Abstract:
A control apparatus for an industrial vehicle is disclosed. The vehicle has a rear axle that is swingable during straight travel of the vehicle and can be fixed during turning of the vehicle. A damper looks or unlocks the rear axle. A controller has a memory that stores a first value and a second value of an angular velocity rate that represents an angular velocity per unit time. The controller activates or deactivates the damper. The controller activates the damper to lock the rear axle when the angular velocity rate is greater than the first value. The controller deactivates the damper to unlock the rear axle when the angular velocity rate is kept smaller than the second value for a predetermined time period after the angular velocity rate has become smaller than the second value.

Description:
BACKGORUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to axle supporting apparatus for industrial vehicles, and more particularly, to an apparatus for locking swingable axles of industrial vehicles. 
     2. Description of the Related Art 
     A typical forklift truck has a rear axle that is supported by a body frame. The rear axle is swung with respect to the body frame by lateral acceleration, or centrifugal force, that is produced when the traveling forklift truck changes directions. This degrades the traveling stability of the forklift truck. Thus, the traveling speed of the forklift truck must be decreased when changing directions. 
     Japanese Unexamined Patent Publication No. 58-211903 describes a forklift truck that locks the swinging of an axle when the truck turns. The forklift truck has a detecting means that detects centrifugal force when the truck is turned. When the centrifugal force becomes greater than a predetermined level, the swinging of the axle with respect to the body frame is restricted. In other words, the axle is locked. This enables stable turning of the forklift truck without having a slow down the vehicle. However, when the forklift truck successively changes direction within a short period of time, for example, when the vehicle turns right and then turns left immediately afterward, the centrifugal force, for an instant, is less than the predetermined level. This releases the locking of the axle regardless of the vehicle changing directions. As a result, the forklift truck becomes unstable when successively turned in different directions. 
     Japanese Unexamined patent Publication No. 58-214406 described a forklift truck that locks its axle when the rotated angle of the steering wheel and the speed of the vehicle become greater than predetermined values. However, when the vehicle is successively turned in different directions, for an instant, the rotated angle of the steering wheel becomes smaller than the predetermined angle. Thus, in the same manner as the forklift truck of Publication No. 58-211903, this releases the locking of the axle and causes instability. 
     Among the four wheels (two front wheels and two rear wheels) of a forklift truck, one of the wheels may be lifted from the ground depending on the weight of the object held by the forks when the axles are locked during turning. For example, when a heavy object is held on the forks, the center of gravity of the vehicle is displaced toward the front of the forklift truck. This may lift one of the rear wheels from the ground if one of the axles are locked. If the axles are locked when a relatively light object is held on the forks, the balance weight provided at the rear of the vehicle may cause one of the front wheels to raise from the ground. 
     Typically, torque is transmitted to the front wheels to drive forklift trucks. The steering angle is transmitted to the rear wheels to steer the vehicle. Accordingly, if the rear axle is locked when a relatively light object is held on the forks, one of the front wheels, which function as the drive wheels, may be lifted away from the ground. This decreases traction between the front wheels and the ground. As a result, the driving force of the vehicle becomes insufficient. This hinders smooth operation of the forklift truck. 
     If a heavy object is held on the forks, the center of gravity, which is near the front of the vehicle, increases the traction force of the front wheels. Accordingly, the locking of the axle does not affect the driving force of the vehicle. However, when the center of gravity is near the front of the vehicle, the vehicle tends to swing in a forward direction. As a result, the forklift truck becomes unstable when lifting and lowering objects and when traveling with the heavy object held on the forks. This tendency is stronger is the object is held at a high position, which raises the center of gravity. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide an industrial vehicle that is capable of traveling in a stable manner. 
     A further objective of the present invention is to provide an industrial vehicle that is capable of lifting and lowering objects in a stable manner. 
     For achieving the above objectives, an improved control apparatus for an industrial vehicle is provided. The vehicle includes a rear axle, said rear axle being swingable during straight travel of the vehicle and lockable during turning of the vehicle. The apparatus comprises holding means for selectively locking and unlocking the rear axle, memory means for storing a first value and a second value of an angular velocity rate, wherein said angular velocity rate represents an angular velocity per unit time, wherein said first value is predetermined to indicate turning of the vehicle when the angular velocity rate is greater than the first value, and wherein said second value is predetermined to indicate the straight travel of the vehicle when the angular velocity rate is smaller than the second value, and control means for selectively activating and deactivating the holding means. The control means activates the holding means to lock the rear axle when the angular velocity rate is greater than the first value, and said control means deactivates the holding means to unlock the rear axle when the angular velocity rate is kept smaller than the second value for a predetermined time period after the angular velocity rate has become smaller than the second value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a side view showing a forklift truck that is provided with a controller according to the present invention; 
     FIG. 2 is a schematic, partial rear view showing the forklift truck together with the rear axle, the shock absorber, and the hydraulic circuit; 
     FIG. 3 is a plan view showing the forklift truck; 
