Patent Publication Number: US-9889882-B2

Title: Dump truck

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to International Application No. PCT/JP2014/054978 filed on Feb. 27, 2014, the contents of which are incorporated herein in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a dump truck, for instance, a large-sized driverless off-road dump truck. 
     BACKGROUND ART 
     A large-sized dump truck working in mines and the like has been typically known. As a steering mechanism of front wheels (steering wheels) of such a dump truck, there has been typically known a steering mechanism including a yoke that is a part of a suspension and has a up-and-down movable proximal end supported on a vehicle body frame, a receiving seat rotatably attached to a distal end of the yoke, a knuckle arm attached to the receiving seat, and a steering cylinder extending between the knuckle arm and the vehicle body frame to connect the knuckle arm and the vehicle body frame (for instance, Patent Literature 1). 
     Moreover, an off-road dump truck has also been known for having steering wheels as all the tires in order to improve delivery performance (for instance, Patent Literatures 2 and 3). 
     CITATION LIST 
     Patent Literatures 
     Patent Literature 1: JP-A-5-193373 
     Patent Literature 2: U.S. Pat. No. 6,578,925 
     Patent Literature 3: U.S. Pat. No. 6,783,187 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In Patent Literature 1, since a proximal end of the steering cylinder is connected to the vehicle body and a distal end of the steering cylinder is connected to the knuckle arm, when the receiving seat moves up and down in conjunction with the yoke in an operation of the suspension, the knuckle arm attached to the receiving seat also moves up and down to slightly change a distance between the vehicle body and the knuckle arm. Accordingly, this change affects an advance and retraction amount of the steering cylinder to cause an unstable steering amount of the tire during the up-and-down motion of the receiving seat. 
     Patent Literatures 2 and 3 in which a four-wheel steering is employed fail to specifically disclose the steering mechanism. 
     An object of the invention is to provide a dump truck having a steering mechanism capable of obtaining a desired steering amount without receiving an influence from an up-and-down motion of a suspension. 
     Means for Solving the Problems 
     According to an aspect of the invention, a dump truck includes a vehicle body frame; a suspension; a steering mechanism; tires suspended from the vehicle body frame via the suspension and configured to be steered by the steering mechanism and travel the dump truck; and a body supported by the vehicle body frame and configured to be raised and lowered, the suspension including: a suspension arm having a up-and-down movable proximal end supported on the vehicle body frame; and a tire support rotatably attached to a distal end of the suspension arm, the steering mechanism including: a steering cylinder having a proximal end attached to the suspension arm and a distal end attached to a knuckle arm provided to the tire support. 
     According to the above aspect of the invention, since the steering cylinder is attached to connect the knuckle arm and the suspension arm, a positional relationship between which is hardly changed in an up-and-down motion direction during the up-and-down motion of the suspension, even when the suspension moves up and down during its operation, such a change does not affect an advance and retraction amount of the steering cylinder, so that a steering amount of the tires can be stabilized to provide a desired steering amount. 
     In the above aspect, it is preferable that the suspension arm includes an upper arm and a lower arm that are attached to the vehicle body frame such that the upper arm is arranged above the lower arm, and the proximal end of the steering cylinder is attached to the upper arm. 
     With this arrangement, since the steering cylinder is disposed at a high position in the same manner as the upper arm, the steering cylinder can be kept from being hit by gravel and the like on a road to prevent damage of the steering cylinder. 
     In the above aspect, it is preferable that the vehicle body frame includes a support that supports the suspension arm such that the suspension arm is movable up and down, the suspension arm includes a steering cylinder attachment arm extending over the support and further extending inward in a vehicle width direction, and the proximal end of the steering cylinder is attached to the steering cylinder attachment arm. 
     With this arrangement, since the proximal end of the steering cylinder is attached to the steering cylinder attachment arm extending inward in the vehicle body frame among the suspension arm, a sufficient distance from this attachment position to an attachment position of the steering cylinder to the knuckle arm can be ensured. Accordingly, since the steering cylinder to be used is sufficiently long in an axial direction, the maximum steering amount (maximum steering angle) can be increased to improve a rotation performance. 
     In the above aspect, it is preferable that, viewed in a travel direction, the steering cylinder has an axial line overlapping a line passing through a rotation center of the suspension arm and the tire support and traversing a motion center of the suspension arm on the vehicle body frame. 
     With this arrangement, respective up-and-down motion regions to be required for respective up-and-down motions of the suspension arm and the steering cylinder can be the same when viewed in the travel direction, so that suspension arm and the steering cylinder can be easily prevented from interfering with other members. 
     In the above arrangement, it is preferable that the vehicle body frame includes a cross member provided in the vehicle width direction, the cross member supports a lower end of a hoist cylinder configured to raise and lower the body, and the steering cylinder is disposed opposite the hoist cylinder across the cross member in the travel direction. 
     With this arrangement, the hoist cylinder body that is requisite for raising and lowering the body can be reliably avoided from interfering with the steering cylinder. 
     According to another aspect of the invention, a dump truck includes a vehicle body frame; a suspension; a steering mechanism; tires suspended from the vehicle body frame via the suspension and configured to be steered by the steering mechanism and travel the dump truck; and a body supported by the vehicle body frame and configured to be raised and lowered, the suspension including: an upper arm having an up-and-down movable proximal end supported on an upper support provided to the vehicle body frame; a lower arm having an up-and-down movable proximal end supported on a lower support provided to the vehicle body frame; and a tire support rotatably attached between respective distal ends of the upper and lower arms, in which the upper arm includes a steering cylinder attachment arm extending over the support and further extending inward in a vehicle width direction, the steering mechanism includes a steering cylinder having a proximal end attached to the steering cylinder attachment arm and a distal end attached to a knuckle arm provided to the tire support, and viewed in the travel direction, the steering cylinder has an axial line overlapping a line passing through a rotation center of the upper arm and the tire support and traversing a motion center of the upper arm on the vehicle body frame. 
