Abstract:
The invention relates to a hydraulic group comprising a reservoir ( 28 ) for oil, in which a multiple piston pump ( 60 ) is arranged. The individual pumps ( 61 ) are cyclically driven by an eccentric ring ( 29 ). In order to ensure a permanent immersion of the individual pumps in the reservoir ( 28 ), an auxiliary reservoir ( 47 ) providing oil for refilling the reservoir ( 28 ) is arranged above the reservoir. The auxiliary reservoir ( 47 ) is connected to an additional tank ( 45 ) by means of a connection line ( 48 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a hydraulic unit comprising an oil-containing reservoir which includes a motor and a pump driven by said motor. 
         [0002]    A hydraulic unit comprises a pump acting as a pressure generator, said pump taking in oil from a reservoir and supplying said oil to a consumer. A return line extends from the consumer back to the reservoir. Hydraulic units are known which comprise a motor configured as a submersible motor arranged in the reservoir and submerged in the hydraulic fluid. The heat generated in the motor is discharged via the hydraulic oil. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is an object of the invention to provide a hydraulic unit which is capable of supplying defined variable volume flows without the risk of air aspiration. 
         [0004]    The hydraulic unit according to the present invention is defined in claim  1 . Said unit comprises a multiple piston pump composed of individual pumps arranged in a specific array, wherein the reservoir has connected therewith an auxiliary reservoir at least partly filled with oil for filling up the reservoir when oil has been withdrawn by the multiple piston pump, thus ensuring a complete submersion of the multiple piston pump. 
         [0005]    A multiple piston pump is a positive displacement pump supplying a continuous volume flow which can be varied by changing the rotational speed. If the motor is a synchronous motor, the magnitude of the volume flow is adapted to be varied in accordance with the load of the respective power consumer using a corresponding motor control. Use of a multiple piston pump, where the individual pumps are arranged in a specific array in the reservoir, involves the risk that, when the oil level decreases, the intake opening of at least one individual pump is temporarily not submerged and takes in air. This must be prevented in view of obtaining a defined volume flow. 
         [0006]    According to the invention, the auxiliary reservoir makes sure that the reservoir is filled up again when a large quantity of oil has been withdrawn from the reservoir, such that a complete submersion of the multiple piston pump is ensured. 
         [0007]    The hydraulic unit according to the invention is in particular suitable in cases where consumers with different consumption capabilities are connected. If a consumer is a hydraulic power wrench for turning screws and bolts, the load to be overcome by the hydraulic pressure is normally high such that the delivery rate and/or the volume flow are small. A different scenario is provided if a connected consumer is a piston-cylinder unit, for example, whose large-volume cylinder includes a piston which is moved against a small load. Here, a resultant rapid piston movement produces a large volume flow. The large volume flow results in a change of the oil level in the reservoir, wherein the upper individual pumps are possibly not submerged for a short time. Such a condition is prevented by the auxiliary reservoir. 
         [0008]    The invention is advantageously applicable to a hydraulic unit comprising a reservoir with a cross section where the width of the reservoir narrows towards the upper end. Such a cross section is offered e.g. by a reservoir having a substantially round cross section. Such a reservoir allows a space-saving placement of the motor and the pump. This configuration offers a hydraulic unit with a small volume and a low weight such that the hydraulic unit can be designed as a portable unit. On the other hand, without an auxiliary reservoir there would be the risk that due to the cross section narrowing in upward direction, the upper region having a small volume would rapidly be emptied and remain so when a large quantity of oil is withdrawn. 
         [0009]    The auxiliary reservoir may be arranged on top of the reservoir either as an additional container or as a cavity which is permanently connected with the reservoir, but has a larger width than the upper end of the reservoir. 
         [0010]    According to a preferred aspect of the invention, the auxiliary reservoir is hermetically sealed above the oil level, and a connecting line connects the auxiliary reservoir with a vented additional tank at a location below the oil level. Here, the auxiliary reservoir and the additional tank define a communicating system. The atmospheric pressure causes oil to be fed from the vented tank to the auxiliary reservoir when oil is withdrawn from the auxiliary reservoir to flow to the reservoir. The additional tank increases the volume of the auxiliary reservoir, but, on the other hand, is spatially separated from the auxiliary reservoir. The auxiliary reservoir is arranged above the reservoir, while the additional tank may be located below the reservoir. The additional tank may thus define a base for the housing of the reservoir. 
