Abstract:
A two-chamber step bearing with hydraulic damping, especially for mounting engines in motor vehicles, has at least one fluid-filled working chamber and at least one compensating chamber connected thereto by an overflow channel. An additional, blockable spring element ( 10 ) is arranged in series after the first hydraulic damping spring element comprising the working chamber ( 1 ), the compensating chamber ( 2 ) and the overflow channel ( 11 ), wherein the spring element ( 10 ) is arranged in a separate pretensioning chamber ( 8 ), which is connected to the working chamber ( 1 ) and the compensating chamber ( 2 ) by at least two switching elements ( 16, 17 ) which can be controlled independently from one another, and wherein the pretensioning chamber ( 8 ) can be filled up with hydraulic fluid for blocking the additional spring element ( 10 ) by the vibrations introduced into the two-chamber step bearing from the engine.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to a two-chamber step bearing with hydraulic damping especially for mounting the engine in motor vehicles. 
     BACKGROUND OF THE INVENTION 
     Various designs of two-chamber step bearings have been generally known from the state of the art. A membrane of varying hardness and ductility may be arranged between the working chamber and the compensating chamber in some hydraulic step bearings. Such a membrane is usually called a coupling membrane, and the degree of mobility of the membrane determines the vibration amplitude which can be influenced, and the stiffness of the membrane determines the frequency of the vibration to be damped. If such a membrane is relatively soft, the vibrations acting on the hydraulic fluid of the working chamber are transmitted without resistance to the adjoining compensating chamber. However, if the membrane is provided with a relatively stiff structure, the vibrations acting on the hydraulic fluid will build up an overpressure in the working chamber, and this overpressure leads to a swelling of the uncoupling membrane and thus guarantees better damping, but a higher dynamic stiffness develops at the same time. It is desirable in the two-chamber step bearings known from the prior art for adaptation to different operating states to correspondingly adapt the vibration behavior of the hydraulic bearing to these operating states. 
     This is of significance especially because the disturbing vibrations generated by the engine during the operation of the vehicle occur essentially in two different forms and therefore require different stiffnesses of the engine mounts used for vibration damping. The different damping properties are of great significance especially in the direct-injection diesel engines that have been used for some time now because the vibrations occurring at idle in these engines are higher than in the diesel and gasoline engines used hitherto in automotive engineering, so that an especially soft engine mount is necessary for the operation at idle for vibration damping in these diesel engines, whereas a substantially stiffer engine mounting is advantageous for vibration damping in the drive mode. 
     Adaptation can be achieved in the above-mentioned two-chamber step bearings provided with an uncoupling membrane between the working chamber and the compensating chamber, e.g., by changing the stiffness of the membrane by, e.g., reducing or increasing the support diameters of the membrane. This can be brought about, e.g., by means of a motor operator with a plunger actuated by same, which presses the uncoupling membrane from below. However, the different spring stiffnesses necessary in the above-mentioned direct-injection diesel engines cannot be achieved with the two-chamber step bearing known from the state of the art because the values of the required spring stiffnesses have an excessively great difference. 
     A prior-art two-chamber step bearing, which is formed by a first hydraulic damping spring element, a spring element, at least one fluid-filled working chamber and at least one compensating chamber connected to same by an overflow channel, wherein an additional, blockable spring element, which is arranged in a separate pretensioning chamber is arranged in series after the said damping spring element, has been known from, e.g., DE 43 22 958 A1. 
     In addition, an additional spring element with a hydraulic functional connection with a first hydraulic damping spring element has been known in a two-chamber step bearing from JP Abstracts 62-270 841 (A), but the spring element disclosed in this document cannot be controlled or blocked. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     The object of the present invention is therefore to improve a two-chamber step bearing with hydraulic damping especially for mounting the engine in motor vehicles such that it is also possible to achieve the very great differences in the spring stiffness of such two-chamber step bearings for the drive mode and operation at idle, which are required in the case of the use of modem diesel engines. In addition, such two-chamber step bearings shall have a compact design and their manufacture shall be inexpensive and they shall operate reliably under all required operating conditions. 
     This object is accomplished corresponding to a first solution variant by the pretensioning chamber having the blockable spring element being hydraulically connected to the working chamber and the compensating chamber via at least two switching elements that can be controlled independently from one another. Due to the vibrations introduced by the engine into the two-chamber step bearing, the pretensioning chamber can be filled up with a hydraulic fluid, which makes possible the blocking of the additional spring element. 
