Abstract:
A hydraulic apparatus is used in an industrial vehicle. The apparatus has a hydraulic cylinder interposed between a vehicle frame and an axle swingably coupled to the frame. A passage connects a first chamber with a second chamber defined by a piston in a cylinder case. The piston is movable based on differential pressure in the chambers. A cylinder rod selectively extends and retracts in respect with the cylinder case to absorb a swinging motion of the axle. The passage is selectively open and closed based on at least one of a traveling state and a loading state of the vehicle. The piston has a first surface defining the first chamber and a second surface defining the second chamber. The first surface has an area equal to that of the second surface.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to axle pivoting controllers and hydraulic cylinders for industrial vehicles. 
     A forklift is generally provided with a rear axle beam and a body frame. In order to improve the riding comfort and enhance the driving performance of the forklift, the center of the rear axle beam can be supported such that the rear axle beam is pivotal with respect to the body frame. The body frame and the rear axle beam are connected to each other by a hydraulic cylinder. 
     Japanese Unexamined Utility Model Publication No. 56-25609 describes a rear axle beam having ends that are each connected to a body frame by a single action hydraulic cylinder. Each hydraulic cylinder has a piston and an oil chamber. The oil chambers of the hydraulic cylinders are connected with each other by a passage. In accordance with the pivotal movement of the rear axle beam relative to the body frame, each piston is moved axially in its associated cylinder. Hydraulic oil flows between the oil chambers through the passage in accordance with the movement of each piston and restricts the pivotal movement of the rear axle beam. 
     When such a forklift carries a cargo, lifts a cargo to a high position, or changes directions at a high speed, the forklift becomes less stable. In order to increase the driving stability, the pivotal movement of the rear axle beam can be locked by restricting the movement of the pistons. An electromagnetic control valve is arranged in the passage to restrict the movement of the pistons by stopping the flow of hydraulic oil in the passage. 
     A forklift that restricts pivotal movements of the rear axle beam by employing two single action hydraulic cylinders requires a large number of components, which are installed on the forklift. In order to reduce the number of components, the owner of the present application has proposed to arrange a multiple action hydraulic cylinder on just one end of the rear axle beam to connect the rear axle beam with the body frame. The multiple action hydraulic cylinder has a piston, which defines a first oil chamber and a second oil chamber. The first and second oil chambers are connected with each other by a passage. The piston is moved axially in accordance with the pivotal movements of the rear axle beam. This moves the hydraulic oil between the first and second oil chambers and restrains the pivotal movement of the rear axle beam relative to the body frame. In this structure, the electromagnetic control valve restricts the movement of the piston by stopping the flow of the hydraulic oil in the passage between the first and second oil chambers. Thus, the pivotal movements of the rear axle beam are also restricted with this structure. In addition, this type of forklift employs only one cylinder. Therefore, the installation of the hydraulic cylinder is facilitated due to the smaller number of components. 
     However, in the multiple action hydraulic cylinder, one end of the piston is connected to a rod, which extends through one of the hydraulic oil chambers. Thus, the cross sectional area of this chamber, on which pressure is applied, is smaller than that of the other hydraulic oil chamber. Accordingly, the pressure applied to the piston differs when the piston moves in opposite directions. As a result, the velocity of the piston differs according to the moving direction. The difference in the velocity of the piston permits the forklift to tilt to the right and to the left in different manners such that the operator can feel the difference. In addition, the volume of oil that flows outward from one oil chamber is not the same as the volume of oil that enters the other oil chamber. It is thus necessary to eliminate the imbalance between the two oil volumes and permit the forklift to tilt to the right and to the left in the same manner. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide an axle pivoting controller for industrial vehicles that uses a reduced number of components to facilitate installation and that has a piston that always moves in the same manner regardless of the direction of movement. 
