Patent Publication Number: US-2021189690-A1

Title: Loading work vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to a loading work vehicle that performs loading work by excavating such as earth and sand and minerals, and loading them into such as a dump truck. 
     BACKGROUND ART 
     As represented by a wheel loader, in the case of a loading work vehicle including a hydraulic circuit for traveling and a hydraulic circuit for a work device that performs excavation, etc., it is important to balance between traction force (traveling drive force) and digging force of the work device. When the traction force is too great against the digging force of the work device, at the time of operating a lift arm to lift a bucket in the upward direction after pushing the bucket into an object to be excavated, slippage of wheels occurs. As a result of the slippage, the traction force becomes rather small, and thus loads such as earth and sand are hardly to be inserted onto the bucket. Furthermore, in this case, since reaction force acting on the lift arm increases at the time of pushing the bucket into the object to be excavated, there are cases where the reaction force becomes resistance which prevents the bucket and the lift arm from being lifted in the upward direction. 
     For example, Patent Literature 1 discloses a wheel loader comprising, as a hydraulic circuit for traveling, a traveling hydraulic pump which is a variable displacement hydraulic pump driven by an engine, and a traveling hydraulic motor which is a variable displacement hydraulic motor driven by hydraulic oil from a hydraulic oil pump, and using an HST circuit in which the traveling hydraulic pump and the traveling hydraulic motor are connected by a pair of main conduits to form a closed circuit. In the case of the wheel loader according to Patent Literature 1, at the time of pushing the bucket into the natural ground as an object to be excavated so as to insert the loads onto the bucket, by setting the maximum tilt of an HST motor to an upper limit value, the traction force is made to exhibit its maximum force to the upper limit value, thereby allowing the loads to be inserted sufficiently onto the bucket. Furthermore, at the time of performing a lifting operation of the lift arm to lift the bucket in the upward direction, by limiting the maximum tilt of the HST motor to about 50% to 70% of the upper limit value, the traction force is suppressed from being excessively increased. Accordingly, balance between lifting operation force of the lift arm (digging force of the work device) and the traction force is excellently maintained, thereby facilitating the lifting operation of the bucket containing the load therein. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-B-5129493 
     SUMMARY OF INVENTION 
     Technical Problem 
     At a work field, on a road surface where slippage of the wheels likely occurs, in other words, where a static friction coefficient between the wheels and the road surface is small, the wheels of the wheel loader slip even with the traction force lower than the set maximum traction force. Accordingly, when the wheel loader according to Patent Literature 1 pushes the bucket into the natural ground while traveling on the slippery road surface, slippage of the wheels likely occurs. When the wheels slip, the traction force is lowered at once while the road surface is dug and recessed, which lowers working efficiency of the wheel loader. 
     In view of the problems above, an object of the present invention is to provide a loading work vehicle capable of improving work efficiency even when working on a slippery road surface. 
     Solution to Problem 
     In order to achieve the object above, the present invention provides a loading work vehicle comprising: a plurality of wheels; an engine; a variable displacement type traveling hydraulic pump driven by the engine; a variable displacement type traveling hydraulic motor that is connected to the traveling hydraulic pump by a closed circuit and transmits drive force of the engine to the wheels; and a work device that is provided on a front portion of a vehicle body and rotatable in a vertical direction, wherein the loading work vehicle further comprises: a traveling state detection sensor configured to detect a traveling state of the loading work vehicle, an operation detection sensor configured to detect an operation of the work device; a mode switch device configured to switch between a limit mode for limiting maximum traction force of the loading work vehicle and a normal mode for not limiting the maximum traction force of the loading work vehicle; and a controller configured to control the traveling hydraulic pump and the traveling hydraulic motor, and the controller is further configured to: determine whether the limit mode is selected based on a mode switch signal from the mode switch device; in a case of determining that the limit mode is selected, specify an operation state of the loading work vehicle based on the traveling state detected by the traveling state detection sensor and the operation of the work device detected by the operation detection sensor; in a case of specifying that the vehicle body is traveling in a state of not performing a lifting operation of the work device, limit the maximum traction force of the loading work vehicle to a first set value which is set based on a static friction coefficient between a road surface and the wheels as well as weight of the vehicle body; and in a case of specifying that the work device starts excavating an object to be excavated, increase the maximum traction force of the loading work vehicle from the first set value. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to improve work efficiency even when working on a slippery road surface. The problems, configurations, and effects other than those described above will be clarified by explanation of the embodiment below. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view illustrating appearance of a wheel loader according to an embodiment of the present invention. 
         FIG. 2A  to  FIG. 2C  explain excavation work performed by a wheel loader. 
         FIG. 3  illustrates a hydraulic circuit and an electric circuit according to traveling drive of a wheel loader. 
         FIG. 4  illustrates a graph showing a relationship between a step-on amount of an accelerator pedal and target engine rotational speed. 
         FIG. 5A  illustrates a graph showing a relationship between engine rotational speed and a displacement volume of an HST pump,  FIG. 5B  illustrates a graph showing a relationship between the engine rotational speed and input torque of the HST pump, and  FIG. 5C  illustrates a graph showing a relationship between the engine rotational speed and a discharge flow rate of the HST pump. 
         FIG. 6  illustrates a hydraulic circuit according to drive of a work device. 
         FIG. 7  illustrates a graph showing a relationship between discharge pressure of a loading hydraulic pump and spool opening area. 
         FIG. 8  is a functional block diagram illustrating functions of a controller. 
         FIG. 9  illustrates a flowchart of processing executed by a controller. 
