Hydraulic steering mechanism and driving-steering-wheel support mechanism

The hydraulic steering mechanism according to the present invention includes an axle case, a first steering case, a second steering case, a first arm, a second arms, a third arm, a hydraulic actuator, and a tie rod. The first, second and third arms are provided to the first steering case, the second steering case and the axle case, respectively. The hydraulic actuator includes a cylinder and a piston for moving with respect to each other by an action of hydraulic pressure. The first and second arms have tie rod mounting portions to which the tie rod is mounted. At least one of the first and second arms has a first hydraulic actuator mounting portion to which one of the cylinder and the piston in the hydraulic actuator is mounted. The third arm has a second hydraulic actuator mounting portion to which the other of the cylinder and the piston in the hydraulic actuator is mounted. The first hydraulic actuator mounting portion is at a longer distance from axis lines of the differential yoke shafts than the second hydraulic actuator mounting portion in a plan view when the vehicle is traveling substantially straight.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydraulic steering mechanism for steering drive wheels by utilizing hydraulic pressure.

2. Related Art

There is conventionally known a hydraulic steering mechanism for steering a pair of drive wheels by connecting a pair of steering cases that are disposed at both ends of an axle case through a tie rod and by turning one of the steering cases about a kingpin shaft by a hydraulic actuator.

However, in the conventional hydraulic steering mechanism, sufficient consideration has not been given to a viewpoint of a reduction in steering force of the hydraulic actuator.

The present invention has been accomplished with the above conventional technique in view, and it is an object of the invention to provide a hydraulic steering mechanism with a simple structure in which a hydraulic actuator for turning a steering case about a kingpin shaft can be miniaturized.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a hydraulic steering mechanism that includes an axle case, a first steering case, a second steering case, a first arm, a second arms, a third arm, a hydraulic actuator, and a tie rod. The axle case has a bearing portion for supporting an input shaft extending in a longitudinal direction of a vehicle in a rotatable manner and accommodates a pair of differential yoke shafts operatively connected to the input shaft and extending in a width direction of the vehicle. The pair of first and second steering cases is respectively mounted to ends of the axle case in a turnable manner about a corresponding kingpin shaft axis extending substantially in a vertical direction. The first and second steering cases respectively support first and second drive axles operatively connected to the corresponding differential yoke shafts in a rotatable manner. The first, second and third arms are provided to the first steering case, the second steering case and the axle case, respectively. The hydraulic actuator includes a cylinder and a piston for moving with respect to each other by an action of hydraulic pressure. The tie rod links the first and second steering cases with each other.

The first and second arms have tie rod mounting portions to which the tie rod is mounted. At least one of the first and second arms has a first hydraulic actuator mounting portion to which one of the cylinder and the piston in the hydraulic actuator is mounted. The third arm has a second hydraulic actuator mounting portion to which the other of the cylinder and the piston in the hydraulic actuator is mounted. The first hydraulic actuator mounting portion is at a longer distance from axis lines of the differential yoke shafts than the second hydraulic actuator mounting portion in a plan view when the vehicle is traveling substantially straight.

With this structure, it is possible to miniaturize the hydraulic actuator for turning the steering case.

Preferably, the second hydraulic actuator mounting portion may be disposed on an opposite side of the input shaft to the first hydraulic actuator mounting portion in the width direction of the vehicle.

With this structure, a swinging angle of the hydraulic actuator about the second hydraulic actuator mounting portion can be reduced to thereby smoothly carry out turning of the steering case by the hydraulic actuator.

In one embodiment, preferably, the tie rod mounting portions, the first and second hydraulic actuator mounting portions and the bearing portion may be disposed on the same side of the axle case in the longitudinal direction of the vehicle. The tie rod mounting portions may be closer to the axis lines of the differential yoke shafts than the first and second hydraulic actuator mounting portions in a plan view when the vehicle is traveling substantially straight. The tie rod mounting portions and the first and second hydraulic actuator mounting portions may be disposed so that the tie rod and the hydraulic actuator are positioned below the input shaft and that an outer end of a movable member of the hydraulic actuator in the width direction of the vehicle is positioned above the tie rod in a front view.

With this structure, it is possible to effectively suppress a length and a height of the vehicle.

