Patent Publication Number: US-9422677-B2

Title: Self-propelled construction machine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a self-propelled construction machine, in particular road milling machine, recycler or stabiliser, with a machine frame and an operating mechanism, wherein the operating mechanism comprises an operating drum and a drum housing surrounding the operating drum. 
     2. Description of the Prior Art 
     With the known road milling machines, the road surface can be milled true to contour and evenly. The known road milling machines have a milling mechanism, which comprises a milling drum for milling off the material. With a rear loader road milling machine, the milled material is fed to the following truck over the rear of the milling machine. 
     The so-called stabilisers or recyclers should be differentiated from road milling machines; by addition of binding agents to unstable ground, for example loose soil (stabiliser) or a damaged roadway (recycler), these produce a stable base that is suitable for later overlaying with a roadway. 
     Road milling machines and stabilisers or recyclers have in common an operating mechanism with an operating drum and a drum housing surrounding the operating drum, that is closed by at least one sealing element, that is also described as stripping element, located behind the operating drum when seen in the operating direction. Apart from the rear sealing element, road milling machines have further a sealing element, also described as hold-down device, located in front of the milling drum. In addition to the hold-down device and the stripper, road milling machines comprise a right and left edge protector extending in the operating direction, which seal the drum housing laterally. 
     In practice, all sealing elements basically confront the problem that the sealing element can strike obstacles in uneven areas. The sealing element must therefore be adjustable in height. This problem is encountered particularly with front and rear sealing elements extending transverse to the operating direction. The problem of tilting is encountered, especially with the rear sealing element, since the rear sealing element is employed in the operating direction and is generally fitted with disc-shaped hard metal elements at the lower edge. This problem is made still worse with the rear sealing element by the fact that this sealing element is generally subjected to a pressure. 
     The road milling machine with a front hold-down device and a rear stripper element is known, for example, from EP 2 050 875 A2. The road milling machine has a tracking member for adjusting the height of the hold-down device, with which the hold-down device is connected by means of a control lever to an articulated joint so that, when the tracking member strikes an obstacle, the tracking member is raised. 
     U.S. Pat. No. 4,723,867A describes a road milling machine, the drum housing of which has a front and a rear sealing element. Both sealing elements can be adjusted in height, so that the milling drum is accessible. 
     SUMMARY OF THE INVENTION 
     The invention has the object of creating a self-propelled construction machine, with which the drum housing is sealed at the front and/or rear and/or laterally in the operating direction without risk of a blockage due to a sealing element striking an obstacle. 
     According to the invention, this object is achieved with the features of the independent claims. The dependent claims relate to preferred embodiments of the invention. 
     The invention relates to a self-propelled construction machine, in particular a road milling machine, stabiliser or recycler, that has at least one front and/or at least one rear sealing element and/or at least one lateral sealing element. 
     The sealing element is any element with which the drum housing is closed off to the ground. However, this does not mean that the drum housing is tightly sealed. A mechanism for raising and lowering is associated with the respective sealing element, or the respective sealing elements, with which the sealing element or the sealing elements rest on the ground with a predetermined force or are pressed on the ground. Where a construction machine has a front and a rear sealing element, two mechanisms are provided, for example, for raising and lowering. 
     It is irrelevant to the principle of the function of the invention, how the mechanism for raising and lowering the at least one sealing element is provided, as long as the sealing element rests on the ground with a predetermined force, if this sealing element is not raised. The contact force of the sealing element can be the force due to the weight of the sealing element. However, the sealing element can be pressed on the ground with a contact force which is greater than the force due to the weight of the sealing element. 
     The construction machine in accordance with the invention is characterised in that the mechanism for raising and lowering the sealing element has a measuring unit, which is configured so that the measuring unit measures the force acting on the sealing element when the measuring unit comes into contact with an obstacle. Furthermore, the mechanism comprises a control unit which is configured so that the control unit generates a control signal for raising the sealing element. When the force measured by the measuring unit is greater than a predetermined limit value, that the sealing element is raised. 