     To achieve the above objective, the present invention provides a hydraulic apparatus used in an industrial vehicle. The industrial vehicle has a hydraulic cylinder interposed between a vehicle frame and an axle swingably coupled to the frame. The hydraulic cylinder has a cylinder case and a piston movable in accordance with hydraulic pressure in the cylinder case. A cylinder rod selectively extends and retracts in respect with the cylinder case to absorb a swinging motion of the axle. A first chamber and a second chamber are defined by the piston in the cylinder case. The piston includes a first surface defining the first chamber and a second surface defining the second chamber. The first surface has a area equal to that of the second surface. A hydraulic passage connects the first chamber with the second chamber. The apparatus includes means for selectively opening and closing the hydraulic passage based on at least one of a traveling state and a loading state of the industrial vehicle. 
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     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 schematic view showing a structure of an axle pivoting controller arranged on a rear axle beam; and 
     FIG. 2 is a cross sectional view showing a hydraulic cylinder. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An axle pivoting controller according to the present invention, which is applied to a pivotal rear axle beam of a forklift, will now be described with reference to FIGS. 1 and 2. 
     FIG. 1 shows a schematic rear view of a forklift having a body frame  1  and a rear axle beam  2 . The rear axle beam  2  is pivotally supported by a central pin  3 , which is arranged in a lower portion of the body frame  1 . A shock absorbing elastic body  4  is provided between the body frame  1  and the rear axle beam  2  to absorb shocks produced by the pivotal movement of the rear axle beam  2 . A steered wheel  5  is mounted on each end of the rear axle beam  2  and supported such that the wheel  5  can be pivoted to steer the forklift. 
     A multiple action hydraulic cylinder  6  is arranged on one end of the rear axle beam  2 . The hydraulic cylinder  6  connects the rear axle beam  2  to the body frame  1 . An electromagnetic control valve  7  and an accumulator  8  are arranged on the body frame  1 . The hydraulic cylinder  6  is connected to the electromagnetic control valve  7  and the accumulator  8  through passages  9   a ,  9   b ,  10   a ,  10   b . Therefore, the passages  9   a ,  9   b ,  10   a ,  10   b , the hydraulic cylinder  6 , the electromagnetic control valve  7 , and the accumulator  8  form a hydraulic circuit. An axle pivoting controller  11  is provided on the body frame  1 . 
     FIG. 2 is a cross sectional view showing the hydraulic cylinder  6 . The cylinder  6  includes a cylindrical tube  12 , a piston  13 , a piston rod  14 , and a guide rod  15 . The upper end of the piston  13  defines a head portion, and the lower end of the piston  13  defines a rod portion. The “upper” and “lower” directions referred to are taken from the upper and lower directions of FIG.  2 . The upper end of the tube  12  is closed by a head piece  16  and a guide piece  17 . The lower end of the tube  12  is closed by a rod piece  18 . The piston  13  defines a rod chamber  19  and a head chamber  20  in the tube  12 . The piston rod  14  extends through the rod chamber  19  and is fixed to the piston  13 . The guide rod  15  extends through the head chamber  20  and is fixed to the piston  13 . The piston rod  14  has a threaded male end  14   a , which extends through the piston  13  and into the head chamber  20 . The guide rod  15  has a threaded female end  15   a , which engages the male end  14   a . The other end of the piston rod  14  (the lower end) extends through the rod piece  18  and out of the tube  12 . The other end of the guide rod  15  (the upper end) is slidably supported by a bearing  16   a  in the head piece  16  and is accommodated in a cavity  17   a , which is defined in the guide piece  17 . The threaded engagement between the male end  14   a  of the piston rod  14  and the female end  15   a  of the guide rod  15  forms a single integral rod. The piston  13  is located at the middle of the single rod. The cross sectional area of the guide rod  15  is equal to that of the piston rod  14 . Therefore, the cross sectional area (or the piston area) that receives pressure is the same in the head side and in the rod side of the piston  13 . 
     The lower end of the piston rod  14  is pivotally supported about a pin  22  on a bracket  21 , which is fixed to the rear axle beam  2 . The upper end of the guide piece  17  is pivotally supported about a pin  24  on a bracket  23 , which is fixed to the body frame  1 . 
