Patent Publication Number: US-9835185-B2

Title: Industrial vehicle

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an industrial vehicle equipped with a hydraulic actuating device. 
     As this type of industrial vehicles, a forklift is known. The forklift includes an engine, a hydraulic pump driven by the engine, and a hydraulic actuating device actuated by hydraulic oil discharged from the hydraulic pump. The forklift has a hydraulic cylinder for moving a fork upward or downward and a hydraulic cylinder for tilting a mast. When the hydraulic pump is driven by the engine, engine torque may become insufficient as the load of the hydraulic pump increases, which may cause an engine stall. To address this, Japanese Laid-Open Patent Publication No. 2012-62137 proposes a configuration for preventing occurrence of an engine stall. 
     However, with the configuration of Japanese Laid-Open Patent Publication No. 2012-62137, while it is possible to prevent occurrence of an engine stall, it is necessary to add a structure for unloading a hydraulic circuit to restrict operation of a hydraulic actuating device. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an industrial vehicle capable of performing unloading when necessary while preventing occurrence of an engine stall. 
     To solve the above-described problem, according to the first aspect of the present invention, an industrial vehicle is provided which includes an engine, a hydraulic pump driven by the engine, a hydraulic actuating device actuated by hydraulic pressure, a connection oil passage connecting the hydraulic pump and the hydraulic actuating device, a supply oil passage through which hydraulic oil to be supplied to the hydraulic actuating device passes, a discharge oil passage through which hydraulic oil to be discharged to an oil tank passes, an unloading valve connecting the supply oil passage and the discharge oil passage, a relief pressure valve connected to the supply oil passage and actuated by pressure of the hydraulic oil passing through the supply oil passage, a timer circuit unit connected to the supply oil passage and opening the supply oil passage when a certain time period has elapsed, and a control unit controlling a state of the unloading valve between an open state and a closed state. The control unit switches the state of the unloading valve to an open state when load is applied to the engine in a state where operation of the hydraulic actuating device is allowed, and switches the state of the unloading valve to an open state while the operation of the hydraulic actuating device is restricted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an overall configuration of a forklift; 
         FIG. 2  is a hydraulic circuit diagram explaining a pressure compensation circuit having an unloading valve; 
         FIG. 3  is a hydraulic circuit diagram explaining the pressure compensation circuit; 
         FIG. 4  is a hydraulic circuit diagram explaining the pressure compensation circuit; 
         FIG. 5  is a timing chart illustrating change of pressure and engine speed when cargo handling operation is started; 
         FIG. 6  is a timing chart illustrating change of pressure when cargo handling operation is restricted; and 
         FIG. 7  is a hydraulic circuit diagram explaining a pressure compensation circuit in another example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment in which an industrial vehicle of the present invention is embodied as a forklift will be described below according to  FIG. 1  to  FIG. 6 . 
     As illustrated in  FIG. 1 , a forklift  10  includes a vehicle body and a cargo handling device  11  mounted on the vehicle body. The cargo handling device  11  includes a multistage mast  14 . The multistage mast  14  is constructed of a pair of right and left masts: an outer mast  12  and an inner mast  13 . A hydraulic tilt cylinder  15  is coupled to the outer mast  12  as a hydraulic actuating device. A hydraulic lift cylinder  16  is coupled to the inner mast  13  as a hydraulic actuating device. When hydraulic oil is supplied to a tilt cylinder  15  or hydraulic oil is discharged from the tilt cylinder  15 , the mast  14  tilts in a longitudinal direction of the vehicle body. When hydraulic oil is supplied to the lift cylinder  16  or hydraulic oil is discharged from the lift cylinder  16 , the inner mast  13  moves in a vertical direction of the vehicle body. A fork  18  as a cargo handling tool is attached to the inner mast  13  via a lift bracket  17 . When the lift cylinder  16  is actuated and the inner mast  13  moves upward or downward along the outer mast  12 , the fork  18  moves upward or downward along with the lift bracket  17 . 
