Patent Publication Number: US-11377334-B2

Title: Industrial truck with at least one hydraulic mast lift cylinder

Description:
CROSS REFERENCE TO RELATED DISCLOSURE 
     This application is based upon and claims priority to, under relevant sections of 35 U.S.C. § 119, German Patent Application No. 10 2018 104 586.7, filed Feb. 28, 2018, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure relates to an industrial truck having at least one hydraulic mast lift cylinder which is connected to a hydraulic block via a hydraulic arrangement. The hydraulic arrangement limits a lowering speed of the mast lift cylinder. 
     Lift frames on an industrial truck usually have at least one mast lift cylinder and one free lift cylinder. Mast sections in the lift frame are displaced telescopically relative to one another via the mast lift cylinder. The free lift cylinder moves a load-carrying means relative to an inner mast section. The maximum lowering speed of a lift frame is limited for the purpose of risk reduction and as specified by standards. The limit is achieved via the hydraulic arrangement by an appropriate choke. A lowering speed of 0.6 m/s, for instance, is provided for a loading state close to the nominal load. For example, a vehicle operator uses a control lever to specify a desired lowering speed, which is checked for admissibility. For reasons of safety, the hydraulic arrangement is additionally equipped with a line rupture safety valve which performs the task of preventing the load from falling in the event of a ruptured line. 
     In known industrial trucks, lowering the load carrier from high lifting heights after depositing the load takes a significant amount of the time. Since the industrial truck may be displaced only after the load carrier has been lowered, a waiting time arises that can add up when working with high lifting heights. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     The industrial truck according to the disclosure has at least one hydraulic mast lift cylinder, which is connected to a hydraulic block via a hydraulic arrangement. This hydraulic arrangement limits the lowering speed of the mast lift cylinder. According to the disclosure, the hydraulic arrangement is configured to limit the lowering speed in a load-dependent manner to one of at least two maximum values. A first maximum value is given for the lowering speed at a particular load value, in particular at the nominal load. The second maximum value is given for the lowering speed in connection with a second load, which is smaller than the first load. The first maximum value here is smaller than the second maximum value. The solution according to the disclosure provides the possibility of lowering the mast lift cylinder in a load-dependent manner with at least two different lowering speeds. Thus, a greater load on the mast lift cylinder is lowered at a lower lowering speed. When there is a smaller load or simply its own weight, the mast lift cylinder can be lowered at a greater speed. With the embodiment of the hydraulic arrangement according to the disclosure, it is therefore possible to lower the lift mast at a fast speed again after depositing a load at a height. With continuous operation of an industrial truck, in particular at high lifting heights, this results in fast lowering when empty and significantly improved performance of the industrial truck. 
     In one embodiment, the hydraulic arrangement is equipped with at least two lowering paths, which are separate from each other. Each of the lowering paths has a load-break safeguard. By using two lowering paths, it is possible to switch between two lowering speeds. The use of two lowering paths does not necessarily mean that the operator can switch only between two lowering speeds in a discrete manner. It is entirely possible to provide a continuous transition between the two separate lowering paths. Each of the lowering paths is equipped with a load-break safeguard so that it is ensured that a load-break safeguard is provided for each of the lowering speeds. 
     In another embodiment, each of the two lowering paths defines a maximum volume flow for the lowering movement. The volume flow in this instance substantially determines the lowering speed of the mast lift cylinder. 
     In yet another embodiment, switching between the first and the second maximum value occurs with the aid of a pressure balance. A pressure from the mast lift cylinder is applied to the pressure balance as a control pressure. This applied pressure can be reduced or increased so that the pressure balance responds. 
     To switch between the two maximum values of the lowering speeds, either a check valve or a changeover valve can be provided. In the former case, the pressure balance actuates a check valve, with which one of the lowering paths (Q 1 ) is blocked or the other lowering path (Q 2 ) is connected, so that the first maximum value is the lowering speed for the mast lift cylinder when the lowering path is blocked and the second maximum value is the lowering speed of the mast lift cylinder when the lowering path is connected. With the check valve, a second lowering path is selectively blocked or connected in parallel with the first lowering path. With a parallel connection, the volume flows of the two lowering paths are added together such that the volume flow is added and the lowering speed is increased. 
     In the described embodiment, the check valve has a valve spool that is pre-tensioned against a spring force in a valve block and that, in response to a pressure in the first lowering path, is displaced against a spring pre-load in a position that closes the second lowering path. Due to the spring force, the valve spool is pre-tensioned into a position wherein the two lowering paths are connected in parallel. If the valve spool is in its spring-loaded position, then the two lowering paths are connected. If the pressure in the first lowering path rises, then this may be interpreted as a sign of a heavy load, and the valve spool is displaced into its blocking position. 