         [0011]    Further, a plurality of additional tanks of different sizes may be provided which are optionally adapted to be attached to the housing. In this manner, the user can select the size of the respective additional tank. 
         [0012]    Alternatively, the auxiliary reservoir can be used without any additional tank. In this case, a ventilation opening must be provided above the maximum oil level such that the withdrawal of oil from the reservoir is not affected. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]    An embodiment of the invention will now be described in greater detail with reference to the drawings in which: 
           [0014]      FIG. 1  shows a schematic perspective representation of the hydraulic unit with a portion cut away, 
           [0015]      FIG. 2  shows a perspective view of the overall hydraulic unit including the auxiliary reservoir and the additional tank, 
           [0016]      FIG. 3  shows a sectional view along line III-III of  FIG. 2 , and 
           [0017]      FIG. 4  shows a perspective rear view of the hydraulic unit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    As shown in  FIG. 1 , the hydraulic unit comprises a housing  10  which is a conventional motor housing of an electric motor. The housing  10  has a cylindrical inner wall  11  and comprises on its outside numerous longitudinal ribs  12  defining cooling ribs. The housing  10  is configured as a profile body composed of an extruded profile. At one location on its circumference the housing  10  is provided with a longitudinal mounting plate  13  attached thereto, and diametrically opposed a fastening profile  14  for attaching components to the housing is located. 
         [0019]    The housing  10  comprises an electric motor  15 . Said electric motor  15  includes a stator  16  and a rotor  17 . The motor is a permanently excited synchronous motor whose stator comprises a rotating field-generating stator winding  18 . The rotor  17  includes a motor shaft  19  and permanent magnets  20  fastened thereto. The motor shaft  19  is supported in bearings  21 , 22  located in the front walls (not shown) of the housing  10 . 
         [0020]    The stator winding  18  is enclosed by a hoop  25  defining a closed ring and surrounding the stator winding. At the cylindrical inner wall  11  of the housing  10  spacers  26  are fastened which extend radially inwardly and center the hoop  25  in the housing. In this manner, the stator  16  is centered in the housing. The spacers  26  are bars extending in the longitudinal direction of the housing. At least three such bars are provided, but in the present embodiment 4 bars are arranged. The spacers  26  are shorter than the axial length of a space  27  such that they do not impair the circulation of hydraulic oil in the space  27 . 
         [0021]    The spacers  26  provide an annular space  27  between the stator  16  and the inner surface  11  of the housing, said annular space  27  constituting the major portion of a reservoir  28 . The reservoir is defined by the housing  10 . The stator  16  and the rotor  17  are submerged in hydraulic fluid. 
         [0022]    The motor shaft  19  of the motor  15  comprises an eccentric ring  29  driving a pump. The pump is a multiple piston pump composed of a plurality of individual pumps arranged in a star-like array about the motor shaft  19 , the pistons of said pumps being driven by the eccentric ring  29  which is permanently connected in an eccentric manner with the motor shaft. Each individual pump takes in oil from the reservoir via a non-return valve, and supplies said oil to the pump outlet via another non-return valve. The multiple piston pump is a volumetric pump. 
         [0023]    At the end of the housing  10  opposite the pump, a fan  30  is located which comprises a housing  31  radially projecting beyond the housing  10 . In the housing  31  a fan wheel rotates which produces an airflow  32  along the ribs  12 . The fan wheel is connected with the motor shaft  19  and is driven by said shaft. An effective heat dissipation results in a good cooling effect. Since the hydraulic unit heats up only to a small extent, its efficiency is increased. Omission of a separate motor housing allows for a compact design and a low weight of the hydraulic unit. 
         [0024]      FIG. 2  shows the overall hydraulic unit. In the Figure, the housing  10  comprising the longitudinal ribs  12  is represented. At one end, the fan  30  is located which produces an air flow along the outside of the housing. At the opposite end of the housing, a pump portion  40  having a pressure connector  41  and a return port  42  on the front side is arranged. Further, a pressure gauge  43  indicating the pressure is disposed at the same end. 
