The disclosure relates to a road finishing machine with a chassis and a pushing device. The pushing device comprises at least one docking assembly and at least one absorber unit. The absorber unit is fixed to the chassis of the road finishing machine and comprises at least one piston-cylinder unit which comprises a cylinder and a working piston movable therein which subdivides the piston-cylinder unit into at least one first and one second chambers and is coupled to the docking assembly. The docking assembly may be positioned, by the movably mounted working piston, relative to the chassis between an extended position and at least one retracted position. In the piston-cylinder unit, at least one flow channel is provided between the first and the second chambers for letting a fluid enclosed in the piston-cylinder unit pass through, the fluid being prestressed in the extended position.

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims foreign priority benefits under 35 U.S.C. §119(a)-(d) to European patent application number EP 13 004 086.8, filed Aug. 16, 2013, which is incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The disclosure relates to a road finishing machine. 
     BACKGROUND 
     A road finishing machine may be used in practice for docking to a truck for material transfer. In the process, it is important that no jerky accelerations are transmitted from the material supply vehicle to the road finishing machine, which could lead to damages in the paving. Consequently, in practice road finishing machines are employed which comprise a pushing device at their chassis in front, seen in the paving direction, by which they may cushion the docking of a material supply vehicle. 
     The applicant's EP 2 527 534 A1 discloses a road finishing machine with a pushing device movably mounted at the chassis which preferably comprises at least one friction spring as absorbing member. With the latter, the shock-absorption of a truck that docks to it works well, while it turned out that an absorber unit including a friction spring is expensive. 
     DE 10 2011 120 161 A1 discloses a road finishing machine with a pushing device movably mounted to the chassis, the pushing device being spring-mounted by at least one elastomer structural absorbing body. A disadvantage of this, however, consists in that the elastomer structural absorbing body gets easily soiled and may embrittle by heat or insolation prevailing on the construction site. Once the elastomer structural absorbing body is embrittled, its absorbing properties will also deteriorate. 
     U.S. Pat. No. 5,004,394 A discloses a road finishing machine having a pushing device mounted to it which, according to a first embodiment, is mounted in an absorbing manner relative to the chassis of the road finishing machine by means of a leaf spring. In another embodiment, the pushing device is fixed to two hydraulic cylinders. The hydraulic cylinders are each connected to a hydraulic circuit which supplies a hydraulic medium to the hydraulic cylinders from outside or which withdraws the hydraulic medium from the hydraulic cylinders. A disadvantage of this consists in that the hydraulic circuit has a complex design and requires a lot of space on the road finishing machine. Moreover, the components employed in the hydraulic circuit, e.g., the hydraulic pump or the control valves, must be serviced at regular intervals. Such a pushing device is expensive to manufacture and difficult to be retrofitted at existing road finishing machines. 
     SUMMARY 
     Therefore, it is an object of the disclosure to improve a road finishing machine such that the docking to a material supply vehicle, in particular a truck, may be cushioned such that no jerky impacts are transmitted to the road finishing machine during docking and during material transfer, while the means employed for this may be easily manufactured and attached to the road finishing machine without requiring a lot of space. 
     According to the disclosure, a road finishing machine is provided with a pushing device which comprises at least one docking assembly and at least one absorber unit. The absorber unit is fixed to the chassis of the road finishing machine and has at least one piston-cylinder unit which comprises a cylinder and a working piston movable at the latter which subdivides the piston-cylinder unit into at least one first and one second chambers and is coupled to the docking assembly. By the movably mounted working piston, the docking assembly may be positioned relative to the chassis between an extended position and at least one retracted position. In accordance with the disclosure, at least one flow channel is provided in the piston-cylinder unit between the first and the second chambers for letting a fluid enclosed in the piston-cylinder unit pass through it, the fluid being prestressed in the extended position. 
     Moreover, embodiments according to the disclosure may also be employed in a charger vehicle with the pushing device described herein. 