     The force measured with the measuring unit is preferably the essentially horizontal force component acting on the sealing element when it strikes an obstacle. However, it is also possible that the measured force has a vertical component. It is also not necessary to determine the absolute force. It is sufficient for the force to be measured quantitatively. The force also need not be directly measured as its actual physical unit but, converted by means of any desired physical principles, can be measured as another physical unit, such as pressure, distance or the like if it is simpler to record these physical variables. 
     The advantage of the sealing element in accordance with the invention is that obstacles in the operating direction of the construction machine are detected where the force acting on the sealing element exceeds a limit value. When this is the case, the sealing element is automatically raised. The sealing element is only raised until the measured force is again below the limit value. In this case, it is assumed that the obstacle has been negotiated and it is possible to return to the original operating state or a preselected other operating state. The limit value for the measured force should be calculated so that the sealing element is not raised where forces are very small. When the sealing element is raised, the sealing element can remain in the raised position. For example, the sealing element remains in the raised position when the obstacle is a step. However, the sealing element can be lowered again if the obstacle is not a step. If, before raising, the mechanism for raising and lowering the sealing element has found the so-called floating state in the operating mode in which the sealing element is held on the ground with a predetermined bearing force, the mechanism for raising and lowering the sealing element can return to the floating state, for example, when the measured force is below the predetermined limit value again. Then the sealing element can automatically move downwards when the height of the terrain decreases, i.e. the sealing element can follow the contour of the terrain again. However, the mechanism for raising and lowering the sealing element can also switch to a preselected alternative operating state when the measured force is below the predetermined limit value again, e.g. to an operating state in which the sealing element is lowered with the assistance of a restoring force. 
     In a preferred embodiment, the control unit generates a second control signal if the force is lower than the predetermined limit value, so that the mechanism for raising and lowering switches to an operating state in which the sealing element maintains a position or can be lowered. The lowering of the sealing element can take place solely under the action of gravity or can be assisted by the mechanism for raising and lowering sealing element with an additional restoring force. The decisive factor is that the sealing element rests on the ground again with the predetermined contact force. 
     In a preferred embodiment, the mechanism for raising and lowering the sealing element comprises one or more piston/cylinder arrangements where their cylinders have an articulated connection to the machine frame and their pistons have an articulated connection to the sealing element or their cylinders have an articulated connection to the sealing element and their pistons have an articulated connection to the machine frame. The piston/cylinder arrangement can be operated hydraulically or pneumatically. However, an electric motor drive is also possible. The sub-assemblies required for this purpose are state of the art. 
     The automatic raising and lowering of the sealing element relieves the machine driver of a task. Furthermore, the stability of the machine is improved and it is able to move forward at a constant rate without there being a risk that the machine will be damaged by obstacles. In addition, wear on the sealing element is reduced. Control of the sealing element in accordance with the invention can always be switched off during operation of the construction machine or also by the machine driver, so that raising and lowering of the sealing element can be controlled manually. 
     A further preferred embodiment of the invention provides an impact element, in particular on the front or rear sealing element, which extends downwards beyond the lower edge of the sealing element. The impact element is preferably a plate-like element, which extends over the width of the sealing element. However, it is also possible for the impact element to extend only over part of the width of the sealing element. 
     In a particularly preferred embodiment, particularly of the front or rear sealing element, an upper part of the impact element is fastened to the sealing element under a resilient preload so that, on impact with an obstacle, the impact element is displaced from a first position in which the upper part of the impact element abuts the sealing element, to a second position in which the upper part of the impact element is spaced from the sealing element. The impact element can either be guided linearly or can be fastened so as to pivot on the sealing element. It is preferred that the impact element alters its position so that the impact on an obstacle can be detected. 
