Abstract:
A hydraulic unit ( 1 ) mounted on a vehicle with an adjustable platform which is supported by two mechanically separate arms actuated by a separate hydraulic lifting device ( 5, 6 ). The hydraulic unit has the supply and return and preferably includes a balance valve ( 21 ) for controlling lowering of the platforms; a flow divider ( 22 ) enabling a division of the supply fluid into two flows having an identical flow rate, and each supplying one of the hydraulic lifting devices; and a resetting solenoid valve ( 23 ) that, when the operator controls the resetting of the hydraulic lifting devices, regardless of the operation direction and position thereof, and isolates one of the hydraulic devices ( 6 ) in order to immobilize the same while the other hydraulic lifting device ( 5  or  6 ) is adjusted.

Description:
This application is a national stage completion of PCT/FR2010/000507 filed Jul. 13, 2010 which claims priority from French application Ser. No. 09/03522 filed Jul. 17, 2009. 
     FIELD OF THE INVENTION 
     The present invention concerns a hydraulic balancing and resetting unit for a lifting assembly with two independent powered hydraulic elements which can be placed in identical positions at any given moment. 
     BACKGROUND OF THE INVENTION 
     Certain vehicles, particularly flat-bed carriers for transporting automobiles, are equipped with platforms or plates for carrying cargo, which are adjustable in height to facilitate loading and unloading operations. 
     These platforms are supported by an assembly of lifting frames that vary in number depending on the location and length of the platform. Each of these frames comprises two posts or dual lifting arms placed on either side of the vehicle, one on the right and one on the left. 
     Each of the posts or lifting arms is equipped with a hydraulic lifting device, conventionally one with a hydraulic auger motor or a hydraulic cylinder, for modifying the angle of the lifting arm or the height of the platform&#39;s recovery point and thereby varying the level of the platform supported. 
     To prevent undesirable torsion from damaging either the cargo being transported or the supporting platform itself, it is imperative that the movement of the hydraulic lifting devices be synchronized between both arms on a single lifting frame. 
     Synchronizing the positioning of the two left and right hydraulic devices on each lifting frame is necessary in order to keep the raised platform straight and horizontal and prevent it from slanting laterally. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to propose a device for resetting the two right and left hydraulic lifting devices for each lifting frame relative to one another if they are not perfectly horizontal because their movement is poorly synchronized. 
     When the right and left hydraulic lifting devices are hydraulic auger motors, they are conventionally supplied in series. Thus, it is fairly easily to reset the two motors relative to each other, at the will of operator, by cutting the supply to one of them using a simple bypass. The unsupplied motor then stops, remaining immobilized in the same position and maintaining the load it supports, while the second auger motor continues moving until it is positioned identically to the first one. The supply to the two auger motors can then be reestablished so that synchronized movement resumes. 
     When the right and left hydraulic lifting devices are hydraulic cylinders supplied in parallel, the situation is more delicate. If the supply to one hydraulic cylinder stops, for example, because of a bypass, the other cylinder actually cannot remain in position and it fails. 
     Since hydraulic lifting cylinders are conventionally supplied in parallel and not in series like hydraulic auger motors, it is necessary to divide the hydraulic fluid in order to supply each of the cylinders separately. At the present time, no satisfactory device exists for dividing the flow of hydraulic fluid in a stable and completely equal way without a difference in the rate of flow between the two branches occurring at a moment&#39;s notice. This difference translates automatically to a shifting between the two lifting cylinders, which are no longer synchronized, and the platform begins to slant. 
     A device to reposition of both hydraulic cylinders relative to each other when such a failure occurs, regardless of the cylinders&#39; position at that time, is highly desirable and even crucial. 
     This is the problem which the invention addresses. 
     To eliminate synchronization failures between the lifting arms, the prior art has attempted to join them mechanically using a torsion bar type of connector. This consists of a connecting tube extending transverse to the vehicle and joining the two lifting arms. This mechanical connection forces the movement of the two arms to be globally synchronized. However, it remains possible for there to be a slight, acceptable offset in amplitude, induced by torsion deformation. 
     Unfortunately, this prior art system of mechanical joining is not satisfactory because it is difficult to put in place and especially because it consumes considerable space on the vehicle. It is well known that the space available for operating systems is particularly limited on vehicles of this type, since a maximum amount of free space needs to be reserved for the cargo being transported. The space consumed by functional devices on the vehicle poses a critical problem; the concurrent challenge of reducing this wasted space is an important one. 