In the one embodiment, more preferably, the first and second hydraulic actuator mounting portions may be disposed so that the hydraulic actuator is inclined downward from a portion of the hydraulic actuator supported by the first hydraulic actuator mounting portion toward a portion of the hydraulic actuator supported by the second hydraulic actuator mounting portion in a front view.

With this structure, the input shaft can be positioned in as low a position as possible to thereby increase a degree of freedom of design.

According to another aspect of the present invention, there is provided a driving-steering-wheel mechanism for supporting a driving-steering wheel that can be operatively driven by a driving source and steered. The driving-steering-wheel mechanism includes a fixed case member, a movable member, a power transmission shaft, a gear train, an upper bearing and a lower bearing.

The fixed case member accommodates a differential yoke shaft operatively connected to the driving source, and includes a hollow kingpin shaft portion extending downward at an outer end of the fixed case member in a width direction of a vehicle. The movable member supports an axle operatively connected to the driving-steering wheel, and includes a hollow turning portion into which the kingpin shaft portion can be inserted from above. The power transmission shaft is formed to be inserted into the kingpin shaft portion so that an upper end thereof is operatively connected to the differential yoke shaft and that a lower end thereof extends below the kingpin shaft portion. The gear train connects the power transmission shaft and the axle. The upper bearing and a lower bearing are disposed between the kingpin shaft portion and the turning portion so that the turning portion can turn about the kingpin shaft portion, each of the upper bearing and lower bearing having an outer ring disposed to the turning portion in a non-movable manner in an axial direction, an inner ring, and rolling elements disposed between the outer ring and the inner ring. The kingpin shaft portion includes inner ring locking portions to be engaged with the inner rings of the upper bearing and the lower bearing when inserted into the turning portion from above.

In one embodiment, the driving-steering-wheel support mechanism may further include a lower bearing fixing member detachably mounted to the kingpin shaft portion so as to be engaged with a lower surface of the inner ring of the lower bearing.

In another embodiment, instead of or in addition to the lower bearing fixing member, the driving-steering-wheel support mechanism may further include a second upward step provided to the turning portion so as to be engaged with a lower surface of the outer ring of the lower bearing; a spacer extending between the upper bearing and the lower bearing; and an upper bearing fixing member detachably mounted to the turning portion so as to be engaged with an upper surface of the outer ring of the upper bearing. The upper bearing and the lower bearing are prevented from moving in the axial direction with respect to the turning portion by means of the second upward step, the spacer and the upper bearing fixing member.

In another aspect, preferably, a first upward step may be provided to the turning portion so as to be engaged with a lower surface of the outer ring of the upper bearing.

In the various embodiments, the driving-steering-wheel support mechanism may further include a first downward step and a second downward step provided to the kingpin shaft portion so as to be engaged with upper surfaces of the inner rings of the upper bearing and the lower bearing, respectively.

In the still another embodiment, the driving-steering-wheel support mechanism may further include a first upward step and a second upward step provided to the turning portion so as to be engaged with lower surfaces of the outer rings of the upper bearing and the lower bearing, respectively; a spacer detachably mounted to the kingpin shaft so as to be engaged with a lower surface of the inner ring of the upper bearing in a non-movable manner in an axial direction; an upper bearing fixing member detachably mounted to the turning portion so as to be engaged with an upper surface of the outer ring of the upper bearing; and a first downward step and a second downward step provided to the kingpin shaft portion so as to be engaged with upper surfaces of the inner rings of the upper bearing and the lower bearing, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, description will be made on a preferred embodiment of the present invention with reference to the accompanying drawings.

FIG. 1is a perspective view of a hydraulic steering mechanism1A according to this embodiment when viewed diagonally from behind.FIGS. 2 and 3are respectively a partial cross-sectional plan view and a back view of the hydraulic steering mechanism1A according to this embodiment.

As shown inFIGS. 1 to 3, the hydraulic steering mechanism1A according to this embodiment includes an axle case10, first and second steering cases20aand20brespectively provided to both ends of the axle case in a longitudinal direction, first and second arms30aand30brespectively provided to first and second steering cases20aand20b, a third arm40provided to the axle case10, a hydraulic actuator50including a cylinder51and a piston52, and a tie rod60for interlocking and linking the first and second steering cases20aand20bwith and to each other.

The axle case10is formed to be able to accommodate a pair of differential yoke shafts130extending in a width direction of a vehicle (seeFIG. 2).