     In a further, particularly preferred embodiment, the upper part of the impact element can be displaced on an axis which is perpendicular to the plane of the sealing element. However, the impact element can also be guided on an axis that is oblique to the plane of the sealing element. 
     The impact element can be guided on the sealing element by at least one guide pin, which extends through a bore in the impact element. Preferably, several guide pins are provided, spaced over the width of the sealing element. The guide pin preferably has a screw thread and the impact element is preferably screwed on with a screw, whereby a spring is interposed between the screw and the impact element, so that the impact element is spring-loaded against the sealing element. The spring load can be adjusted by tightening and untightening the screw. 
     The measuring unit has at least one sensor detecting the position of the impact element, preferably a distance sensor, with which the deflection or displacement of the impact element can be detected when it strikes an obstacle. In the simplest case, the distance sensor can be a contact switch that is actuated by the impact element. 
     In a further preferred embodiment of the invention, the drum housing is closed off by two sealing elements located behind the operating drum when seen in the operating direction of the construction machine, wherein two mechanisms are provided for raising and lowering the sealing elements, so that the two sealing elements is can be raised independently of one other on impact with an obstacle. In this embodiment, the sealing elements each extend over half of the operating width of the operating drum. An impact element is again associated with each sealing element to detect the impact force on an obstacle. 
     The embodiment with two sealing and two impact elements has the advantage that only one of the two sealing elements is raised when an obstacle is encountered, so that the one on the other side of the drum housing remains closed. This is a particular advantage when there is a risk of the impact element striking an obstacle on the outside or inside of the bend, when a turning operation is performed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, an example of an embodiment of the invention is explained in detail with reference to the drawings. 
       These show: 
         FIG. 1  a self-propelled construction machine in accordance with the invention in a perspective view, 
         FIG. 2  a simplified schematic representation of the drum housing surrounding the operating drum of the construction machine, together with the machine frame, wherein a sealing element closing off the drum housing in front of the operating drum, when seen in the operating direction, is in a first operating position, 
         FIG. 3  the drum housing, wherein the front sealing element is in a second operating position, 
         FIG. 4  the drum housing, wherein the front sealing element is in a third operating position, 
         FIG. 5  a schematic representation of the drum housing, together with the operating drum, wherein the front sealing element is in a raised position, 
         FIG. 6  a schematic representation of the drum housing, together with the operating drum, wherein the sealing element is in a lowered position, 
         FIG. 7  a section through a guide element and a mounting element of the guide of the front sealing element, 
         FIG. 8  the device for raising and lowering the front sealing element in schematic representation, 
         FIG. 9A  a schematic representation of a sealing element sealing the drum housing behind the operating drum in the operating direction and a device for raising and lowering the rear sealing element, wherein the sealing element rests on the ground, 
         FIG. 9B  a schematic representation of the rear sealing element, wherein the sealing element strikes an obstacle, 
         FIG. 9C  a schematic representation of the rear sealing element, wherein the sealing element is raised, 
         FIG. 10A  a schematic representation of a sealing element closing off the drum housing laterally, wherein the sealing element rests on the ground, 
         FIG. 10B  a schematic representation of the lateral sealing element, wherein the sealing element strikes an obstacle, 
         FIG. 11  a simplified hydraulic circuit, which shows the hydraulic cylinder of the front or lateral sealing element, 
         FIG. 12  a simplified hydraulic circuit, which shows the hydraulic cylinder of the rear sealing element, 
         FIG. 13  the control system for the mechanisms for raising and lowering the front and rear sealing element, together with the lateral sealing elements, in very simplified representation. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows in perspective a road milling machine, as an example of a construction machine, specifically a rear loader road milling machine. The road milling machine comprises a machine frame  1 , which is supported by a chassis  2 . The chassis  2  has a front wheel  2 A and two rear wheels  2 B, when seen in the operating direction. The operator&#39;s platform  3  is in the rear part of the machine frame. The milling mechanism  4  of the road milling machine is underneath the operator&#39;s platform  3 . 