     FIG. 4 is a diagrammatic drawing showing the front wheels the driving mechanism of the front wheels, and sensors; 
     FIG. 5 is a diagrammatic drawing showing the forks that are mounted on the outer masts at the front of the forklift truck together with sensors; 
     FIG. 6 is a diagrammatic drawing showing the relationship between the position of the center of gravity and the traction force of the front and rear wheels; 
     FIG. 7 is a block diagram showing the electrical structure of the forklift truck; 
     FIGS.  8 (A) and  8 (B) are flowcharts showing the operation of the controller; 
     FIG. 9 is a graph showing the timing of the output of the locking signal in relation to changes in the yaw rate altering rate; 
     FIG. 10 is a graph showing the timing of the output of the locking signal in relation to changes in the computed centrifugal force; 
     FIG. 11 is a graph showing changes of the yaw rate altering rate, the yaw rate, the steering angle, the actual centrifugal force, and the computed centrifugal force with respect to time when the forklift truck turns from a state in which it is traveling straight; 
     FIG. 12 is a graph showing changes of the yaw rate altering rate, the yaw rate, the steering angle, the actual centrifugal force, and the computed centrifugal force with respect to time when the forklift truck changes directions in a successive manner; 
     FIG. 13 is a flowchart showing the operation of the controller according to a second embodiment of the present invention; 
     FIG. 14 is a cross-sectional drawing showing the structure of an electromagnetic switching valve according to a third embodiment of the present invention; 
     FIG. 15 is a timing chart showing changes in the opening of the electromagnetic switching valve with respect to time; 
     FIG. 16 is a schematic, partial rear view showing a hydraulic system that employs a switching valve provided with restricting passages that have different diameters; 
     FIG. 17 is a timing chart showing the operation of the switching valve shown in FIG. 16; 
     FIG. 18 is a schematic, partial rear view showing the forklift truck according to a further embodiment of the present invention; 
     FIG. 19 is a diagrammatic explanatory view showing the centrifugal force when the forklift truck of FIG. 18 changes directions; and 
     FIG. 20 is a timing chart showing changes with respect to time of the centrifugal force illustrated in FIG.  19 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An industrial vehicle, or forklift truck  1  having a body frame  1   a  is shown in FIG. 1. A pair of left and right outer masts  2  are mounted on the front of the forklift truck  1 . A pair of inner masts  3  are provided between the outer masts  2 . A fork  4  is mounted on each inner mast  3 . The fork  4  is lifted and lowered by the associated inner mast  3 . A tilting cylinder  5  having a body  5   a  and a cylinder rod  5   b  is provided for each outer mast  2 . The body  5   a  is coupled to the body frame  1   a  while the cylinder rod  5   b  is coupled to the associated outer mast  2 . The outer masts  2  and the forks  4  are tilted by the tilting cylinders  5 . A lifting cylinder  6  having a body  6   a  and a rod  6   b  is provided for each inner mast  3 . The body  6   a  is coupled to the body frame  1   a  while the cylinder rod  6   b  is coupled to the associated inner mast  3 . The inner masts  3  are lifted and lowered by the lifting cylinders  6 , on which the forks  4  are mounted. 
     A pair of front wheels  7  are mounted at the front portion of the frame  1   a.  As shown in FIG. 4, the front wheels  7  are connected to an engine E by means of a differential gear U and a transmission D. Hence, each front wheel  7  is driven by the engine E. 
     A pair of rear wheels  8  are mounted at the rear portion of the frame  1 . A structure for connecting the rear wheels  8  is shown in FIG. 2. A rear axle  11  is provided at the lower rear portion of the frame  1   a  extending between the left and right sides of the frame  1   a.  The rear axle  11  is pivotal about a center pin  11   a.  The rear wheels  8  are mounted on the ends of the rear axle  11 . The direction of the rear wheels  8  is changed by turning a steering wheel  10 , which is installed in a driver&#39;s compartment  9 . 
     A damper, or shock absorber  12  is connected between the body frame  1   a  and the rear axle  11 . The shock absorber  12  is a double action hydraulic cylinder and absorbs the force that is applied to the rear wheels  8 . The shock absorber  12  includes a cylindrical body  12   a,  a piston  12   b  accommodated in the body  12   a,  and a piston rod  12   c  connected to the piston  12   b.  The distal end of the piston rod  12   c  is connected to the rear axle  11 . 
     a first compartment R 1  and a second compartment R 2  are defined in the shock absorber  12  by the piston  12   b.  A first oil passage P 1  is connected to the first chamber R 1  while a second oil passage P 2  is connected to the second chamber R 2 . The first and second oil passages P 1 , P 2  are connected to an electromagnetic switching valve  13 . The switching valve  13  includes a body and a spool. The spool has a connecting portion  14  and a disconnecting portion  15 . A third oil passage P 3  and a fourth oil passage P 4  are connected to the switching valve  13 . The third oil passage P 3  is connected with the fourth oil passage P 4 . The fourth oil passage P 4  is connected to an accumulator  16  in which hydraulic oil is reserved. A throttle valve  17  is provided in the second oil passage P 2 . 
     In the switching valve  13 , the spool moves with respect to the body and selectively connects the connecting portion  14  and the disconnecting portion  15  with the oil passages P 1  to P 4 . When the connecting portion  14  is connected to the oil passages P 1  to P 4 , the first oil passage P 1  is communicated with the third oil passage P 3  and the second oil passage P 2  is communicated with the fourth oil passage P 4 . In this state, the first and second chambers R 1 , a R 2  are communicated with the accumulator  16 . This permits hydraulic oil to flow in and out of the first and second chambers R 1 , R 2 . Thus, the shock absorber  12  enables pivoting of the rear axle  11 . When the disconnecting portion  15  is connected with the oil passages P 1  to P 4 , the oil passages P 1  to P 4  are disconnected from one another. In this state, the piston  12   b  of the shock absorber  12  is locked. Thus, the hock absorber  12  prohibits pivoting of the rear axle  11 . 