     According to the above aspect of the invention, the same advantages as those of the above-described invention having the same arrangement can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a partially-exploded perspective view of a dump truck according to an exemplary embodiment of the invention. 
         FIG. 2  is a side view of the dump truck. 
         FIG. 3  is an illustration of the dump truck in a travel direction, which is viewed in a direction indicated by an arrow III in  FIG. 2 . 
         FIG. 4  is a plan view of the dump truck. 
         FIG. 5  is a cross-sectional view showing a suspension, which is viewed in a direction indicated by an arrow V-V in  FIG. 4 . 
         FIG. 6  is a schematic illustration for explaining an intersection angle. 
         FIG. 7  is a cross-sectional view showing a steering mechanism, which is viewed in a direction indicated by an arrow VII-VII in  FIG. 4 . 
         FIG. 8  is a cross-sectional view showing a support structure and a cooling structure of an electric motor. 
         FIG. 9  is a plan view showing a layout of devices. 
         FIG. 10  is a perspective view showing an overall support frame. 
         FIG. 11  is an illustration of an attachment position of a hoist cylinder in the travel direction, which is viewed in a direction indicated by an arrow XI-XI in  FIG. 4 . 
         FIG. 12  is a side view showing the attachment position of the hoist cylinder. 
     
    
    
     DESCRIPTION OF EMBODIMENT(S) 
     Exemplary embodiment(s) of the invention will be described below with reference to the attached drawings. 
       FIGS. 1 to 4  are respectively a partially-exploded perspective view, a side view, an illustration of the dump truck in a travel direction, which is viewed in a direction indicated by an arrow III in  FIG. 2 , and a plan view of a dump truck according to an exemplary embodiment of the invention. 
     X, Y and Z axes in the drawings are orthogonal to each other in the exemplary embodiment. For convenience of the explanation, in the exemplary embodiment, with reference to  FIG. 1 , a first travel direction of the dump truck  1  is defined as an arrow direction of the X axis while a second travel direction thereof is defined as an opposite direction of the arrow direction of the X axis, a first vehicle width direction of the dump truck  1  is defined as an arrow direction of the Y axis while a second vehicle width direction thereof is defined as an opposite direction of the arrow direction of the Y axis, and a first vertical direction of the dump truck  1  is defined as an arrow direction of the Z axis while a second vertical direction thereof is defined as an opposite direction of the arrow direction of the Z axis. Moreover, in the following exemplary embodiment, the first travel direction, the second travel direction, the first vehicle width direction and the second vehicle width direction are sometimes respectively referred to as “front,” “rear (back),” “right” and “left”. 
     Overall Description of Dump Truck 
     As shown in  FIG. 1 , a dump truck  1  is a driverless off-road dump truck configured to travel by remote control. For instance, the dump truck  1  is a vehicle working at a mining site for developing mines. The remote control is performed fully using information communication technology such as a communication means set at a control center and the dump truck  1  and GPS (Global Positioning System). 
     The dump truck  1  includes a vehicle body  10  configured to travel with use of a pair of left and right tires  11 ,  11  and a pair of left and right tires  12 ,  12 , the tires  11 ,  11  being set on both vehicle-width-directional sides of the vehicle body  10  in the first travel direction, the tires  12 ,  12  being set on both vehicle-width-directional sides of the vehicle body  10  in the second travel direction. The vehicle body  10  includes: a vehicle body frame  20  extending along the travel direction and to which the tires  11 ,  12  are provided; a load-carrying body  30  that is supported by the vehicle body frame  20  and configured to be raised and lowered (see a two-dot chain line in  FIG. 2 ; devices  41  to  49  mounted on the vehicle body frame  20 ; a suspension  50  by which the tires  11 ,  12  are suspended from the vehicle body frame  20 ; and a steering mechanism. The dump truck  1 , which is a vehicle dedicated for remote control, does not include a cab for a driver to operate, which is provided in a typical dump truck. 
     Explanation of Vehicle Body Frame 
     The vehicle body frame  20  will be described in details below. 
     As shown in  FIGS. 2 to 4 , the vehicle body frame  20  includes: a lower cross member  201  (i.e., a lower one of cross members) provided at positions of the right and left tires  11  in the first travel direction (i.e., the right and left tires  11  provided to a first side of the vehicle body frame  20  in the travel direction) and extending in the vehicle width direction; a pair of right and left vertical members  202 ,  202  standing upward on both ends of the lower cross member  201 ; and an upper cross member  203  (i.e., an upper one of the cross members) extending in the vehicle width direction in a manner to connect upper ends of the respective vertical members  202 . Among the above members, the pair of vertical members  202  and the upper cross member  203  define a first vertical frame  21  that vertically stands at the positions of the right and left tires  11  and has a portal shape viewed in the travel direction of the vehicle body  10  (see  FIG. 3 ). 
     In other words, the vehicle body frame  20  in a side view includes the first vertical frame  21  that vertically stands at the positions of the tires  11  provided to the first side of the vehicle body frame  20 . 