     The milling mechanism  4  comprises a milling drum  5 , with cutting tools  5 A spaced around its periphery. The milling drum  5  is positioned in a milling drum housing  7 A to rotate about an axis  6  mounted transverse to the operating direction of the milling machine. The milling drum  5  rotates in the milling drum housing  7 A in a predetermined direction of rotation D. In the present example, the milling drum  5  rotates in a counter-clockwise direction. The housing  7 A enclosing the milling drum  5  has a discharge opening at the rear, when seen in the operating direction. The milling drum housing is closed off by side plates  8  on the longitudinal sides. The transport arrangement  9  on the milling drum housing  7 A comprises a conveyor belt  10  for conveying the milled material, which can be received by a truck driven behind the milling machine. 
     In the following, the milling drum housing  7 A accommodating the milling drum  5  is described in detail with reference to  FIGS. 2 to 8 . 
     The milling drum housing  7 A is a fixed housing part  7 A that is permanently attached to the machine frame  1 . The fastening members for the milling drum housing  7 A are not shown in the Figures. In the Figures, the milling drum  5  is represented schematically by a cylindrical body that encloses the tips of the tools  5 A of the milling drum  5 . The milling drum housing  7 A extends beyond the width of the milling drum  5  on both sides. It encloses the milling drum  5  up to an aperture  11 A in front of the milling drum when seen in the operating direction and an aperture  11 B behind the milling drum when seen in the operating direction ( FIG. 5 ). The front aperture  11 A in the operating direction is closed by a sealing element, which is called hold-down device  7 B in the following. The rear aperture  11 B is closed by a rear sealing element located behind the milling drum when seen in the operating direction but not shown in  FIGS. 1 to 8 . This sealing element is also described as stripper.  FIGS. 1 to 8  also do not show the lateral sealing elements, which are known by the name of edge protector. 
     The height of the hold-down device  7 B can be adjusted according to the milling depth.  FIGS. 2 to 4  show how the milling drum penetrates into the material to be removed in the vertical direction. While the milling drum is penetrating into the material, the hold-down device  7 B is moved from a first position, shown in  FIG. 2 , in which the hold-down device  7 B is fully lowered, into a second position, in which the hold-down device is fully raised ( FIG. 4 ). The maximum milling depth is obtained in this position.  FIG. 3  shows a middle position of the hold-down device  7 B with a smaller milling depth. In the present embodiment, the closed milling drum housing  7 A along with the hold-down device  7 B completely surrounds the milling drum  5  over a circumferential angle of approximately 180°. 
       FIGS. 5 and 6  show a sectional view in which the hold-down device  7 B is in the raised position ( FIG. 5 ) and in the lowered position ( FIG. 6 ). The hold-down device  7 B closes the aperture pointing in the operating direction between the lower edge  27  of the hold-down device  7 B and the surface of the road pavement material  13  to be removed. 
     On both sides of the hold-down device  7 B, there is a guide rail  15 A,  15 B on the outer side, extending upwards over the periphery  15 A,  15 B. The guide rails  15 A and  15 B are guided in mounting elements  16 A and  16 B, which are fastened on the machine frame  1 . The fastening for the mounting elements is not shown in  FIG. 5  or  FIG. 6 . 
       FIG. 7  shows a section through the guide rails  15 A,  15 B and mounting elements  16 A,  16 B. The mounting elements  16 A,  16 B have a U-shaped cross-section, in which the guide rails  15 A,  15 B are longitudinally displaceable. Since the mounting elements  16 A,  16 B enclose the guide rails  15 A,  15 B, the guide rails are secured in the axial and radial directions. If the hold-down device  7 B is in the lowered position, the portions of the guide rails  15 A,  15 B extending upwards are supported on the milling drum housing  7 A. This allows larger forces to be absorbed. 