         FIG. 10  illustrates a graph showing a relationship between discharge pressure of a loading hydraulic pump and a maximum displacement volume of an HST motor. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a wheel loader will be described as one of the aspects of a loading work vehicle according to an embodiment of the present invention. 
     (Overall Configuration of Wheel Loader  1 ) 
     Firstly, an overall configuration of a wheel loader  1  according to the embodiment of the present invention will be described with reference to  FIG. 1  and  FIG. 2 . 
       FIG. 1  is a side view illustrating appearance of the wheel loader  1  according to the embodiment of the present invention.  FIG. 2  explains excavation work performed by the wheel loader  1 . 
     The wheel loader  1  includes a vehicle body which is provided with a front frame  1 A and a rear frame  1 B, and a work device  2  for loading work which is provided on the front of the vehicle body. The wheel loader  1  is an articulated type work vehicle which is swiveled on a central portion of the vehicle body and steered thereby. Specifically, the front frame  1 A and the rear frame  1 B are connected to each other by a center joint  10  to swivel in the left and right direction so that the front frame  1 A is bent in the left and right direction with respect to the rear frame  1 B. 
     The wheel loader  1  includes a pair of left and right front wheels  11 A which is provided on the front frame  1 A, and a pair of left and right rear wheels  11 B which is provided on the rear frame  1 B.  FIG. 1  illustrates, among the pair of left and right front wheels  11 A and the pair of left and right rear wheels  11 B, only the left front wheel  11 A of the pair of left and right front wheels  11 A and the left rear wheel  11 B of the pair of left and right rear wheels  11 B. In the following, there are cases where the “front wheels  11 A and rear wheels  11 B” are collectively referred to as “wheels  11 A,  11 B”. 
     The rear frame  1 B is provided with an operator&#39;s cab  12  to be boarded by an operator, a mechanical room  13  in which devices such as an engine, a controller, and a hydraulic pump are accommodated, and a counterweight  14  for maintaining balance between the vehicle body and the work device  2  to prevent the vehicle body from tilting. In the rear frame  1 B, the operator&#39;s cab  12  is disposed on the front, the counterweight  14  is disposed on the rear, and the mechanical room  13  is disposed between the operator&#39;s cab  12  and the counterweight  14 , respectively. 
     The work device  2  includes a lift arm  21  attached to the front frame  1 A, a pair of lift arm cylinders  22  configured to expand and contract to rotate the lift arm  21  in the vertical direction with respect to the front frame  1 A, a bucket  23  attached to the front end portion of the lift arm  21 , a bucket cylinder  24  configured to expand and contract to rotate the bucket  23  in the vertical direction with respect to the lift arm  21 , a bell crank  25  that is rotatably connected to the lift arm  21  and constitutes a link mechanism between the bucket  23  and the bucket cylinder  24 , and a plurality of conduits (not illustrated) for leading hydraulic oil to the pair of lift arm cylinders  22  and the bucket cylinder  24 .  FIG. 1  illustrates only one of the pair of lift arm cylinders  22 , which is disposed on the left side, by indicating it by a broken line. 
     The lift arm  21  is rotated in the upward direction by expansion of a rod  220  of each of the lift arm cylinders  22 , and rotated in the downward direction by contraction of each rod  220 . The bucket  23  is tilted (rotated in the upward direction with respect to the lift arm  21 ) by expansion of a rod  240  of the bucket cylinder  24 , and dumped (rotated in the downward direction with respect to the lift arm  21 ) by contraction of the rod  240 . 
     The wheel loader  1  is a loading work vehicle configured to perform loading work by excavating such as earth and sand and minerals in a strip mine, etc., and loading them into such as a dump truck. In the wheel loader  1 , the bucket  23  can be replaced with various attachments such as a blade, and in addition to general loading work, various work such as dozing work and snow removing work can be performed. 
     Next, excavation work of the wheel loader  1  will be described with reference to  FIG. 2A  to  FIG. 2C . 
     Firstly, the wheel loader  1  travels forward toward a natural ground  100  which is an object to be excavated at full throttle, and pushes the bucket  23  into the natural ground  100  (state illustrated in  FIG. 2A ). Next, an operator performs the lifting operation of the lift arm  21  while tilting the bucket  23 , or performs the lifting operation of the lift arm  21  after tilting the bucket  23 , to make the wheel loader  1  scoop up loads such as earth and sand and minerals (state illustrated in  FIG. 2B ). Then, due to the lifting operation of the lift arm  21  performed by the operator, the bucket  23  containing the loads therein is lifted further in the upward direction (state illustrated in  FIG. 2C ). 
     The series of operations illustrated in  FIG. 2A  to  FIG. 2C  is referred to as “excavation work”. In the excavation work, the operation of the wheel loader  1  illustrated in  FIG. 2A  is referred to as “pushing bucket into ground”. Furthermore, in the operations of the wheel loader  1  illustrated in  FIG. 2B  and  FIG. 2C , a period during which the lifting operation of the lift arm  21  is performed by the operation of the operator is referred to as “excavation”. Accordingly, at the time of scooping up the loads, when the lifting operation of the lift arm  21  is performed after tilting the bucket  23 , the period after the time when the lifting operation of the lift arm  21  is started is referred to “excavation”. 
     (Traveling Drive System) 
     Next, a traveling drive system of the wheel loader  1  will be described with reference to  FIG. 3  to  FIG. 5 . 
       FIG. 3  illustrates a hydraulic circuit and an electric circuit of the wheel loader  1 .  FIG. 4  illustrates a graph showing a relationship between a step-on amount of an accelerator pedal and target engine rotational speed.  FIG. 5A  illustrates a graph showing a relationship between engine rotational speed and a displacement volume of an HST pump  41 ,  FIG. 5B  illustrates a graph showing a relationship between the engine rotational speed and input torque of the HST pump  41 , and  FIG. 5C  illustrates a graph showing a relationship between the engine rotational speed and a discharge flow rate of the HST pump  41 . 