Furthermore, the axle case10has a bearing portion15on its side surface oriented inward in a longitudinal direction of the vehicle (seeFIGS. 1 and 2).

The bearing portion15supports an input shaft110operatively connected to a driving source (not shown) of the vehicle along the longitudinal direction of the vehicle so that the input shaft110can rotate about an axis line. The bearing portion15permits transmission of driving force from the input shaft110to the pair of differential yoke shafts130.

The hydraulic steering mechanism1A according to this embodiment is formed to steer a pair of front wheels of the vehicle. Therefore, the bearing portion15is provided to the side surface of the axle case10oriented rearward in the longitudinal direction of the vehicle.

In this embodiment, transmission of power from the input shaft110to the pair of differential yoke shafts130is carried out through a differential gear120accommodated in the axle case10.

Specifically, the axle case10includes a main body11accommodating the first and second differential yoke shafts130aand130b, and first and second fixed cases12aand12bprovided to both ends of the main body11.

In this embodiment, the main body11has hollow first and second portions11aand11brespectively for accommodating the first and second differential yoke shafts130aand130b.

The first and second portions11aand11bhave openings with larger diameters than the differential gear120at opposed ends. By butt-coupling the opposed ends with the differential gear120sandwiched between them, the first and second portions11aand11bcan accommodate the differential gear120.

FIG. 4is a vertical sectional back view of a vicinity of the first fixed case12a.

As shown inFIG. 4, the first fixed case12ahas a hollow first base end portion13aconnected to an outer end of the first portion11ain the width direction of the vehicle and a hollow first kingpin shaft portion14aextending substantially vertically from the first base end portion13a.

Similarly, the second fixed case12bhas a hollow second base end13band a hollow second kingpin shaft14b(seeFIG. 1).

Into the first and second kingpin shaft portions14aand14b, first and second power transmission shafts140aand140bconnected to the first and second differential yoke shafts130aand130bare respectively inserted in a rotatable manner about the axis line (the second transmission shaft140bis not shown).

More specifically, to an upper end of each of the first and second power transmission shafts140aand140b, a bevel gear141engaged with a bevel gear131provided to the corresponding differential yoke shaft130so as not to be rotatable with respect to the differential yoke shaft130is provided in a non-rotatable manner with respect to the upper end.

The first and second power transmission shafts140aand140bhave upper ends that are respectively and rotatably supported by the first and second kingpin shaft portions14aand14bthrough bearings201, and have lower ends that extend respectively downward from the first and second kingpin shaft portions14aand14b.

As shown inFIGS. 1 to 3, the first and second steering cases20aand20bare respectively connected to the both ends of the axle case10in a turnable manner about the kingpin shafts.

The first and second steering cases20aand20bhave substantially the same structures.

Therefore, the following description of the first steering case20ais also applied to the second steering case20b.

FIG. 5is a perspective view of the first steering case20a.

As shown inFIGS. 4 and 5, the first steering case20ahas a first turning portion21afitted over the first kingpin shaft portion14aof the first fixed case12afrom outside in a rotatable manner and a first outer portion22aextending outward in the width direction of the vehicle from the first turning portion21a.

The first turning portion21ahas an opening at its upper portion. In other words, the first turning portion21ais fitted over the first kingpin shaft portion14afrom below.

A lower portion of the first turning portion21ais closed and supports the lower end of the first power transmission shaft140athrough a bearing202in a rotatable manner.

The first outer portion22asupports a corresponding first axle150ain a rotatable manner and accommodates a power transmission member for interlocking and linking the first power transmission shaft140aand the first axle150awith and to each other.

In this embodiment, the first outer portion22ahas a base end portion23aformed integrally with the first turning portion21aand a lid portion24adetachably connected to the base end portion23a.

The base end portion23aand the lid portion24aare formed to support a final gear160forming the power transmission member.

The final gear160is operatively connected to the first power transmission shaft140aand supported on the first axle150ain a non-rotatable manner with respect to the axle150a.

Here, description will be made on a structure for preventing withdrawal of the first steering case20afrom the first fixed case12a.

As shown inFIG. 4, the first turning portion21aof the first steering case20ais fitted over the first kingpin shaft portion14aof the first fixed case12aso as to be in a rotatable manner relative to the first kingpin shaft portion14avia an upper bearing210and a lower bearing220spaced from each other in a vertical direction.