     At its lower edge  27 , the hold-down device  7 B has a sliding element  18 , extending along the lower edge, which can be a sliding bar. The hold-down device  7  slides with the sliding element  18  on the surface of the road surface cover  13 . In doing so, the hold-down device  7 B is supported on the road pavement, solely due to its weight. When the milling drum  5  penetrates into the road surface in a vertical direction, the hold-down device  7 B moves upwards in the guide. 
     The road milling machine has a mechanism  19  for raising and lowering the hold-down device  7 B, comprising a piston/cylinder  20 . The piston/cylinder arrangement  20  is operated by a hydraulic unit  21 , shown only in outline, which supplies a hydraulic fluid to the cylinder  20 A of the piston/cylinder arrangement  20  ( FIG. 8 ). 
     The cylinder  20 A of the piston/cylinder arrangement  20  is flexibly connected to the machine frame  1  and the piston  20 B is flexibly connected to the upper end of a U-shaped profile element  22 , which is fastened to the hold-down device  7 B. The hold-down device  7 B can be raised and lowered by admitting hydraulic fluid to the cylinder  20 A. 
     The mechanism  19  for raising and lowering the hold-down device  7 B further has a control unit  23  and a processing unit  24 , which are connected together by means of a data line  25 . The control unit  23 , which is connected to the hydraulic unit  21  by a control line  26 , controls the hydraulic unit, so that the piston/cylinder arrangement  20  keeps the hold-down device  7 B in contact with the ground with a predetermined downwards force. For example, the hydraulic unit  21  can release the piston in the cylinder, so that the hold-down device  7 B rests on the ground with its weight if the hold-down device  7 B is not raised when it strikes an obstacle. 
     The mechanism  19  for raising and lowering the hold-down device  7 B further comprises a measuring unit  26  for measuring the force exerted on the hold-down device  7 B on impact with an obstacle. Preferably, only the horizontal force component acting on the hold-down device is measured by the measuring unit  26 . 
     The processing unit  24  compares the impact force measured by the measuring unit  26  with a predetermined limit value. When the impact force is greater than the limit value, the control unit  23  generates a first control signal for the hydraulic unit  21  to raise the hold-down device  7 B, so that the hydraulic unit  21  actuates the piston  20 B of the piston/cylinder unit  20 . The hold-down device  7 B is raised by the piston/cylinder unit  20  until the measured impact force is again less than the predetermined limit value. When the impact force is smaller than the limit value, the control unit  23  generates a second control signal for the hydraulic unit  21 , with which the piston/cylinder arrangement  20  is actuated once more to lower the hold-down device  7 B again until the lower edge  27  the hold-down device  7 B again rests on the ground with the predetermined downwards force, or the hold-down device maintains its current position, for example, if the obstacle is a step. Alternatively, the piston/cylinder arrangement  20  can also release the hold-down device  7 B, so that the hold-down device moves downwards in the guide under its own weight or rests on the step under its weight. Since the force acting on the hold-down device is compared with a predetermined limit value, this completely prevents the height of the hold-down device being adjusted due to smaller impacts with the material to be milled off. 
     The measuring unit  26  has two sensors  26 A,  26 B, for measuring the impact force, positioned between the mounting elements  16 A,  16 B and the guide rails  15 A,  15 B, in the area in which the guide rails extend upwards beyond the hold-down device  7 B. The sensors  26 A,  26 B are connected to the processing unit  24  by signal lines  26 A′ and  26 B′. When an essentially horizontal force acts on the hold-down device, the ends of the guide rails exert a contact pressure on the ends of the mounting elements or a slight tilting movement within the existing clearance, which is measured by the two sensors  26 A,  26 B. The processing unit  24  processes the measurement signals of the two sensors. Either only one or the other measurement signal can be processed, or both measurement signals together. For example, the two measurement signals can be averaged. Suitable pressure sensors and the processing of the measurement signals are part of the state of the art. However, it is also possible for the sensors to be positioned, not between the mounting elements  16 A,  16 B and guide rails  15 A,  15 B, but on the outside of the mounting elements  16 A,  16 B, in order to detect the tilting movement of the mounting elements  16 B. 
