Abstract:
A device recovers energy in working machines with at least one power drive actuated to move a load mass back and forth and with an energy storage system ( 16 ) absorbing the energy released in the movement of the load mass in one direction and making it available for a subsequent movement in the other direction. The energy storage system includes an accumulator cylinder ( 16 ) mechanically coupled to the load mass and storing pneumatic pressure energy for movement in one direction. For movement in the other direction, the accumulator cylinder acts as an auxiliary working cylinder supporting the power drive and converting the stored pressure energy into driving force.

Description:
FIELD OF THE INVENTION 
     The invention relates to a device for recovering energy in working machines. The device has at least one power drive actuatable to move a load mass back and forth, and an energy storage system absorbing the energy released in the movement of the load mass in one direction and permitting a subsequent movement in the other direction. 
     BACKGROUND OF THE INVENTION 
     Devices of this type for recovery of potential energy in working machines are prior art; see, for example, WO 93/11363 or EP 0 789 816 B1. As energy storage systems, such devices have pressure accumulators storing the released potential energy as pressure energy of a working gas. It is crucial for the efficiency of these devices that the lowest possible energy losses occur in operation. The losses involve primarily losses of thermal energy of the accumulator gas. Generally, a large part of the thermal energy forming when the working gas is compressed is released via the outer walls of the hydraulic accumulator used in the prior art as an energy storage system. The large-area contact region between the working gas and the exterior can lead to considerable heat losses for the relatively large surface of the accumulator housing (preferably of steel) under consideration. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved energy recovery device characterized by a greatly improved energy balance with an especially simple and money-saving design. 
     According to the invention, this object is basically achieved by a device comprising an energy storage system in the form of an accumulator cylinder. The accumulator cylinder is mechanically coupled to the load mass and stores pneumatic pressure energy for movement in one direction. For movement in the other direction, the accumulator cylinder acts as an auxiliary working cylinder supporting the power drive and converting the stored pressure energy into driving force. 
     Preferred, the accumulator cylinder as the auxiliary working cylinder is coupled to a load mass to be raised and lowered and stores potential energy released in lowering processes in the form of pneumatic pressure energy. 
     The use of an energy storage system in the form of an accumulator cylinder as a replacement of conventional hydraulic accumulators improves the energy balance in more than one respect. On the one hand, the direct mechanical coupling of the accumulator cylinder to the load mass allows the stored pressure energy to be converted directly into lifting force so that the accumulator cylinder acts as an additional power drive, and results in the elimination of the hydraulic system required in the prior art between the hydraulic accumulator and power drive. The associated energy losses, which otherwise occur, are then eliminated. Furthermore, an accumulator cylinder, when compared to a hydraulic accumulator, affords considerably more design options for reducing the direct heat loss of the working gas. 
     This direct heat loss can be reduced quite significantly and especially advantageously, when the piston rod of the accumulator cylinder is a hollow, end-side open part forming the piston whose cavity in the position fully retracted into the cylinder contains essentially the entire volume of the working gas. In this construction of the piston, generation of heat takes place when the piston rod is lowered within the piston, that is, in a region isolated from the cylinder wall by the wall of the hollow piston. The piston is dimensioned such that in its cavity it contains essentially the entire volume of the working gas. When the piston is fully retracted in this operating state corresponding to the strongest compression, and to the greatest generation of heat, the piston wall extends over the entire length of the cylinder so that it is double walled in this state of greatest generation of heat. Heat loss is thus minimized. 
     On the other hand, in this construction, as a result of the specific overall length of the piston, in the fully extended position its wall with a corresponding flat portion is outside the cylinder wall. In this fully extended position, the working gas has cooled in response to the expansion. At the same time, for this piston position the wall surface exposed to the exterior and formed from a cylinder surface and the exposed jacket surface of the piston, has a maximum value. Accordingly, the thermal resistance of the total wall area is minimal so that a relatively large amount of thermal energy is absorbed from the ambient air and is released to the cooled working gas. This construction results in an optimal energy balance. 
     Not only does the double wall arrangement present in certain sections contribute to optimization of the thermal energy balance, but also of the working or operating medium enclosed in the double wall, for example, in the form of a working gas and/or in the form of hydraulic oil. 