     The electromagnetic control valve  7  has associated ports a, c, and associated ports b, d. The electromagnetic control valve  7  incorporates an electromagnetic solenoid  25 , which shifts the control valve  7  between positions  7   a  and  7   b . At position  7   a , the electromagnetic control valve  7  disconnects port a from port c, and it disconnects port b from port d. At position  7   b , the control valve  7  connects port a with port c, and port b with port d. A spring  26  is arranged in the control valve  7 . When the electromagnetic solenoid  25  is de-excited, the control valve  7  is held at position  7   a  by the force of the spring  26 . When the solenoid  25  is excited, the valve  7  is shifted to position  7   b  against the force of the spring  26 . Therefore, the electromagnetic control valve  7  is normally closed. Port a is connected with the rod chamber  19  by way of the passage  9   a . Port b is connected with the head chamber  20  by way of the passage  9   b . Port c is connected with the accumulator  8  by way of the passage  10   a . Port d is connected with the accumulator  8  by way of the passage  10   b.    
     Therefore, when the electromagnetic control valve  7  is shifted to position  7   a , the rod chamber  19  is disconnected from the head chamber  20 . Thus, the hydraulic cylinder  6  restricts the pivotal movement of the rear axle beam  2  relative to the body frame  1  by stopping the flow of the hydraulic oil between the chambers  19 ,  20 . On the other hand, when the control valve  7  is shifted to position  7   b , the rod chamber  19  and the head chamber  20  are connected to each other by way of the passages  9   a ,  9   b  and the accumulator  8 . The hydraulic cylinder  6  permits the pivotal movement of the rear axle beam  2  relative to the body frame  1  by permitting the hydraulic oil to flow between the chambers  19 ,  20 . When the chambers  19 ,  20  are connected to each other, the hydraulic cylinder  6  restrains the pivotal movement of the rear axle beam  2  by the resistance generated as hydraulic oil passes through passages  9   a  and  9   b.    
     When predetermined conditions are satisfied, the axle pivoting controller  11  excites the electromagnetic control valve  7 . When the predetermined conditions are unsatisfied, the axle pivoting controller  11  de-excites the electromagnetic control valve  7  and restricts the pivotal movement of the rear axle beam  2 . The predetermined conditions are unsatisfied when the height to which the cargo is lifted, the angle of the steered wheels  5 , and the traveling speed of the vehicle exceed certain values. The height of the cargo is detected by a height sensor H. The angle of the wheels  5  is detected by a steering angle sensor S. The traveling speed of the vehicle is detected by a velocity sensor V. 
     The operation of the axle pivoting controller  11  and the hydraulic cylinder  6  will now be described. 
     When the controller  11  excites the electromagnetic control valve  7 , the control valve  7  is shifted to position  7   b  and the head chamber  20  is communicated with the rod chamber  19 . In this state, the hydraulic cylinder  6  is actuated and pivoting of the rear axle beam  2  relative to the body frame  1  is permitted in accordance with driving conditions. Therefore, pivoting of the rear axle beam  2  causes hydraulic oil to enter the rod chamber  19  or the head chamber  20  and move out of the rod chamber  19  or the head chamber  20 . 
     When the rear axle beam  2  pivots clockwise relative to the body frame  1 , as viewed in FIG. 1, the piston rod  14  retracts into the cylinder  6 . The piston  13  thus moves toward the body frame  1  and receives pressure from the hydraulic oil in the head chamber  20 . This restrains the axial movement of the piston rod  14  and the pivotal movement of the rear axle beam  2  relative to the body frame  1 . As a result, the rear axle beam  2  pivots clockwise at an appropriate velocity. 
     If the rear axle beam  2  pivots counterclockwise relative to the body frame  1 , the piston rod  14  projects from the cylinder  6 . The piston  13  thus moves toward the rear axle beam  2  and receives pressure from the hydraulic oil in the rod chamber  19 . This restrains the axial movement of the piston rod  14  and the pivotal movement of the rear axle beam  2  relative to the body frame  1 . As a result, the rear axle beam  2  pivots counterclockwise at an appropriate velocity. Since the cross sectional area of the piston  13  is equal in the head chamber  20  and the rod chamber  19 , the piston  13  receives the same pressure in the head chamber  20  and the rod chamber  19  when the piston  13  moves axially. Therefore, the retraction velocity of the piston  13  is equal to the projection velocity of the piston  13 . As a result, the pivoting velocity of the rear axle beam  2  relative to the body frame  1  is the same when the rear axle beam  2  tilts clockwise and counterclockwise, as viewed in FIG.  2 . 