     The vehicle body frame  20  also includes: the lower cross member  201  provided at positions of the right and left tires  12  in the second travel direction (i.e., the right and left tires  12  provided to a second side of the vehicle body frame  20  in the travel direction) and extending in the vehicle width direction; a pair of right and left vertical members  202 ,  202  standing upward on both ends of the lower cross member  201 ; and an upper cross member  203  extending in the vehicle width direction in a manner to connect upper ends of the respective vertical members  202 . Among the above members, the pair of vertical members  202  and the upper cross member  203  define a second vertical frame  22  that vertically stands at the positions of the right and left tires  12  and has a portal shape viewed in the travel direction of the vehicle body  10 . 
     In other words, the vehicle body frame  20  in a side view includes the second vertical frame  22  that vertically stands at the positions of the tires  12  provided to the second side of the vehicle body frame  20 . 
     The first vertical frame  21  and the second vertical frame  22  have substantially the same shape. 
     Ends of the front lower cross member  201  are connected to ends of the back lower cross member  201  by a pair of right and left lower side members  23 ,  23  disposed in parallel to each other in the travel direction and spaced from each other in the vehicle width direction. A vertical middle of the first vertical frame  21  is connected to a vertical middle of the second vertical frame  22  by a pair of right and left upper side members  24 ,  24  positioned above the lower side members  23 ,  23  (see  FIG. 2 ). 
     Side members  25 ,  25  of a short length, which are respectively positioned on extension lines of the lower side members  23 ,  23 , extend in the first travel direction from a lower portion of the first vertical frame  21 . Ends of the respective side members  25 ,  25  are connected by a cross member  26 . Side members  27 ,  27  of a short length, which are respectively positioned on extension lines of the lower side members  23 ,  23 , extend in the second travel direction from a lower portion of the second vertical frame  22 . Ends of the respective side members  27 ,  27  are connected by a cross member  28  along the vehicle width direction (see  FIG. 4 ). 
     As shown in  FIGS. 3 and 5 , the lower cross member  201  on which the first vertical frame  21  stands is shaped in a hollow cylinder. Electric motors  43 ,  43  configured to independently respectively drive the tires  11 ,  11  via a drive shaft  18  are housed at both sides of an inside of the lower cross member  201 . In the exemplary embodiment in which all the tires  11 ,  12  are to be driven, a pair of electric motors  43 ,  43  are also housed in the same manner as the above within the lower cross member  201  on which the second vertical frame  22  stands and are configured to independently respectively drive the tires  12 . A final reduction gear  14  (a planet gear mechanism) is disposed between an end of the drive shaft  18  and a tire wheel. 
     An upper surface of the upper cross member  203  defining an upper portion of the first vertical frame  21  and an upper portion of the second vertical frame  22  is defined as a mount portion  204  that is a concave curve with a predetermined curvature. The body  30  is mounted only on the mount portion  204 . A suspension support  205  supporting an upper end of a suspension cylinder  53  (a part of the suspension  50 ) is provided at each end of the upper cross member  203 . A lower end of the suspension cylinder  53  is connected to an upper arm  51  (a part of the suspension  50 ). With this arrangement, the mount portion  204  is positioned on an axial line  53 A of the suspension cylinder  53  configured to transmit a load downward (see  FIG. 3 ). 
     The suspension  50  will be described later. 
     Herein, the load to be transmitted to a road surface through the tires  11 ,  12  includes a carrying load and a vehicle body load. The carrying load means a load defined by a weight of the body  30  loaded with goods. The vehicle body load means a load defined by a weight of the vehicle body  10  excluding weights of the tires  11 ,  12  and the body  30 . In the exemplary embodiment, the vehicle body load and the carrying load are sometimes collectively referred to as an entire load. 
     Accordingly, the carrying load is transmitted from the mount portion  204  to the road surface through the suspension  50  (including the suspension cylinder  53 ) beneath the mount portion  204  and the tires  11 ,  12 . Thus, the carrying load is transmitted through a short and simple transmission path (see dotted lines in  FIGS. 2 and 3 ). In other words, the carrying load is transmitted without acting on the upper side member  24 , the lower side member  23  and the like. 
     A shape of the entire vehicle body frame  20  is substantially plane-symmetrical to a vertical plane including a first center line  10 A passing through the middle between the front tires  11  and the rear tires  12  and extending in the vehicle width direction, while being substantially plane-symmetrical to a vertical plane including a second center line  10 B being orthogonal to the first center line  10 A and extending in the travel direction through the middle between the tires in the vehicle width direction (see  FIGS. 1 and 4 ). 
     Moreover, a support frame  81  is provided on the first center line  10 A in the vehicle body frame  20  in a manner to bridge over the vehicle body frame  20  in the vehicle width direction. The support frame  81  is provided for supporting the devices  44  to  48  on the vehicle body frame  20 . As shown in  FIGS. 1, 9 and 10 , the support frame  81  includes a pair of front and back sub frames  82  spaced from each other in the travel direction. The entire support frame  81  is shaped in a form of a saddle. Both sides of the support frame  81  project between the pair of front and back tires  11  and  12  on both the right and left sides of the vehicle body frame  20 . Accordingly, the devices  44  to  48  supported by the support frame  81  are also disposed between the pair of front and back tires  11  and  12 . 
     Specific disposition of the devices  44  to  48  will be described later. 