     Another solution variant for the object is disclosed by the technical teaching of the pretensioning chamber being connected to the compensating chamber by a first line, in which a pumping device is arranged, which is driven by the vibrations introduced into the two-chamber step bearing and is provided for filling up the pretensioning chamber with hydraulic fluid, and by a second line with an electromagnetic on-off valve inserted therein. 
     Due to the embodiments according to the present invention corresponding to the two possible solutions, blocking of the second spring element, which is connected in series with the first damping spring element, is brought about as needed by the vibrations of the engine only, whose vibrations are to be damped by the two-chamber step bearing, and which vibrations are introduced into the two-chamber step bearing, without an external power source. 
     Due to the blocking of the additional spring element, the overall stiffness of the engine mount is substantially greater than when both spring elements connected in series are active. As a result, both a soft damping characteristic for the idle operation of a motor vehicle engine and a stiff damping characteristic for the normal drive mode are provided in the two-chamber step bearing according to the present invention. The blocking of the additional spring element can be eliminated via the switching element, which is present between the working chamber and the compensating chamber and which is preferably an electromagnetic on-off valve by a pressure equalization being brought about between the pretensioning chamber and the working chamber or the compensating chamber. 
     Special other embodiments of the first solution according to the present invention providing one switching element as the nonreturn valve arranged between the working chamber and the pretensioning chamber and providing one switching element as an electromagnetic on-off valve arranged between the compensating chamber and the pretensioning chamber. Special embodiments according to the second solution include providing the pretensioning chamber connected to the compensating chamber by a first line, in which a pumping device, which is driven by the vibrations introduced into the two-chamber step bearing and is provided for filling up the pretensioning chamber with hydraulic fluid, is arranged, and by a second line with an electromagnetic on-off valve inserted into it and by providing the pumping device with a plunger piston and a nonreturn valve, wherein the said nonreturn valve is arranged between the plunger piston and the compensating chamber and provides a possibility of flow from the compensating chamber to the pretensioning chamber. 
     In addition, additional advantageous embodiments of both solutions according to the present invention may employ the blockable spring element arranged between a partition, which forms a wall of the compensating chamber, and a movable bottom plate, which forms a wall of the pretensioning chamber. The said blockable spring element may comprise at least two, preferably three coil springs arranged concentrically to the central longitudinal axis of the two-chamber step bearing. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a sectional view of the two-chamber step bearing with the features corresponding to claim  1  in the switching position with soft damping characteristic; 
     FIG. 2 is the two-chamber step bearing from FIG. 1 in the switching position with hard damping characteristic; 
     FIG. 3 is a sectional view of a two-chamber step bearing with the features corresponding to a second solution of the invention in the switching position with soft damping characteristic; 
     FIG. 4 is a two-chamber step bearing from FIG. 3 in the switching position with hard damping characteristic; 
     FIG. 5 a  is an enlarged view of the pumping device of the two-chamber step bearing according to FIG. 3 in a pumping position; 
     FIG. 5 b  is an enlarged view of the pumping device of the two-chamber step bearing according to FIG. 3 in another pumping position; 
     FIG. 5 c  is an enlarged view of the pumping device of the two-chamber step bearing according to FIG. 3 in another pumping position; and 
     FIG. 5 d  is an enlarged view of the pumping device of the two-chamber step bearing according to FIG. 3 in still another pumping position. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings in particular, all drawings show sectional views and details of the two-chamber step bearing according to the present invention in the installed, i.e., loaded state. 
     FIG. 1 shows a two-chamber step bearing, which has a working chamber  1 , which is limited by a rubber wall  3  on its upper side facing the engine to be mounted. The lower limitation of the working chamber  1  is formed by an intermediate plate  4 , in the middle area of which an uncoupling membrane  5  made of an elastic material is recessed. A compensating chamber  2  is located beneath the uncoupling membrane  5  and the intermediate plate  4 , and a partition  6  is located on the underside of the compensating chamber  2 . The compensating chamber  2  is sealed against the fixed partition  6  with a membrane  7  made of an elastic material. 