     The invention responds to this space problem by eliminating this very bulky mechanical torsion connection and furnishing a particularly compact, less voluminous system. Advantageously, with the system of the invention, the lifting arms remain mechanically independent. 
     Another resetting system, without a mechanical torsion connection developed in the prior art, allows the two arms to be mechanically independent. It consists of a system internal to the hydraulic lifting cylinders. These cylinders comprise a hydraulic fluid discharge track opening into their cylinder wall through a perforation accessible only when the cylinder is in the upper position. Therefore, when one of the cylinders is offset and arrives in advance at the upper position, the hydraulic fluid being supplied to it flows out through the discharge track through the perforation, which has become accessible, while the second cylinder continues to ascend until it also reaches the upper position. In this way, the two cylinders are resynchronized. 
     However, this prior art system only allows synchronization to be reestablished when the platform is in the final upper position, which is at the end of the course followed by the two hydraulic lifting cylinders. No regulation is possible when the platform is in the intermediate position because the perforations providing access to the discharge circuits are covered at that time. The operator can only observe powerlessly the appearance of a synchronization fault when it occurs during the ascent or the descent of the cargo being lifted. 
     Conversely, the resetting device of the invention can be actuated at any time by the operator and, therefore, it advantageously allows an error in synchronizing the movement of the two right and left lifting devices to be corrected regardless of the position of the platform being raised. 
     Furthermore, with the prior art system, the cylinder pistons are equipped with a peripheral gasket that must pass regularly over the inlet perforation in the discharge path, causing the gasket to degrade progressively. If the gasket is not replaced in time, micro leaks may appear in this area and there is no guarantee the load will be maintained. Such a situation is not acceptable for cylinders that must guarantee safe operation. 
     Advantageously, the resetting device of the invention does not have these drawbacks. 
     The device of the invention performs several functions simultaneously. It controls the descent of the load being supported, it splits the flow of hydraulic fluid, and it performs the resetting of the hydraulic lifting devices upon demand by the operator, regardless of the position of these hydraulic devices at that moment. 
     Additionally, all of these functions are integrated within one compact case called the hydraulic unit. It is, therefore, easy to install on a vehicle, despite the space problems that have always existed with this type of application. Moreover, it is easy to connect to the hydraulic circuit, as it is limited in the number of connections used. Assembly is simple and costs are reduced. 
     To resolve this technical problem, the invention furnishes a hydraulic unit for installation on a vehicle, especially a vehicle for transporting automobiles, comprising at least one plate or platform for transporting a load, adjustable in height, said plate or platform being supported by at least one lifting frame formed of two mechanically independent lifting arms, right and left, respectively, each of said lifting arms being equipped with an independent hydraulic lifting device for varying the height of the plate or platform it supports. 
     According to the invention, this hydraulic unit comprises the following hydraulic components: 
     a stream splitter having an inlet track and two outlet tracks and which regulates the flow of fluid to obtain two streams flowing at the identical rate on the two outlet tracks, regardless of the direction in which the fluid is flowing; and 
     a dual-position solenoid valve having one inlet track and two outlet tracks which, in the first position, is passable from its inlet track to its first outlet track, as its second outlet track is blocked; and which, in its second position, is passable from its inlet track to its second outlet track, as its first inlet track is blocked. 
     These hydraulic components are placed in a hydraulic circuit that comprises: 
     a first branch comprising a first inlet conduit splitting at the level of a dividing point and a first outlet conduit and a second outlet conduit; and 
     a second branch comprising a second inlet conduit ending at the inlet track of the stream splitter and extending at the two outlet tracks of the stream splitter on one side to a third outlet conduit; and on the other side, to a conduit leading to the inlet track of the solenoid valve and continuing at the first outlet track of the solenoid valve through a fourth outlet conduit, and at the second outlet track of the solenoid to a connecting conduit joining the second outlet track of the solenoid valve to the first inlet conduit. 
     The hydraulic unit of the invention is connected: 
     to the hydraulic fluid reservoir via the hydraulic control unit for the hydraulic lifting devices, at its first inlet conduit and at its second inlet conduit; 
     to one of the hydraulic lifting devices at its first outlet conduit and at its third outlet conduit; and 
     to the other hydraulic lifting device at its second outlet conduit and its fourth outlet conduit. 