     As shown in FIG. 1, a balance weight  19  is mounted on the rear of the forklift truck  1 . A piezo-electric sensor, or gyroscope  20 , is arranged on the balance weight  19 . The gyroscope  20  detects the angular velocity, or yaw rate ω of the forklift truck  1  when changing directions. As shown in FIGS. 1 and 3, an acceleration sensor  21  is arranged on an instrument panel W provided in the driver&#39;s compartment  9 . The acceleration sensor  21  is located midway between the front wheels  7 . Actual lateral acceleration, or actual centrifugal force Fa, that is produced when the forklift truck  1  changes directions is detected by the acceleration sensor  21 . 
     As shown in FIGS. 1 and 5, a limit switch  24  is arranged at the upper portion of the outer mast  2  to detect the vertical position of the associated fork  4 . The limit switch  24  is located at a position that is lower than the top end of the outer mast  2  by one fourth the length of the mast  2 . A pressure sensor  25  is provided at the bottom end of the lifting cylinder  6  to detect the hydraulic pressure of the hydraulic oil that acts on the lifting cylinder  6 . 
     As shown in FIG. 2, a steering angle sensor  22  is arranged on one of the rear wheels  8  to the detect the steering angel θ of the rear wheels  8 . As shown in FIG. 4, a vehicle speed sensor  23  is provided in the forklift truck  1  to detect the rotating speed of the differential gear U, or the traveling speed v of the forklift truck  1 . 
     The electrical structure of a controller  31  employed in the above industrial vehicle will now be described with reference to an electric block diagram illustrated in FIG.  7 . 
     The controller  31  has a memory  31   a  and a timer  31   b  that are included in a central processing unit (CPU), a read only memory (ROM), and other parts. The controller  31  also has an input terminal and an output terminal. The gyroscope  20 , the acceleration sensor  21 , the steering angle sensor  22 , the vehicle speed sensor  23 , the limit switch  24 , and the pressure sensor  25  are connected to the input terminal. The electromagnetic switching valve  13  is connected to the output terminal of the controller  31 . 
     When the forklift truck  1  changes direction, the gyroscope  20  sends an angular velocity signal, which corresponds to the detected yaw rate ω, to the controller  31 . When the forklift truck  1  changes direction, the acceleration sensor  21  sends an actual centrifugal force signal, which corresponds to the detected actual centrifugal force Fa, to the controller  31 . The steering angle sensor  22  sends a steering angle signal, which corresponds to the detected steering angle θ of the rear wheels  8 , to the controller  31 . The vehicle speed sensor  23  sends a speed signal, which corresponds to the detected speed v of the forklift truck  1 , to the controller  31 . 
     The limit switch  24  sends an activated signal to the controller  31  when actuated as the associated for  4  is lifted above a reference height Zref. The reference height Zref is located at a position that is lower than the top end of the outer mast  2  by approximately one fourth the length of the outer mast  2 . The pressure sensor  25  sends a hydraulic pressure signal, which corresponds to the detected hydraulic pressure y applied to the lifting cylinder  6 , to the controller  31 . 
     Based on the angular velocity signal sent from the gyroscope  20 , the controller  31  computes the angular velocity altering rate, or the yaw rate altering rate Δω/ΔT. The altering rate Δω/ΔT corresponds to the angular acceleration. The controller  31  obtains the altering rate Δω/ΔT by differentiating the yaw rate ω with respect to time. 
     The controller  31  determines lateral acceleration based on centrifugal force. The controller  31  computes centrifugal force based on the traveling speed signal from the vehicle speed sensor  23  and the angular velocity signal from the hyroscope  20 . The computed centrifugal force Fb differs from the actual centrifugal force detected by the acceleration sensor  21 . Thus, there is a slight difference between the value of the actual centrifugal force Fa and the computed centrifugal force Fb. The computed centrifugal force Fb is obtained by multiplying the traveling speed v by the annular velocity ω, as represented by the following equation (1): 
     
       
         Fb=v×ω  (1)  
       
     
     Reference values of the yaw rate altering rate Δω/ΔT are stored in the memory  31   a  of the controller  31 . The reference rate values include a maximum reference value Kmax, which is referred to when the rate Δω/ΔT increases, and a minimum reference value Kmin, which is referred to when the rate Δω/ΔT decreases. The controller  31  starts the output of a locking signal when the value of the computed altering rate Δω/ΔT changes from a value equal to or smaller than the maximum reference value Kmax to a valve greater than the maximum reference value Kmax. When the locking signal is being output, the controller  31  compares the altering rate Δω/ΔT with the minimum reference value Kmin. If the value of the altering rate Δω/ΔT changes from a valve equal to or greater than the minimum reference value Kmin to a valve smaller than the minimum reference value Kmin, the controller  31  stops the output of the locking signal. 
     The controller  31  waits for a predetermined time length T to elapse before stopping the output of the locking signal. More specifically, a timer  31   b  starts measuring time when the value of the altering rate Δω/ΔT changes from a value equal to or greater than the minimum reference valve Kmin to a valve smaller than the minimum reference value Kmin. When the measure dtime reaches the time length T, the controller  31  stops the output of the locking signal. The time  31   b  is controlled by the controller  31  so as to stop measuring time when the value of the altering rate Δω/ΔT becomes equal to or greater than the minimum reference valve Kmin. 