     On the vehicle body of the forklift  10 , an engine  19 , a hydraulic pump  20  which is driven by the engine  19  and a hydraulic mechanism  21  are mounted. The engine  19  is a drive source for travelling operation and cargo handling operation of the forklift  10 . Hydraulic oil ejected from the hydraulic pump  20  is supplied to the hydraulic mechanism  21 . The hydraulic mechanism  21  controls supply and discharge of hydraulic oil to and from the cylinders  15  and  16 . An oil passage  23  is connected to the hydraulic pump  20  to supply the hydraulic oil pumped from the oil tank  22  to the hydraulic mechanism  21 . The oil passage  23  is connected to a discharge port of the hydraulic pump  20 . To the hydraulic mechanism  21 , a discharge oil passage  24  through which the hydraulic oil to be discharged to the oil tank  22  passes is connected. 
     On the vehicle body of the forklift  10 , a vehicle control device  25  as a control unit and an engine control device  26  are mounted. The engine control device  26  is electrically connected to the vehicle control device  25 . To the vehicle control device  25 , a tilt sensor  28  detecting an operating state of a tilting operating member  27  and a lift sensor  30  detecting an operating state of a lifting operating member  29  are electrically connected. The tilting operating member  27  is a member for giving an instruction of operation of the tilt cylinder  15 , while the lifting operating member  29  is a member for giving an instruction of operation of the lift cylinder  16 . Further, an accelerator sensor  32  detecting an accelerator opening degree and an operator detecting sensor  33  detecting whether or not there is an operator are electrically connected to the vehicle control device  25 . An accelerator operating member  31  is operated when the operator gives an instruction to accelerate the forklift  10 . The tilting operating member  27 , the lifting operating member  29  and the accelerator operating member  31  are disposed in an operating room of the forklift  10 . The operator detecting sensor  33  is disposed at an operator&#39;s seat. The vehicle control device  25  detects whether or not an operator exists at a right operation position based on the detection result of the operator detecting sensor  33 . When an operator does not exist at a right operation position, the vehicle control device  25  restricts cargo handling operation and travelling operation of the forklift  10 . 
     Further, the vehicle control device  25  controls engine speed by outputting a speed instruction of the engine  19  to the engine control device  26 . The engine control device  26  controls the engine  19  based on the speed instruction input to the engine control device  26 . The engine control device  26  outputs the actual speed of the engine  19  detected by a speed sensor  34  to the vehicle control device  25 . Because the hydraulic pump  20  is driven by the engine  19 , the tilt cylinder  15  and the lift cylinder  16  are actuated when the operator steps on the accelerator operating member  31  and operates the tilting operating member  27  and the lifting operating member  29 . 
     The hydraulic mechanism  21  has a control circuit  36  for controlling supply and discharge of hydraulic oil, and a pressure compensation circuit  37  for compensating pressure within the hydraulic mechanism  21 . The control circuit  36  has control valves  39  and  41 . The control valve  39  is connected to an oil chamber of the tilt cylinder  15  via an oil passage  38 . The control valve  41  is connected to an oil chamber of the lift cylinder  16  via an oil passage  40 . The control valves  39  and  41  are connected to the oil passage  23  and the discharge oil passage  24 , respectively. The oil passages  23 ,  38  and  40  constitute a connection oil passage connecting the hydraulic pump  20 , the tilt cylinder  15  and the lift cylinder  16 . 
     The tilting operating member  27  is mechanically coupled to the control valve  39 . Therefore, when the tilting operating member  27  is operated, a state of the control valve  39  is switched between an open state and a closed state. The lifting operating member  29  is mechanically coupled to the control valve  41 . Therefore, when the lifting operating member  29  is operated, a state of the control valve  41  is switched between an open state and a closed state. 
     The hydraulic oil is discharged from the hydraulic pump  20  and flows into the control valves  39  and  41  through the oil passage  23 . The hydraulic oil is supplied to the oil chambers of the cylinders  15  and  16  respectively through the oil passages  38  and  40 . For example, when the tilting operating member  27  is operated, the hydraulic oil is discharged from the hydraulic pump  20  and supplied to the oil chamber of the tilt cylinder  15  through the oil passage  38  connected to the control valve  39 . The hydraulic oil is discharged from the oil chambers of the cylinders  15  and  16  and discharged to the oil tank  22  through the discharge oil passage  24 . 