     Each of the sub frames  82  includes: a pair of right and left L-shaped frames  85  each including vertical portions  83  that are each fixed to the lower side member  23  and the upper side member  24  at both sides of the vehicle width direction and extensions  84  respectively horizontally extending toward the outside of the vehicle body frame  20  from lower ends of both the vertical portions  83  in the vehicle width direction; an upper connecting portion  86  connecting upper ends of the vertical portions  83  of the pair of right and left L-shaped frames  85  above the upper side member  24 ; and a lower connecting portion  87  connecting the respective lower ends of the vertical portions  83  of the pair of right and left L-shaped frames  85  under the lower side member  23 . 
     The L-shaped frames  85  are detachably fixed to the lower side members  23  and the upper side member  24   s  with an unillustrated fastening unit (e.g., a bolt). The upper ends of the vertical portions  83  in each of the L-shaped frames  85  are detachably connected to the upper connecting portion  86  with a pin. The lower ends of the vertical portions  83  in each of the L-shaped frames  85  are detachably connected to the lower connecting portion  87  with a pin. Accordingly, the connection of each of the L-shaped frames  85  to the connecting portions  86  and  87  is provided by a flexible connection with a pin while each of the L-shaped frames  85  to the vehicle body frame  20  is kept firmly fixed, so that the L-shaped frames  85  can favorably tolerate torsion or the like of the vehicle body frame  20 . 
     Explanation of Body 
     As shown in  FIGS. 1 to 3 , a depth of the body  30  is the maximum at a middle portion thereof in the travel direction and is decreased toward both the sides thereof in the travel direction and both the sides thereof in the vehicle width direction. Specifically, the body  30  include: a slant bottom  31  that defines the depth deeper toward the middle of the body  30 ; and side faces  32 ,  32  that guard respective edges of longer sides of the bottom  31  in the travel direction. On slant parts, which are slant in different directions, of a lower surface of the bottom  31 , horizontal ribs  33 ,  33  are provided in a manner to traverse the slant parts in the vehicle width direction and so that ends of each of the horizontal ribs  33 ,  33  extend to outer faces of the side faces  32 ,  32 . 
     The horizontal ribs  33  are mounted on the respective mount portions  204  of the first vertical frame  21  and the second vertical frame  22 . Each of the horizontal ribs  33  is curved at the same curvature as that of the mount portion  204  so that the horizontal ribs  33  are respectively in close contact with the mount portions  204 . Moreover, a pair of vertical ribs  34 ,  34  are provided in parallel in the travel direction on the lower surface of the bottom  31 . The body  30  is mounted on the mount portion  204  at positions where the vertical ribs  34  intersect with the horizontal ribs  33  (see  FIGS. 2 and 3 ). 
     Respective attachment portions  36 ,  36  to which upper ends of a pair of hoist cylinders  35 ,  35  are attached are provided on one of the slant parts of the lower surface of the bottom  31  (see  FIGS. 1, 11 and 12 ). The hoist cylinders  35  are hydraulic actuators for raising and lowering the body  30 . Lower ends of the hoist cylinders  35  are attached to the lower cross member  201  on which the second vertical frame  22  stands. In the middle of the one of the slant parts, a pair of pivot shafts  37 ,  37  connecting the body  30  to the vehicle body frame  20  so that the body  30  is pivotable are provided (only one of the pivot shafts  37 ,  37  is shown in  FIGS. 2 and 12 ). The pivot shafts  37  are respectively supported on body supports  206 ,  206  extending from upper portions of the vertical members  202  to the upper cross member  203 . Since the second vertical frame  22  stands at the position of the right and left tires  12  in a side view, in terms of the support position of the body  30  in the travel direction, the body  30  is supported on the vehicle body frame  20  at the position of the right and left tires  12  through the body support  206 . 
     A shape of the entire body  30  is also substantially plane-symmetrical to the above-described vertical plane including the first center line  10 A and substantially plane-symmetrical to the above-described vertical plane including the second center line  10 B (see  FIG. 1 ). The body  30  is mounted in the middle of the vehicle body frame  20  that is plane-symmetrical to the vertical plane including the first center line  10 A and the second center line  10 B. Consequently, a load distribution of the carrying load to be transmitted from the mount portion  204  of the vehicle body frame  20  to the tires  11  and  12  becomes equal. In other words, an entire load obtained by combining the vehicle load and the carrying load is transmitted to the tires  11 ,  12  at an even load distribution. 
     Explanation of Devices 
     Main devices shown in  FIG. 1  are an engine  41 , a generator motor  42  and a hydraulic pump (not shown) configured to be driven by an output of the engine  41 , an electric motor  43  configured to be driven by electric energy generated by the generator motor  42  (see  FIGS. 2 and 3 ), a first radiator  44  configured to radiate heat of a cooling water of the engine  41 , a first cooling fan  45  configured to supply a cooling air to the first radiator  44 , a second radiator  46  for a water-cooling type aftercooler  41 A (see  FIG. 9 ) configured to cool charge air delivered from an air cleaner through a supercharger to the engine  41 , a second cooling fan  47  configured to supply a cooling air to the second radiator  46 , a pair of brake resistors  48 ,  48  configured to convert kinetic energy of the tires  11 ,  12  to electric energy to generate Joule heat, a hydraulic fluid tank (not shown) configured to store a hydraulic fluid pumped from the hydraulic pump, and a pair of front and rear controllers  49 ,  49  configured to totally control travel of the dump truck  1 . 
     Specific disposition of the devices  41  to  49  will be described later. 