     By a prestressing force, i.e., a compression force, acting on the fluid already in the extended position in the disclosure, the working piston will remain in the extended position until a force pressing from outside onto the docking assembly, e.g., by a docking truck, exceeds this compression force. The compression force prevailing in the extended position and caused by the pressure of the compressed fluid acting on the working piston is simultaneously the minimum restoring force which is required for returning the working piston from its retracted position back to its extended position. 
     If in operation of the disclosure, a force is applied from outside to the pushing device mounted to the road finishing machine, e.g., by a docking truck, which force counteracts and exceeds the compression force prevailing in the piston-cylinder unit, the working piston will be pushed into the pre-compressed fluid. Since the pre-compressed fluid may not be displaced out of the closed piston-cylinder unit, it is forced to flow through the flow channel which is provided in the piston-cylinder unit. The retraction of the working piston into the pre-compressed fluid causes an additional compression of the fluid by the volume of the retracting working piston, because the fluid is locked in the cylinder of the piston-cylinder unit and cannot exit from it. This additional compression provides an increase in the compression force along the damping path and results in the spring rate R which will be described below more in detail with reference to  FIG. 7 . 
     The highest compression force within the piston-cylinder unit is reached when the working piston comes to a standstill in its retraction movement. While a portion of the force acting on the pushing device from outside is converted into friction and heat, the rest of it will be stored as compressive pressure in the fluid within the piston-cylinder unit. 
     The compressive pressure on the working piston present in the retracted position causes the working piston to press against the load applied from outside and to finally return to its extended position when the force acting from outside diminishes. 
     While the working piston is being returned, the fluid flows again through the flow channel, whereby the excessive additionally built-up compressive energy may be slowly relieved. Finally, the compressed fluid emits as much energy as is required for returning the working piston. When the working piston is in its original position again, i.e., in the extended position (position as before retraction), the originally prestressed compression force is applied to it again. Then, the pushing device is ready for action again. 
     The disclosure also relates to a method for cushioning a load acting on a pushing device of a road finishing machine, where a prestressing force caused by a fluid locked in the absorber unit is acting on a working piston of a closed piston-cylinder absorber unit in an extended position, which force pushes the working piston in the direction of the extended position, while the working piston is being pushed into the absorber unit by a load counteracting and exceeding the prestressing force, so that the prestressing force present in the fluid increases, and wherein the fluid flows through at least one flow channel provided in the piston-cylinder absorber unit, so that it is displaced from a first chamber, in which the fluid is nearly completely in the extended position, into a second chamber of the piston-cylinder absorber unit which is formed by the working piston driving into the piston-cylinder absorber unit, where the working piston automatically returns to its extended position when the load on the working piston is relieved. The working piston is slowly returned to the extended position when the load is removed. 
     Preferably, the prestressed fluid is a highly compressible fluid, in particular a liquid, such as silicone oil. The silicone oil offers excellent compression properties and may be compressed to about 20% of the present volume. The compressibility of silicon oil permits to manufacture very strong absorber units of small dimensions. According to a particular embodiment, the silicone oil is a polydimethylsiloxane (PDMS) which exhibits high compressibility and also excellent lubrication properties. 
     It is also advantageous for the flow channel, or a plurality of flow channels, to be embodied in the working piston. Preferably, the flow channel is provided in a piston plate of the working piston. The flow channel may be oriented in the piston plate along an adjustment travel of the working piston, where it is also possible for the flow channel to be embodied obliquely in the piston plate in order to extend the passage section for the fluid. This permits to achieve different absorption degrees or speeds. 