     On the lower side of the partition  6  facing away from the compensating chamber  2 , an additional hollow space is arranged as a pretensioning chamber  8 , whose side walls are formed by a pot-shaped housing  9  and inside which side walls three coil springs  10  located on a circle concentric to the central longitudinal axis are arranged. Only one of the coil springs  10  is shown in the view in FIG.  1 . The three coil springs  10 , of which only one is shown in FIG. 1, form a common, additional spring element under the first hydraulic damping spring element, which is formed by the rubber wall  3 , the working chamber  1 , the uncoupling membrane  5 , the compensating chamber  2  as well as an overflow channel  11  located between the working chamber  1  and the compensating chamber  2 . The coil springs  10  are supported on their top side at the partition  6 , and the opposite end of the coil springs  10  is in contact with a bottom plate  12  of the two-chamber step bearing, which is arranged in the axial direction of the principal longitudinal axis of the two-chamber step bearing opposite the housing  9 , displaceably in the said housing, and forms the bottom of the pretensioning chamber  8 . The pretensioning chamber is sealed by an additional rolling membrane  19  made of an elastic material against the walls of the housing  9  as well as the bottom plate  12 . On its underside facing away from the pretensioning chamber  8 , the bottom plate  12  has a threaded hole  20  for fixing the two-chamber step bearing to the body. 
     FIG. 1 shows that a gap  21  of about 3-5 mm is present between the bottom plate  12  and the lower, body-side area of the housing  9 . Based on the fact that the rubber wall  3  fixed to the engine and the bottom plate  12  fixed to the body have fixed positions, the introduction of vibrations into the two-chamber step bearing leads to a movement of the partition  6  between the first rubber spring element, which is hydraulically damped by the hydraulic fluid present in the working chamber  1 , the compensating chamber  2  and the overflow channel, and the second spring element, which is formed by the coil springs  10  made of steel. The interaction of the two spring elements guarantees an effective damping of low-frequency vibrations of high amplitude due to its soft overall spring stiffness characteristic. 
     FIG. 1 also shows that the partition  6  has in its middle a tubular partial area  15 , which extends upward into the working chamber  1  and which forms a connection between the working chamber  1  and the pretensioning chamber  8 . The lower opening of the tubular partial area  15  is closed by a switching element  16  designed as a nonreturn valve. Another switching element  17 , designed as an electromagnetic on-off valve, which is shown in the right-hand area of FIG. 1, is located between the pretensioning chamber  8  and the compensating chamber  2 . 
     The interaction of the switching elements  16  and  17  as well as the pretensioning chamber  8  and the bottom plate  12  makes possible the build-up of a hydraulic pressure cushion within the pretensioning chamber  8  solely due to the vibrations introduced into the two-chamber step bearing via the engine. Due to the build-up of such a pressure cushion in the pretensioning chamber  8 , the entire hydraulic bearing is raised except for the bottom plate  12  in relation to the body and the bottom of the housing  9  is thus pressed upward against the bottom plate  12 , so that the force no longer flows via the coil springs  10 , which are thus not functioning. The putting of the coil springs  10  out of operation leads to a substantial hardening of the damping characteristic of the two-chamber step bearing, because only the first rubber spring element acts. 
     The requirement for the build-up of the pressure cushion within the pretensioning chamber  8  is the severing of the connection between the pretensioning chamber  8  and the compensating chamber  2  by the closing of the on-off valve  17 . Hydraulic fluid is prevented by this measure from flowing back into the compensating chamber  2 , as is indicated by arrow P in the opened on-off valve  17  shown in FIG.  1 . The pressure cushion is built up by an overpressure, which is generated by the introduction of vibrations into the working chamber  1  and which causes hydraulic fluid to enter the pretensioning chamber  8  through the nonreturn valve  16  opened by the overpressure from the working chamber  1  via the tubular partial area  15 . 
     The hydraulic fluid having entered the pretensioning chamber presses the hydraulic bearing against the bottom plate  12  and brings about a bridging over of the coil springs  10 . 
     The lowering of the hydraulic bearing can be supported by connecting a vacuum line to a connecting fitting  22 . The connecting fitting  22  opens inside the housing  9  into the gap  21  under the bottom plate, so that the connected vacuum exerts a suction effect on the lower part of the housing  9 . 
     The position of the bottom plate  12  and of the on-off valve  17  are illustrated in FIG. 2, which shows the two-chamber step bearing in its operating state with hard damping characteristic. The coil springs  10  are prevented from slipping in this position by a respective mandrel  13  and  14  each engaging the upper and lower ends of the particular coil spring  10 . 