     Explanation of Suspension 
       FIG. 5  is a cross-sectional view showing the suspension  50 , which is viewed in a direction indicated by an arrow V-V in  FIG. 4   
     As shown in  FIGS. 3 to 5 , an independent double-wishbone-type suspension is employed as the suspension  50 . The suspension  50  includes: substantially horizontal upper arm  51  and lower arm  52  each having up-and-down movable proximal ends supported by the vehicle body frame  20 ; a tire support in a form of a cylindrical casing  56  having an upper portion rotatably connected to a distal end of the upper arm  51  and a lower portion rotatably connected to a distal end of the lower arm  52 ; and a suspension cylinder  53  having an upper end rotatably connected to the vehicle body frame  20  and a lower end rotatably connected to the upper arm  51 . The suspension cylinder  53  absorbs and attenuates impacts to the tires  11 ,  12  while transmitting the vehicle body load and the carrying load to the tires  11 ,  12 . The casing  56  rotates and supports the tires  11 ,  12  through a final reduction gear  14 . In the exemplary embodiment, the upper arm  51  and the lower arm  52  define a suspension arm according to the invention. 
     Specifically, a pair of proximal ends of the upper arms  51  having a bifurcated shape in a plan view are rotatably supported by the support in a form of an upper support  207  provided on a lower portion of the vertical member  202  of each of the first vertical frame  21  and the second vertical frame  22 . A pair of proximal ends of the lower arm  52  having a bifurcated shape in a plan view are rotatably supported by a lower support  208  provided on a lower end of the lower cross member  201  on which each of the first vertical frame  21  and the second vertical frame  22  stands. 
     A distal end of the upper arm  51  is connected to an upper ball joint  57  provided to the upper portion of the casing  56 . A distal end of the lower arm  52  is connected to a lower ball joint  58  provided to the lower portion of the casing  56 . A top of the upper ball joint  57  is covered with a connecting bracket  54  fixed to an upper surface of the upper arm  51 . The connecting bracket  54  is connected by the suspension cylinder  53  to the suspension support  205  of each of the first vertical frame  21  and the second vertical frame  22 . At this time, the lower end of the suspension cylinder  53  is connected to the connecting bracket  54  at a position very close to the upper ball joint  57 . 
     A king pin shaft  56 A connecting a rotation center  57 A of the upper ball joint  57  to a rotation center  58 A of the lower ball joint  58  intersects with an axial line  53 A of the suspension cylinder  53  at a connecting portion of the upper ball joint  57  of the casing  56  and the upper arm  51 , specifically, within a range of a ball diameter of the upper ball joint  57 , more specifically at a rotation center  57 A of the upper ball joint  57  and the upper arm  51 . Accordingly, the vehicle body load and the carrying load to be transmitted through the suspension cylinder  53  hardly act on the upper arm  51 , but are transmitted to the tires  11 ,  12  through the casing  56  to which the upper ball joint  57  is provided. Consequently, since the carrying load is not transmitted to the upper arm  51  and the lower arm  52 , respective structures of the upper arm  51  and the lower arm  52  can be simplified. 
     Herein, when the vehicle body  10  is moved up and down along with the up-and-down motion of the upper arm  51  and the lower arm  52 , a positional relationship between the tires  11 ,  12  and the electric motor  43  is slightly shifted. The drive shaft  18  is connected to an output shaft  43 A of the electric motor  43  and an input shaft  14 A of the final reduction gear  14  via a universal joint in order to absorb the shift of the tires  11 ,  12  from the electric motor  43 , and is structured in such a slide type that the drive shaft  18  is extendable and contractible in an axial direction in order to absorb a changed amount of a distance between the vehicle body frame  20  and the casing  56  to be generated when the tires  11 ,  12  move up and down. 
     Although the drive shaft  18  is shown horizontal in  FIG. 5  for convenience, when no load is placed in the body  30 , as shown in  FIG. 6 , the drive shaft  18  is actually inclined at an intersection angle α 1  relative to the horizon with a distal end of the drive shaft  18  near the tire  11  facing downward. On the other hand, when the maximum load within the allowable load is placed in the body  30 , the drive shaft  18  is inclined at an intersection angle α 2  relative to the horizon with the distal end of the drive shaft  18  near the tire  11  facing upward. The intersection angles α 1  and α 2  are preferably in a range from 2.5 degrees to 3.5 degrees. In the exemplary embodiment, both of the intersection angles α 1  and α 2  are approximately 3 degrees. The intersection angles α 1  and α 2  are set by adjusting a strength of the suspension cylinder  53  or adjusting a distance between the connecting bracket  54  and the suspension support  205  that are connected by the suspension cylinder  53 . 
     With the above arrangement, a change in the inclination angle of the drive shaft  18  relative to the horizon between when the load is placed and when no load is placed can be decreased, thereby inhibiting torsional vibration of the drive shaft  18  during travelling. For instance, if the drive shaft  18  is arranged to be horizontal at the intersection angle α 1  of zero degree when no load is placed, the drive shaft  18  becomes largely inclined relative to the horizon at an angle close to the intersection angle α 2  of about 6 degrees when the maximum load is placed, so that the torsional vibration during travelling is increased to reduce durability. In other words, when the drive shaft  18  is largely inclined, even during a constant speed travelling in which an angular speed ω 1  of the output shaft  43 A and an angular speed ω 3  of the input shaft  14 A are kept constant, an angular speed ω 2  of the drive shaft  18  is changed depending on the size of the intersection angles α 1  and α 2 , resulting in generation of torsional vibration. In the exemplary embodiment, such generation of torsional vibration can be inhibited to improve durability and the above-described universal joint can be employed well instead of an expensive constant-velocity joint capable of absorbing a large inclination angle. 