     According to the invention, the solenoid valve is in its first position while the two hydraulic lifting devices are operating normally and simultaneously allowing the plate or platform to be raised or lowered, and it passes into its second position and proceeds to reset the hydraulic lifting devices, relative to each other, by immobilizing one of the hydraulic lifting devices while the other one continues to move, such resetting being possible at any moment regardless of the direction of operation or the position of the hydraulic lifting devices. 
     The invention also concerns a vehicle, specifically a vehicle for transporting automobiles, comprising at least one plate or platform for transporting the cargo and which is adjustable in height, said plate or platform being supported by at least one lifting frame formed of two mechanically independent lifting arms, right and left, respectively, each of said lifting arms being equipped with an independent hydraulic lifting device for varying the height of the plate or platform it supports, in which vehicle each of the lifting frames is equipped with a hydraulic unit, according to the invention, connected to the hydraulic lifting devices of the lifting frame concerned. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and features of the invention will become apparent from reading the following detailed description, with reference with the attached drawings, in which: 
         FIG. 1  is a perspective of the rear of an automobile transport vehicle equipped with a hydraulic unit according to the invention; 
         FIGS. 2 and 3  are front and rear perspectives, respectively, of a hydraulic unit according to the invention; 
         FIG. 4  is a simplified hydraulic schematic of a hydraulic unit according to the invention connected to two simple hydraulic cylinders; 
         FIGS. 5 and 6  are simplified hydraulic schematics illustrating operation of the hydraulic unit of  FIG. 4  while the shafts of the hydraulic cylinders are extended and retracted, respectively; 
         FIGS. 7 and 8  are simplified hydraulic schematics illustrating operation of the hydraulic unit of  FIG. 4  when resetting is requested by the operator, said resetting being accomplished by extension or retraction of the left cylinder shaft; 
         FIG. 9  is a hydraulic schematic of a preferred embodiment of the hydraulic unit, according to the invention, which will be connected to two secured hydraulic cylinders; 
         FIGS. 10 and 11  are hydraulic schematics illustrating the operation of the hydraulic unit of  FIG. 9  while the shaft of the secured hydraulic cylinders is extended and retracted, respectively; 
         FIGS. 12 and 13  are hydraulic schematics illustrating the operation of the hydraulic unit of  FIG. 9  when resetting is requested by the operator, said resetting being accomplished by extension or retraction of the left secured cylinder shaft. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The hydraulic unit of the present invention will now be described in detail with reference to  FIGS. 1-13 . Equivalent elements shown in different drawings will bear the same reference numerals. 
     In  FIG. 1  hydraulic unit  1  of the invention is shown in its environment and during use, installed on the rear of a vehicle  2 , specifically an automobile-transporting vehicle according to a preferred, but non-limiting, exemplary application of the invention. 
     Hydraulic unit  1  is installed on the chassis  3  of vehicle  2  below the lower plate  4  of the vehicle. It supplies and controls two hydraulic lifting devices, left device  5  and right device  6 , respectively, which actuate the left and right lifting arms of a lifting frame that is not shown. 
     Depending upon the applications, hydraulic lifting devices  5  and  6  may actuate arms, posts, upright elements, supports or any other elements of a lifting device that supports a variable height plate or platform. For purposes of simplification, all these elements, whatever their exact nature, will be designated by the term “arm” for the remainder of this description and in the claims, with no limitations intended. 
     Hydraulic lifting devices  5  and  6 , shown in this drawing, are hydraulic cylinders  7  and  8  and more specifically, secured hydraulic cylinders  9  and  10 . As described below, the use of hydraulic unit  1  of the invention is not limited to only this type of secured hydraulic cylinders  9  and  10 . Hydraulic unit  1  of the invention can also be used with conventional hydraulic cylinders  7 ,  8  or even with a different type of hydraulic lifting device  5 ,  6 , such as auger motors, for example. 
     The movement of shafts  11  and  12  of cylinders  9  and  10  makes it possible to vary the height of the upper platform of the vehicle (not shown). 
     A hydraulic unit  1 , according to the invention, is preferably connected to hydraulic lifting devices  5  and  6  on each of the vehicle&#39;s lifting frames. In order to minimize the length of the hydraulic connectors to be used, hydraulic unit  1  is preferably located between the two lifting arms of the frame concerned, for example, generally at the level of the vehicle&#39;s longitudinal axis and thus more or less in the middle of the two lifting arms or, for example as shown, on one side of the vehicle, preferably the side where the manual or the electric hydraulic control unit for the two hydraulic lifting devices  5 ,  6  is located. 