     If the pressure cushion present in the pretensioning chamber  8 , which makes the damping possibility of the coil springs  10  ineffective, is to be eliminated, it is necessary for the bottom plate  12  to be able to return into its original position, as is shown in FIG.  1 . The pressure cushion within the pretensioning chamber  8  is eliminated by opening the electromagnetic on-off valve  17 , as a result of which hydraulic fluid can flow through the connection hole  18  into the compensating chamber  2 . The return into the raised position of the bottom plate  12  causes both the rubber damping spring element in the upper area of the two-chamber step bearing and the additional spring element formed by the coil springs  10  to be effective again, so that the damping characteristic for the two-chamber step bearing is, on the whole, softer than that seen when only the upper hydraulic rubber damping spring element acts. 
     Thus, FIGS. 1 and 2 show that a rigidity of the two-chamber step bearing, which differs greatly depending on the spring properties of the coil springs  10 , can be brought about during blocking and non-blocking of the additional spring element formed by the coil springs  10 . The coil springs  10  are blocked here only by the pumping action brought about as a consequence of the vibrations introduced into the two-chamber step bearing by the working chamber  1  and by the opening and closing of the connection hole  18  by the actuation of the electromagnetic on-off valve  17 . It is possible to provide a possibility of achieving different damping properties by means of a single two-chamber step bearing in an extremely compact manner. 
     Another solution variant for the object according to the present invention is embodied in FIGS. 3 and 4 by the two-chamber step bearing shown there. Just as the step bearing according to the solution variant already described in detail above, this two-chamber step bearing also has a working chamber  1 , which is limited by a rubber wall  3  on its upper side facing the engine to be mounted. The lower limitation of the working chamber  1  is formed by an intermediate plate  4 , in the middle area of which an uncoupling membrane  5  made of an elastic material is recessed. A compensating chamber  2  is located under the uncoupling membrane  5  and the intermediate plate  4 , and a fixed partition  6  is located on the underside of the compensating chamber  2 . The compensating chamber  2  is sealed against the fixed partition  6  with a membrane  7  made of an elastic material. 
     On the side of the partition  6  facing away from the compensating chamber  2 , a pretensioning chamber  8  is arranged, whose side walls are formed by a pot-shaped housing  9  and inside which three coil springs  10  located on a circle concentric to the central longitudinal axis are arranged. These coil springs  10  together form an additional spring element, which is connected in series with the first hydraulic rubber spring damping element formed by the working chamber  1 , the compensating chamber  2 , the rubber membrane  3 , the membrane  4  and the overflow channel  11 . The coil springs  10  are supported on their top side at the fixed partition  6 , and the opposite end of the coil springs  10  is in contact with a bottom plate  12  of the two-chamber step bearing, which is arranged in the axial direction of the principal longitudinal axis of the two-chamber step bearing against the housing  9 , displaceably in the said housing, and forms the bottom of the pretensioning chamber  8 . The pretensioning chamber is sealed against the walls of the housing  9  as well as the bottom plate  12  by another membrane  19  made of an elastic material. On its underside facing away from the pretensioning chamber  8 , the bottom plate  12  likewise has a threaded hole  20  for fixing the two-chamber step bearing to the body. 
     FIG. 3 shows that analogously to the exemplary embodiment shown in FIGS. 1 and 2, a gap  21  of about 3-5 mm is located between the bottom plate  12  and the lower, body-side area of the housing  9 . Based on the fact that the rubber wall  3  fixed to the engine and the bottom plate  12  fixed to the body have fixed positions, the introduction of vibrations into the two-chamber step bearing leads to a movement of the partition  6  between the first rubber spring element, which is hydraulically damped by the hydraulic fluid present in the working chamber  1 , the compensating chamber  2  and the overflow channel  1 , and the second spring element formed by the coil springs  10  made of steel. The interaction of the two spring elements guarantees an effective damping of low-frequency vibrations of high amplitude due to the overall soft spring stiffness characteristic of the two-chamber step bearing in this mode of operation. 
     Compared with the first variant, the exemplary embodiment according to FIGS. 3 and 4 is characterized in that the partition  6  has in its middle a tubular partial area  15 , which is connected in its upper end facing away from the partition  6  to the intermediate plate  4 . As can be determined from FIGS. 3 and 4, the tubular partial area  15  has two connection holes  30  and  31  to the compensating chamber  2 . A hollow cylindrical projection  32  is arranged on the underside of the partition  6 . A first switching device  33 , which comprises a nonreturn valve  34  and a plunger cylinder located thereunder, is located within the projection  32 . The plunger cylinder  35  forms a pumping device, by means of which hydraulic fluid can be pumped from the compensating chamber into the pretensioning chamber  8  via the connection holes  30  and  31 , the tubular partial area  15  and the nonreturn valve  34 . Next to the switching device  33 , the two-chamber step bearing according to the present invention shown in FIGS. 3 and 4 has a second switching device  36 , which comprises an electromagnetic on-off valve and is suitable for closing a connection hole  37  between the pretensioning chamber  8  and the compensating chamber  2 . 