Specifically, the first kingpin shaft portion14ahas a first downward step18to be engaged with an upper surface of an inner ring of the upper bearing210and a second downward step19to be engaged with an upper surface of an inner ring of the lower bearing220on an outer peripheral surface of the first kingpin shaft portion14a.

On the other hand, the first turning portion21ahas a first upward step28to be engaged with a lower surface of an outer ring of the upper bearing210and a second upward step29to be engaged with a lower surface of an outer ring of the lower bearing220on an inner peripheral surface of the first turning portion21a.

In other words, the upper bearing210is sandwiched between the first downward step18and the first upward step28, and the lower bearing220is sandwiched between the second downward step19and the second upward step29.

The first downward step18, the first upward step28, the second downward step19, and the second upward step29are preferably formed substantially throughout circumferences.

With such a structure, loads applied to the upper bearing210and the lower bearing220can be made uniform to thereby effectively prevent breakage and the like.

In this embodiment, in order to prevent withdrawal of the first turning portion21afrom the first kingpin shaft portion14a, a spacer71disposed to be engaged with a lower surface of the inner ring of the upper bearing210and a fastening member72for retaining the spacer71are provided.

The spacer71is preferably in a ring shape to be fitted over the first kingpin shaft portion14aso as to be outside of the first kingpin portion14a.

With this preferable structure, the inner ring of the upper bearing210can be retained throughout its circumference.

The fastening member72is detachably secured to a peripheral wall of the first kingpin shaft portion14a. The fastening member72is fastened to the first kingpin shaft portion14aso as to engage with and retain a lower surface of the spacer71.

The fastening member72is accessible from outside in a state where the first turning portion21ais fitted over or inserted around the first kingpin shaft portion14a.

Concretely, an access opening73for access to the fastening member72is formed in a peripheral wall of the first turning portion21a.

In this embodiment, the fastening member72is fastened to an outer surface of the first kingpin shaft portion14a.

Therefore, the access opening73is formed in the outer surface of the first turning portion21aso that the outer surface of the first kingpin shaft portion14ais accessible from outside.

More specifically, in this embodiment, the access opening73communicates with an axle insertion opening25formed in the first outer portion22aand an operator can gain access to the fastening member72through the axle insertion opening25and the access opening73.

Though various structures of the fastening member72are conceivable, the fastening member72is preferably a bolt from a viewpoint of workability and the like.

A reference numeral75inFIG. 4designates a snap ring for preventing an upward movement of the upper bearing210.

The snap ring75is disposed in advance in the first turning portion21abefore insertion of the first kingpin shaft portion14ainto the first turning portion21aof the first steering case20a.

In other words, upon connection between the first turning portion21aand the first kingpin shaft portion14a, the lower bearing220is first inserted into the first turning portion21aso as to be placed on the second upward step29and the spacer71is then inserted. At this time, the spacer71is retained by the lower bearing220.

Then, the upper bearing210is inserted into the first turning portion21aso as to be engaged with the first upward step28and is prevented from withdrawal by means of the snap ring75.

In this state, the first kingpin shaft portion14ais inserted into the first turning portion21aand the fastening member72is fastened through the access opening73to fix the spacer71to a predetermined position.

The first and second arms30aand30brespectively extend inward in the width direction of the vehicle from the first and second steering cases20aand20b.

As shown inFIGS. 1,5and the like, the first and second arms30aand30bare formed integrally with the first and second steering cases20aand20b, respectively, in this embodiment.

The first and second arms30aand30brespectively have tie rod mounting portions31to which the tie rod60is mounted.

In other words, in the hydraulic steering mechanism1A according to this embodiment, the first and second steering cases20aand20bare turned in synchronization and about the corresponding kingpin shaft portion by the tie rod60mounted to the first and second arms30aand30b.

The hydraulic actuator50has the cylinder51and the piston52as described above.

The piston52has a piston main body accommodated in the cylinder51in a slidable manner and a piston rod53projecting outside the cylinder51from the piston main body.

In the hydraulic actuator50, pressure oil is supplied and discharged to and from an oil chamber in the cylinder51, so that the cylinder51and the piston52move with respect to each other.

In other words, in the hydraulic actuator50, one of the cylinder51and the piston52(piston rod53) operates as a fixed member and the other operates as a movable member for reciprocating with respect to the fixed member according to control of supply and discharge of pressure oil to and from the oil chamber.