     The pressure compensation circuit  37  will be described next with reference to  FIG. 2  to  FIG. 4 . 
     As illustrated in  FIG. 2 , the pressure compensation circuit  37  has a first supply oil passage  45  connected to the tilt cylinder  15  and the lift cylinder  16 , and a second supply oil passage  46 . The first supply oil passage  45  introduces sensing pressure of the cylinders  15  and  16  into the pressure compensation circuit  37 . By this means, the first supply oil passage  45  becomes a supply oil passage through which hydraulic oil being supplied to the hydraulic actuating device passes. Further, the first supply oil passage  45  is connected to the discharge oil passage  24 . 
     The second supply oil passage  46  diverges at a diverging point P 1  on the connection oil passage including the oil passage  23 . Because the second supply oil passage  46  is connected to the oil passage  23 , the second supply oil passage  46  becomes a supply oil passage through which hydraulic oil being supplied to the hydraulic actuating device passes. Further, the second supply oil passage  46  is connected to a connection point P 2  located in the middle of the first supply oil passage  45 . 
     On the first supply oil passage  45 , a side connected to the cylinders  15  and  16  is defined as an upstream side, and a side connected to the discharge oil passage  24  is defined as a downstream side. To the first supply oil passage  45 , a check valve  47 , a relief pressure valve  48 , a filter  49  and an unloading valve  50  are connected in this order from the upstream side toward the downstream side. Predefined operating pressure is set at the relief pressure valve  48 . The unloading valve  50  is an electromagnetic valve and is switched between an open state and a closed state. An ON/OFF state of a solenoid of the unloading valve  50  is controlled by the vehicle control device  25 . 
       FIG. 2  illustrates the pressure compensation circuit  37  when the unloading valve  50  is put into a closed state. In this state, the discharge oil passage  24  is not connected to the first supply oil passage  45 .  FIG. 3  illustrates the pressure compensation circuit  37  when the unloading valve  50  is put into an open state. In this state, the discharge oil passage  24  is connected to the first supply oil passage  45 . In this way, the unloading valve  50  is a valve for connecting the first supply oil passage  45  which is a supply oil passage to the discharge oil passage  24 . 
     A diverging oil passage  51  is provided on the first supply oil passage  45 . The diverging oil passage  51  diverges from the first supply oil passage  45  and is connected to the first supply oil passage  45 . The diverging oil passage  51  is connected to a connection point P 3  between the relief pressure valve  48  and the check valve  47  and a connection point P 4  between the relief pressure valve  48  and the filter  49 . The diverging oil passage  51  can divert the hydraulic oil away from the relief pressure valve  48 . An on-off valve  52  is connected to the diverging oil passage  51 . The on-off valve  52  operates by a spring force. The on-off valve  52  opens or closes the diverging oil passage  51  by a state of the on-off valve  52  being switched between an open state and a closed state. 
     Between the on-off valve  52  and a connection point P 5  of the second supply oil passage  46 , an oil passage  53  is connected. A check valve  54  is connected to the oil passage  53 . The check valve  54  allows hydraulic oil to flow from the second supply oil passage  46  toward the on-off valve  52 . Further, an oil passage  55  diverging from the oil passage  53  to divert hydraulic oil away from the check valve  54  is connected to the oil passage  53 . An orifice  56  is connected to the oil passage  55 . As illustrated in  FIG. 2 , when the unloading valve  50  is put into a closed state, the on-off valve  52  is put into a closed state by receiving pressure against the spring force through the first supply oil passage  45 , the second supply oil passage  46  and the oil passage  53  (check valve  54 ). In other words, when the unloading valve  50  is put into a closed state, pressure of the second supply oil passage  46  is applied to the on-off valve  52  by the check valve  54 . On the other hand, as illustrated in  FIG. 3  and  FIG. 4 , when the unloading valve  50  is put into an open state, the above-described pressure applied through the first supply oil passage  45 , the second supply oil passage  46  and the oil passage  53  (check valve  54 ) is lowered, and the on-off valve  52  is put into an open state. As illustrated in  FIG. 4 , when the on-off valve  52  is put into an open state, the above-described pressure applied to the on-off valve  52  is released to the first supply oil passage  45  through the second supply oil passage  46  via the orifice  56  of the oil passage  55 . Further, between the diverging point P 1  and the connection point P 5  on the second supply oil passage  46 , a reducing valve  57  and an orifice  58  are connected. 