         [0025]    Below the housing  10  of the reservoir an additional tank  45  is arranged which is defined by a closed box provided with a stub  46  for venting and filling in hydraulic oil. 
         [0026]    On the upper side of the housing, an auxiliary reservoir  47  is disposed which is connected with the additional tank  45  via a connecting line  48 . On top of the auxiliary reservoir a motor control unit  50  is accommodated in a housing. Said motor control unit  50  is connected with the electric motor  15  via electric lines (not shown). 
         [0027]    The pressure at the pressure connector  41  is detected by a pressure sensor (not shown). This pressure is a measure of the load of the connected consumer. The motor control  50  controls the electric motor  15  in accordance with the pressure value such that at a higher pressure the rotational speed of the motor is reduced. When the pressure decreases, the motor rotational speed increases. In this manner, the power input of the motor substantially remains constant and to a large extent independent of the respective load condition of the consumer. 
         [0028]      FIG. 3  shows a schematic representation of a multiple piston pump  60  arranged in the space  27  and comprising a plurality of individual pumps  61  disposed in a star-shaped array. Each individual pump  61  is a piston pump whose piston rod  62  is pressed against the circumference of the eccentric ring  29  by a spring (not shown). The piston rods  62  are cyclically actuated by the eccentric ring  29 . Each individual pump  61  comprises an inlet and an outlet (not shown). The outlets are connected with each other and extend to the pressure connector  41 . The inlets are openings through which oil from the reservoir  28  is taken in and fed to the individual pump. The oil, which is supplied to the consumer connected with the hydraulic unit, is withdrawn from the reservoir  28 . Since the housing  10  has a round shape, withdrawal of oil results in a rapid level drop. Consequently, the inlets of the upper individual pumps  61  are possibly no longer submerged. This condition is prevented by the auxiliary reservoir  47 . 
         [0029]    As shown in  FIG. 3 , the auxiliary reservoir  47  has a width exceeding that of the upper end of the reservoir  28 . Said reservoir  47  is filled with oil up to a level  65 . Above the level  65 , the auxiliary reservoir  47  is hermetically sealed such that a trapped air cushion  66  is defined. The lower end of the auxiliary reservoir  47  is connected with the space  27  via a passage  67 . The auxiliary reservoir  47  thus provides an unthrottled amount of oil for refilling the reservoir  28 . 
         [0030]    The connecting line  48  connects the additional tank  45  with the auxiliary reservoir  47 . Said connecting line  48  extends into the auxiliary reservoir at a location below the level  65 . In the additional tank  45  the connecting line is configured as a submerged tube which ends directly above the bottom. When a negative pressure is generated by the multiple piston pump  60  in the auxiliary reservoir  47 , oil from the additional tank  45  is supplied in upward direction through the connecting line  48 . This supply in upward direction is caused by the atmospheric pressure entering into the additional tank through the stub  46 . In this manner, a very large amount of oil for refilling the reservoir  28  is provided. 
         [0031]    Alternatively, the auxiliary reservoir  47  can be used without the additional tank  45 . In this case, the auxiliary reservoir must be vented. 
         [0032]      FIG. 4  shows that the auxiliary reservoir  47  is provided with an inspection glass  68  through which the level can be checked for determining whether the auxiliary reservoir operates properly. 
         [0033]    As shown in  FIG. 4 , a motor control unit  50  is arranged on top of the auxiliary reservoir  47 . Said motor control unit  50  includes the required electrical components not shown here. The motor control unit  50  comprises on its lower side a plate  70  provided with numerous longitudinal cooling ribs  71 . The cooling ribs  71  cover that portion of the housing  10  which is not covered by the auxiliary reservoir  47 . The motor control unit  50  projects beyond the auxiliary reservoir  47  in a cantilevered fashion. The cooling ribs  71  are surrounded by an air guide housing  72  provided with a plurality of openings containing fans  73  for blowing cooling air into the air guide housing. The cooling air is discharged through openings  74 .