     The accumulator cylinder can be formed in the shape of a cup having closed bottom with a filler port for the working gas, such as N 2 . 
     In especially advantageous exemplary embodiments, on the open end of the accumulator cylinder, opposite the bottom, a guide is formed to guide the outside of the piston at a distance from the inner wall of the cup, which distance forms an oil gap. 
     Preferably, on the open end of the piston, a second guide is formed guiding the end of the piston while maintaining the oil gap. In this way, the piston can be guided without problems. 
     In particular, together with an oil charge located in the oil gap, a high pressure sealing system can be formed working reliably in long-term operation even in applications with high pressures, for example, of more than 100 bar. 
     To accommodate the oil that is displaced when the piston is extended and with the resulting reduction of the length of the oil gap and to make it available again upon retraction, a hydraulic accumulator can be connected to the oil gap. The accumulator then compensates for changes of the volume of the oil gap when the piston moves. 
     In especially advantageous exemplary embodiments, the accumulator cylinder is used as an auxiliary working cylinder mechanically shunted to a hydraulic working cylinder which can be actuated by the hydraulic system and which is used as a power drive. This structure enables an especially simple construction, especially for hoists, crane booms, and the like, where hydraulic cylinders are provided as a power drive acting directly on the load mass. 
     Since the prior art recovered energy is available in the form of hydraulic pressure energy from a hydraulic accumulator, the recovered energy can be used only for hydraulic power drives such as working cylinders or hydraulic motors. In contrast, the invention can be used in conjunction with any power drives which need not be able to be hydraulically actuated, for example, in spindle drives, cable pulls, or the like, which are activated by an electric motor and which are provided for the lifting of loads. 
     Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings which form a part of this disclosure: 
         FIG. 1  is a schematically simplified, side elevational view of a crane boom, provided with one exemplary embodiment of the device according to the invention for recovering potential energy; 
         FIG. 2  is a side elevational view, symbolically representing an accumulator cylinder in mechanical shunting to a working cylinder for explaining the operating principle of the invention; 
         FIG. 3  is a schematically simplified, side elevational view in section of an accumulator cylinder according to a first exemplary embodiment of the invention; 
         FIG. 4  is a schematically simplified, side elevational view in section of an accumulator cylinder a second exemplary embodiment of the invention; 
         FIG. 5  is a schematically simplified, side elevational view in section of an accumulator cylinder according to a third exemplary embodiment of the invention; and 
         FIG. 6  is a schematically simplified, side elevational view in section of an accumulator cylinder according to a fourth exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is explained below using exemplary embodiments in which a crane boom  2  forms a load mass  4  ( FIG. 2 ). The boom  2  can be raised by a power drive in the form of a hydraulic working cylinder  6 . More specifically, boom  2  can be pivoted around a coupling point  8 . The working cylinder  6  is a hydraulic cylinder which can be actuated by a hydraulic system  10  symbolically represented only in  FIG. 2 . The hydraulic system  10  is only shown with a control valve arrangement  12  and a hydraulic pump  14  in  FIG. 2 , can be, in particular, of a design that is conventional for working machines, so that it need not be described in detail. 
     An accumulator cylinder  16  is mechanically shunted to the working cylinder  6  forming the power drive. Specifically the piston rod  18  of the accumulator cylinder  16 , like the piston rod  20  of the working cylinder  6 , acts directly on the load mass  4  (boom  2 ). 
       FIG. 3 , in a separate representation, shows details of the accumulator cylinder  16 . As is apparent, the accumulator cylinder has the shape of a cup  22  with a closed bottom  24 . Bottom  24  has a filler port, (not shown) for a working gas, in this example N 2 . In the illustrated example, the end of the piston rod  18  forms the piston  26  in the form of a hollow body with an inner cavity  30  opening on the piston end  28 . In the fully retracted position of the piston  26 , when the piston end  28  is on the bottom  24  of the cup  22 , piston  26  contains the entire volume of working gas.  FIG. 3  shows the piston  26  more or less in the middle position in which the gas volume is composed of the inner space of the cup  22  free of the piston  26  and the cavity  30  of the piston  26 . 