     A skid  34  can also be provided on the hold-down device, to support the upwards movement and to introduce the force on impact with an obstacle, pushing the hold-down device upwards. 
     Apart from the mechanism  19  described above for raising and lowering the front sealing element, the milling machine also has a mechanism for raising and lowering of the rear sealing element or the lateral sealing elements, not shown in  FIGS. 1 to 8 , which has the same structure. 
     An alternative embodiment of the mechanism  19  for raising and lowering a sealing element is described in the following, with reference to  FIGS. 9A to 9C . The sealing element  7 A′ can be a stripper element of a milling machine, which closes off the milling drum housing behind the milling drum, when seen in the operating direction. However, the sealing element can also be a stripper element of a stabiliser or recycler, which closes off the mixing drum housing behind the mixing drum when seen in the operating direction. A stabiliser is known, for example, from EP 1 012 396 B1. 
     Parts which correspond to the embodiment of  FIGS. 1 to 8  are given the same reference symbols in the embodiment described with reference to  FIGS. 9A to 9C . 
     The sealing element  7 A′, which is described in the following as stripper element, is shown only in a very simplified representation in  FIGS. 9A to 9C , together with the piston/cylinder arrangement  20 . The mechanism  19  for raising and lowering the stripper element  7 A′ comprises the control unit  23 , the processing unit  24 , and the measuring unit  26 ′ as well as the hydraulic unit  21 , which are connected to one another by data and control lines  25 ,  26 . 
     In the embodiment of  FIG. 9A to 9C , a plate-like impact element  28  is fastened to the stripper element  7 A′, and can be a metal plate, which preferably extends over the whole width of the stripper element. 
     The impact element  28  has several spaced holes  29  in the upper part, through which guide pins  30  extend, at equal distances, each having an external thread  31 . The impact element  28  is bolted to the stripper element  7 A′ with nuts  33 , whereby compression springs  32  are positioned between the impact element  28  and the nuts  33 , so that the impact element  28  is pre-loaded against the stripper element  7 A′. The guide pins  30  with the nuts  33  and the springs  32  form a linear guide for the impact element  28 , so that the impact element  28  can deviate from the position shown in  FIG. 9A  on impact with an obstacle in a direction opposite to the operating direction A. 
       FIG. 9B  shows the instant in which the impact element  28  strikes an obstacle. On impact with the obstacle, the impact element  28  is displaced against the force of the compression springs  31 , so that a gap  35  is formed between the stripper element and the impact element. 
     The measuring unit  26 ′ has one or more sensors  26 A′, spaced apart from one other. The sensors  26 A′ are distance sensors, which detect if the impact element  28  is being moved backwards against the distance of travel A. In doing so, the force of the compression springs  32  determines the limit value of the force which must act during impact of the impact element with an obstacle in order to produce a control signal to raise the stripper element  7 A′. At the instance of impact, the control unit  23  generates a first control signal for the hydraulic unit  21 , which actuates the piston/cylinder arrangement  20 , so that the stripper element  7 A′ is raised immediately. 
       FIG. 9C  shows the position in which the lower edge of the stripper element  7 A′ is precisely at the height of the obstacle. At this instant, the compression springs  32  can force the impact element  28  against the stripper element  7 A′ again. 
     When the sensor or sensors  26 A′ detect again that the impact element  28  is in contact with the stripper element  7 A′ ( FIG. 9A ), the contact unit  22  generates a second control signal for the hydraulic unit  21 , so that the piston/cylinder arrangement  20  presses the stripper element  7 A′ on the ground with a predetermined force. 