     Operation of the above-described hydraulic mechanism  21  will be described below with reference to  FIG. 2  to  FIG. 6 . 
     As illustrated in  FIG. 2 , in the pressure compensation circuit  37  when a load is applied, because the unloading valve  50  is put into a closed state, pressure of the oil passage  23  is not released to the discharge oil passage  24 . Therefore, hydraulic oil discharged from the hydraulic pump  20  flows into the tilt cylinder  15  and the lift cylinder  16  through the control valves  39  and  41 . That is,  FIG. 2  illustrates a state of the pressure compensation circuit  37  during normal cargo handling operation. In this case, the on-off valve  52  is put into a closed state by receiving pressure through the first supply oil passage  45 , the second supply oil passage  46  and the oil passage  53  (check valve  54 ). 
     The forklift  10  sometimes performs cargo handling operation in a state where a load is applied while pressure inside the hydraulic mechanism  21  is lowered, for example, when the accelerator operating member  31  is not operated and speed of the engine  19  is restricted to be speed for an idle state. In such a case, when the hydraulic actuating device is activated, the load of the hydraulic pump  20  rapidly increases, which may result in deficiency of torque of the engine  19  and may cause an engine stall. Therefore, the vehicle control device  25  controls the engine  19  to avoid an engine stall in a state where rapid fluctuation of the load occurs. 
     The cargo handling operation includes operation of the tilt cylinder  15  and operation of the lift cylinder  16 . Such cargo handling operation becomes a load operation which applies a load to the engine  19 . Further, the vehicle control device  25 , when detecting that the operator exists at a right operation position based on the detection result of the operator detecting sensor  33 , allows cargo handling operation. This state refers to a state where operation of the tilt cylinder  15  and operation of the lift cylinder  16  are allowed. 
     Upon instructed to perform a load operation that applies a load to the engine  19 , as illustrated in  FIG. 3 , the vehicle control device  25  switches a state of the unloading valve  50  to an open state. By this means, pressure inside the pressure compensation circuit  37  is released to the discharge oil passage  24 . At this time, the hydraulic oil is discharged from the hydraulic pump  20  and flows through the oil passage  23 . Meanwhile, the pressure of the oil passage  23  is lowered as the pressure of the pressure compensation circuit  37  is lowered. Further, supply of the hydraulic oil to the tilt cylinder  15  or the lift cylinder  16  through the control valves  39  and  41  increases the pressure of the first supply oil passage  45 . Further, regardless of cargo handling operation, the pressure is compensated so that the pressure of the oil passage  23  is slightly higher than the pressure of the first supply oil passage  45 . 
     In the pressure compensation circuit  37 , when the pressure of the first supply oil passage  45  reaches operating pressure of the relief pressure valve  48 , the relief pressure valve  48  is opened. By this means, the pressure of the first supply oil passage  45  is released to the discharge oil passage  24  through the unloading valve  50 , so that the pressure of the first supply oil passage  45  does not increase higher than the operating pressure of the relief pressure valve  48 . Further, the pressure of the oil passage  23  is also maintained at pressure slightly higher than the pressure of the first supply oil passage  45 . At this time, pressure against the spring force is applied to the on-off valve  52 . Therefore, the on-off valve  52  maintains a closed state. 