     In another embodiment, a changeover valve is provided, wherein it may be possible to switch between the first lowering path and the second lowering path such that either the first maximum value occurs in the first lowering path or the second maximum value occurs in the second lowering path. The changeover valve also functions with a pressure balance. The changeover valve has a valve spool that is pre-tensioned against a spring force in a valve block and that selectively blocks one of the lowering paths depending upon its position. Here, the valve spool is structurally formed such that the two lowering paths can be blocked only alternatively to each other. 
     In the described embodiment, a switching load value is preferably provided that is less than or equal to the nominal load, and the lowering speed is switched to the first maximum value when the switching load value is exceeded. A performance such as this is standard-compliant, since it relates to the maximum lowering speed at the nominal load. 
     In one configuration, a free lift cylinder of the industrial truck is provided with a further hydraulic arrangement, which can limit the lowering speed to at least two maximum values depending upon the load. As with the mast lift cylinder, a lower lowering speed can be defined for a greater load than for a smaller load in the free lift cylinder, as well, which also permits a greater lowering speed in the free lift. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is explained in greater detail on the basis of two exemplary embodiments. The following is shown: 
         FIG. 1  depicts a hydraulic circuit plan with a check valve between a first and a second lowering path, 
         FIG. 2  illustrates a second exemplary embodiment with a changeover valve between the first and the second lowering path, 
         FIG. 3  shows a schematic view of a hydraulic arrangement with a check valve, 
         FIG. 4  depicts a schematic view of a hydraulic arrangement with a changeover valve, and 
         FIG. 5  illustrates a schematic view of a hydraulic arrangement with another check valve, which is different from the valve in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
       FIG. 1  shows two mast lift cylinders  10  and a free lift cylinder  12  in a schematic view. The differentiation between the mast lift cylinder and the free lift cylinder arises from the arrangement and function of the hydraulic cylinders in a lift frame. The lift frame comprises a plurality of telescoping mast sections, wherein the mast sections are displaced relative to one another through the mast lift cylinders. The free lift cylinder lifts the load-carrying means relative to the mast section that can be lifted the farthest. 
     The mast lift cylinder  12  and free lift cylinder  10  are jointly provided with hydraulic fluid via a hydraulic block (not shown). The cylinders are connected to a hydraulic block, the outlet line  14  of which has the inlet line to the hydraulic arrangements  16  and  18 . The hydraulic arrangements  16  have two lowering paths  20 ,  22 , whereas the free lift cylinder  12  in the exemplary embodiment has only one single lowering path  24 . Each of the lowering paths  20 ,  22 ,  24  has a load-break safeguard (LBS), which ensures a slow controlled lowering in the event of a fault, even if there is a load. The load-break safeguard is schematically shown as a valve that is connected via a pressure balance. Here, the cylinder-side pressure  26  is compared with a pressure  28  that is choked upstream. If the difference in pressure is not too severe, then lowering is permitted downstream. On the other hand, if the difference in pressure is significant, i.e., too great as a result of a line break for instance, then a further choke  30  is activated, by means of which the lowering process is continued with a substantial restriction. 
     In the embodiment shown in  FIG. 1 , a check valve  30  is provided for the two mast lift cylinders  10 . The check valve  30  is pre-tensioned by a spring  32  into the position shown for a pressure in the mast lift cylinder that is not too great. In this position, the check valve  30  is open and the mast lift cylinder  10  is lowered via the two lowering paths  36 ,  38 . The lowering paths  36  and  38  are connected in parallel, such that their volume flows may be added together and the lowering speed may be increased. 
     If the load applied to the hydraulic cylinder  10  is too great, then the pressure on the control line  34  increases, and the check valve  30  switches into its blocking position. The first lowering path  36  is thereby blocked, and a lowering of the mast lift cylinder  10  takes place only via the second lowering path  38 . 
     In practice, the check valve  30  is dimensioned such that, when the nominal load approaches, it closes, and the load and/or the section of the lift frame is lowered at an admissible lowering speed via second lowering path  38 . 