     Several, preferably two, sub-assemblies described in  FIGS. 9A to 9C  can also be positioned next to one another over the whole width of the operating drum, which can be a milling or mixing drum. A sub-division of the stripper element  27  into several segments offers advantages, in particular when entering bends, when only one of the two segments has to be raised. 
     The sealing element that is automatically adjustable in height can also be one or both of the lateral sealing elements, which are described as edge protectors.  FIGS. 10A and 10B  show, in very simplified representation, the left or right edge protector, which extends in the operating direction. The edge protector  36  is a plate-like element, which is adjustable in height and is guided, slightly oscillating between two lateral stops  37 . In  FIGS. 10A and 10B , the lateral stops  37 , which touch the lateral guides  38  of the machine frame, are shown only in outline. 
     The mechanism for raising and lowering the edge detector has a piston/cylinder arrangement  39 , which is operated by the hydraulic unit, not shown in the Figures, to admit the hydraulic fluid to the cylinder  39 A of the piston/cylinder arrangement  39 . The cylinder  39 A of the piston/cylinder arrangement  39  has an articulated connection to the machine frame, not shown, and the piston  39 B has an articulated connection to the edge protector  36 . When hydraulic fluid is admitted to the cylinder  39 A, the edge protector can be raised and lowered. 
     The edge protector  36  is put under a spring preload in the operating direction A by a preloading device. The preloading device  40  comprises a guide  41  provided on the edge protector  36  and element  42  provided on the machine frame, whereby the element  42  provided on the machine frame is guided longitudinally with the guide  41  in or opposite to the operating direction. The edge protector  36  is preloaded in the operating direction with a compression spring  43 , which is supported with one end on the edge protector  36  and the other end on the element  42  provided on the machine frame. 
       FIG. 10A  shows the edge protector  36  in the preloaded initial position before impact with an obstacle. An essentially horizontal force F, which can have a frontal or lateral force component, is exerted on the edge protector on impact with an obstacle. The edge protector  36  then moves under spring preload opposite to the operating direction A so that the compression spring  43  is compressed ( FIG. 10B ). The edge protector  36  is thereby displaced by a certain distance. The displacement by the predetermined distance is detected by a sensor  44 , so that a control signal is generated for the hydraulic unit, which actuates the hydraulic cylinder  39  to raise the edge protector. The edge protector  36  is raised until the obstacle has been negotiated. When the obstacle has been negotiated, the edge protector is displaced to return to its initial position, due to the restoring force of the compression spring  43 , so that the edge protector is lowered again. Thus the restoring force of the compression spring  43  determines the impact force at which the edge protector is automatically raised. 
       FIG. 11  shows a simplified hydraulic circuit, which shows the hydraulic cylinder  45  for raising or lowering a hold-down device or edge protector (not shown). During the forward movement of the construction machine, the hold-down device or edge protector is in a floating position, so that the hold-down device or edge protector rests on the ground with a predetermined force. In the floating position, the hydraulic valve  46  of the hydraulic unit connects the upper and lower cylinder chamber  45 A and  45 B of the hydraulic cylinder  45  by means of the hydraulic lines  47 ,  48  connected to the cylinder ports, for raising and lowering the hold-down device or edge protector, with a hydraulic tank (not shown), so that the chambers are not subjected to the system pressure. The hydraulic valve  46  is a 4/3 directional control valve. For simplicity, the hydraulic lines leading to the valve are not shown in  FIG. 11 . Since no specific hydraulic force acts on the cylinder, the piston can be displaced in the cylinder, so that the hold-down device or edge protector moves downwards due to its weight. When pressure is the same in both cylinder chambers, this movement downwards can still be supported through an appropriate configuration of the active contact surfaces of the hydraulic cylinder, when both chambers are subjected to a pressure in the floating position that preferably does not correspond to the system pressure, however. By switching over the hydraulic valve  46 , one or the other hydraulic line  47 ,  48 , can be subjected to system pressure (pressure line) or can be connected to the tank (tank line) so that the piston moves upwards or downwards. The hydraulic valve  46  is actuated depending on the measured impact force by the control unit, which is not shown in  FIG. 11 . On impact with an obstacle, the control unit generates a first control signal to activate the hydraulic valve  46 , so that the lower cylinder chamber  45 B is connected to the pressure line and the upper cylinder chamber  54 A is connected to the tank line, raising the hold-down device or edge protector. If the measured force is less than the predetermined limit value, the control unit generates a second control signal, so that the hydraulic valve  46  is switched back to the floating position, which is shown in  FIG. 11 , so that the hold-down device or edge protector drops again. In an alternative embodiment, the hydraulic valve  46  connects the upper cylinder chamber  45 A to the pressure line and the lower cylinder chamber  45 B to the tank line, so that the hold-down device or edge protector is forced downward until the hold-down device or edge protection meets the ground. Only then does the control unit switch the hold-down device back to the floating position. 