     On the other hand, when the vehicle control device  25  switches the state of the unloading valve  50  to an open state, the vehicle control device  25  maintains an open state of the unloading valve  50  for a predetermined time period (for example, several hundred ms). When the predetermined time period has elapsed since the unloading valve  50  was controlled to be put into an open state, the vehicle control device  25  switches the state of the unloading valve  50  to a closed state. As a result, as illustrated in  FIG. 2 , the pressure compensation circuit  37  is put into a state where the pressure of the first supply oil passage  45  is not released to the discharge oil passage  24 . Therefore, the pressure compensation circuit  37  cannot release the pressure outside the circuit. Accordingly, the pressure inside the circuit increases beyond the operating pressure of the relief pressure valve  48 . Further, the pressure of the first supply oil passage  45  and the pressure of the second supply oil passage  46  also increase. As a result, pressure required for actuating the tilt cylinder  15  and the lift cylinder  16  is supplied to the control valves  39  and  41  through the oil passage  23 , which allows the tilt cylinder  15  and the lift cylinder  16  to be actuated according to the operation of the tilting operating member  27  and the lifting operating member  29 . 
       FIG. 5  illustrates change of pressure and engine speed caused by the above-described control. Solid line in  FIG. 5  indicates pressure, and dashed-dotted line in  FIG. 5  indicates engine speed. 
     Upon instructed to perform cargo handling operation, the hydraulic pump  20  is driven and pressure of the hydraulic mechanism  21  increases, while the engine speed is lowered from speed X (speed for an idle state). Therefore, as illustrated in  FIG. 3 , when the unloading valve  50  is put into an open state (“ON” in  FIG. 5 ), as described above, the pressure increases to pressure Y which is equal to the operating pressure of the relief pressure valve  48  (time T 1  in  FIG. 5 ). Then, the pressure is maintained at pressure Y. Therefore, an increase of the load of the hydraulic pump  20  is stopped once, so that an engine stall is avoided. Further, at time T 2 , the engine  19  can recover to increase the engine speed. Then, at time T 3 , as illustrated in  FIG. 2 , when the state of the unloading valve  50  is switched to a closed state (“OFF” in  FIG. 5 ), the pressure increases beyond the operating pressure of the relief pressure valve  48 . The pressure then reaches pressure Z required for actuating the tilt cylinder  15  and the lift cylinder  16 . 
     As illustrated in  FIG. 5 , the vehicle control device  25  switches the state of the unloading valve  50  to an open state when the predetermined time period, which is the time period until time T 3  has elapsed. Pressure Y, which is the operating pressure of the relief pressure valve  48 , is preferably the maximum pressure at which an engine stall can be avoided, and can be calculated through a simulation, or the like. When pressure Y is set too high, as is clear from the change of the engine speed illustrated in  FIG. 5 , an engine stall is more likely to occur. On the other hand, when the predetermined time period is set too long or pressure Y is set too low, as is clear from  FIG. 5 , a time period required for the pressure to reach pressure Z may be longer. That is, if the time period required for the pressure to reach pressure Z becomes longer, there is a possibility that even if it is instructed to perform a cargo handling operation, the tilt cylinder  15  and the lift cylinder  16  may not respond for a longer period. As illustrated in  FIG. 5 , in order to prevent occurrence of an engine stall, the vehicle control device  25  opens or closes the unloading valve  50  of the pressure compensation circuit  37  to increase the pressure within the circuit in two stages. 
     Next, a control process for restricting the cargo handling operation will be described with reference to  FIG. 2  to  FIG. 4 . 
     The vehicle control device  25 , when detecting that the operator is not located at the right operation position when a load is applied as illustrated in  FIG. 2 , switches the state of the unloading valve  50  to an open state as illustrated in  FIG. 3 . By this means, because the relief pressure valve  48  is actuated, the pressure of the first supply oil passage  45  is released to the discharge oil passage  24  through the unloading valve  50 . Therefore, the pressure of the first supply oil passage  45  is lowered. 