At least one of the first and second arms30aand30b(first arm30ain this embodiment shown in the drawings) has a first hydraulic actuator mounting portion35to which one of the cylinder51and the piston rod53(piston rod53in this embodiment shown in the drawings) of the hydraulic actuator50is mounted in addition to the tie rod mounting portion31.

The other of the cylinder51and the piston rod53(cylinder51in this embodiment shown in the drawings) of the hydraulic actuator50is mounted to a second hydraulic actuator mounting portion45provided to the third arm40.

With this structure, by controlling supply and discharge of the pressure oil to and from the hydraulic actuator50, the corresponding steering case20(first steering case20ain this embodiment shown in the drawings) is turned about the kingpin shaft by the movable member of the hydraulic actuator50.

In the hydraulic steering mechanism1A according to this embodiment, as is well shown inFIG. 2, the first hydraulic actuator mounting portion35is at a longer distance from axis lines of the differential yoke shafts130than the second hydraulic actuator mounting portion45in a plan view when the vehicle is traveling substantially straight to thereby miniaturize the hydraulic actuator50.

In other words, from a viewpoint of strength required of at least one of the first and second arms30aand30b(first arm30ain this embodiment shown in the drawings) provided with the first hydraulic actuator mounting portion35and the third arm40provided with the second hydraulic actuator mounting portion45, it is preferable to dispose the first and second hydraulic actuator mounting portions35and45close to the axis lines of the differential yoke shafts130.

On the other hand, from a viewpoint of force required to turn the steering case20on a side connected to the hydraulic actuator50(first steering case20ain this embodiment shown in the drawings and hereinafter referred to as an operating-side steering case), it is preferable to dispose the first hydraulic actuator mounting portion35away from the axis lines of the differential yoke shafts130.

Specifically, the operating-side steering case is turned about the kingpin shaft by reciprocation of the movable member of the hydraulic actuator50. An axis line of the kingpin shaft is positioned substantially in the same position as the axis lines of the differential yoke shafts130in the longitudinal direction of the vehicle. Therefore, by disposing a connection position (position of the first hydraulic actuator mounting portion35) of the movable member of the hydraulic actuator50and the operating-side steering case away from the kingpin shaft, it is possible to turn the operating-side steering case about the kingpin shaft with small force and, as a result, the hydraulic actuator can be miniaturized.

Moreover, in this embodiment, as shown inFIGS. 1 to 3, the second hydraulic actuator mounting portion45is on an opposite side of the input shaft110to the first hydraulic actuator mounting portion35in the width direction of the vehicle to thereby facilitate smooth turning of the operating-side steering case.

In other words, with this structure, it is possible to suppress a swinging angle of the hydraulic actuator50about the second hydraulic actuator mounting portion upon turning of the operating-side steering case about the kingpin shaft. Therefore, the operating-side steering case can be turned more smoothly by the hydraulic actuator50.

Furthermore, in this embodiment, the tie rod mounting portions31, the first and second hydraulic actuator mounting portions35and45, and the bearing portion15are on the same side of the axle case10in the longitudinal direction of the vehicle (seeFIG. 2).

In other words, in this embodiment, the tie rod mounting portions31and the first and second hydraulic actuator mounting portions35and45are provided on the same side as the input shaft110(i.e., inner side in the longitudinal direction of the vehicle) to thereby reduce a longitudinal length of the whole vehicle including the hydraulic steering mechanism1A.

Moreover, in this embodiment, in the form in which the tie rod mounting portions31, the first and second hydraulic actuator mounting portions35and45, and the bearing portion15are disposed on the same side with respect to the axle case10in the longitudinal direction of the vehicle, the following structure is provided so as to prevent interference between the respective members and to minimize a height of the vehicle.

In other words, in this embodiment, in a plan view when the vehicle is traveling substantially straight, the tie rod mounting portions31are closer to the axis lines of the differential yoke shafts130than the first and second hydraulic actuator mounting portions35and45(seeFIG. 2). The tie rod mounting portions31and the first and second hydraulic actuator mounting portions35and45are disposed so that the tie rod60and the hydraulic actuator50are positioned below the input shaft110and that an outer end of the movable member of the hydraulic actuator50in the width direction of the vehicle is positioned above the tie rod60in a front view (seeFIG. 3).