     According to a further embodiment of the disclosure, at least one one-way valve, i.e., a check valve, is provided in the piston-cylinder unit for letting the fluid locked in the piston-cylinder unit pass when the working piston presses in the direction of the retracted position. When the working piston is returned in the direction of the extended position, the one-way valve shuts off and prevents a passage of the fluid, so that the fluid may only flow through the flow channel. This causes the effect that an insertion of the working piston into the cylinder in the direction of the refracted position causes a lower resistance to the working piston than during the return of the working piston in the direction of the extended position. This has the advantage that the absorber unit or the pushing device according to the disclosure may excellently absorb impacts of a docking load, e.g., a truck, at the road finishing machine according to the disclosure, and that the docking assembly may slowly return to the extended position, whereby no accelerations are transmitted to the road finishing machine. 
     It may in particular be desired that the one-way valve lets more fluid pass than the flow channel when the working piston presses in the direction of the retracted position. This permits a quick and soft cushioning of the working piston into the prestressed fluid. 
     It is preferably possible for a flow rate through the one-way valve and/or the flow channel to be variable, for example by an adjusting screw. In such an embodiment, it is useful to provide the one-way valve and/or the flow channel within the cylinder of the absorber unit, so that the adjusting screw is easily accessible from outside. Thereby, the absorber unit could be adjusted to different loads. 
     A further embodiment of the disclosure provides for the piston-cylinder unit to comprise a guiding section with a guiding piston which is fixed to the working piston. The guiding section takes care that the working piston may be exactly and repeatedly uniformly adjusted within the piston-cylinder unit. The working piston may be excellently guided by the guiding section even if high, non-uniformly distributed loads are acting on the pushing device from outside. 
     For the guiding piston to easily slide within the guiding section, at least one vent hole is preferably provided in the guiding section. Through this vent hole, air may escape or be sucked in when the guiding piston travels into a retracted position or returns from the latter to its original position. The vent hole may be provided, for example, in an external wall of the guiding section which is embodied as extension of the cylinder of the piston-cylinder unit. As an alternative or in addition, the vent hole may also be provided in the guiding piston to release air from the guiding section to the outside while the working piston is retracting. 
     Preferably, a mounting bore is provided in the working piston for fixing the working piston for mounting the guiding piston on it. According to a further embodiment, a further bore, preferably a tapped bore, is provided in an external wall of the guiding section which bore is, in the extended position, aligned with the mounting bore provided in the working piston, so that a fixing pin may be inserted through these bores to fix the working piston for mounting the guiding piston. Thus, it is possible to firmly fix the guiding piston on the working piston. Optionally, the guiding piston may be fixed and aligned on the working piston by a tension pin. 
     According to a preferred embodiment, the guiding section has a guiding bushing within which the guiding piston is movable. The guiding bushing may, according to one embodiment, be made at least partially from polyoxymethylene (POM), making the guiding piston slide easily in the guiding bushing. 
     For the guiding bushing not to slip out of the guiding section, according to a further embodiment, a scraper may be provided in the guiding section which secures the guiding bushing in the guiding section. The scraper is preferably provided at the edge of the guiding section, so that it is easily accessible and removable to replace the guiding bushing if required. Preferably, the scraper is at least partially made of a thermoplastic elastomer, whereby it exhibits improved abrasion properties and the guiding piston may easily slide along it. 
     Preferably, a pressure within a range of 30 bar-120 bar, preferably 70 bar-110 bar, and in particular within a range of 80 bar-100 bar, prevails in the already prestressed fluid. Thereby, excellent absorbing properties result from the prestressed fluid. 
     Preferably, the piston-cylinder unit has a piston bearing which guides the working piston. The cylinder is closed by the piston bearing and the cylinder bottom. For a particularly stable mounting of the piston bearing at the cylinder of the piston-cylinder unit, the piston bearing is screwed into the cylinder by means of a thread. Preferably, a valve is provided in the piston bearing through which the piston-cylinder unit may be filled with the fluid. 