     Hydraulic unit  1  is connected by two supply connectors  13  to the hydraulic fluid reservoir, via the hydraulic control unit, for the lifting devices concerned. It is also connected by a unit of distribution connectors  14  to the two hydraulic lifting devices  5  and  6 . 
     In the instance shown where the lifting arms are equipped with secured hydraulic cylinders  9  and  10 , the distribution connectors  14  number three per cylinder and they end at the level of the securing device  15  or  16  for each cylinder  10 . 
     These connectors  13  and  14  may be made either entirely or partially in the form of either flexible or rigid hydraulic tubes. 
     The hydraulic unit  1  of the invention is shown alone in  FIGS. 2 and 3 . 
     It preferably comprises a compact body  17  that surrounds the hydraulic circuit and the hydraulic components of unit  1 . 
     This body  17  may take the form of a generally parallelepipedal block, for example, in which various perforations have been drilled to form housings for receiving the hydraulic components  18  necessary for the operation of hydraulic unit  1 , as well as conduits  19  for the passage of hydraulic fluid. 
     The unit of conduits  19  forms a hydraulic circuit that will be described in detail below. Conduits  19  open outside body  17  through orifices  20 , which are of the appropriate size and shape for introduction of the extremity of a connector  13  or  14 , forming a sealed connection in this area between the connector and the concerned conduit  19 . 
     In order to facilitate the hydraulic connections and thus simplify installation of hydraulic unit  1 , each inlet conduit and/or outlet conduit on the hydraulic circuit of unit  1 , or only certain ones, may open outside body  17  through several equivalent orifices  20  situated at different locations on unit body  17  and preferably on different surfaces thereof in order to ensure that at least one of these orifices  20  always remains physically accessible. This makes it possible to form easily the hydraulic connections regardless of the position and the size of the installation area for unit  1 , or the orientation of the unit in the assembled position. Unused orifices are blocked using stoppers or some other means of tightly sealing them. 
     The shown hydraulic unit  1  contains three principal hydraulic components  18 : a balance valve  21 , a stream splitter  22  and a solenoid resetting valve  23 . 
     Balance valve  21  is not vital for all applications. When it is present, its function is to brake the descent of the platforms, which is a powered descent, and to control their descent so that it is progressive and not too rapid. Balance valve  21  does not open until there is sufficient incoming fluid pressure. Thus, an automatic and progressive equilibrium is established, in the area of valve  21 , between the incoming fluid pressure and the weight of the descending load. 
     Balance valve  21  fulfills a supplementary function when hydraulic lifting devices  5 ,  6  are secured cylinders  9 ,  10  comprising a securing device  15 ,  16  with gates as shown schematically in  FIGS. 10 through 13 . 
     In this case, to ensure that the platforms are maintained in position, balance valve  21  keeps the return closed as long as the gates on securing devices  15 ,  16 , for secured hydraulic cylinders  9 ,  10 , are closed, thus providing additional security. To do this, the pressure applied to open it must be greater than the pressure that opens the safety gates on the cylinders. The gates on securing devices  15 ,  16  of the cylinders open, therefore, before the load is allowed to descend when balance valve  21  opens. 
     Stream splitter  22  is a static stream splitter that balances the passage of hydraulic fluid passing through it by creating two outgoing streams with identical flow rates, from a single incoming stream. This component functions regardless of the direction in which the fluid circulates. In the reverse direction, it regulates the flow rate of the incoming streams and allows two incoming streams with the identical flow rate to pass through, reuniting them into a single outgoing stream. The stream splitter fulfills its balance function regardless of the load on the two cylinders and even when the two loads are not identical. 
     Solenoid resetting valve  23  is a triple track, dual position solenoid valve. It is passable as long as the operator does not order resetting of hydraulic lifting valves  5 ,  6 , for example, by pressing on a button provided for this purpose. It is preferably a solenoid valve with gates that produces a tighter seal than a wedge gate valve. 
     The operation of hydraulic unit  1 , according to the invention, will now be described in detail with reference to the hydraulic schematics in  FIGS. 4 through 13 . 
     In these drawings the following conventions have been adopted: the conduits through which fluid flows are shown by solid lines, in bold when the fluid is pressurized and in light type when there is no pressure. The conduits through which no fluid flows are shown by broken lines, in bold when the fluid is pressurized and in light type when there is no pressure. 