     Incidentally, for the sake of an easy understanding of the intersection angles α 1  and α 2 , the intersection angles α 1  and α 2  are exaggeratingly shown in  FIG. 6  to be larger than the actual angles. 
     Explanation of Steering Mechanism 
       FIG. 7  is a cross-sectional view showing the steering mechanism, which is viewed in a direction indicated by an arrow VII-VII in  FIG. 4 . 
     As shown in  FIGS. 4 and 7 , the steering mechanism is configured to move all the tires  11  and  12  using respective steering cylinders  61 . The steering mechanism includes the steering cylinders  61  each having a proximal end attached to the upper arm  51  and a distal end attached to the casing  56 . 
     Specifically, the bifurcated upper arm  51  integrally includes a cylinder attachment arm  55  having an L shape in a plan view. The cylinder attachment arm  55  horizontally extends inward from one of the proximal ends of the upper arm  51  to extend over the upper support  207  on the vertical member  202 . Moreover, the casing  56  integrally includes a knuckle arm  56 B extending in the same direction as the distal end of the cylinder attachment arm  55  in the plan view. The proximal end of the steering cylinder  61  is attached to the cylinder attachment arm  55  while the distal end of the steering cylinder  61  is attached to the knuckle arm  56 B. 
     Moreover, a steering arm  56 C extending in the travel direction in the plan view is integrally formed to the lower portion of the casing  56 . The steering arm  56 C includes first and second steering arms  56 C in the vehicle width direction. The first and second steering arms  56 C are connected by a pair of tie rods  62 ,  62  (on the sides) and a bell crank  63  (in the middle). By advancing and retracting each of the steering cylinders  61 , the tires  11 ,  12  with the respective casings  56  are steered around the king pin shaft  56 A via the knuckle arm  56 B. This movement is mutually transmitted to the casings  56  in pair via the tie rods  62  and the bell crank  63 , so that both the tires  11  ( 12 ) are steered in conjunction with each other. 
     Further, a distal end of the knuckle arm  56 B is bent upward. A height of a connecting portion between the knuckle arm  56 B and the steering cylinder  61  is set substantially the same as a height of the rotation center of the upper arm  51  and the casing  56 , specifically, a height of the rotation center  57 A of the upper ball joint  57 . Accordingly, viewed in the travel direction, an axial line  61 A of the steering cylinder  61  overlaps a line  51 A passing through the rotation center  57 A and traversing a motion center  207 A of the upper arm  51  on the upper support  207 . The respective up-and-down motions of the upper arm  51  and the steering cylinder  61  are exactly the same. Accordingly, respective up-and-down motion regions to be required for the upper arm  51  and the steering cylinder  61  to move up and down are the same when viewed in the travel direction (see  FIGS. 5 and 7 ). 
     In this arrangement, the steering cylinder  61  is disposed adjacent to the lower cross member  201  along the vehicle width direction. On the lower cross member  201  near the second vertical frame  22 , in other words, on the lower cross member  201  supporting the lower end of the hoist cylinder  35 , the steering cylinder  61  is disposed opposite the hoist cylinder  35  in the travel direction across the lower cross member  201 , in order to avoid interference with the hoist cylinder  35 . 
     In the exemplary embodiment, since the proximal end of the steering cylinder  61  is attached not to the vehicle body frame  20  but to the cylinder attachment arm  55  integrated with the upper arm  51 , even when the suspension  50  including the upper arm  51  is operated, a distance between the knuckle arm  56 B and the cylinder attachment arm  55  of the upper arm  51  is hardly changed. Accordingly, a relationship between steering amounts of the respective tires  11 ,  12  and advancing and retracting amounts of the respective steering cylinders  61  becomes clear, so that advancing or retracting of the steering cylinders  61  for obtaining desired steering amounts of the steering cylinders  61  can be easily controlled. 
     Explanation of Support Structure and Cooling Structure of Electric Motor 
       FIG. 8  is a cross-sectional view showing a support structure and a cooling structure of the electric motor  43 . 
     As shown in  FIG. 8 , the electric motors  43  are housed in both sides of a hollow portion of the lower cross member  201  on which each of the first vertical frame  21  and the second vertical frame  22  stands. Openings  209  are provided on both sides of the lower cross member  201 . Each end of a body of the electric motors  43  close to the output shaft  43 A is fixed around each of the openings  209  by an appropriate fastening unit. 
     Inside the lower cross member  201 , a projection  210  projects from an inner surface of the hollow portion toward each of the electric motors  43 . Accordingly, an end of each of the electric motors  43  opposite the output shaft  43 A is supported by the inner surface of the hollow portion through the projection  210 . The projection  210  is provided by a plurality of projections spaced from each other in a circumferential direction. Since the electric motors  43  are housed inside the lower cross member  201  and fixed to the lower cross member  201 , the lower cross member  201  itself is reinforced by the electric motors  43 , so that rigidity of the lower cross member  201  is improved. 
     An inflow port  211  for taking in a cooling air is provided on a center top of the lower cross member  201  in the vehicle width direction. A cooling blower  71  is attached at a position corresponding to the inflow port  211 . Moreover, a predetermined gap (not shown) for discharging the cooling air to the outside is formed at each of fastening parts between the electric motors  43  and both the sides of the lower cross member  201 . The cooling air supplied from the cooling blower  71  flows from the inflow port  211  to a space between a pair of electric motors  43  inside the lower cross member  201 , and is subsequently branched toward the electric motors  43 . The branched cooling air passes between the projections  210  to enter a space between the electric motors  43  and the lower cross member  201 , flows to the ends while cooling the electric motors  43  from an outer circumference thereof, and flows to the outside from the gaps on both the sides of the lower cross member  201 . 