     Preferably, at least one sensor unit or one sensor is installed at the road finishing machine which monitors a position of the pushing device. This is preferably a contactless sensor which detects the position of the docking assembly relative to the chassis and/or the position of the working piston and/or the position of the guiding piston to detect the docking of a truck. The sensor may be fixed, for example, directly to the chassis and/or to the pushing device, in particular, however, to the absorber unit. It would also be conceivable to provide, as an alternative or supplement to the sensor unit, a sensor within the absorber unit which is configured to detect the pressure acting on the fluid to determine, in relation to it, a position of the working piston and thus also the covered damping path. 
     According to a further embodiment of the disclosure, the road finishing machine provides a signal device which is operatively linked to the sensor to indicate an operator of the road finishing machine whether a truck has docked to the pushing device for material transfer. The signal device is preferably provided in the control panel of the road finishing machine and/or at an external control platform which is available as control panel for the operators at the rear at the screed of the road finishing machine. It is also possible to provide the signal device or an additional signal device at one or both outside mirrors of the road finishing machine to indicate the preceding truck driver the docking status. For example, the signal device may comprise a traffic light display which lights up in different colors depending on the docking status. It is also conceivable for the signal device to emit acoustic signals as an alternative or supplement to the optical display. This permits a secure docking or undocking of a truck for material transfer to or from the road finishing machine according to the disclosure. 
     A further embodiment of the disclosure provides that, depending on the position of the pushing device, in particular of the docking assembly relative to the chassis of the road finishing machine, maneuvering instructions may be displayed to the road finishing machine's driver and/or the truck driver by means of the signal device. For example, the signal device could include a separate optical and/or acoustic signal transmission by means of which it may indicate the road finishing machine's driver and/or the truck driver that the pushing device or the docking assembly is positioned obliquely relative to the chassis, meaning that the truck has docked to the road finishing machine obliquely. Consequently, the truck driver may perform a corresponding compensation maneuver with the truck to orient the truck exactly in the paving direction of the road finishing machine. This permits the paving material to be uniformly poured into the material bunker and the pushing device to be shifted parallel to the chassis during docking, so that docking forces may be optimally absorbed. 
     Another embodiment of the disclosure provides for the road finishing machine to be configured such that at least one paving parameter and/or at least one paving drive position automatically changes from a paving mode to a docking mode as soon as the truck has docked to the pushing device, and/or the pushing device, in particular the docking assembly, has assumed a predetermined docking position. For example, it is conceivable for the road finishing machine to interrupt its paving drive. On the other hand, it is also possible that only at least one paving speed and/or velocity is decelerated during the paving drive, and/or at least one linear drive travels from the paving position to a docking position when a truck has docked. It is also possible that the road finishing machine according to the disclosure automatically returns from the docking mode back to the normal paving mode if the signal device detects that the truck has undocked from the pushing device. This automatic adaptation of different parameters or driving positions of the road finishing machine during the docking operation takes care that an optimal paving result is achieved and no visible traces remain in or on the road pavement due to the docking procedure. 
     Embodiments of the disclosure will be illustrated with reference to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a road finishing machine with a pushing device for docking to a truck; 
         FIG. 2  shows an enlarged perspective representation of the pushing device for docking to a truck; 
         FIG. 3  shows an absorber unit according to the disclosure in an extended position; 
         FIG. 4  shows an absorber unit according to the disclosure in a retracted position; 
         FIG. 5  shows an embodiment of the absorber unit according to the disclosure; 
         FIG. 6  shows a working piston of the absorber unit according to the disclosure; and 
         FIG. 7  shows a characteristic force-displacement graph of the absorber unit according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Detailed embodiments according to the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples, and that other embodiments may take various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
       FIG. 1  shows a road finishing machine  1  with a screed  2  and a material bunker  3  provided in the front, seen in the paving direction F, for receiving paving material. A pushing device  4  in accordance with the disclosure is provided in front of the material bunker  3 . The pushing device  4  is mounted at the chassis  5  of the road finishing machine  1 . In particular, the pushing device  4  is fixed to a transverse member  6  of the chassis  5 . 