     In addition, the volume of the hydraulic oil discharged from one of the oil chambers  19 ,  20  is the same as the volume of the hydraulic oil that enters the other oil chamber  19 ,  20 . 
     The preferred and illustrated embodiment has the advantages described below. 
     (a) In the hydraulic cylinder  6  that connects the body frame  1  and the rear axle beam  2 , the volume of the hydraulic oil that flows out of one of the oil chambers  19 ,  20 , is equal to the volume of the hydraulic oil that flows into the other oil chamber  19 ,  20 . As a result, the retraction velocity and the projection velocity of the piston  13  are the same. Thus, the hydraulic cylinder  6  operates in the same manner when the piston rod  14  retracts and projects. Accordingly, the forklift tilts to the right and to the left in the same manner. 
     The volume of the hydraulic oil that flows out of one of the oil chambers  19 ,  20 , and the volume of the hydraulic oil that flows into the other oil chamber  19 ,  20 , are the same. Therefore, it is not necessary to arrange a special structure to compensate for volume differences. As a result, the hydraulic circuit can be constructed in a simpler form. 
     (b) When the rear axle beam  2  pivots relative to the body frame  1 , the clockwise pivoting velocity of the rear axle beam  2  is equal to the counterclockwise pivoting velocity of the rear axle beam  2 , assuming the same amount of force is applied to each end of the rear axle beam  2 . 
     (c) The electromagnetic valve  7  is normally closed. When the controller  11  malfunctions, the valve  7  cannot be controlled. Thus, the hydraulic oil does not flow between the passages  9   a ,  9   b . In such case, the rear axle beam  2  is locked. This stabilizes the forklift even when carrying a cargo. Therefore, the transportation of the cargo can be completed before servicing the controller. 
     (d) The cross sectional area of the head side of the piston  13  and that of the rod side of the piston  13  are equal. Accordingly, the pressure applied to the piston  13  is the same when the piston rod  14  retracts and projects. Then, the velocity of the piston rod  14  is equal regardless of whether the piston rod  14  retracts or projects. As a result, the piston rod  14  operates in the same manner when it projects and retracts. 
     (e) The piston rod  14  and the guide rod  15  form a single rod by fastening the male end  14   a  of the piston rod  14  to the female end  15   a  of the guide rod  15 . The piston  13  is located at the middle of the single rod. The piston rod  14 , the guide rod  15 , and the piston  13  are assembled by connecting the piston rod  14  and the guide rod  15 . Thus, the rod is assembled with precision in a facilitated manner in comparison with the other assembling methods, such as welding. 
     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. More particularly, the preferred and illustrated embodiment may be modified as described below. 
     The electromagnetic control valve  7  may be normally open. In this case, if the controller  11  malfunctions and the electromagnetic control valve  7  cannot be controlled, the rod chamber  19  and the head chamber  20  are connected. This permits pivoting of the rear axle beam  2 . Therefore, the rear axle beam  2  is permitted to pivot, even if the controller  11  malfunctions. Accordingly, traction is maintained by all four wheels of the forklift and the ground as the forklift travels along unpaved roads. Thus, the forklift can be driven even if the road is unpaved before being serviced. 
     The piston rod  14  may be connected to the body frame  1  and the guide rod  15  may be connected to the rear axle beam  2 . In other words, the cylinder  6  may be inverted from the orientation shown in FIG.  2 . 
     The piston rod  14  and the guide rod  15  may be fixed to the piston  13  by welding, fasteners such as bolts, press fitting, and other appropriate means. 
     The cross sections of the piston rod  14  and the guide rod  15  do not have to be circular and may be polygonal. 
     The head piece  16  and the guide piece  17  may be fixed to each other in an integral manner. 
     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 and equivalence of the appended claims.