     In the operating state of the two-chamber step bearing shown in FIG. 3, both the spring elements  10  and the upper hydraulic rubber spring damping element are active. The hydraulic fluid being transported by means of the plunger cylinder  35  from the compensating chamber into the pretensioning chamber can again move back into the compensating chamber  2  without problems because the on-off valve  36  is opened and the hydraulic fluid can flow through the connection hole  37 . 
     If stiffening of the damping characteristic of the two-chamber step bearing is desired due to the operating state of the connected engine, the on-off valve  36  is actuated and it closes the connection hole  37 . The flowing back of hydraulic fluid into the compensating chamber  2  is thus ruled out. The hydraulic fluid being transported by the plunger cylinder  35  from the compensating chamber  2  into the pretensioning chamber  8  is now used to build up a pressure cushion within the pretensioning chamber  8 , which causes, analogously to the above-described mode of operation of the first solution variant, the bottom plate  12  to come into contact with the housing  9 . The contact is via a cylindrical stop face  38  as well as a conical stop face  39 . This double interlocking between the bottom plate  12  and the housing  9  guarantees that tilting of the two-chamber step bearing is prevented even at very strong radial forces because lateral forces occurring are reliably transmitted by the stop faces. 
     If the two-chamber step bearing is to be returned from the operating state with the hard damping characteristic, as is shown in FIG. 4, into the operating state with soft damping characteristic, it is necessary to eliminate the pressure cushion present in the pretensioning chamber  8 , which makes the damping possibility of the coil springs  10  ineffective. The pressure cushion within the pretensioning chamber  8  is eliminated by opening the electromagnetic on-off valve  36 , so that hydraulic fluid can flow back into the compensating chamber  2  due to the reopening of the connection hole  37 . 
     The mode of operation of the plunger cylinder  35  provided as a pumping device will be explained in greater detail below on the basis of FIGS. 5 a-d . FIGS. 5 a - 5   d  show an enlarged sectional view of the nonreturn valve  34  as well as of the plunger cylinder  35  which is likewise present in the projection  32 . 
     The pumping effect of the plunger cylinder  35  is based on the movements of the partition  6  in the “soft” state of the hydraulic bearing, i.e., in the cases in which vibration can be introduced into the two-chamber step bearing. Based on the fact that the rubber wall  3  fixed to the engine and the bottom plate  12  fixed to the body assume fixed positions, the introduction of vibrations into the two-chamber step bearing leads to an up-and-down movement of the partition  6  corresponding to arrow B in FIG. 5 a . A pumping cycle of the plunger cylinder  35  is brought about by an upward and downward movement corresponding to a vibration of the partition  6 . FIGS. 5 a - 5   d  show different stages of the pumping cycle. 
     The plunger cylinder  35  comprises essentially a piston  43 , which is displaceable axially in the direction of the principal axis of the two-chamber step bearing within the cylindrical projection  32  of the partition  6 , a spacing spring  42 , which is arranged in the intermediate chamber  48  with a variable volume V 1  between the top side of the piston  43  and the underside of the nonreturn valve housing  46 , as well as a nonreturn valve, which is arranged centrally inside and comprises the valve seat  44  and the pressure spring  45 . FIG. 5 a  shows the position of the two-chamber step bearing in which both the hydraulic rubber spring damping element and the coil spring element are active. This means that no pressure cushion is built up in the pretensioning chamber. The view in FIG. 5 a  shows that the piston  35  of the plunger cylinder has moved into the cylindrical projection  32  of the partition  6 , both the nonreturn valve  34  and the nonreturn valve  47  located in the piston  43  are opened. The chamber volume V 1  between the nonreturn valve and the piston  43  has a low value. 
