Patent Description:
Conveyance devices for carrying products such as workpieces or packages are conventionally known. For example, Patent Literature <NUM> discloses a stacker crane for carrying packages in a warehouse. This stacker crane comprises a lower frame which moves along rails on the floor, a support column which protrudes upwardly from the lower frame, an upper frame which is installed at the top of the support column and which moves along a ceiling guide rail, and a loading platform which moves upward and downward along the support column. The loading platform has a pair of guide rollers, and the support column is provided with a pair of inwardly-facing tracks which face each other. The guide rollers of the loading platform are arranged inside the support column so as to roll on the tracks of the support column.

<CIT> features a conveyance device for carrying a product, the conveyance device comprising, a carriage which travels in horizontal forward and rearward directions, a support column which protrudes upwardly from the carriage, a lifting device which moves in the upward and downward directions along the support column, comprising, an upper guide, a lower guide arranged below the upper guide, having a second wheel which rolls on a front surface of the support column and a lower member which supports the second wheel, and a linking member which links the upper member and the lower member to each other, and a loading platform which is moved in the upward and downward direction by the lifting device, which is arranged in front of the second wheel, and which is linked to a front side of the linking member, wherein the lower guide has a pair of fourth wheels which roll on the left and right surfaces of the support column, the lower member comprises a lower main member which is arranged in front of the support column and which supports the second wheel, and a pair of lower auxiliary members which protrude rearwardly from the lower main member so as to face each other with the support column interposed there between, and which support the pair of fourth wheels, the lower member is open toward the rear.

In the conveyance device of Patent Literature <NUM>, since it is necessary that the guide rollers of the loading platform be arranged inside the support column, as described above, the structure is complex and it is difficult to access the guide rollers during assembly and maintenance, etc..

The present invention aims to provide a conveyance device having suitable maintainability with a simple structure.

One aspect of the present disclosure provides a conveyance device according to claim <NUM>. The dependent claims set out particular embodiments of the invention.

In the conveyance device according to the aspect of the present disclosure, the first wheel and second wheel of the lifting device are arranged so as to roll on the rear surface and the front surface of the support column, respectively. Thus, the first wheel and the second wheel are arranged outside of the support column, and thus, the structure can be simplified, and the accessibility of the first wheel and the second wheel can be improved. Therefore, a conveyance device having suitable maintainability with a simple structure can be provided.

The support column may be a square column having the front surface, the rear surface, and left and right surfaces, the upper guide may have a pair of third wheels which roll on the left and right surfaces of the support column, the upper member comprises an upper main member which is arranged behind the support column and which supports the first wheel, and a pair of upper auxiliary members which protrude forwardly from the upper main member so as to face each other with the support column interposed therebetween, and which support the pair of third wheels, the upper member may be open toward the front, the lower guide has a pair of fourth wheels which roll on the left and right surfaces of the support column, the lower member comprises a lower main member which is arranged in front of the support column and which supports the second wheel, and a pair of lower auxiliary members which protrude rearwardly from the lower main member so as to face each other with the support column interposed therebetween, and which support the pair of fourth wheels, the lower member is open toward the rear, and the linking member, in a side view, extents diagonally and linearly so as to link the upper main member and the lower main member. In this case, in a right-side view, a substantially Z-shaped structure is obtained by means of the upper member, the lower member, and the linking member. In this Z-shaped structure, since the upper main member supporting the first wheel and the lower main member supporting the second wheel are diagonally and linearly linked by the linking member, the weight of the loading platform and product in front of the second wheel can be transmitted to the upper rear first wheel by the shortest distance. Thus, the force can be efficiently distributed to the lower front second wheel and the upper rear first wheel. Furthermore, the third and fourth wheels can improve the stability in the left-right directions. As described above, both the distribution of the force in the front-rear directions and stability in the left-right directions can be efficiently achieved by the Z-shaped structure, as described above.

The lifting device may be driven by a gear which is rotated by a motor affixed to the lifting device and a linear guide member which is affixed to the rear surface of the support column and which meshes with the gear. In this case, since the linear guide member is affixed to the rear surface of the support column, a compact structure as compared with a lifting device driven using a moving member such as a chain can be obtained.

The conveyance device may further comprise a safety device which detects inclination of the lifting device when an upward force is exerted on the loading platform. In the conveyance device according to the aspect of the present disclosure, when an upward force is exerted on the loading platform, a moment is generated in the direction in which the first wheel and the second wheel are separated from the support column. Thus, by detecting inclination of the lifting device due to such a moment, safety can be improved.