     Reference values of the centrifugal force acting on the forklift truck  1  are stored in the memory  31   a  of the controller  31 . The reference valves include a centrifugal force maximum reference valve Hmax, which is referred to when the centrifugal force increases, and a centrifugal force minimum reference value Hmin, which is referred to when the centrifugal force decreases. The controller  31  starts outputting a locking signal when the absolute value of the computed centrifugal force Fb changes from a value equal to or smaller than the maximum reference valve Hmax to a valve greater than the maximum reference value Hmax. During the output of the locking signal, the controller  31  stops the looking signal if the absolute value of the computed centrifugal force Fb becomes smaller than the minimum reference value Hmin. 
     The controller  31  determines the weight of the load on the forks  4  based on the signal from the pressure sensor  25 . The controller  31  further computes and locates the center of gravity G (FIG. 6) of the forklift truck  1  based on the weight of the load. The center of gravity G corresponds to the center of gravity of the combined mass of the vehicle and the load. In this case, the controller computes the center of gravity G under the assumption that the inclination of the outer masts  2  is maximum as shown in the dotted lines of FIG. 5. A reference pressure value Nref of the hydraulic pressure y applied to the lifting cylinder  6  is stored in the memory  31   a  of the controller  31 . 
     The controller  31  computes the hydraulic pressure y applied to the lifting cylinder  6  based on the detecting signal from the pressure sensor  25  when receiving an activated signal from the limit switch  24 . If the hydraulic pressure y is equal to or greater than the reference pressure value Nref, the controller  31  sends alocking signal to the electromagnetic switching valve  13 . 
     More specifically, the controller  31  sends a locking signal to the switching valve  13  when any one of the following six conditions is satisfied: 
     (a) The yaw rate altering rate Δω/ΔT is grater than the maximum reference value Kmax. 
     (b) the yaw rate altering rate Δω/ΔT becomes less than the maximum reference value Kmax from a valve greater than the maximum reference value Kmax but is grater than the minimum reference valve Kmin. 
     (c) The yaw rate altering rate Δω/ΔT is less than the minimum reference value Kmin within the predetermined time length T, which is measured from when the altering rate Δω/ΔT falls below the minimum reference valve Kmin from the state where condition (b) is satisfied. 
     (d) The absolute valve of the computed centrifugal force Fb is grater than the maximum reference valve Hmax. 
     (e) The absolute value of the computed centrifugal force Fb becomes equal to or less than the maximum reference valve Hmax form a valve greater than the maximum reference valve Hmax but is greater than the minimum reference value Hmin. 
     (f) The activated signal is sent from the limit switch  24  and the hydraulic pressure y applied to the lifting cylinder  6  is equal to or greater than the reference pressure value N. 
     The electromagnetic switching valve  13  has a solenoid that is excited by the locking signal. This causes the disconnecting portion  15  of the switching valve  13  to be selectively connected with the oil passages P 1  to P 4 . As a result, the switching valve  13  closes the first and second oil passages P 1 , P 2  and locks the rear axle  11  with the shock absorber  12 . When not receiving the locking signal, the solenoid is de-excited. This causes the connecting portion  14  to be selectively connected to the oil passages P 1  to P 4 . As a result, the switching valve  13  permits hydraulic oil to flow in and out of the first and second chamber R 1 , R 2 . In such state, the body frame  1   a  is swingable with respect to the rear axle  11 . 
     The controller  31  locks the rear axle  11  with the switching valve  13  in accordance with the flowchart illustrated in FIGS.  8 (A) and  8 (B). It is presumed here that the locking signal is not being output from the controller  31  and that the rear axle  11  is swingable when the controller  31  enters the control processing of FIG.  8 (A). 
     In the flowcharts, the characters “S” stands for “steps.” At step  101 , the controller  31  reads the traveling speed v of the forklift truck  1  based on the traveling speed signal sent from the vehicle speed sensor  23 . At step  102 , the controller  31  reads the yaw rate ω based on the angular velocity signal sent from the gyroscope  20 . At step  103 , the controller  31  obtains the computed centrifugal force Fb by applying the traveling speed v and the yaw rate ω to the equation (1). 
     At step  104 , the controller  31  computes the yaw rate altering rate Δω/ΔT based on the yaw rate ω. The controller  31  has a low pass filter function, which is used in step  105  to eliminate high frequency noise from the altering rate Δω/ΔT. 
     At step  106 , the controller  31  determines whether the absolute value of the computed centrifugal force Fb (|Fb|) is greater than the maximum reference valve Hmax. If it is determined that the altering rate Δω/ΔT is greater than the maximum reference valve Hmax, the controller  31  proceeds to step  107  and continuously outputs a locking signal to the electromagnetic switching valve  13  and maintains the solenoid thereof in an excited state. This employees the disconnecting portion  15  of the switching valve  13  and causes the shock absorber  12  to hold the rear axle  11  in a locked state. Thus, the rear axle  11  is held in a manner such that swinging is restricted. The controller  31  then returns to step  101  and repeats the execution of the above steps. 
     When it is determined that the absolute value |Fb| is equal to or smaller than the maximum reference value Hmax, the controller  31  proceeds to step  108 . At step  108 , the controller  31  determines whether the absolute value |Fb| is smaller than the minimum reference value Hmin. If, in step  108 , it is determined that the absolute value |Fb| is smaller than the minimum reference value Hmin, the controller  31  proceeds to step  109  and stops the output of the locking signal. The controller  31  than proceeds to step  110 . If, in step  108 , it is determined that the absolute valve |Fb| is equal to or greater than the minimum reference value Hmin, the controller  31  proceeds to step  110  without outputting the locking signal. 