     If the partition  6  is moved upward in the direction of the compensating chamber corresponding to arrow P in FIG. 5 a , the volume V 1  of the intermediate chamber  48  increases. Thus, a vacuum is generated in the chamber  48 , which causes the valve seat  44  of the nonreturn valve  47  to be moved upward corresponding to arrow Q 2  under the effect of the vacuum and the spring forces of the pressure spring  45 , which means that the nonreturn valve  47  closes. At the same time, hydraulic fluid is drawn by the overpressure in the chamber  48  into the intermediate chamber  48  through the opened nonreturn valve  34  from the compensating chamber  2  via the connection holes  30  and  31  as well as the tubular partial area  15  due to the overpressure in the chamber  48 . 
     FIG. 5 b  shows the intermediate stage of the upward movement of the partition  6  at which the nonreturn valve  34  is opened and the nonreturn valve  47  is closed. 
     Due to the vibrating movement of the partition  6 , the latter will subsequently perform a downward movement from the upward movement shown in FIGS. 5 a  and  5   b  corresponding to arrow P shown in FIG. 5 c , as a result of which the largest possible volume V 1  of the intermediate chamber will again decrease at the moment of the reversal of the movement, The reduction in the volume leads to an increase in the pressure within the intermediate chamber  48 , as a result of which the valve seat  40  of the nonreturn valve will move upward corresponding to arrow Q 1  because of the increasing pressure and the force of the pressure spring  41  and the nonreturn valve  34  will close as a consequence of this. The further movement of the partition  6  in the direction of arrow P leads to a further increase in the pressure within the intermediate chamber  48 , as a result of which the nonreturn valve  47  arranged within the piston  43  will open. The hydraulic fluid present in the intermediate chamber  48  is pressed into the pretensioning chamber  8  through the opened nonreturn valve  47  as a consequence of the further downward movement of the partition  6 . 
     FIG. 5 d  shows an intermediate stage during the downward movement of the partition  6 , at which the nonreturn valve  34  is closed and the nonreturn valve  47  is opened, so that the hydraulic fluid present within the intermediate chamber  48  can escape downward into the pretensioning chamber  8 . 
     If the partition  6  has passed through the bottom dead center of its movement within the framework of its vibration amplitude, at which the intermediate chamber  48  has assumed its smallest volume V 1 , a new vibration cycle begins due to the repeated upward movement of the partition  6  corresponding to arrow P in FIG. 5 a . The repeated upward movement of the partition  6  leads to a vacuum within the intermediate chamber  48 , so that the nonreturn valve  47  closes, as it already happened in the explanation of FIG. 5 a , while the nonreturn valve  34  opens, so that hydraulic fluid can again escape from the compensating chamber  2  into the intermediate chamber. 
     Thus, hydraulic fluid is pressed into the pretensioning chamber during each vibration cycle of the partition  6  located within the two-chamber step bearing, which leads to the build-up of the pressure cushion as long as the on-off valve  36  closes the connection hole  37  and no hydraulic fluid can thus escape from the pretensioning chamber. 
     Another advantage of this arrangement according to the present invention is that the piston  43  is also pretensioned by the gradual build-up of the pressure cushion within the pretensioning chamber  8 , so that the vibrations fade out slowly and the plunger cylinder is no longer actuated. The bottom plate  12  is in contact with the housing  9  in this state corresponding to FIG. 4, so that the coil springs arranged between the bottom plate  12  and the partition  6  are ineffective. 
     A return into the state of the two-chamber step bearing in which the coil springs  10  are again active is brought about by the connection opening  37  being opened by the on-off valve  36 . The overpressure in the pretensioning chamber  8  can be eliminated by the opening, so that the interlocking between the bottom plate  12  and the housing  9  is eliminated. 
     Corresponding to a special embodiment, the build-up of the pressure cushion or the interlocking between the bottom plate  12  and the housing  9  can be supported by a vacuum tube, which additionally supports the locking movement between the bottom plate  12  and the housing  9 , being connected to a connecting fitting  22  introduced into the housing wall. 
     It appears clearly from the explanations given above that a two-chamber step bearing with hydraulic damping and two greatly different damping spring rates is created, in which the transition from a soft damping behavior to a hard damping behavior can be brought about only by the vibrations introduced into the two-chamber step bearing. An external drive is thus unnecessary for building up a corresponding pressure cushion, so that a very compact and inexpensive solution is possible. Since the fluid for building up a corresponding pressure cushion is incompressible, the entire system can absorb very strong axial forces. In addition, the solution variants described have the advantage that no loose components are present within the two-chamber step bearing and the bearing forms, moreover, a hermetically, tightly sealed system. 
     While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.