The conveyance may further comprise an adjustment mechanism for adjusting at least one of a position of the first wheel with respect to the upper member and a position of the second wheel with respect to the lower member. The first wheel and the second wheel may wear with use. As the first wheel and second wheel wear, the posture of the lifting device with respect to the support column changes, which can cause the lifting device to incline. In order to maintain a constant posture of the lifting device with respect to the support column, at least one of the position of the first wheel with respect to the upper member and the position of the second wheel with respect to the lower member must be adjusted. In the present aspect, the adjustment mechanism enables adjustment of at least one thereof. Thus, a constant posture of the lifting device with respect to the support column can be maintained over the long term.

The conveyance device may further comprise a reinforcing member which links the carriage and the rear surface of the support column. In the aspect described above, in which the lower guide of the lifting device opens rearwardly, even if such a reinforcing member is provided on the rear surface of the support column, the lifting device and the reinforcing member do not interfere with each other. Thus, the strength of the conveyance device can be improved.

According to the aspect of the present disclosure, a conveyance device having suitable maintainability with a simple structure can be provided.

The conveyance device according to an embodiment will be described below with reference to the attached drawings. Identical or corresponding elements have been assigned the same reference sign, and duplicate descriptions thereof have been omitted. In order to facilitate understanding, the scales of the drawings have been modified in some cases.

<FIG> is a top view showing a system <NUM> comprising a conveyance device <NUM> according to an embodiment, and <FIG> is a side view showing the system <NUM> of <FIG>. Referring to <FIG>, the conveyance device <NUM> can travel between a plurality of locations along rails R, and can carry various products. For example, in the present embodiment, the conveyance device <NUM> is used in a factory for machining workpieces W. Specifically, in the system <NUM>, the conveyance device <NUM> conveys workpieces W between processing machines 1A, 1B, a pallet stocker (shelf) <NUM>, and workpiece setup stations (WSS, hereinafter referred to simply as "stations") 3A, 3B. Referring to <FIG>, the workpieces W may be attached to, for example, pallets P and the pallets P may be carried by the conveyance device <NUM>. In another embodiment, the workpieces W may be carried directly by the conveyance device <NUM>. Furthermore, in another embodiment, the conveyance device <NUM> may carry other products. For example, the conveyance device <NUM> may carry packages between a plurality of shelves in a warehouse. Note that in <FIG>, to facilitate understanding, the processing machines 1A, 1B and the stations 3A, 3B are not shown.

Referring to <FIG>, the processing machines 1A, 1B can be any of various machine tools such as machining centers. For example, each of the processing machines 1A, 1B comprises a pallet changer PC which is capable of exchanging a pallet P having a non-machined workpiece W with a pallet P having a machined workpiece W. In the stations 3A, 3B, an operator can attach a workpiece W to a pallet P and can remove a workpiece W from a pallet P. Referring to <FIG>, the pallet stocker <NUM> can house pallets P with or without a workpiece W. The pallet stocker <NUM> has a plurality of stock areas S for housing pallets P (<NUM> rows × <NUM> columns in <FIG>). The conveyance device <NUM>, processing machines 1A, 1B, and stations 3A, 3B may comprise individual controllers (local controllers), and the system <NUM> may comprise a controller (main controller) which is capable of communicating with these local controllers.

<FIG> is a side view showing the conveyance device <NUM> of <FIG>, and <FIG> is a rear view showing the conveyance device <NUM> of <FIG>. Referring to <FIG>, the conveyance device <NUM> comprises a carriage <NUM>, a support column <NUM>, a lifting device <NUM>, and a loading platform <NUM>. The conveyance device <NUM> may further comprise other constituent elements (for example, a controller).

Referring to <FIG>, the carriage <NUM> is configured so as to travel horizontally along the rails R. In the present embodiment, the carriage <NUM> travels along two rails R. In another embodiment, the carriage <NUM> may travel along a single rail R. The travelling of the carriage <NUM> may be controlled by the local controller of the conveyance device <NUM> described above.

Regarding the coordinate axes with respect to the conveyance device <NUM>, the axis parallel to the direction in which the carriage <NUM> moves (also referred to as the "forward-rearward directions") is the X-axis (also referred to as the "travel axis"). With respect to the support column <NUM>, the side on which the loading platform <NUM> is present is the "front", and the side opposite thereto is the "rear. " Among horizontal directions, the direction orthogonal to the front-rear direction is the left-right direction, and the axis parallel to the left-right direction (i.e., the horizontal axis orthogonal to the X-axis) is the Z-axis (also referred to as the "transfer axis"). The axis parallel to the vertical direction (also referred to as the "up-down directions") is the Y-axis (also referred to as the "elevation axis").