     At step  110 , the controller  31  determines whether the yaw rate altering rate Δω/ΔT is greater than the maximum reference value Kmax. When it is determined that the altering rate Δω/ΔT is greater than the maximum reference value Kmax, the controller  31  proceeds to step  111  and outputs the locking signal to the switching valve  13 . This fixes the shock absorber  12  and holds the rear axle  11  in a locked state. The controller  31  then returns to step  101  and repeats the execution of the above steps. If it is determined that the altering rate Δω/ΔT is equal to or less than the maximum reference value Kmax in step  110 , the controller  31  processes to step  112 . 
     At step  112 , the controller determines whether the altering rate Δω/ΔT is smaller than the minimum reference value Kmin. When it is determined that the altering rate Δω/ΔT is smaller than the minimum reference valve Kmin, the controller  31  proceeds to step  113  and determines whether the time measured by the timer  31   b  has reached the predetermined time length T. When it is determined that the time length T has not yet elapsed, the controller  31  returns to step  101  and repeats the above steps. If it is determined that the time length T has elapsed, the controller  31  proceeds to step  114  and stops the output of the locking signal. Afterwards, the controller  31  returns to step  101 . 
     The controller  31  then proceeds to step  112  and determines whether the value of the hydraulic pressure y, which is obtained based on the signal from the pressure sensor  25 , is equal to or greater than the reference pressure value Nref. When it is determined that the hydraulic pressure y is equal to or greater than the reference value Nref, the controller  31  proceeds to step  116  and determines whether the activated signal is being output from the limit switch  24 . If it is determined that the activated signal is being output from the limit switch  24 , the controller  31  proceeds to step  118  and continues to output the locking signal. 
     If it is determined that the value of the hydraulic pressure y is smaller than the reference value Nref in step  115 , the controller  31  proceeds to step  117  and stops outputting the locking signal. The controller  31  also proceeds to step  117  and stops outputting the locking signal if it is determined that the activated signal is not being output from the limit switch  24  in step  116 . 
     The processing performed by the controller  31  when the forklift truck  1  is turned to the right from a state in which it is traveling straight will now be described with reference to FIG.  11 . When the forklift truck  1  turns right, the value of the steering angle θ becomes positive and when turning left, the value of the steering angle θ becomes negative. In the same manner, the value of the yaw rate ω, the actual centrifugal force Fa, and the computed centrifugal force Fb become negative when the forklift truck  1  turns left, and positive when the truck  1  turns right. 
     When the operator of the forklift truck  1  steers the steering wheel  10  and turns the forklift truck  10  to the right, the valve of the steering angle θ increases. As the value of the steering angle θ increases, the values of the yaw rate ω, the computed centrifugal force Fb, the actual centrifugal force Fa, and the yaw rate altering rate Δω/ΔT are each increased accordingly. The altering rate Δω/ΔT is the first to increase among these values and becomes greater than the maximum reference valve Kmax. The controller  31  locks the rear axle  11  when the altering rate Δω/ΔT becomes greater than the maximum reference valve Kmax. At this point, the valve of the computed centrifugal force Fb is smaller than the maximum reference valve Hmax. Accordingly, when the turning of the forklift truck  1  begins, the rear axle  11  is locked by the altering rate Δω/ΔT before being locked by the computed centrifugal force Fb. 
     As the value of the steering angle θ further increases, the valves of the yaw rate ω, the computed centrifugal force Fb, and the actual centrifugal force Fa are each increased accordingly. This causes the absolute valve of the computed centrifugal force |Fb| to become greater than the maximum reference valve Hmax. At this point, the altering rate Δω/ΔT is greater than the maximum reference valve Kmax and the rear axle  11  is held in a locked state. 
     When the operator continues to turn the forklift truck  1  by holding the steering wheel  10  at the same predetermined angle, resulting in continuation of the same steering angle θ, the yaw rate ω, the computed centrifugal force Fb, and the actual centrifugal force Fa and maintained at constant values. When the value of the steering angle θ becomes constant, the valve of the altering rate Δω/ΔT starts decreasing and becomes smaller than the minimum reference value Kmin. At this point, the computed centrifugal force Fb is greater than the maximum reference value Hmax. Thus, the controller  31  outputs the locking signal since the computed centrifugal force Fb is greater than the maximum reference valve Hmax. Accordingly, the rear axle  11  remains in a locked state. 
     The above processing is executed in the same manner when the forklift truck  1  is steered to the left from a state in which it is traveling straight. Thus, when the forklift truck  1  is driven in a straight direction and then steered either to the right or tho the left, the altering rate Δω/ΔT first becomes greater than the maximum reference value Kmax and locks the rear axle  11 . The altering rate Δω/ΔT then becomes smaller than the minimum reference valve Kmin. However, since the absolute value of the centrifugal force |Fb| is greater than the maximum reference valve Hmax, the rear axle  11  remains in a locked state. 
     The processing performed by the controller  31  when the forklift truck  1  is steered to the right and then successively steered to the left will now be described with reference to FIG.  12 . When the forklift truck  1  is steered to the right in a manner such that the value of the steering angle θ is constant and is then steered to the left by turning the steering wheel  10  so as to decreases the valve of the steering angle θ, the yaw rate ω and the computed centrifugal force Fb start to decrease from the predetermined valves. When the computed centrifugal force Fb corresponds to the predetermined value, the centrifugal force Fb exceeds the maximum reference valve Hmax. Thus, the controller  31  outputs the locking signal and locks the rear axle  11 . The value of the altering rate Δω/ΔT starts to increase as the steering angle θ decreases and becomes greater than the maximum reference value Kmax. At this point, the computed centrifugal force Fb is greater than the minimum reference value Hmin. 