     The wheel loader  1  according to the present embodiment is provided with an HST traveling drive device including a hydraulic circuit which is a closed circuit. As illustrated in  FIG. 3 , the HST traveling drive device includes an engine  3 , a hydraulic oil tank  40  that stores hydraulic oil, an HST pump  41  as a traveling hydraulic pump driven by the engine  3 , an HST charge pump  41 A that supplies hydraulic oil for controlling the HST pump  41 , an HST motor  42  as a traveling hydraulic motor connected to the HST pump  41  via a pair of conduits  400 L,  400 R by a closed circuit, a forward/reverse changeover valve  43  for switching forward and reverse movement of the vehicle body, and a controller  5  for controlling each device such as the HST pump  41  and the HST motor  42 . 
     The HST pump  41  is a swash plate type or swash shaft type variable displacement hydraulic pump of which displacement volume is controlled in accordance with a tilt angle. The tilt angle is adjusted by a tilt cylinder  44  which is driven by the hydraulic oil discharged from the HST charge pump  41 A and acting thereon. 
     The HST motor  42  is a swash plate type or a swash shaft type variable displacement hydraulic motor of which displacement volume is controlled in accordance with a tilt angle, and transmits drive force of the engine  3  to the wheels  11 A,  11 B. The tilt angle is adjusted by a regulator  420  in accordance with a command signal output from the controller  5 . 
     In the HST traveling drive device, firstly, the engine  3  is rotated when the operator steps on an accelerator pedal  61  provided in the operator&#39;s cab  12 , and the HST pump  41  is driven by the drive force of the engine  3 . Furthermore, the HST charge pump  41 A is also driven by the drive force of the engine  3 , and the hydraulic oil discharged from the HST charge pump  41 A is guided to the tilt cylinder  44  via the forward/reverse changeover valve  43 . 
     The forward/reverse changeover valve  43  is provided between the charge pump  41 A and the tilt cylinder  44 . The forward/reverse changeover valve  43  is connected to a discharge conduit  800  connected to a discharge side of the HST charge pump  41 A by a pair of main conduits  800 A,  800 B. Furthermore, the forward/reverse changeover valve  43  is connected to left and right oil chambers  44 L,  44 R of the tilt cylinder  44  by a pair of pilot conduits  800 L,  800 R. 
     The forward/reverse changeover valve  43  includes a forward position  43 A for making the vehicle body travel in the forward direction, a reverse position  43 B for making the vehicle body travel in the reverse direction, and a neutral position  43 N for stopping the vehicle body, and is operated by the forward/reverse changeover lever  62  provided in the operator&#39;s cab  12 . 
     As illustrated in  FIG. 3 , when the forward/reverse changeover valve  43  is in the neutral position  43 N, the hydraulic oil discharged from the HST charge pump  41 A acts equally on the left and right oil chambers  44 L,  44 R of the tilt cylinder  44  via a throttle  401  provided on the main conduit  800 B which is one of the main conduits  800 A,  800 B, respectively. Thus, since a rod of the tilt cylinder  44  is positioned in the neutral, the displacement volume of the HST pump  41  becomes 0 and a discharge amount of the HST pump  41  becomes 0. In this way, the vehicle body is brought to a stopped state. 
     On the other hand, when the forward/reverse changeover valve  43  is switched to the forward position  43 A, the pressure on the upstream side of the throttle  401  acts on the left side oil chamber  44 L of the tilt cylinder  44 , and the pressure on the downstream side of the throttle  401  acts on the right side oil chamber  44 R. Due to the pressure difference generated between the left and right oil chambers  44 L,  44 R, the rod of the tilt cylinder  44  is actuated in the right direction of  FIG. 3 . Accordingly, a tile angle of the HST pump  41  becomes wise and the hydraulic oil from the HST pump  41  is guided to the HST motor  42  through the conduit  400 L, whereby the HST motor  42  is rotated in the forward direction to make the vehicle body travel in the forward direction. 
     On the other hand, when the forward/reverse changeover valve  43  is switched to the reverse position  43 B, the pressure on the downstream side of the throttle  401  acts on the left side oil chamber  44 L of the tilt cylinder  44 , and the pressure on the upstream side of the throttle  401  acts on the right side oil chamber  44 R. Due to the pressure difference generated between the left and right oil chambers  44 L,  44 R, the rod of the tilt cylinder  44  is actuated in the left direction of  FIG. 3 . Accordingly, the tilt angle of the HST pump  41  becomes wide and the hydraulic oil from the HST pump  41  is guided to the HST motor  42  via the conduit  400 R, whereby the HST motor  42  is rotated in the reverse direction to make the vehicle body travel in the reverse direction. 
     Since the HST motor  42  is rotated by the hydraulic oil guided from the HST pump  41  as described above, output torque from the HST motor  42  is transmitted to the front wheels  11 A and the rear wheels  11 B via an axle  15  so as to make the wheel loader  1  travel. Therefore, the output torque of the HST motor  42  serves as the traveling drive force of the wheel loader  1 , that is, the traction force of the vehicle body. 
     The output torque of the HST motor  42  is expressed by the product of the displacement volume (tilt angle) of the HST motor  42  and the traveling load pressure (=pressure of conduit  400 L minus pressure of conduit  400 R). Accordingly, it is possible to control the traction force of the vehicle body by controlling the displacement volume of the HST motor  42 . 
