. 
     According to FIG. 1, the evaporated water enters at steam outlet  20  into steam conduit  21  and, at the end of conduit  21 , is conducted to steam nozzle  7 . In steam nozzle  7 , the speed of the steam is considerably increased and, simultaneously, air is aspirated from the side which, when the steam nozzle  7  is immersed into a container with milk, heats the milk, on the one hand, and passes through it, on the other hand, so that the milk is frothed. This process is continued until the milk is adequately hot and sufficiently frothed. 
     Now the two-way valve  45  is moved by hand by means of an actuating button (not shown) to adopt the zero position so that simultaneously the switch  32  is deactivated by way of the mechanical coupling  46 . When the two-way valve is changed over to the zero position, the conduit portion  11   a  to the conduit portion  11   b  will be shut off and, at the same time, the conduit portion  11   b  to the relief conduit  47  opened. Because water no longer flows via the second portion  17  into the steam pipe  18 , with the steam nozzle  7  acting as a throttle, steam within the steam pipe  18  is forced via conduit  17  into the chamber  53 , forcing water accumulated within this chamber back to water tank  2  via conduit portion  11   b , two-way valve  45 , conduit  47  and conduit end  48 . 
     According to FIGS. 1 to  3 , the hollow chamber  58  designed in the valve piston  57  is preferably used to prevent the steam flowing back to the chamber  53  from condensing in the water and at the walls of chamber  53  (therefore, the intermediate tank  5  and the valve piston  57  are made of a plastic material storing only little thermal energy), with the result that pressure would not be provided for a time sufficiently long for the return of the water from chamber  53  of the intermediate tank  5 . The hollow chamber  58  takes up the steam that flows back from the chamber port  54  of the pipe  52  without losing substantial heat and condensing. This ensures that pressure is still at disposal in chamber  58  for a sufficiently long time which permits displacing the water from the chamber  53  and returning it into the water tank  2 . The intermediate tank  5  is emptied upon completion of the steam generation process this way, and the procedure described hereinabove may recommence. The water tank  2  may be emptied manually after return of the water. 
     The purpose of the pressure relief valve  84  according to FIGS. 2 and 3 is that in case the pressure in the chamber  53  rises excessively, for example due to calcification in the steam pipe  18  or because the steam nozzle  7  or the steam pipe are clogged, this pressure will be decreased by control. This means that if the force acting on the valve piston  57  increases, the valve piston  57  according to FIGS. 2 and 3 will move in an upward direction after having overcome the friction forces between the seals  62 ,  64  and the bore  56  and after having overcome the biassing force of the spring  66  in the bore  56 . In this arrangement, seal  64  initially overrides the outlet  82  and, upon further displacement of the valve piston  57  in the direction X upwards, the ring seal  62  will override the outlet  82  in part or entirely, so that steam may exit from chamber  53  and propagate via the longitudinal grooves  60  and past the ring seal  62  into the outlet  82 , as is indicated in FIG.  3 . It is this way prevented that the parts of the steam generating device  1  are unnecessarily stressed or damaged by the rising pressure. 
     The special purpose of the second ring seal  64  is to prevent water and, possibly, even steam from flowing from the annular chamber  91  into the chamber  90  provided between the end surface  86  of the valve piston  57  and the cover  67  in the open position of the pressure relief valve  84  according to FIG.  3 . After pressure reduction in the chamber  53 , the valve piston  57  is urged by the force of spring  66  to adopt its closed position again, as shown in FIG. 2, when the force of the valve piston  57 , which develops due to the pressure in chamber  53 , is lower than the force of the spiral spring  66  which is applied to the valve piston  57  from the other side. Friction forces at the seals  62 ,  64  will be ignored in this respect. 
     It should still be mentioned that, admittedly, a certain time will lapse for a user after switch-on of the steam generating device  1  until steam exits from the steam nozzle  7 . However, this delay time is acceptable because the pump  3  is already activated, and its operation is perceived by a user thus knowing that the steam generating device  1  is working and will deliver steam shortly afterwards. If, on the other hand, as is known from the state of the art, the pump  3  would be set in action only when the continuous flow heater  6  has reached its operating temperature, a display would have to be provided on the appliance indicating that the appliance is not yet ready for operation, or is being preheated. Such costly provisions can be economized by pump  3  working already and making corresponding noise. 
     For a possible cleaning of the intermediate tank  5  in a servicing station, the cover  67  can be removed from the cylindrical housing  55  by detaching the retaining arms  73 ,  74  from the detents  75 ,  76 . Subsequently, the valve piston  57  along with the spring  66  can be withdrawn from the bore  56  so that replacement of e.g. the seals  62 ,  64  or other maintenance measures such as greasing the bore  56  and the valve piston  57  can be effected.