     In these drawings certain hydraulic components  18  and conduits  19  have been arbitrarily placed on the left side and others on the right side. Obviously, in other embodiments of the invention, this arrangement could just as well be reversed without affecting the operation of the device. 
     First,  FIGS. 4 through 8  show a basic embodiment of the invention. 
     In this base variation, the inlets and outlets of hydraulic unit  1  have not been doubled and hydraulic unit  1  has been designed specifically to cooperate with hydraulic devices  5 ,  6 , each requiring only two tracks for the passage of fluid, used alternately in both directions depending on the direction in which hydraulic devices  5  and  6  are operating. 
     These hydraulic devices  5 ,  6  are conventional hydraulic cylinders, for example, without any securing system. In this instance, maintaining the platform in position once the height has been regulated, notably during travel, is not accomplished by hydraulic blocking in the area of hydraulic lifting devices  5 ,  6 . It must be maintained in some other way, for example, by the operator positioning lateral pins in the area of cylinders  7  and  8  or in the area of the lifting arms, or by any other mechanical or other type of blocking means. 
     The hydraulic devices  5  and  6 , shown in  FIGS. 4 through 8 , are two-way cylinders  7 ,  8  with conventional cylinder bodies  24 ,  25  surrounding a large chamber  26 ,  27  and a small chamber  28 ,  29  separated by a piston  30 ,  31  from which the rod  11 ,  12  of the corresponding cylinder extends. 
     Hydraulic unit  1  is connected to the hydraulic fluid reservoir via the hydraulic control unit by two supply connectors  13 , one bringing fluid to the system inlet and the other alternately returning fluid to the reservoir, depending upon the direction in which cylinders  7 ,  8  are operating. 
     First, the normal operation of the device will be described, when the shafts  11 ,  12  of the two cylinders  7 ,  8  extend or retract simultaneously without the operator performing any resetting. 
     When the operator controls the ascent of the platform without performing any resetting, the device is in the situation depicted in  FIG. 5 . 
     Hydraulic unit  1  is supplied with hydraulic fluid through its orifice B, with the pressurized fluid entering through conduit  32 . 
     The fluid encounters a first conduit  33  which has a closed extremity in the area of solenoid valve  23  when the device is in this configuration. 
     The fluid then progresses toward balance valve  21  which is in the closed position. It short-circuits this valve through a bypass conduit  34  which has a gate  35  inserted in it, through which the fluid passes in the direction of travel, allowing it to reach a T-shaped division point  36  where it separates into two streams progressing to conduits  37  and  38 , each one supplying a large chamber,  26  or  27 , respectively, in one of cylinders  7 ,  8 . 
     The hydraulic fluid entering large chambers  26 ,  27  of cylinders  7 ,  8  causes pistons  30 ,  31  to ascend and thus shafts  11 ,  12  to extend outside cylindrical bodies  24 ,  25  of cylinders  7 ,  8  thereby making the corresponding platform ascend. 
     The hydraulic fluid located in small chambers  28 ,  29  of cylinders  7 ,  8  is expelled from the cylinders through connectors  14  and returns to hydraulic unit  1  via conduits  39  and  40 . 
     The fluid progressing through conduit  39  arrives directly to one of the inlet paths of stream splitter  22 . The fluid progressing through conduit  40  first encounters solenoid valve  23 . In this operational mode, when the operator has not ordered repositioning, solenoid valve  23  is passable; the fluid passes through and travels through conduit  41  to the other inlet track of stream splitter  22 . 
     Stream splitter  22  operates here to recompose the stream and it functions in such a way that the two arriving streams flow at identical rates on each of its inlet tracks, regardless of the load on the two cylinders. This synchronizes the operation of the two cylinders  7 ,  8 . 
     From these two incoming streams that it has formed with identical flow rates, stream splitter  22  forms a single outgoing stream which leaves through conduit  42  and exits hydraulic unit  1  through its orifice A to return to the reservoir through one of the supply connectors  13 , via the hydraulic control unit. 
     Before leaving unit  1 , the hydraulic fluid progresses through conduit  42  to which a conduit  43  is attached for controlling balance valve  21 . However, in this case, the fluid pressure is not sufficient to force balance valve  21  into the open position. 
     When the operator orders the descent of the platform without ordering resetting, as shown in  FIG. 6 , hydraulic unit  1  is then supplied through orifice A and the fluid penetrates conduit  42 . 
     When the fluid reaches control conduit  43 , this time it has enough pressure to push balance valve  21  into open position. 