     A further decrease of the steering angle θ causes the computed centrifugal force Fb to become smaller than the minimum reference value Kmin. However, the controller  31  continues to output the locking signal since the altering rate Δω/ΔT is greater than the maximum reference valve Kmax. 
     The altering rate Δω/ΔT is maximum when the steering angle θ becomes close to zero degrees. The forklift truck  1  begins to turn left as the valve of the steering angle θ reaches zero degrees and then further decreases. The decrease in the value of the steering angle θ causes further decrease of the valves of the yaw rate ω and the computed centrifugal force Fb. The values of the yaw rate ω and the computed centrifugal force Fb become negative and continues to further decrease. This increases the absolute values of the yaw rate ω and the computed centrifugal force Fb (|Fb|, |ω|). Thus, the absolute value of the computed centrifugal force |Fb| becomes greater than the maximum reference value Hmax. At this point, the altering rate Δω/ΔT is equal to or greater than the minimum reference value Kmin. Since the conditions of |Fb|&gt;Hmax and Δω/ΔT&gt;Kmin are satisfied, the controller  31  continues to output the locking signal. Thus, the rear axle  11  remains locked. 
     A further decrease of the steering angle θ causes the altering rate Δω/ΔT to become smaller than the minimum reference value Kmin. However, the controller  31  continues to output the locking signal and keeps the rear axle  11  in a locked state. The locking signal is continued prior to the expiration of the predetermine dtime length T, which is measured from when the altering rate Δω/ΔT became smaller than the minimum reference value Kmin. Also, the locking signal is continues as long as the absolute value of the computed centrifugal force |Fb| is greater than the maximum reference value Hmax. When the operator stops turning the steering wheel  10  and holds the steering angle θ at a predetermined angle, the valves of the yaw rate ω and the computed centrifugal force Fb become constant. Furthermore, the valve of the altering rate Δω/ΔT becomes zero. 
     When the predetermined time length T elapses, the conditions related to the altering rate Δω/ΔT for outputting the locking signal fail to be met. However, the conditions related to the computed centrifugal force Fb remain satisfied. Accordingly, the controller  31  continues to output the locking signal and keeps the rear axle  11  in a locked state. 
     When the forklift truck  1  turns right and then left successively, there is a period Y 1  during which the conditions for outputting the locking signal related to the computed centrifugal force Fb are unsatisfied when the steering angle θ is in the vicinity of zero degrees. However, the conditions for outputting the centrifugal force Fb related to the altering rate Δω/ΔT are satisfied. Thus, the rear axle  11  is constantly in a locked state when the forklift truck  1  turns right and then left successively. Holding the steering angle θ at a predetermined angle to steer the forklift truck  1  further to the left ends a period Y 2 , at which the conditions for outputting the locking signal related to the altering rate Δω/ΔT are satisfied. However, since the conditions for outputting the locking signal related to the computed centrifugal force Fb are satisfied, the rear axle  11  remains in a locked state. Accordingly, when the forklift truck  1  turns right and then left successively, the rear axle  11  remains in a locked state. 
     The above processing is carried out in the same manner when the forklift truck  1  turns left and then right successively. 
     FIG. 12 shows the roll angle of the forklift truck  1  when in a rolling state. The roll angle is detected by a rolling sensor (not shown) provided on the body frame  1   a.    
     As described above, in addition to the locking signal based on the altering rate Δω/ΔT, the controller  31  outputs a locking signal based on the computed centrifugal force Fb. Accordingly, during successive turning of the forklift truck  1 , the locking signal based on the altering rate Δω/ΔT is output even when the locking signal based on the computed centrifugal force Fb is not output when the steering angle θ is close to zero degrees. Thus, when the forklift truck  1  turns right and then left successively, the rear axle  11  is constantly maintained in a locked state. This enables the forklift truck  1  to change directions in a stable manner. 
     The control performed by the controller  31  when the forklift truck  1  transports loads will now be described. 
     FIG. 6 is an explanatory diagram that shows how a center of gravity G of the forklift truck  1  changes in accordance with the weight of the load carried on the forks  4  of the forklift truck  1 . Diagonal lines have been drawn between the left front wheel  7  and the left rear wheel  8 . The center of gravity, when there is no load carried on the forks  4 , is denoted as G 1  and is located rearward with respect to the intersecting point X of the two diagonal lines. The weight of a load carried by the forks  4  causes the center of gravity G to move forward between the center of gravity G 1  and the intersecting point X. Thus, the traction force of the rear wheels  8  increases, and the traction force of the front wheels  7  decreases. In such state, one of the front wheels  7  may be lifted from the ground. Accordingly, the controller  31  does not output the locking signal. This allows swinging of the rear axle  11 . In this case, swinging of the rear axle  11  decreases the traction force of the rear wheels  8  and increases the traction force of the front wheels  7 . The value of the hydraulic pressure y applied to the lifting cylinder  6 , when the load on the forks  4  causes the center of gravity G to coincide with the intersecting point X, is set as the reference pressure value Nref. 