     The rotational speed of the engine  3  is adjusted by a step-on amount of the accelerator pedal  61 , and a discharge amount of the HST charge pump  41 A connected to the engine  3  is proportional to the rotational speed of the engine  3 . Therefore, the differential pressure between the front and the rear of the throttle  401  is proportional to the rotational speed of the engine  3 , and the tilt angle of the HST pump  41  is also proportional to the rotational speed of the engine  3 . 
     The step-on amount of the accelerator pedal  61  is detected by a step-on amount sensor  610  mounted on the accelerator pedal  61 . The rotational speed of the engine  3  is controlled in accordance with target engine rotational speed corresponding to the step-on amount detected by the step-on amount sensor  610 . 
     As illustrated in  FIG. 4 , the step-on amount of the accelerator pedal  61  is proportional to the target engine rotational speed. As the step-on amount of the accelerator pedal  61  increases, the target engine rotational speed also increases. When the step-on amount of the accelerator pedal  61  reaches S 2 , the target engine rotational speed becomes maximum rotational speed Nmax 1 . 
     In  FIG. 4 , a range where the step-on amount of the accelerator pedal  61  is 0 to S 1  (for example, a range of 0% to 20% or 30%) is set as a dead zone in which the target engine rotational speed becomes constant at predetermined minimum rotational speed Nmin regardless of the step-on amount of the accelerator pedal  61 . Furthermore, a range where the step-on amount of the accelerator pedal  61  is S 2  to 100 (for example, a range of 70% or 80% to 100%) is set such that the target engine rotational speed is maintained at the maximum target engine rotational speed Nmax regardless of the step-on amount of the accelerator pedal  61 . These ranges can be arbitrarily set and changed. 
     Next, the relationship between the engine  3  and the HST pump  41  is as illustrated in  FIG. 5A  to  FIG. 5C . 
     As illustrated in  FIG. 5A , in a range where the engine rotational speed is between N 1  and N 2 , the rotational speed N of the engine  3  and the displacement volume q of the HST pump  41  are in a proportional relationship, and as the rotational speed of the engine  3  increases from N 1  to N 2  (N 1 &lt;N 2 ), the displacement volume increases from 0 to a predetermined value qc. When the engine rotational speed is N 2  or higher, the displacement volume of the HST pump  41  becomes constant at the predetermined value qc regardless of the engine rotational speed. 
     The input torque of the HST pump  41  is obtained by multiplying the displacement volume and the discharge pressure (input torque=displacement volume×discharge pressure). As illustrated in  FIG. 5B , in the range where the engine rotational speed is between N 1  and N 2 , the rotational speed N of the engine  3  and the input torque T of the HST pump  41  are in a proportional relationship, and as the rotational speed of the engine  3  increases from N 1  to N 2 , the input torque increases from 0 to a predetermined value Tc. When the engine rotational speed is N 2  or higher, the input torque of the HST pump  41  becomes constant at the predetermined value Tc regardless of the engine rotational speed. 
     As illustrated in  FIG. 5C , in the range where the engine rotational speed is between N 1  and N 2 , the discharge flow rate Q of the HST pump  41  and the rotational speed N of the engine  3  are in a quadratically proportional relationship, and as the rotational speed of the engine  3  increases from N 1  to N 2 , the discharge flow rate of the HST pump  41  increases from 0 to Q 1 . When the engine rotational speed is N 2  or higher, the rotational speed N of the engine  3  and the discharge flow rate Q of the HST pump  41  is in a proportional relationship. 
     As the rotational speed N of the engine  3  increases, the discharge flow rate Q of the HST pump  41  increases and a flow rate of the hydraulic oil flowing from the HST pump  41  into the HST motor  42  increases, and accordingly, the rotational speed of the HST motor  42  increases and the vehicle speed increases. The vehicle speed is detected as the rotational speed of the HST motor  42  by a motor rotational speed sensor  72  (see  FIG. 3 ). 
     As described above, in the HST traveling drive device, the vehicle speed is controlled by continuously increasing or decreasing the discharge flow rate of the HST pump  41  (speed change), thereby making it possible for the wheel loader  1  to start traveling smoothly and stop with less impact. 
     As illustrated in  FIG. 3 , the hydraulic oil discharged from the HST charge pump  41 A is also guided to the conduits  400 L,  400 R via the throttle  401  and check valves  402 A,  402 B. The pressure on the downstream side of the throttle  401  is limited by a charge relief valve  403  provided on a conduit connecting the forward/reverse changeover valve  43  and the hydraulic oil tank  40 , and the maximum pressure in the conduits  400 L,  400 R is limited by a relief valve  404 . 
     The HST traveling drive device according to the present embodiment is provided with a high-pressure selection valve  405  for selecting the higher pressure out of the conduit pressure in the conduits  400 L,  400 R that guide the hydraulic oil from the HST pump  41  to the HST motor  42 . The pressure selected by the high-pressure selection valve  405  is input to the controller  5 . 
     Furthermore, the wheel loader  1  includes a mode switch device  60  provided in the operator&#39;s cab  12  for switching between a limit mode for limiting the maximum traction force (traveling drive force) of the wheel loader  1  and a normal mode for not limiting the maximum traction force of the wheel loader  1 . A switch signal from the mode switch device  60  is input to the controller  5 . 
     (Drive System of Work Device  2 ) 
     Next, the drive system of the work device  2  will be described with reference to  FIG. 3 ,  FIG. 6 , and  FIG. 7 . 
       FIG. 6  illustrates a hydraulic circuit according to drive of the work device  2 .  FIG. 7  illustrates a graph showing a relationship between the discharge pressure of a loading hydraulic pump  45  and spool opening area. 