     The hydraulic fluid reaches the entry to stream splitter  22 , which separates it into two streams with an identical flow rate sent through conduits  39  and  41 . 
     The fluid progressing through conduit  39  proceeds directly to small chamber  28  of left cylinder  7  and fills it, while the fluid progressing through conduit  41  first passes through solenoid valve  23 , which is in passable position, before going on to supply small chamber  29  of right cylinder  8  through conduit  40 . 
     The entry of hydraulic fluid into small chamber  28 ,  29  provokes retraction of cylinder shafts  11  and  12  and thus the descent of the corresponding platform. Since the fluid flows at an identical rate through conduits  39  and  40  because of stream splitter  22 , the operation of the two cylinders is synchronized. 
     The hydraulic fluid in large chambers  26 ,  27  of cylinders  7 ,  8  is expelled toward hydraulic unit  1  and through its conduits  37  and  38 . 
     It recombines into a single stream at division point  36  and passes through balance valve  21  which is in the passable position this time, to escape hydraulic unit  1  through orifice B via conduit  32  and return to the reservoir by means of one of the supply connectors  13  via the hydraulic control unit. 
     In this basic embodiment of the hydraulic unit of the invention, it is equally possible to place the stream splitter either in the hydraulic circuit near large chambers  26 ,  27  or near small chambers  28 ,  29  of cylinders  7  and  8 . It is therefore possible to exchange its position with that of division point  36 . 
     Since the operation of stream splitter  22  is by nature less than perfect, it happens that a positioning problem may result in lack of synchronization between cylinders  7  and  8 . In this case the operator controls resetting of the device by activating solenoid valve  23 , for example, by pressing a control button and keeping it depressed until the positioning problem is resolved and the two cylinders are again synchronized. This resetting procedure can be performed regardless of the direction in which the cylinders are operating and regardless of their position. 
       FIGS. 7 and 8  show the configuration of the next stage. Resetting consists of stopping one of the cylinders by isolating it while the second one continues to operate, with the operator selectively controlling shaft extension or retraction according to the situation until resetting of the two cylinders has been established. 
     In the embodiment shown in  FIGS. 7 and 8 , the cylinder isolated during the resetting operation is right cylinder  8 . A person skilled in the art can easily imagine a variation using left cylinder  7  by simply reversing the positions of certain hydraulic components on the circuit. Generally, the cylinder that is isolated during resetting is preferably the one located on the side opposite the manual controls so the operator is located beside the active cylinder for better visibility and control over its movement. 
     When the operator orders resetting, solenoid valve  23  is fed and placed in reset position as shown in  FIGS. 7 and 8 . In this position, conduit  33  is no longer blocked and it is placed in communication with conduit  41  through solenoid valve  23 . 
     Conversely, conduit  40 , which communicates with small chamber  29  in right cylinder  8 , terminates at solenoid valve  23  in a gate that is in closed position. The fluid contained in small chamber  29  can no longer escape, thereby making it impossible to displace piston  31  and or move shaft  12  of cylinder  8 , which is isolated and therefore immobilized. 
     At the point during the resetting process when the operator needs to order extension of left cylinder  7  (upward height adjustment), the system is in the configuration shown in  FIG. 7 . 
     As before, the hydraulic unit is supplied through orifice B by conduit  32 . 
     A portion of the fluid passes through conduit  33  and traverses solenoid valve  23 , reaching one of the two inlet tracks of stream splitter  22  via conduit  41 . 
     The remaining fluid short-circuits balance valve  21 , in the closed position, through bypass conduit  34  and reaches division point  36 . 
     Since the path of piston  31  is blocked, the fluid can no longer supply large chamber  27  of right cylinder  8 . Therefore it will only fill the large chamber  26  of left cylinder  7 , passing through conduit  37 . 
     Hydraulic fluid entering large chamber  26  causes the extension of shaft  11  from cylinder  7 , as well as the expulsion of hydraulic fluid located in small chamber  28  toward conduit  39  of hydraulic unit  1 . 
     The expelled fluid arrives directly at the other inlet track of stream splitter  22 , which reforms a single outgoing stream from the two streams arriving through conduits  39  and  41 . This outgoing stream exits through conduit  42 , which communicates with conduit  43 . This outgoing fluid lacks sufficient pressure to force balance valve  21  to open. It escapes from hydraulic unit  1  to return to the reservoir via the hydraulic control unit. 