       FIG. 12  shows the hydraulic circuit of a further embodiment of the hydraulic control system. This embodiment differs from the example embodiment according to  FIG. 11 , in that two cylinder-piston arrangements  49  and  50  are provided for raising and lowering the sealing element (not shown). A further difference lies in an additional hydraulic unit  51 , with which a defined applied force is exerted on the sealing element, greater than the weight of the sealing element but less than the maximum operating force of the respective piston-cylinder arrangement. This applied force, with which the sealing element is pressed on the ground, has proved to be advantageous, in particularly with a stripper, since the stripper should remain in contact with the ground, even when it is irregular. In an alternative embodiment, the upper cylinder chambers  49 A,  50 A of the two piston-cylinder arrangements  49 ,  50  are short-circuited by means of a first hydraulic line  51  and lower cylinder chambers  49 B,  50 B of the piston-cylinder arrangements  49 ,  50  by means of a second hydraulic line  52 . A third hydraulic line  53  leads from the first hydraulic line  51  and a fourth hydraulic line  54  leads from the second hydraulic line  52  to a hydraulic valve  55 . To raise the hold-down device (not shown), the third hydraulic line  53  is connected by a tank line (not shown) and the fourth hydraulic line  54  to a pressure line (not shown). For this purpose, the control unit (not shown) actuates the hydraulic valve  55 . During the forward movement of the construction machine, the ends of the third and fourth hydraulic lines  53 ,  54  are closed, and the hydraulic valve is in the position shown in  FIG. 12 . For this, the third hydraulic line  53  is connected to the pressure line  56  and the fourth hydraulic line  54  to the tank line  57  of the additional hydraulic unit  51 , so that the hold-down device is pressed on the ground with the predetermined contact force. Since the contact force will be less than the maximum operating force of the piston-cylinder arrangement, the pressure in the pressure line  56  is less than the system pressure with which the piston-cylinder arrangements are operated. When an obstacle is detected, the control unit (not shown) again generates a control signal to actuate hydraulic valve  55 , so that the hold-down device is released until the obstacle has been negotiated. 
       FIG. 13  shows, in greatly simplified schematic representation, an embodiment of the control system for an overload protection for a stripper  58 , a hold-down device  59  and an edge protector  60 . A measuring unit  58 A,  59 A,  60 A is assigned to each of the stripper, hold-down device or edge protector, each of which is connected by means of a signal line  61  to a central control and processing unit  65 . The control and processing unit  65  actuates the hydraulic valve associated with the stripper, hold-down device or edge protector by means of signal lines  62  as a function of the contact force measured by the respective measuring unit  58 A,  59 A,  60 A, with which the piston-cylinder arrangement (not shown in  FIG. 13 ) associated with the stripper, hold down device and edge protector is actuated. Furthermore, an operating unit  63  is provided, which is connected by means of data line  64  to the control and processing unit  65 . The machine driver can switch off the automatic overload protection with the operating unit  63  and can adjust the height of the sealing elements  58 ,  59 ,  60  manually.