     As illustrated in  FIG. 3  and  FIG. 6 , the wheel loader  1  includes the loading hydraulic pump  45  that is driven by the engine  3  to supply the hydraulic oil to the work device  2 , a control valve  46  that is provided between each of the lift arm cylinders  22  and the bucket cylinder  24  and the loading hydraulic pump  45  to control a flow of the hydraulic oil supplied from the loading hydraulic pump  45  to the lift arm cylinders  22  and the bucket cylinder  24 , respectively, a lift arm operation lever  210  for operating the lift arm  21 , and a bucket operation lever  230  for operating the bucket  23 . 
     In the present embodiment, a fixed hydraulic pump is used as the loading hydraulic pump  45 . As illustrated in  FIG. 6 , the loading hydraulic pump  45  is connected to the control valve  46  by a first conduit  801 . The discharge pressure from the loading hydraulic pump  45  is detected by a discharge pressure sensor  75  provided on the first conduit  801 , and a signal relating to the detected discharge pressure is input to the controller  5 . The discharge pressure sensor  75  is one of the aspects of a discharge pressure detection sensor for detecting the discharge pressure of the loading hydraulic pump  45 . 
     Each of the lift arm operation lever  210  and the bucket operation lever  230  is one of the aspects of an operation device for operating the work device  2 , and provided in the operator&#39;s cab  12  (see  FIG. 1 ). For example, when the operator operates the lift arm operation lever  210 , pilot pressure proportional to its operation amount is generated as an operation signal. 
     As illustrated in  FIG. 6 , the generated pilot pressure is guided to a pair of pilot conduits  64 L,  64 R connected to a pair of pressure receiving chambers of the control valve  46  to act on the control valve  46 . Thus, the spool in the control valve  46  strokes in accordance with the pilot pressure, and a flowing direction and a flow rate of the hydraulic oil is determined. The control valve  46  is connected to a bottom chamber of each of the lift arm cylinders  22  by a second conduit  802 , and is connected to a rod chamber of each of the lift arm cylinders  22  by a third conduit  803 . 
     The hydraulic oil discharged from the loading hydraulic pump  45  is guided to the first conduit  801 , and is further guided to the second conduit  802  or the third conduit  803  via the control valve  46 . When being guided to the second conduit  802 , the hydraulic oil flows into the bottom chamber of each of the lift arm cylinders  22 , whereby the rod  220  of each of the lift arm cylinders  22  is extended to raise the lift arm  21 . On the other hand, when being guided to the third conduit  803 , the hydraulic oil flows into the rod chamber of each of the lift arm cylinders  22 , whereby the rod  220  of each of the lift arm cylinders  22  is contracted to lower the lift arm  21 . 
     As illustrated in  FIG. 6 , the lift arm operation amount of the lift arm operation lever  210  is detected by an operation amount sensor  76  mounted on the lift arm operation lever  210 . The operation amount sensor  76  is one of the aspects of an operation amount detection sensor for detecting an operation amount of the lift arm operation lever  210  which is an operation device. 
     In the present embodiment, each of the lift arm operation lever  210  and the bucket operation lever  230  is a hydraulic lever, respectively. Meanwhile, an electric lever may be used therefor, and in such a case, a current value corresponding to an operation amount is generated as an operation signal. 
     As illustrated in  FIG. 7 , the lifting operation amount of the lift arm operation lever  210  and the spool opening area of the control valve  46  are in a proportional relationship, and as the lifting operation amount of the lift arm operation lever  210  increases, the spool opening area also increases. Accordingly, when the lift arm operation lever  210  is operated largely in the direction of raising the lift arm  21 , the amount of the hydraulic oil flowing into the bottom chamber of each of the lift arm cylinders  22  increases and the rod  220  is extended quickly. That is, as the operation amount of the lift arm operation lever  210  increases, operation speed of the lift arm  21  also increases. 
     In  FIG. 7 , a range where the lifting operation amount of the lift arm operation lever  210  is 0% to 10% is set as a dead zone in which the spool does not open and the opening area becomes 0% even when the lift arm operation lever  210  is operated. Furthermore, a range where the lifting operation amount of the lift arm operation lever  210  is 85% to 100%, the spool opening area is constant at 100% and a full lever operation state is maintained. These ranges can be arbitrarily set and changed. 
     Here, the discharge pressure of the loading hydraulic pump  45  and the operation amount of the lift arm operation lever  210  are indices for an operation of the lift arm  21 , respectively. Each of the discharge pressure sensor  75  and the operation amount sensor  76  is one of the aspects of an operation detection sensor for detecting an operation of the lift arm  21  by the lift arm operation lever  210 . 
     In order to accurately detect the operation of the lift arm  21 , it is preferable to use the respective values detected by both the discharge pressure sensor  75  and the operation amount sensor  76 . Meanwhile, as the operation detection sensor, it is sufficient to use at least one of the discharge pressure sensor  75  and the operation amount sensor  76 . 
     Similarly to the operation of the lift arm  21 , as for the operation of the bucket  23 , the pilot pressure generated in accordance with the operation amount of the bucket operation lever  230  acts on the control valve  46 , whereby the spool opening area of the control valve  46  is controlled to adjust the amount of the hydraulic oil flowing into and out of the bucket cylinder  24 . Although not illustrated in  FIG. 6 , sensors for detecting an operation of the bucket  23  and the like are provided on each conduit of the hydraulic circuit. 
     (Configuration of Controller  5 ) 
     Next, the configuration of the controller  5  will be described with reference to  FIG. 8 . 
       FIG. 8  is a functional block diagram illustrating the functions of the controller  5 . 