     In the exemplary embodiment, the hollow portion of the lower cross member  201  defines a duct  72  configured to circulate the cooling air. 
     The arrangement for flowing the cooling air to the outside is not limited to the arrangement for flowing the cooling air to the outside from the gaps on the fastening units between the lower cross member  201  and the electric motors  43 . A plurality of outflow openings, through which the cooling air is discharged, may be provided on both the sides of the lower cross member  201 . 
     Explanation of Layout of Devices 
       FIG. 9  is a plan view showing a layout of the devices  41  to  49 . 
     As shown in  FIG. 9 , the devices  41  to  49  are disposed on the vehicle body frame  20  as follows in consideration of a weight balance and maintenance capability of the vehicle body  10 . Specifically, in the order from the first travel direction of the vehicle body frame  20  (in the order from the left to the right in  FIG. 9 ), the controller  49 , a pair of electric motors  43 ,  43  configured to drive the tires  11 , the engine  41 , the generator motor  42 , a pair of electric motors  43 ,  43  configured to drive the tires  12 , and a second controller  49  are disposed substantially in alignment. The engine  41  is the heaviest device among the above devices and is disposed closer to the middle of the vehicle body frame  20  than the first vertical frame  21 . 
     In the middle of the vehicle body frame  20  in the travel direction, at a position outwardly away from the vehicle body frame  20  in the first vehicle width direction, the first radiator  44  for the engine  41  is disposed and the first cooling fan  45  is disposed to an inner side of the first radiator  44 . At a position outwardly away from the vehicle body frame  20  in the second vehicle width direction, the second radiator  46  for the water-cooling type aftercooler  41 A is disposed and the second cooling fan  47  is disposed to an inner side of the second radiator  46 . 
     The first and second radiators  44  and  46  are substantially the same in size and the first and second cooling fans  45  and  47  are substantially the same in size. The first and second radiators  44  and  46  are disposed symmetrically with respect to the above-described second center line  10 B and the first and second cooling fans  45  and  47  are disposed symmetrically with respect to the above-described second center line  10 B (see  FIG. 4 ). The first and second cooling fans  45  and  47  are suction fans. A cooling air sucked from the outside and having been subjected to heat exchange with the cooling water of the engine  41  at the first and second radiators  44  and  46  and a cooling air having been subjected to heat exchange with the cooling water of the aftercooler  41 A are delivered toward the engine  41  and the generator motor  42  provided in the middle of the vehicle body frame  20  to cool the engine  41  and the generator motor  42  from outer sides thereof. 
     A pair of brake resistors  48 ,  48  covered with respective exterior covers are disposed on the second radiator  46  and the second cooling fan  47  (see  FIG. 1 ). Cooling fans (not shown) configured to respectively cool the brake resistors  48  are housed inside the respective exterior covers. Such cooling fans are discharge fans. The brake resistors  48  are collectively disposed on one side of the vehicle body frame  20  in order to put priority on maintenance capability. Since the brake resistors  48  have a light weight compared with those of the other devices, a weight balance of the vehicle body  10  is less affected even by the disposition of the brake resistors  48  only on the one side of the vehicle body frame  20 . 
     The first and second radiators  44  and  46 , the first and second cooling fans  45  and  47 , and the brake resistor  48  are mounted on the support frame  81 . The support frame  81  is fixed to the lower side member  23  and the upper side member  24  of the vehicle body frame  20  by a fastening unit (e.g., a bolt). 
     At the outside of the vehicle body frame  20  in the first vehicle width direction, the first radiator  44  and the first cooling fan  45  bridge over a pair of L-shaped frames  85  juxtaposed in the travel direction to be supported by the frames  85  and are disposed in an area between the tires  11  and  12 . The first radiator  44  and the first cooling fan  45  are mounted in the middle of the vehicle body frame  20  in the travel direction via the support frame  81  (see the first center line  10 A in  FIG. 1 ). 
     Similarly, at the outside of the vehicle body frame  20  in the second vehicle width direction, the second radiator  46  and the second cooling fan  47  bridge over the pair of L-shaped frames  85  juxtaposed in the travel direction to be supported by the frames  85 . The second radiator  46  and the second cooling fan  47  are disposed in an area between the tires  11  and  12 . The second radiator  46  and the second cooling fan  47  are mounted in the middle of the vehicle body frame  20  in the travel direction via the support frame  81  (see the first center line  10 A in  FIG. 1 ). 
     Attachment Position of Hoist Cylinder and Relationship Between Hoist Cylinder and Engine 
       FIG. 11  is an illustration of an attachment position of the hoist cylinder  35  in the travel direction, which is viewed in a direction indicated by an arrow XI-XI in  FIG. 4 .  FIG. 12  is a side view showing the attachment position of the hoist cylinder  35 . However, the steering mechanism is not shown in  FIG. 11 . 
     As shown in  FIGS. 11 and 12 , upper ends of the pair of hoist cylinders  35  are rotatably attached to the attachment portions  36  provided in the middle of the lower surface of the body  30 . At the second side of the vehicle body frame  20 , a pair of hoist supports  212  are juxtaposed in the vehicle width direction on the lower cross member  201  on which the second vertical frame  22  stands. The lower ends of the pair of hoist cylinders  35  are rotatably supported by the respective hoist supports  212  to be supported near the set positions of the tires  12  in the travel direction. The hoist cylinders  35  set at such positions are supported with a large distance from the engine  41  and the generator motor  42  connected to the engine  41  near the hoist cylinders  35 . 