     The center of gravity of the forklift truck  1 , when the weight of the load carried by the forks  4  becomes equal to the maximum payload of the forklift truck  1 , is denoted as G 2  and is located in front of the intersecting point X. When the center of gravity G of the forklift truck  1  is located between the center of gravity G 2  and the intersecting point X, the traction force of the front wheels  7  increases. In such state, one of the rear wheels  8  may be lifted away from the ground. However, since the traction force of the front wheels  7 , which are the drive wheels, is satisfactory, the rear axle  11  may be locked without interfering with the travel of the forklift truck  1 . 
     However, lifting the forks  4  to a position above the reference height 2ref moves the center of gravity G upward. This causes the forklift truck  1  to become unstable laterally. At this point, the controller  31  outputs the locking signal to lock the rear axle  11  to enhance the stability of the forklift truck  1 . 
     A second embodiment of the present invention will now be described. In this embodiment, the forklift truck  1  is not equipped with the gyroscope and the acceleration sensor. An assumed yaw rate ωx is obtained based on the steering angle θ and the traveling speed v. The objective of this embodiment is to provide a structure that is capable of obtaining the same advantages of the first embodiment while having a decreased number of parts and a more cost-efficient structure. 
     With reference to the flowchart illustrated in FIG. 13, the controller  31  reads the traveling speed v at stop  201  and the steering angle θ at step  202 . At step  203 , the controller  31  obtains the inverse of a turning radius r that is computed based on the steering angle θ. 
     At step  204 , the controller  31  computes an assumed centrifugal force Fx. The assumed centrifugal force Fx corresponds to the computed centrifugal force Fb of the first embodiment. However, since the forklift truck  1  does not have a gyroscope, the yaw rate is not detected. Instead, an assumed yaw rate value ωx is obtained by dividing the traveling speed v by the turning radius r (ωx=v/r). Thus, the assumed centrifugal force Fb is computed using the equation of Fx=2v/r, whereas, in the first embodiment, the computed centrifugal force is computed using the equation of Fb=v×ω. At step  205 , the controller  31  obtains the assumed yaw rate altering rate Δωx1/ΔT. The purposes of executing these steps are the same as these in the first embodiment and will not be described for the sake of brevity. 
     The absolute value of the assumed centrifugal force Fx is compared with the maximum reference value Hmax in step  206  and with the minimum reference value Hmin in step  208 . The comparison is carried out with reference to a map illustrated in FIG.  10 . The altering rate Δω1/ΔT is compared with the maximum reference value Kmax in stop  210  and the minimum reference value Kmin in step  212 . The purposes of executing these steps are the same as those of the first embodiment. After execution of steps  207 ,  211 ,  213 , and  214 , the controller  31  proceeds to step  115 . 
     A third embodiment of the present invention will now be described with reference to FIGS. 14 to  16 . In this embodiment, the rear axle  11  is gradually released from the locked state by a hydraulic unit that operates the shock absorber  12  in a gradual manner. When the rear axle  11  is locked, the interior of the shock absorber  12  is highly pressurized. If the pressure in the shock absorber  12  is suddenly decreased, the rear axle  11  may start to swing in a sudden manner. The operating mechanism of the shock absorber  12  employed in this embodiment regulates such undesirable swinging of the rear axle  11 . 
     In this embodiment, an electromagnetic proportional valve  130  is employed in lieu of the electromagnetic switching valve  13 . The controller  31  sends a duty signal to the proportional valve  130  by way of a driving circuit (not shown) to adjust the opening area of the proportional valve  130 . The structure of the valve  130  will now be described. 
     As shown in FIG. 14, the proportional valve  130  includes a cylindrical body  14  and a spool  15 , which is slidably accommodated in the body  14 . The spool  15  is driven by an electromagnetic solenoid  13   a  and a spring  13   b.  The spool  15  is constantly urged toward an opening position by the spring  13   b.  When a duty signal from the controller  31  excites the solenoid  13   a,  the spool  15  is moved toward a closing position. The first, second, third, and fourth oil passages P 1 , P 2 , P 3 , P 4 , are respectively connected with first, second, third, and fourth holes E 1 , E 2 , E 3 , E 4 , which extend through the wall of the body  14 . The spool  15  is provided with a first groove F 1  and a second groove F 2 . When the spool  15  is at the opening position, the first groove F 1  is aligned with the first and third holes E 1 , E 3 , and the second groove F 2  is aligned with the second and fourth holes E 2 , E 4 . Thus, the first groove F 1  connects the first oil passage P 1  to the third oil passage P 3 , and the second groove F 2  connects the second oil passage P 2  to the fourth oil passage P 4 . 
     The spool  15  is moved for a distance corresponding to the duty ratio of the duty signal sent from the controller  31 . The movement of the spool  15  alters the aligned area between the first groove F 1  and the corresponding holes E 1 , E 3 , and between the second groove F 2  and the corresponding holes E 2 , E 4 . In other words, the movement of the spool  15  alters the opening area of the proportional valve  130 . This adjusts the flow rate of the hydraulic oil flowing through the oil passages P 1 -P 4 . 
     Hydraulic oil is supplied to the oil passages P 1 -P 4  from the accumulator  16 . The accumulator  16  also supplies oil to the lifting cylinder  6 . 