     The controller  5  is configured such that a CPU, a RAM, a ROM, an HDD, an input I/F, and an output I/F are connected to each other via a bus. Various operation devices such as the mode switch device  60  and the forward/reverse changeover lever  62  and various sensors such as the discharge pressure sensor  75  are connected to the input I/F. The regulator  420  of the HST motor  42  and the like is connected to the output I/F. 
     In this hardware configuration, the CPU reads out an arithmetic program (software) stored in a recording medium such as the ROM, the HDD, or an optical disk, expands it on the RAM, and executes the expanded arithmetic program. Thereby, the control program and the hardware are operated in cooperation, which realizes the functions of the controller  5 . 
     In the present embodiment, the controller  5  is described by a combination of software and hardware. Meanwhile, the present invention is not limited thereto, but an integrated circuit that realizes the functions of a control program executed on the side of the wheel loader  1  may be used. 
     As illustrated in  FIG. 8 , the controller  5  includes a data acquisition section  51 , a mode determination section  52 , an operation state specification section  53 , a motor volume calculation section  54 , a storage section  55 , and a traction force control section  56 . 
     The data acquisition section  51  is configured to acquire data relating to a mode switch signal output from the mode switch device  60 , an operation direction of the forward/reverse changeover lever  62 , and the discharge pressure Pa of the loading hydraulic pump  45  detected by the discharge pressure sensor  75 , respectively. 
     The mode determination section  52  is configured to determine whether the limit mode is selected based on the mode switch signal acquired by the data acquisition section  51 . 
     The operation state specification section  53  is configured to specify an operation state of the wheel loader  1  based on the operation direction of the forward/reverse changeover lever  62  and the discharge pressure of the loading hydraulic pump  45  acquired by the data acquisition section  51  when the mode determination section  52  determines that the limit mode is selected. Specifically, the operation state specification section  53  specifies a traveling state of the wheel loader  1  based on the operation direction of the forward/reverse changeover lever  62 , and specify an operation state of the work device  2  based on the discharge pressure of the loading hydraulic pump  45  so as to specify the operation state of the wheel loader  1  as a whole. 
     In the present embodiment, the traveling state of the wheel loader  1  is detected based on the operation direction of the forward/reverse changeover lever  62 . In this connection, for example, the operation direction of the forward/reverse changeover lever  62  can be detected by detecting a forward/reverse changeover signal corresponding to the operation direction output from the forward/reverse changeover lever  62 , or detecting which direction, namely in the forward direction or the reverse direction, the traveling direction of the vehicle body is based on the rotation direction of a propeller shaft. In this connection, the sensor for detecting the operation direction of the forward/reverse changeover lever  62  is one of the aspects of a traveling state detection sensor for detecting the traveling state of the wheel loader  1 . Meanwhile, the present embodiment is not limited thereto, and the traveling state of the wheel loader  1  may be specified by using data detected by other traveling state detection sensors mounted on the vehicle body (for example, step-on amount of the accelerator pedal  61  detected by the step-on amount sensor  610 ). 
     The motor volume calculation section  54  is configured to calculate a maximum displacement volume q 1  (hereinafter referred to as “first maximum displacement volume q 1 ”) of the HST motor  42  at which the maximum traction force of the wheel loader  1  becomes a first set value when the operation state specification section  53  specifies that the vehicle body is traveling in a state of not performing the lifting operation of the work device  2 . Furthermore, when the operation state specification section  53  specifies that the work device  2  starts excavating the natural ground  100 , the motor volume calculation section  54  calculates a maximum displacement volume q 2  (hereinafter referred to as “second maximum displacement volume q 2 ”) of the HST motor  42  at which the maximum traction force of the wheel loader  1  becomes a second set value. 
     Here, the case where “it is specified that the vehicle body is traveling in a state of not performing the lifting operation of the work device  2 ” corresponds to the case where the bucket  23  is pushed into the natural ground  100  or the case where the dozing operation (soil pushing operation) for pushing the road surface to level the natural ground by using the work device  2  (bucket  23 ) is performed. 
     In the present embodiment, the “state of not performing the lifting operation of the work device  2 ” indicates a state where the lifting operation of the lift arm  21  is not performed, but does not include the presence or absence of the operation of the bucket  23 . For example, in the actual dozing work, the operator operates the bucket operation lever  230  to slightly change the angle of the bucket  23 , but this operation of the bucket  23  is specified as “a state of not performing the lifting operation of the work device  2 ” by the operation state specification section  53 . 
     The “first set value” is a value set based on a static friction coefficient μ between the road surface and the wheels  11 A,  11 B, as well as the weight of the vehicle body (hereinafter referred to as “vehicle weight”). The second set value is a value set based on the static friction coefficient μ between the road surface and the wheels  11 A,  11 B, the vehicle weight, and digging force of the work device  2  (lift arm  21 ). The second set value is greater than the first set value (second set value&gt;first set value). 
     For example, in the case where the wheel loader  1  works on a slippery road surface of which the static frictional coefficient μ with the wheels  11 A,  11 B is 0.4 to 0.5 (μ=0.4 to 0.5), at the time of pushing the bucket  23  into the natural ground  100  or performing the dozing operation, the maximum traction force of the wheel loader  1  is set to 40% to 50% of the vehicle weight as the first set value. 
     On the other hand, during the excavation operation, when the digging force of the work device  2  is 70% of the vehicle weight, the maximum traction force of the wheel loader  1  is set to 50% to 70% of the vehicle weight as the second set value. That is, the maximum traction force of the wheel loader  1  during the excavation operation is set greater than the maximum traction force of the wheel loader  1  at the time of pushing the bucket  23  into the natural ground  100  or performing the dozing work. This is because, by performing the lifting operation of the lift arm  21  at the time of excavation, the digging force of the lift arm  21  acts against the wheels  11 A,  11 B in addition to its own weight (vehicle weight). Accordingly, even when increasing the maximum traction force during the excavation operation more than that at the time of pushing the bucket  23  into the natural ground  100  or performing the dozing work, slippage of the wheels  11 A,  11 B hardly occurs, thereby realizing the excavation operation without lowering its efficiency. 