With this structure, it is unnecessary to secure a space for disposing the hydraulic steering mechanism1A above the input shaft110; therefore, the height of the vehicle can be minimized.

Furthermore, it is possible to effectively prevent interference between the hydraulic actuator50and the tie rod60when the operating-side steering case (first steering case20ain this embodiment shown in the drawings) makes the largest turn (seeFIG. 6).

More preferably, the first and second hydraulic actuator mounting portions35and45can disposed so that the hydraulic actuator50is inclined downward from a portion of the hydraulic actuator50supported by the first hydraulic actuator mounting portion35toward a portion of the hydraulic actuator50supported by the second hydraulic actuator mounting portion45in a front view (seeFIG. 3).

With this structure, the input shaft110can be disposed in as low as possible while preventing interference with the hydraulic actuator50, thereby increasing a degree of freedom of design with regard to the vehicle height and the like.

Hereinafter, description will be made on another preferred embodiment of the present invention with reference toFIGS. 7 and 8.

FIG. 7is a vertical sectional back view of a vicinity of a fixed case in a hydraulic steering mechanism1B according to this embodiment.FIG. 8is an exploded vertical sectional back view of the fixed case and a steering case.

In the drawings, members similar or corresponding to those in the first embodiment are provided with the same reference numerals; therefore, description thereof will not be made.

The hydraulic steering mechanism1B according to this embodiment is different from the first embodiment only in that the structure for preventing withdrawal of the first steering case20afrom the first fixed case12aand is substantially the same in other structures.

In other words, the hydraulic steering mechanism1B according to this embodiment includes a spacer76extending between the upper bearing210and the lower bearing220, and a snap ring77for preventing movements of the first kingpin shaft portion14aand the first turning portion21awith respect to each other in the vertical direction in stead of the spacer71and the fastening member72in the first embodiment.

Specifically, in this embodiment, when the first fixed case12aand the first steering case20aare connected to each other (seeFIG. 7), a lower end of the first kingpin shaft portion14aextends below a lower surface of the lower bearing220, so that the lower end of the kingpin shaft portion14ais accessible from outside.

In other words, as shown inFIGS. 7 and 8, the first steering case20ais provided with an opening78oriented outward so as to permit power transmission from the first power transmission shaft140ato the final bevel gear160.

The lower end of the first kingpin shaft portion14ais accessible from outside through the opening78in a state where the first kingpin shaft portion14aand the first steering case20aare connected to each other.

The snap ring77is connected to a lower extension portion of the first kingpin shaft portion14aso as to be engaged with a lower surface of the inner ring of the lower bearing220.

The spacer76is disposed in the first turning portion21aof the first steering case20aso that an upper end surface of the spacer76is engaged with the lower surface of the outer ring of the upper bearing210and a lower end surface of the spacer76is engaged with the upper surface of the outer ring of the lower bearing220.

The spacer76is preferably engaged with the upper bearing210and the lower bearing220throughout the circumference.

Here, with reference toFIG. 8, description will be made on a procedure for mounting the first steering case20ato the fixed case12a.

First, the lower bearing220is disposed on the second upward step29of the first turning portion21a. Then, the spacer76is placed on an upper surface of the lower bearing220.

At this time, the spacer76also functions as a jig for properly placing the lower bearing220on the second upward step29.

Then, the upper bearing210is disposed on the spacer76and the first upward step28, and the snap ring75is disposed.

In this state, the first kingpin shaft portion14ais inserted from an upper portion of the first turning portion21aand the snap ring77is connected to the lower extension portion of the first kingpin shaft portion14a.

Then, the lid portion24asupporting the final bevel gear160is connected to the base end portion23a.

According to the above hydraulic steering mechanism1B having the structure for preventing withdrawal, a high load from the axle case10can be supported more reliably.

In other words, though a load applied to the spacer71is supported at one point by the fastening member72in the first embodiment, the load applied to the spacer76is supported by the whole lower bearing220to thereby support the high load more reliably in this embodiment.

This specification is by no means intended to restrict the present invention to the preferred embodiments set forth therein. Various modifications to the hydraulic steering mechanism and the driving-steering-wheel support mechanism may be made by those skilled in the art without departing from the spirit and scope of the present invention as define in the appended claims.