       FIG. 2  shows the pushing device  4  separate from the road finishing machine  1  in an enlarged perspective representation. The pushing device  4  has a docking assembly  7  and first and second absorber units  8 . The two absorber units  8  are fixed to the transverse member  6  of the chassis  5 . The docking assembly  7  is mounted to be movable relative to the transverse member  6  of the chassis  5 . The docking assembly  7  may be moved towards the transverse member  6  and away from it. If a load L is acting on the docking assembly  7  from the front on one side, the assembly may also be inclined with respect to the transverse member  6 , as is shown in  FIG. 2 . 
     According to  FIG. 2 , the docking assembly  7  is embodied as transverse beam which supports two rotary pushing rollers  9 . The pushing rollers  9  may dock at rear wheels of a truck which pours paving material into the material bunker  3  of the road finishing machine  1 . Thanks to the pushing device according to the disclosure, such material transfer is possible during a paving drive without any paving defects occurring in the paving section. 
       FIG. 2  also schematically shows a sensor unit  38  which is operatively linked to a signal device. The sensor unit  38  is configured to detect a position of the pushing device  4  and forward it to the signal device  39  from which a docking position and/or docking maneuvering instructions may be displayed to the operator of the road finishing machine and/or the truck. 
       FIG. 3  shows the assembly of the absorber unit  8  according to the disclosure in an extended position. The absorber unit  8  is in  FIG. 3  fixed to the transverse member  6  of the chassis  5  and includes a piston-cylinder unit  10 . The piston-cylinder unit  10  is a closed differential cylinder with a cylinder  11  and a working piston  12  mounted in a prestressed manner in the cylinder  11 . In the cylinder  11 , a pre-compressed fluid  13  is provided which prestresses the working piston  12  in the extended position. The prestressed fluid  13  pushes the working piston  12  against a piston bearing  14 . The piston bearing  14  is screwed into the cylinder  11  and closes the cylinder  11  to the outside. 
     A valve  15  is provided in the piston bearing  14 . Through the valve  15 , the cylinder  11  of the piston-cylinder unit  10  may be filled with the fluid  13 . By filling in the fluid  13 , a predetermined pressure may be applied to it, so that it is present in the cylinder  11  in a prestressed or compressed state and thus stores a compression force by which the working piston  12  may be held in the extended position. 
     The working piston  12  has a piston rod  16  and a piston plate  17 . The piston rod  16  is guided in the piston bearing  14  and seals the piston-cylinder unit  10  together with the piston bearing  14 . The piston plate  17  has a slightly smaller diameter than the inner diameter of the cylinder  11 , a plate seal  18  being provided at the periphery of the piston plate  17 . The plate seal  18  prevents the prestressed fluid  13  from flowing, during a movement of the working piston  12 , into the cylinder  11  or out of it externally past the piston plate  17 . The piston-cylinder unit  10  moreover has a mounting flange  19  which is provided for fixing the piston-cylinder unit  10  at the transverse member  6  of the chassis  5 . 
     The docking assembly  7  is fixed to the piston rod  16  of the working piston  12 . The distance between the docking assembly  7  and the mounting flange  19  represents a damping path X.  FIG. 3  shows that the docking assembly is positioned in an extended position in which the working piston  12  is pressed against the piston bearing  14  by the prestressed fluid  13 . If the working piston  12  is pressed into the cylinder  11  by a force L applied externally at the docking assembly  7 , the docking assembly  7  will move towards the mounting flange  19 . Simultaneously, the compression force present in the cylinder  11  due to the prestressed fluid  13  will increase because the volume of the working piston  12  in the cylinder  11  will increase. 
       FIG. 3  also shows that two flow channels  21  are provided in the piston plate  17 . When the working piston  12  is pressed into the cylinder  11 , the fluid  13  will be forced to flow through the flow channels  21 . In the process, the fluid will be displaced from the first chamber right of the piston plate  17  into the second chamber left of the piston plate  17 . When the working piston  12  is returned, the fluid  13  will flow back into the first chamber. 