     The storage section  55  is configured to store a first threshold P 1  and a second threshold P 2  which relate to the operation amount (discharge pressure of the loading hydraulic pump  45 ) necessary for the lifting operation of the lift arm  21 , respectively. The first threshold P 1  is the discharge pressure of the loading hydraulic pump  45  when the wheel loader  1  starts excavation by the work device  2 . The second threshold P 2  is the discharge pressure of the loading hydraulic pump  45  when the lift arm  21  is in a horizontal posture during the excavation operation. The storage section  55  stores the first set value and the second set value, respectively. 
     The traction force control section  56  is configured to output, to the regulator  420  of the HST motor  42 , a command signal based on the first maximum displacement volume q 1  calculated by the motor volume calculation section  54  so as to limit the maximum displacement volume qmax of the HST motor  42  to the first maximum displacement volume q 1  when the operation state specification section  53  specifies that the vehicle body is traveling in a state of not performing the lifting operation of the work device  2 . 
     On the other hand, the traction force control section  56  outputs, to the regulator  420  of the HST motor  42 , a command signal based on the second maximum displacement volume q 2  calculated by the motor volume calculation section  54  so as to increase the maximum displacement volume qmax of the HST motor  42  to the second maximum displacement volume q 2  as the discharge pressure Pa of the loading hydraulic pump  45  increases when the operation state specification section  53  specifies that the work device  2  starts excavating the natural ground  100 . 
     In the case where the operation state specification section  53  specifies that the work device  2  is excavating the natural ground  100 , when the discharge pressure Pa of the loading hydraulic pump  45  is equal to or more than the second threshold P 2 , the traction force control section  56  maintains the maximum displacement volume qmax of the HST motor  42  at the second maximum displacement volume q 2  (constant value), regardless of increase in the discharge pressure Pa of the loading hydraulic pump  45 . 
     (Processing in Controller  5  and Operation of Wheel Loader  1 ) 
     Next, a specific flow of processing executed in the controller  5  and the operation of the wheel loader  1  in accordance with control of the controller  5  will be described with reference to  FIG. 9  and  FIG. 10 . 
       FIG. 9  illustrates a flowchart of the processing executed by the controller  5 .  FIG. 10  illustrates a graph showing a relationship between the discharge pressure Pa of the loading hydraulic pump  45  and the maximum displacement volume qmax of the HST motor  42 . 
     Firstly, the data acquisition section  51  acquires a mode switch signal output from the mode switch device  60  (step S 501 ). Next, the mode determination section  52  determines whether the limit mode is selected based on the mode switch signal acquired in step S 501  (step S 502 ). 
     When it is determined in step S 502  that the limit mode is selected (step S 502 /YES), the data acquisition section  51  acquires the operation direction of the forward/reverse changeover lever  62  and the discharge pressure Pa of the loading hydraulic pump  45  output from the discharge pressure sensor  75 , respectively (step S 503 ). 
     Next, the operation state specification section  53  determines whether the vehicle body is in the traveling state based on the operation direction of the forward/reverse changeover lever  62  acquired in step S 503 , and determines whether the discharge pressure Pa is smaller than the first threshold P 1  based on the discharge pressure Pa acquired in step S 503  (step S 504 ). 
     When it is determined in step S 504  that the vehicle body is in the traveling state and the discharge pressure Pa is smaller than the first threshold P 1  (Pa&lt;P 1 ) (step S 504 /YES), in other words, when it is specified that the vehicle body is traveling in the state of not performing the lifting operation of the work device  2 , the motor volume calculation section  54  calculates the first maximum displacement volume q 1  at which the maximum traction force of the wheel loader  1  becomes the first set value (step S 505 ). 
     Then, the traction force control section  56  outputs, to the regulator  420  of the HST motor  42 , a command signal based on the first maximum displacement volume q 1  calculated in step S 505  so as to limit the maximum displacement volume qmax of the HST motor  42  to the first maximum displacement volume q 1  (step S 506 ). Thus, the maximum traction force of the wheel loader  1  is limited to the first set value. 
     When it is not determined in step S 504  that the vehicle body is traveling and the discharge pressure Pa is smaller than the first threshold P 1  (step S 504 /NO), the operation state specification section  53  determines whether the discharge pressure Pa acquired in step S 503  is equal to or greater than the first threshold P 1  as well as is smaller than the second threshold P 2  (step S 507 ). 
     When it is determined in step S 507  that the discharge pressure Pa is equal to or greater than the first threshold P 1  and is smaller than the second threshold P 2  (P 1 ≤Pa&lt;P 2 ) (step S 507 /YES), in other words, when it is specified that the work device  2  starts excavating the natural ground  100 , the motor volume calculation section  54  calculates the second maximum displacement volume q 2  at which the maximum traction force of the wheel loader  1  becomes the second set value (step S 508 ). 
     Then, the traction force control section  56  outputs, to the regulator  420  of the HST motor  42 , a command signal based on the second maximum displacement volume q 2  calculated in step S 508  so as to increase the maximum displacement volume qmax of the HST motor  42  to the second maximum displacement volume q 2  as the discharge pressure Pa increases (step S 509 ). Thus, the maximum traction force of the wheel loader  1  increases from the first set value to the second set value. 