     The hoist supports  212  are respectively provided at the positions where the electric motors  43  are housed in the lower cross member  201 , in other words, at the positions reinforced by the electric motors  43  in the lower cross member  201 . Moreover, the lower cross member  201  is a member on which the second vertical frame  22  provided with the body support  206  stands. Accordingly, the portion to receive the carrying load of the standing body  30  is focused on the second vertical frame  22  and the lower cross member  201  positioned between the right and left tires  12 . The carrying load is transmitted from the body supports  206  and the hoist supports  212  of the hoist cylinders  35  to the road surface immediately under the tires  12  through the suspension  50  and the tires  12 . The carrying load does not act on the lower side member  23  and the upper side member  24  (see  FIG. 12 ). 
     As shown by solid lines in  FIG. 12 , the body  30  is raised in the second travel direction by extending the hoist cylinder  35 , thereby performing an unloading operation. When the body  30  is sufficiently raised at or exceeding a predetermined angle, the hoist cylinders  35  substantially vertically stand. In such a condition, a large open-top space is defined from the first vertical frame  21  to the middle of the vehicle body frame  20 , in other words, above the portion where the engine  41  is mounted. Since the body  30  and the hoist cylinders  35  are not present in this space, the engine  41  disposed near the middle of the vehicle body frame  20  can be suspended with a wire or the like with use of this space and is configured to be moved up and down while being suspended when the engine  41  is detached and attached for maintenance. 
     In addition, the engine  41  is disposed in a region defined by the first vertical frame  21 , the second vertical frame  22 , a pair of right and left lower side members  23 , and a pair of right and left upper side members  24 . As shown in  FIG. 12 , there is an open space between the lower side members  23  and the upper side members  24  so that the engine  41  is accessible from the outside of the vehicle body frame  20 . With this arrangement, even while the engine  41  is mounted on the vehicle body frame  20 , maintenance of the engine  41  can be easily conducted from the right and left sides of the vehicle body frame  20 . 
     Travelling of Driverless Dump Truck 
     The above-described dump truck  1  travels between a loading site where dug minerals are taken in the dump truck  1  as a load and an unloading site where the load is unloaded. At this time, when the dump truck  1  travels on an outward road toward the unloading site, the side of the dump truck  1  supporting the body  30 , in other words, near the second vertical frame  22  is defined as the back (rear) of the dump truck  1  and the side thereof near the first vertical frame  21  is defined as the front of the dump truck  1 . When the dump truck  1  travels on a return road after the unloading, since the dump truck  1  is not turned around, the side of the dump truck  1  near the second vertical frame  22  is defined as the front of the dump truck  1  and the side thereof near the first vertical frame  21  is defined as the back (rear) of the dump truck  1  (shuttle travelling). 
     However, the dump truck  1  may be turned around as needed and travel with either the first vertical frame  21  or the second vertical frame  22  constantly defined as the front of the dump truck  1 . 
     It should be understood that the scope of the invention is not limited to the above-described exemplary embodiment(s) but includes modifications and improvements as long as the modifications and improvements are compatible with the invention. 
     For instance, in the above exemplary embodiment, the electric motor  43  is configured to drive the tires  11  and  12 . However, a driving unit for the tires  11  and  12  is not limited to the electric motor  43 , but a hydraulic motor may replace the electric motor  43 . 
     Moreover, instead of housing the hydraulic motor or the electric motor in the lower cross member  201 , the hydraulic motor or the electric motor may be disposed inside the tires  11  and  12  to be defined as an in-wheel motor. 
     Further, a driving force of the engine  41  may be transmitted via a differential device and a drive shaft to drive the tires  11  and  12 . 
     In the above exemplary embodiment, the suspension  50  is provided by a double wishbone type suspension including the upper arm  51  and the lower arm  52 . However, the suspension of the invention is not limited to the double wishbone type suspension. For instance, the suspension  50  may be provided by a McPherson strut type suspension including an up-and-down movable arm supported by the lower portion of the vehicle body frame  20  and a suspension cylinder connecting the upper portion of the vehicle body frame  20  to the arm. In this arrangement, the proximal end of the steering cylinder is supported by the arm. 
     In the above exemplary embodiment, the body support  206  supporting the body  30  in a manner to raise or lower the body  30  is provided on the second vertical frame  22  disposed in the second travel direction, and the lower ends of the hoist cylinders  35  for raising and lowering the body  30  are supported by the lower cross member  201  on which the second vertical frame  22  stands. However, the lower ends of the hoist cylinders  35  may be supported by the lower cross member  201  on which the first vertical frame  21  stands. In this arrangement, on the lower cross member  201  near the first vertical frame  21 , it is only necessary to dispose the steering cylinder  61  opposite the hoist cylinders  35  in the travel direction across the lower cross member  201 , in order to avoid interference between the steering cylinder  61  and the hoist cylinders  35 . 
     In addition, the above exemplary embodiment is not exhaustive, but it may be determined accordingly in each case which portion of the upper arm  51  is attached with the proximal end of the steering cylinder  61  or which position is set as the connecting position between the steering cylinder  61  and the knuckle arm  56 B. 
     The invention is also applicable to a manned off-road dump truck including a cab.