       FIG. 4  shows the absorber unit  8  according to the disclosure in a retracted position, wherein the docking assembly  7  is pushed against the mounting flange  19 . In  FIG. 4 , the working piston  12  has passed the complete damping path X, whereby the docking assembly  7  is lying against the mounting flange  9 . Here, the working piston  12  is inserted in the cylinder  11  corresponding to the length of the damping path X. The compression force onto the fluid  13  prestressed in the cylinder  11  has thereby increased, whereby additional energy is stored in the fluid  13 . 
     In  FIG. 4 , the load L applied from outside is higher than the force F originally applied by the prestressed fluid  13 . Thereby, the working piston  12  is pushed into the cylinder  11 . The working piston  12  is completely returned to its original position according to  FIG. 3  if the force L acting from outside falls below the compression force F originally applied by the fluid  13 . The working piston  12  will then travel back to the extended position, as is shown in  FIG. 3 . 
       FIG. 5  shows a particular embodiment of the absorber unit  8  according to the disclosure. The absorber unit  8  has a guiding section  22 . The guiding section  22  forms an extension of the cylinder  11 . The guiding section  22  has a guiding piston  23  which is screwed onto the working piston  12 . The guiding piston  23  has a mounting piece  24  which may be coupled to the docking assembly  7 . The guiding section  22  furthermore has a cylinder section  25  which is an extension of the cylinder  11  and receives the guiding piston  23  so that the latter is movable within it. In the cylinder section  25 , a guiding bushing  26  is provided. The guiding bushing  26  lines the cylinder section  25  and takes care of a uniform sliding motion of the guiding piston  23 . 
     Furthermore, a vent hole  27  is provided in the guiding section  22  which is formed at the bottom of the guiding section  22  in the cylinder section  25 . As an alternative or in addition, the vent hole  27  could also be formed in the guiding piston  23 . According to  FIG. 5 , the guiding bushing  26  is secured in the guiding section  22  by a scraper  28 . The scraper  28  takes care that the guiding bushing  26  does not slip out of the guiding section  22  during a movement of the working piston  22 . The scraper  28  may be designed as a shaped ring and engage in a mounting of the cylinder section  25 . 
       FIG. 5  furthermore shows that the mounting flange  19  is welded to the guiding section  22 . By means of several screw connections, the mounting flange  19  is fixed to the transverse member  6 . The mounting flange  19  is made of a rectangular sheet preferably having the following dimensions: 173 mm×160 mm×30 mm. In the mounting flange  19 , 4 bores are preferably provided. These preferably have a diameter of 18 mm and a hole circle diameter of 110 mm. Preferably, the mounting flange  19  is attached to the transverse member  6  by means of hexagon bolts  29 . 
     For assembling the guiding piston  23 , a mounting bore  30  is provided in the piston rod  16  of the working piston  12 , which, in the extended position of the working piston  12 , is aligned with a tapped bore  31  which is provided in the guiding section  22 . During the assembly of the guiding piston  23 , a bolt is pushed through the tapped bore  31  and the mounting bore  30 , so that the working piston  12  is fixed relative to the guiding section  22 . The guiding piston  23  may then be firmly screwed onto the piston rod  16  of the working piston  12 . After screwing and securing, the bolt is removed again. The tapped bore  31  is covered by the transverse member  6  and therefore protected from soiling. 
     In the piston plate  17  of the working piston  12 , two one-way valves  32  are provided. The latter may be transferred to an open position when the working piston  12  pushes into the cylinder  11 . With a restoring movement of the working piston  12 , they will then close automatically, so that the fluid  13  may only flow back through the flow channels  21 . This permits to provide only little resistance against the working piston  12  when it is retracted into the cylinder  11 , where higher resistance will act against the restoring movement of the working piston  12 . Hence, the working piston  12  is slowly moved back into its original position in the extended position. 