     The piston  26  is guided on the inner wall of the cup  22  of the accumulator cylinder  16  such that there is an oil gap  32  on the outside of the piston  26 . For this purpose, a guide  36  for the piston  26  is on the open end  34  of the cup  22 . On the open piston end  28 , a second guide  38  is provided. Both guides  36 ,  38  ensure preservation of the oil gap  32  during piston movements. They are additionally each provided with a seal arrangement  40  so that together with oil filling of the oil gap  32  not only piston lubrication, but also a high pressure sealing system are formed. To compensate for the volume of the oil gap  32 , which varies during piston movements, a hydraulic accumulator  42  is connected to the oil gap  32  and accommodates the oil displaced when the piston  26  is extended and releases it again when the piston  26  is retracted. 
     As mentioned, in  FIG. 3  the piston  26  is in a middle position at which the load mass  4  is partially lowered. If the load mass  4  is completely lowered, the piston  26  moves in the direction of the bottom  24  of the cup  22  so that the piston end  28  in the end position of the lowering motion approaches the bottom  24 . When the piston  26  is retracted, the working gas is compressed to a volume corresponding to the volume of the cavity  30  of the piston  26  in the fully retracted position. In this way, the potential energy of the load mass  4  released during lowering is converted into pressure energy in the accumulator cylinder  16 . The fully retracted position of the piston  26  corresponds to the state of strongest compression and thus to the maximum heating of the working gas. At the same time, in the invention in this operating state, the heated working gas is enclosed double walled, because the piston wall  44  in this position extends over the entire length of the cup  22  along the cup wall  46 . In addition, the medium collected in the oil gap  32  and extending essentially over the entire length of the cup  22  forms an additional insulating layer between the cup wall  46  and piston wall  44 . 
     In the state of maximum heating, the accumulator cylinder  26  is thus at the same time in the state of best heat insulation. On the other hand, in the fully extended position of the piston  26 , that is, a state in which as a result of expansion the working gas is in the most heavily cooled state, the piston  26  with almost the entire length of its piston wall  44  is outside the cup  22 . Specifically, during the “supercooled” operating state, the accumulator cylinder  16  exhibits the highest value of the wall surface exposed to the exterior. The essentially entire surface of the cup wall  46  and the piston wall  44  is exposed so that a relatively large amount of heat can be absorbed from the ambient air. Therefore, the energy balance is good overall due to the low heat release for the “superheated” state and the high heat absorption for the “supercooled” state of the working gas in the invention. 
       FIG. 4  shows a second exemplary embodiment where there is no external hydraulic accumulator connected at the oil gap  32 . Instead, the oil gap  32  does not contain a complete oil charge, but is divided into an oil side  62  containing an oil charge and a gas side  64  filled with nitrogen by a floating, that is, axially movable seal  60 . In the movements of the piston  26 , the oil gap thus forms a type of miniaturized hydraulic accumulator. 
       FIG. 5  shows a further modified example according to a third exemplary embodiment, in which, with the hydraulic accumulator  42  connected to the oil gap  32 , the accumulator&#39;s gas side is connected to the interior of the piston  26  via a charging line  66 . The filling pressure of the accumulator  42  is then automatically held at the pressure level of the working cylinder  16 . Pressure limitation and/or check valves (not shown) can be provided in the charging line  66  to dictate the filling pressure of the hydraulic accumulator  42  or convey it in one direction, if so desired. In a modification of this solution, line  66  can be advantageously connected to the bottom  24  of the accumulator cylinder  16 , and not in the region of the upper, head-side cover of the piston rod  18 , to provide a direct fluid-carrying connection between the interior of the working cylinder  16  and the accumulator  42 , specifically, on the side of the accumulator  42  opposite the outlet site of the line leading to the space  32 . 
       FIG. 6  shows a fourth version in which the interior of the accumulator cylinder  6  is connected to a supply source  70  for working gas via a supply line  68 . Moreover, to further improve heat insulation, the inner cavity  30  of the piston  26  is completely filled with a large-pore foam material  72  which can partially also accommodate the working gas. 
     In the highly schematically simplified representations of  FIGS. 3 to 6 , which illustrate only the operating principle, design details have been omitted. For example, a divided configuration of the open end  34  of the cup  22  enabling installation of the piston  26  or connections for delivery of the media into the oil gap  32  is not shown. 
     While various embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.