     During the resetting process, when the operator controls the return of left cylinder  7  (downward height adjustment), the system is in the configuration shown in  FIG. 8 . 
     The hydraulic fluid enters hydraulic unit  1  through conduit  42  and pushes balance valve  21  into open position via conduit  43 . 
     It then arrives at the entry to stream splitter  22 , which separates it into two streams of identical flow rate moving through conduits  39  and  41 . 
     The fluid progressing through conduit  41  traverses solenoid valve  23  and it is sent outside hydraulic unit  1  toward the hydraulic control unit and the reservoir, via conduits  33  and  32 , while the fluid progressing through conduit  39  continues on to fill small chamber  28  of left cylinder  7  and thus provoke the return of cylinder shaft  11 . 
     The hydraulic fluid present in large chamber  26  of cylinder  7  is expelled through conduit  37  of hydraulic unit  1 . Since right cylinder  8  is blocked, the fluid is forced, at division point  36 , to flow toward balance valve  21 , which it traverses to rejoin conduit  32  and return to the reservoir via the hydraulic control unit using supply connector  13 . 
     When shaft  11  of left cylinder  7  has returned to the same position as shaft  12  of right cylinder  8 , the operator stops the resetting, for example, by releasing the control button. Operation of the two cylinders then continues in the conventional, synchronized fashion according to one of the two normal operational modes described previously. 
       FIGS. 9 through 13  represent a second embodiment of hydraulic unit  1  of the invention, designed specifically for connection to two secured cylinders  9  and  10 . Such hydraulic devices  5 ,  6  each require three tracks for fluid passage, two of them being used alternately in two directions depending on the direction in which the cylinders are operating, and the third used for operation of the securing device  15 ,  16 . 
     This hydraulic unit comprises the same principal hydraulic components  18  as the basic embodiment previously described, as well as a similar hydraulic circuit. However, the following differences are noted: 
     Conduit  33 , which in the basic embodiment begins at a point of intersection  44  with conduit  32  and ends at solenoid valve  23 , proceeds on the side with point of intersection  44  through a conduit  45  opening outside hydraulic unit  1  through orifice T 12 ; and on the other side, through a conduit  46  opening through orifice T 13 . During operation these supplementary conduits  45  and  46  are connected by a distribution connector  14  to securing devices  15 ,  16 , respectively, for secured cylinders  9  and  10 . 
     This arrangement of supplementary outlet conduits  45  and  46  is easy to construct and represent. However, either one these supplementary outlet conduits  45 ,  46  may be connected to any point on connecting conduit  33  or to any point on first inlet conduit  32  that is located before balance valve  21  when first inlet conduit  32  has one. 
     Alternatively, hydraulic unit  1  may contain only a single supplementary outlet conduit joined to connecting conduit  33  or to inlet conduit  32 , while the two connections  14  required for operation of the securing devices for the secured cylinders may be interconnected outside hydraulic unit  1 , for example. 
     In the preferred embodiment shown in  FIG. 9 , each of the outlets on hydraulic unit  1  is advantageously duplicated by another equivalent outlet located on another surface of the unit. 
     Inlet conduit  41  is thus divided into two conduits  47  and  48  opening through orifices A 1  and A 2 , respectively. Inlet conduit  32  is also divided into two conduits  49  and  50 , respectively, opening through orifices B 1  and B 2 . 
     Outlet conduits  37 ,  38 ,  39  and  40  each split into two conduits,  51  and  52 ,  53  and  54 ,  55  and  56 , and  57  and  58 , respectively, which respectively open through orifices B 11  and B 12 , B 13  and B 14 , A 11  and A 12 , and A 13  and A 14 . 
     Likewise, supplementary outlet conduits  45  and  33  each split into two conduits  59  and  60 , and  46  and  61 , respectively, which respectively open through orifices T 11  and T 12 , and T 13  and T 14 . 
     The operator can thus select the orifices to be used according to need and the accessibility to the lifting area on hydraulic unit  1 . The unused orifices are blocked, using simple stoppers, for example. 
     This preferred embodiment of the hydraulic unit can also be used with simple hydraulic devices  5 ,  6 , each of which require only two tracks for fluid passage, such as for example, conventional hydraulic cylinders  7  and  8  without any securing system. It is only necessary to bypass the supplementary outlet conduits that are not necessary for such an application by simply blocking orifices T 11 , T 12 , T 13  and T 14 . 