     With reference to  FIG. 5 , the piston rod  16  of the working piston  12  preferably has a diameter of 30 mm. The damping path X is preferably 80 mm. The initial force onto the working piston  12  applied by the prestressed fluid  13  is preferably within a range of between 200 N and 800 N, but in particular 300 N or 700 N. The blocking force, i.e., the maximum compression force by the fluid  13  which is reached when the working piston  12  has covered the complete damping path X, i.e., when it is completely retracted, is preferably within a range of between 7500 N and 8500 N, in particular, however, 8000 N or 8300 N. 
     The diameter of the guiding piston  23  is preferably 90 mm. The guiding piston  23  is screwed onto the piston rod  16  of the working piston  12 . By means of a tension pin  33 , the guiding piston  23  is secured on the piston rod  16 . The guiding bushing  26  preferably has dimensions of 90 mm×98 mm×118 mm. 
     In operation, when a load L from outside acts on the docking assembly  7 , the guiding piston  23  retracts into the piston-cylinder unit  10  together with the working piston  12 , the working piston  12  immersing into the prestressed fluid  13 . The prestressed fluid  13  originally exerts a pressure within a range of 30 bar to 110 bar onto the working piston  12 , where it in particular applies a pressure of 43 bar (300 N) or 99 bar (700 N). 
     Caused by the one-way valves  32  in the piston plate  17 , the working piston  12  retracts with relatively low resistance and compresses the prestressed fluid  13  across the spring trajectory by a certain bar value until the blocking force is reached. When the load L acting from outside is removed, the one-way valves  32  will be closed and the working piston  12  together with the guiding piston  23  slowly returns to the extended position because the fluid  13  is now, while it is returning, only forced through the smaller flow channels  21 . The returning speed depends on the diameter of the flow channels  21  which may be of different sizes or be variable, depending on the embodiment. 
     According to a particular embodiment, a proximity sensor  34  may be fixed in the above-mentioned tapped bore  31 , which is used for assembling the guiding piston  23 , the proximity sensor being embodied for detecting the docking of a load to the docking assembly  7  by detecting an insertion of the guiding piston  23  into the guiding section  22 . The proximity sensor  34  may be operatively linked to a non-depicted signal device of the road finishing machine  1  by which the docking of a truck to the pushing device  4  according to the disclosure may be displayed to a driver of the road finishing machine  1  and/or a driver of the docked truck. 
       FIG. 6  shows the working piston  12  with the piston rod  16  and the piston plate  17 . In the piston plate  17 , the one-way valves  32  are designed as ball valves. Within the one-way valves  32 , the flow channels  21  are provided. The piston plate  17  comprises the plate seal  18  along its circumference. The piston rod  16  has a first threaded section  35  onto which the guiding piston  23  may be screwed, and a second threaded section  36  provided at the opposed end. A screw nut  37  fixes the piston plate  17  on the threaded section  36 .  FIG. 6  also shows the mounting bore  30  for fixing the working piston  12  during the assembly of the guiding piston  23 . 
       FIG. 7  shows a force-displacement graph for an absorber unit according to the disclosure. An initial force of 300 N acting on the working piston  12  in the extended position and a blocking force of 8000 N acting on the working piston  12  in the retracted position are characteristic when the working piston  12  has covered the complete damping path X. According to  FIG. 7 , the damping path X is 80 mm. Here, a spring rate of 962.5 N/mm results. 
     By the pushing device according to the disclosure, high loads may be excellently cushioned, while an energy increase may be easily stored and released again in a controlled manner by it. With the pushing device according to the disclosure, it is possible to slow down trucks docking to it without any accelerations being transmitted to the road finishing machine by the docking operation. After the material transfer, the truck may be easily undocked, whereby the pushing device according to the disclosure returns to its original position. 
     According to the disclosure, the pushing device  4  may just as well be provided at a charger vehicle which docks to a truck for material transfer. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.