     At a time point at which the state of the unloading valve  50  is switched to an open state, the pressure of the oil passage  53  is also released to the discharge oil passage  24 . Here, because the pressure of the oil passage  53  attached to the on-off valve  52  is released, the state of the on-off valve  52  is switched from the closed state to the open state by the spring force. A time period required to switch the state is determined by a radius of the orifice  56 . As illustrated in  FIG. 4 , when the state of the on-off valve  52  is switched to an open state, the pressure of the first supply oil passage  45  is released to the discharge oil passage  24  through the on-off valve  52 . Meanwhile, the pressure of the oil passage  23  is slightly higher than that of the first supply oil passage  45 . Therefore, when the pressure of the first supply oil passage  45  is released to the discharge oil passage  24 , the pressure of the oil passage  23  is lowered. As a result, pressure applied to the tilt cylinder  15  and the lift cylinder  16  is lowered. Accordingly, when the operator is not located at the right operation position, even if the operator gives an instruction for a cargo handling operation, operation of the tilt cylinder  15  and the lift cylinder  16  is restricted, and the tilt cylinder  15  and the lift cylinder  16  are not actuated. Here, a timer circuit unit for opening the first supply oil passage  45  when a certain time period has elapsed is constructed of the on-off valve  52 , the check valve  54  and the orifice  56 . 
       FIG. 6  illustrates change of pressure caused by control for restricting the cargo handling operation. As illustrated in  FIG. 6 , the vehicle control device  25 , when detecting that the operator is not located at the right operation position at time T 4 , switches the state of the unloading valve  50  to an open state. By this means, the pressure applied to the cylinders  15  and  16  is gradually lowered from pressure Z by being released to the discharge oil passage  24  through the pressure compensation circuit  37 . When the pressure reaches the pressure Y and the state of the on-off valve  52  is switched to an open state, the pressure applied to the cylinders  15  and  16  is further lowered and becomes zero at time T 5 . By this means, even if an instruction for a cargo handling operation is given for some reasons, the cargo handling operation is restricted, and thus is not performed. 
     Therefore, according to the present embodiment, the following effects can be obtained. 
     (1) Upon instructed to perform a cargo handling operation that applies load to the engine  19 , because the state of the unloading valve  50  is switched to an open state, it is possible to suppress rapid increase of pressure, so that it is possible to prevent occurrence of an engine stall. Further, because the state of the unloading valve  50  is switched to an open state, the pressure within the hydraulic circuit is lowered, so that it is possible to restrict operation of the cylinders  15  and  16 . Therefore, it is possible to perform unloading when necessary while preventing occurrence of an engine stall. 
     (2) The vehicle control device  25 , when detecting that the operator is not located at the right operation position, switches the state of the unloading valve  50  to an open state to lower the pressure within the hydraulic circuit, so that it is possible to prevent erroneous operation from occurring for some reasons. 
     (3) By using the on-off valve  52  which mechanically opens or closes the oil passage, it is possible to simplify the structure of the hydraulic circuit, so that it is possible to prevent an increase in cost. 
     (4) The vehicle control device  25  can increase the pressure applied to the cylinders  15  and  16  in a stepwise manner, so that it is possible to prevent occurrence of an engine stall by switching the state of the unloading valve  50  to an open state, and then, actuate the cylinders  15  and  16  by switching the state of the unloading valve  50  to a closed state. 
     The above-described embodiment may be modified as follows. 
     While in the present embodiment, the state of the unloading valve  50  is switched to an open state through an instruction of cargo handling operation which applies load to the engine  19 , there is also a case where load is applied to the engine  19  in a case other than a case where an instruction of cargo handling operation is given, in which case, the engine speed may be lowered. Therefore, when it is detected that the engine speed is lowered, it may be judged that a load is applied to the engine  19 . In this case, the state of the unloading valve  50  may be switched to an open state based on the detection that the engine speed is lowered. 
     As illustrated in  FIG. 7 , the timer circuit unit may be constructed of an electromagnetic valve  59 . When cargo handling operation is restricted, the vehicle control device  25  may switch the state of the electromagnetic valve  59  to an open state after a predetermined time period has elapsed since the state of the unloading valve  50  was switched to an open state, and release the pressure of the first supply oil passage  45  to the discharge oil passage  24 . 
     The forklift  10  may further have a hydraulic cylinder for making an attachment operate as the hydraulic actuating device. 
     The forklift  10  may further include a hydraulic cylinder for making a power steering mechanism operate as the hydraulic actuating device. 
     It is also possible to use electromagnetic valves as the control valves  39  and  41  and control opening and closing of the electromagnetic valves by the vehicle control device  25 . 
     The industrial vehicle may be a vehicle having a hydraulic actuating device, such as a shovel loader, other than the forklift  10 .