       FIG. 2  shows an alternative configuration of the disclosure in a schematic view. Identical components are provided with the same reference numerals. In this configuration, first and second lowering paths  36 ,  38  are provided for each of the two mast lift cylinders  10 . Unlike the first exemplary embodiment, a changeover valve  40  is provided, with which it may now be possible to selectively switch between the first lowering path  36  and second lowering path  38 . In the position shown in  FIG. 2 , the lowering process occurs via lowering path  36 . The check valve  40  is located in the position in which it is pre-tensioned by the spring  42 . If the pressure rises in the mast lift cylinder  10 , then the check valve  40  is switched via the control side  44 , and the lowering process occurs solely via lowering path  38 . In one embodiment according to the disclosure, lowering path  38  is dimensioned such that a maximum admissible lowering speed is not exceeded at the nominal load. On the other hand, if the load is lower than the nominal load, then a switch is made to the other lowering path  36 , which, for example, has a significantly greater volume flow and thus allows for a greater lowering speed. Each of the two lowering paths  36  and  38  has its own appropriately configured load-break safeguard. 
       FIG. 3  shows a schematic view of a hydraulic arrangement according to  FIG. 1  with a practical implementation of the check valve.  FIG. 3  shows an interior space  46  of a mast lift cylinder that is linked to two lowering paths  48 ,  50 . Each of the two lowering paths  48 ,  50  has a schematically represented load-break safeguard  52 . A valve spool  54 , which is pre-tensioned by a spring  56  into a position that releases the lowering path, is arranged in the lowering path or channel  48 . If the pressure in the lowering path  48  increases, then the valve spool  54  is displaced against the tension of the spring  56  and thereby blocks the lowering path  48 . In this case, only lowering path  50  is in operation in order to divert the hydraulic fluid via a connected line  58 . Each of lowering paths  48  and  50  is closed by screw plugs  60 . 
       FIG. 4  shows a schematic view of a changeover valve, in which a switch is made between a first path  64  and a second path  66  by means of a valve spool  62 . Each of paths  64  and  66  has a load-break safeguard  68 ,  70 . The valve spool  62  is pre-tensioned by the spring  72  into a position that blocks path  66 . If the pressure in a line  63  rises, then the valve spool  62  is pushed counter to the spring force  72  into the position in which path  66  is released and which blocks path  65 , which is connected to the ambient pressure and/or from the lowering path. The valve spool  62  adjusts itself depending upon the difference in pressure between the lowering path and the hydraulic cylinder. A pressure difference required for switching is determined by the spring  72 . 
       FIG. 5  shows an embodiment of a check valve  74  in a schematic view. The check valve  74  has a valve block  76 , in which a valve spool  78  is centrally arranged. In a line  81  coming from the mast lift cylinder, the hydraulic fluid exists via an outlet channel  80  and a load-break safeguard  82  through a line  84  to the hydraulic block. In the position of the valve spool  78  shown here, a second lowering path  86  is opened, such that the hydraulic fluid in the second lowering path  86  can exit via the load-break safeguard  88 . The valve spool  78  is pre-tensioned by the spring  92  into its position that opens lowering path  86 . The pressure from the outlet channel  80  is applied to the foot of the valve spool  78  via a choke  90 . If this pressure exceeds a minimum threshold value, then the valve spool  78  is urged against or counter to the force of the spring  92  into a position blocking the lowering path  86 . The valve block  76  is closed by screw plugs  94  and  96 , wherein screw plug  96  has a through-hole for a projection  98  of a valve spool  78 . The position of the valve spool  76  can be then be checked from outside by the projection  98  of the valve spool  78 . Its intact function can thereby be tested. 
     REFERENCE LIST 
     
         
           10  Mast lift cylinder 
           12  Free lift cylinder 
           14  Outlet line 
           16 ,  18  Hydraulic arrangements 
           20 ,  22 ,  24  Lowering paths 
           26  Cylinder-side pressure 
           28  Choked pressure 
           30  Check valve 
           32  Spring/lowering path 
           34  Control line 
           36 ,  38  Lowering paths 
           40  Changeover valve/check valve 
           42  Spring 
           44  Control side 
           46  Interior space 
           48 ,  50  Lowering paths 
           52  Load-break safeguard 
           54  Valve spool 
           56  Spring 
           58  Line 
           60  Screw plugs 
           62  Valve spool 
           64  First path 
           66  Second path 
           68 ,  70  Load-break barrier 
           72  Spring 
           74  Check valve 
           76  Valve block 
           78  Valve spool 
           80  Outlet channel 
           81  Line 
           82  Load-break safeguard 
           84  Line 
           86  Second path 
           88  Load-break safeguard 
           90  Choke 
           92  Spring 
           94 ,  96  Screw plug 
           98  Projection