     When it is not determined in step S 507  that the discharge pressure Pa is equal to or greater than the first threshold P 1  as well as is smaller than the second threshold P 2  (step S 507 /NO), the operation state specification section  53  determines whether the discharge pressure Pa acquired in step S 503  is equal to or greater than the second threshold P 2  (step S 510 ). 
     When it is determined in step S 510  that the discharge pressure Pa is equal to or greater than the second threshold P 2  (Pa≥P 2 ) (step S 510 /YES), similarly to step S 508 , the motor volume calculation section  54  calculates the second maximum displacement volume q 2  at which the maximum traction force of the wheel loader  1  becomes the second set value (step S 511 ). 
     Then, the traction force control section  56  outputs, to the regulator  420  of the HST motor  42 , a command signal based on the second maximum displacement volume q 2  calculated in step S 508  so as to increase the maximum displacement volume qmax of the HST motor  42  to the second maximum displacement volume q 2  regardless of increase in the discharge pressure Pa (step S 512 ). Thus, the maximum traction force of the wheel loader  1  is maintained at the second set value as a constant value. 
     Upon completion of step S 506 , step S 509 , and step S 512 , the controller  5  returns to step S 503 . When it is determined in step S 502  that the limit mode is not selected, in other words, when it is determined that the normal mode is selected (step S 502 /NO), or when it is not determined in step S 510  that the discharge pressure Pa is equal to or greater than the second threshold P 2 , in other words, when both the vehicle body and the work device  2  are out of operation (step S 510 /NO), the processing in the controller  5  is terminated. 
     As described above, when the limit mode is selected by the mode switch device  60 , at the time of pushing the bucket  23  into the natural ground  100  or performing the dozing work in which the discharge pressure Pa of the loading hydraulic pump  45  is 0 or more as well as less than P 1  (0≤Pa&lt;P 1 ), as illustrated in  FIG. 10 , the maximum displacement volume qmax of the HST motor  42  is limited to the first maximum displacement volume q 1 , in other words, the maximum traction force is limited to the first set value. Accordingly, even on a slippery road surface, slippage of the wheels  11 A,  11 B hardly occurs, and thus the working efficiency is improved. 
     Furthermore, at the time of performing excavation of the natural ground  100 , which is an operation of the wheel loader  1  in which the discharge pressure Pa of the loading hydraulic pump  45  is equal to or greater than P 1  (Pa≤P 1 ), the maximum displacement volume qmax of the HST motor  42  is made to increase from the first maximum displacement volume q 1  to the second maximum displacement volume q 2 , that is, the maximum traction force is made to increase from the first set value to the second set value. Accordingly, even on a slippery road surface, the traction force can be maintained while slippage of the wheels  11 A,  11 B is suppressed, and thus it becomes easy to perform excavation. 
     According to the wheel loader  1  of the present embodiment, even in the case of working on a slippery road surface, the maximum traction force can be controlled based on the static friction coefficient μ between the road surface and the wheels  11 A,  11 B as well as the operation content, so that the operator can perform operations of pushing a bucket into the ground, excavating, or dozing without concerning about slippage of the wheels  11 A,  11 B. Accordingly, it is possible to provide the operator with ride comfort and reduction in his or her fatigue. 
     In this connection, as illustrated in  FIG. 10 , on the slippery road surface, even if the discharge pressure Pa of the loading hydraulic pump  45  becomes the relief pressure Pr, slippage of the wheels  11 A,  11 B occurs when the maximum displacement volume of the HST motor  42  is set to a rated value (100%) of the maximum displacement volume in the normal mode. Accordingly, it is preferable to keep the maximum displacement volume of the HST motor  42  at the second maximum displacement volume q 2  or less. 
     In the above, the embodiment of the present invention has been described. It should be noted that the present invention is not limited to the embodiment and modifications described above, and various modifications are included. For example, the embodiment described above has been explained in detail in order to clarify the present invention, but is not necessarily limited to those having all the configurations described. In addition, a part of the configuration of the present embodiment can be replaced with that of other embodiments, and the configuration of other embodiments can be added to the configuration of the present embodiment. Furthermore, it is possible to add, delete, or replace another configuration with respect to a part of the configuration of the present embodiment. 
     For example, in the embodiment described above, as the loading hydraulic pump  45 , a fixed displacement hydraulic pump is used. Meanwhile, the present embodiment is not limited thereto, and a variable displacement hydraulic pump may be used therefor. 
     Furthermore, in the embodiment described above, the maximum traction force of the wheel loader  1  is controlled by adjusting the maximum displacement volume qmax of the HST motor  42 . Meanwhile, the present embodiment is not limited thereto, and for example, the maximum traction force of the wheel loader  1  may be controlled by adjusting the displacement volume of the HST pump  41 . 
     REFERENCE SIGNS LIST 
     
         
           1 : wheel loader (loading work vehicle) 
           2 : work device 
           3 : engine 
           5 : controller 
           11 A: front wheel (wheel) 
           11 B: rear wheel (wheel) 
           21 : lift arm 
           23 : bucket (attachment) 
           41 : HST pump (traveling hydraulic pump) 
           42 : HST motor (traveling hydraulic motor) 
           45 : loading hydraulic pump 
           60 : mode switch device 
           62 : forward/reverse changeover lever (traveling state detection sensor) 
           75 : discharge pressure sensor (discharge pressure detection sensor, operation detection sensor) 
           76 : operation amount sensor (operation amount detection sensor, operation detection sensor) 
           100 : natural ground (object to be excavated) 
           210 : lift arm operation lever (operation device) 
           230 : bucket operation lever (operation device) 
           610 : step-on amount sensor (traveling state detection sensor) 
         μ: static friction coefficient