     The operation of this preferred hydraulic unit is similar to the basic embodiment and can easily be deduced by studying  FIGS. 10 through 13 . In these drawings, the duplication of outlets detailed above has not been shown, in the interests of simplification and to facilitate the reader&#39;s comprehension. 
       FIGS. 10 and 11  show the normal operation of a lifting system, without the operator ordering resetting, in the direction of the simultaneous extension of shafts  11  and  12  of cylinders  9  and  10  in  FIG. 10 , and their simultaneous return in  FIG. 11 . 
     In the case shown in  FIG. 10 , the operation is identical to what is described with reference to  FIG. 5 , apart from the fact that the hydraulic fluid present in outlet conduits  37  and  38  is not sent directly to the large chambers  26  and  27  of cylinders  9  and  10 , but passes first through securing devices  15  and  16 , respectively. 
     During its passage, it encounters two successive securing gates, referenced as  62  and  63  respectively, for the left securing device  15 , and  64  and  65  for right securing device  16 . In this configuration the fluid circulates in the direction allowed by the gates and can therefore pass through, ending at the large chamber of the cylinders and thus causing extension of shafts  11  and  12 . 
     When the supply of hydraulic fluid to hydraulic unit  1  stops, securing devices  15  and  16  ensure that cylinders  9 ,  10  are maintained in position. 
     The effect is to hydraulically lock the cylinders using their successive securing gates  62 ,  63  and  64 ,  65  which prevent hydraulic fluid from flowing out of the large chambers in the cylinders. The risk of leaks is avoided by the succession of two gates in a series which, in addition, are preferably of different types. 
     Securing devices  15 ,  16  for cylinders  9 ,  10  comprise, additionally, a slide valve regulating means  66 ,  67 , the piston  68  and  69 , respectively, of which can mechanically open securing gates  62 ,  63  and  64 ,  65  when there is sufficient pressure in control conduit  70 ,  71 . 
     In order to establish internal equilibrium for the satisfactory operation of these regulatory means  66  and  67 , the latter are also connected via conduits  72  and  73  and a distributing connector  14  to supplementary outlet conduits  45  and  46  on hydraulic unit  1 . 
     When the operator commands shafts  11  and  12  of cylinders  9  and  10  to return as shown in  FIG. 11 , the pressure in control conduits  70  and  71  of the securing devices becomes higher than in conduits  72  and  73 . It provokes the extension of piston  68 ,  69  of regulating means  66  and  67 , which then mechanically opens securing gates  62 ,  63  and  64 ,  65  on securing devices  15   1   n    16 , thus allowing hydraulic fluid to be evacuated from large cylinder chambers  26 ,  27 . 
     The resetting operation, shown in  FIGS. 12 and 13 , takes place as before by isolating one cylinder  9  or  10  and stopping its operation while the other cylinder continues to move. 
     With such secured cylinders  9  and  10 , the hydraulic unit preferably comprises no hydraulic component  18  between balance valve  21  and securing device  15 ,  16  for the secure cylinders. Such hydraulic components could actually interfere with the operation of these securing devices  15 ,  16 . For this reason, stream splitter  22  is preferably placed on the circuit branch that is not connected to securing devices  15 ,  16 . In the example shown, the securing devices are arranged beside large cylinder chamber  26 ,  27  and the stream splitter  22  is then placed on the circuit branch that is connected to small chambers  28  and  29  of the secure cylinders. This arrangement could easily be reversed in another embodiment of the invention. 
     It is obvious that the invention is not limited to the preferred embodiments described previously and shown in the different drawings, since a person skilled in the art might make numerous modifications and conceive of other variations without departing from either the scope or the realm of the invention described in the claims. 
     For example, the simple balance valve shown might be replaced by a double balance valve, or it might be placed on the branch of the circuit that is connected to small cylinder chambers  28 ,  29  (in the case of cylinders that work by “pulling”) and thus on the second inlet conduit  42  of the hydraulic circuit. 
     It is also possible to use the hydraulic unit of the invention with hydraulic auger motors taking the place of hydraulic cylinders, which would allow these hydraulic motors to be supplied in parallel and not in series, and therefore perhaps to be less powerful. In such an application, balance valve  21  is no longer necessary and it may be eliminated from hydraulic unit  1 . 
     Furthermore, it would be possible for the solenoid valve&#39;s passage to the second position be controlled not by the operator, but rather by an automatic device detecting the positions of the two hydraulic lifting devices  5  and  6 .