     The controller  31  locks the shock absorber  12  by means of the proportional valve  130  when any one of the conditions (a) to (f) for outputting the locking signal (described in the first embodiment) is satisfied. This, in turn, locks the rear axle  11 . The controller  31  outputs the duty signal to the proportional valve  130  when none of the signal outputting conditions (a) to (f) is met. When first output, the duty ratio of the duty signal is one hundred percent. The duty ratio decreases gradually until it reaches zero percent. Thus, as shown in FIG. 15, the opening area of the proportional valve is gradually increased. This gradually increases the amount of hydraulic oil that flows through the shock absorber  12 . Accordingly, the rear axle  11  is gradually released from the locked state. Thus, sudden swinging of the rear axle  11  is prevented. 
     In the same manner, the amount of hydraulic oil supplied to the lifting cylinder  6  is controlled by the proportional valve  130 . Thus, when the signal outputting signal (f) is cleared, sudden lifting or lowering of the load on the forks  4  is avoided. Accordingly, the vehicle does not receive a sudden impact from the load on the forks  4 . This enables smooth lifting and lowering of loads. 
     FIG. 16 illustrates a further embodiment according to the present invention. This embodiment employs a first electromagnetic switching valve  41  that is similar to the one used in the first embodiment. Furthermore, a second electromagnetic switching valve  42  substitutes for the throttle valve  17 . The second switching valve  42  has a minimal restricting passage  42   a  and a maximal restricting passage  42   b,  each of which restrict the flow of hydraulic oil. Hydraulic oil selectively flows through either the minimal restricting passage  42   a  or the maximal restricting passage  42   b.  The passage diameter of the minimal restricting passage  42   a  is greater than the passage diameter of the maximal restricting passage  42   b.  Thus, the amount of hydraulic oil that passes through the minimal restricting passage  42   a  is greater than that of the maximal restricting chamber  42   b.  The controller  31  is connected to the second switching valve  42 . The restricting area of the switching valve  42  is adjusted to gradually release the locked state of the rear axle  11 . 
     As shown in FIG. 17, when the locking signal is not output from the controller  31 , the minimal restricting passage  42   a  is selected to allow hydraulic oil flow therethrough. In this state, the shock absorber  12  supports the rear axle  11  in an unlocked state. When the controller  31  outputs the locking signal, the maximal restricting passage  42   b  is selected to allow hydraulic oil flow therethrough. When a predetermined time length J elapses after the controller  31  terminates the output of the locking signal, the minimal restricting passage  42   a  is selected to allow hydraulic oil flow therethrough. This results in the shock absorber  12  releasing the locking of the rear axle  11 . 
     When releasing the rear axle  11  from a locked state, the flow rate of the hydraulic oil supplied to the shock absorber  12  is restricted by the minimal restricting passage  42   a.  This prevents sudden swinging of the rear axle  11  that may be caused by a sudden increase in the pressure within the shock absorber  12  and ensures stability of the traveling forklift truck  1 . 
     In this embodiment, an electromagnetic proportional valve may substitute for the electromagnetic switching valve  42 . Such a structure further prevents sudden swinging of the rear axle  11  when the controller  31  stops the output of the locking signal to have the shock absorber  12  release the rear axle from a locked state. 
     A further embodiment of the present invention will now be described with reference to FIGS. 18 to  20 . In this embodiment, a pair of coil springs B connect the rear axle  11  to the body frame is to absorb the impact transmitted to the frame  1  from the rear axle  11  when the axle  11  swings as the forklift truck  1  changes directions. In addition, the rear axle  11  is released from a locked state when the centrifugal force Fa detected by the acceleration sensor  21  becomes smaller a predetermined value. This structure is described in Japanese Unexamined Patent Publication No. 58-211903. However, since the springs B absorb the impact that is transmitted to the entire vehicle as the vehicle is being steered to change directions, the centrifugal force applied to the vehicle may not be accurately detected. As a result, the locking of the rear axle  11  may be released even when the forklift truck  1  is changing directions. This embodiment solves this problem. 
     As shown in FIG. 19, when the forklift truck  1  changes directions, an actual centrifugal force F swings the vehicle together with the rear axle  11 . During the swinging, the force Fs of the spring B acts on the rear axle  11 . The centrifugal force Fa detected by the acceleration sensor  21  corresponds to the resultant force of the actual centrifugal force F and the spring force Fs. 
     The controller  31  compares the centrifugal force Fa with a predetermined maximum reference value Hmax and a predetermined minimum reference valve Hmin. This comparison is carried out in the same manner as illustrated in the flowchart of FIGS.  8 (A), and  8 (B). In this processing, the computed centrifugal force Fb replaces the centrifugal force Fa. The controller  31  stops the output of the locking signal when the predetermined time length Ta elapses after the centrifugal force Fa decreases from a value equal to or greater than the minimum reference value Hmin to a value smaller than the minimum reference value Hmin. 
     FIG. 20 shows how the centrifugal force Fa detected by the acceleration sensor  21  changes as time elapses when the rear axle  11  is in a locked state during turning of the forklift truck  1 . During the time period X 1 , the value of the centrifugal force Fa is smaller than the minimum reference value Hmin due to the spring force Fs. Since the time period X 1  is shorter than the predetermined time length Ta, the controller  31  continues to output a locking signal to the electromagnetic switching valve. After the vehicle stops turning, the centrifugal force Fa remains smaller than the minimum reference value Hmin during time period X 2 , which is longer than the predetermined time length Ta. Thus, the controller  31  stops the output of the locking signal when the predetermined time length Ta elapses. Accordingly, the rear axle  11  remains locked regardless of the centrifugal force Fa temporarily becoming smaller than the minimum reference value. This ensures stability of the forklift truck  1  when changing directions. 
     Although several embodiments of the present invention have been described herein, it should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.