The support column <NUM> protrudes vertically upward from the carriage <NUM>. The support column <NUM> is a square column, and has a front surface <NUM>, a rear surface <NUM>, a right surface <NUM>, and a left surface <NUM>. Referring to <FIG>, the conveyance device <NUM> comprises a reinforcing member <NUM> which links the carriage <NUM> and the rear surface <NUM> of the support column <NUM>. In the conveyance <NUM>, a lower member <NUM> of the lifting device <NUM>, which will be described in detail later, is open rearwardly, and thus, when the lifting device <NUM> moves downwardly, the reinforcing member <NUM> does not interfere with the lifting device <NUM>.

The lifting device <NUM> moves in the up-down directions along the support column <NUM>. The lifting device <NUM> has an upper guide <NUM>, a lower guide <NUM>, and a pair of linking members <NUM>. The upper guide <NUM> constitutes an upper part of the lifting device <NUM>, and has an upper member <NUM> and a first wheel <NUM>. Furthermore, referring to <FIG>, the upper guide <NUM> has a pair of third wheels <NUM>.

<FIG> is an enlarged perspective view showing portion B of <FIG>, and shows the upper guide <NUM> as viewed from below and behind. Note that in <FIG>, several constituent elements such as the linking members <NUM> are not shown. The upper member <NUM> is substantially U-shaped opening forwardly in a bottom view (or top view). Specifically, the upper member <NUM> includes an upper main member 41a and a pair of upper auxiliary members 41b.

The upper main member 41a is arranged behind the support column <NUM> and extends along the left-right directions (Z-axis). The upper main member 41a supports the first wheel <NUM>. Specifically, the first wheel <NUM> is attached to the bottom surface of the upper main member 41a so as to roll along the rear surface <NUM> of the support column <NUM> in contact with the rear surface <NUM>.

The pair of upper auxiliary members 41b protrude frontward along the X-axis from the vicinity of both ends of the upper main member 41a so as to face each other with the support column <NUM> interposed therebetween. The upper auxiliary members 41b support the third wheels <NUM>. Specifically, one third wheel <NUM> is attached to the front surface of one upper auxiliary member 41b so as to roll on the left surface <NUM> of the support column <NUM> in contact with the left surface <NUM>. Likewise, though not shown in <FIG>, the other third wheel <NUM> is attached to the front surface of the other upper auxiliary member 41b so as to roll on the right surface <NUM> of the support column <NUM> in contact with the right surface <NUM>.

The conveyance device <NUM> comprises a first adjustment mechanism 70A for adjusting the position of the first wheel <NUM>. Specifically, the first adjustment mechanism 70A has a bracket 71a which is affixed to the upper main member 41a and machine bolts 72a. Furthermore, for example, an affixation part 42a of the first wheel <NUM> can be affixed to the upper main member 41a by bolts 42b in slots provided in the affixation part 42a. When the first wheel <NUM> becomes worn, the position of the first wheel <NUM> with respect to the upper member <NUM> can be adjusted along the slots by loosening the bolts 42b and tightening the machine bolts 72a. Once the position has been adjusted, the bolts 42b can be tightened.

Referring to <FIG>, the lower guide <NUM> constitutes a lower part of the lifting device <NUM>, and is arranged below the upper guide <NUM>. The lower guide <NUM> has a lower member <NUM> and a second wheel <NUM>. Furthermore, referring to <FIG>, the lower guide <NUM> has a pair of fourth wheels <NUM>.

<FIG> is an enlarged perspective view showing portion C of <FIG>, and shows the lower guide <NUM> as viewed from above and in front. Note that in <FIG>, several constituent elements such as the linking member <NUM> are not shown. The lower member <NUM> is substantially U-shaped opening to the rear in a top view (or bottom view). Specifically, the lower member <NUM> has a lower main member 51a and a pair of lower auxiliary members 51b.

The lower main member 51a is arranged in front of the support column <NUM> and extends along the left-right directions (Z-axis). The lower main member 51a supports the second wheel <NUM>. Specifically, the second wheel <NUM> is attached to the upper surface of the lower main member 51a so as to roll on the front surface <NUM> of the support column <NUM> in contact with the front surface <NUM>.

The pair of lower auxiliary members 51b protrude rearward along the X-axis from the vicinity of both ends of the lower main member 51a so as to face each other with the support column <NUM> interposed therebetween. The lower auxiliary members 51b support the fourth wheels <NUM>. Specifically, one fourth wheel <NUM> is attached to the rear surface of one lower auxiliary member 51b so as to roll on the left surface <NUM> of the support column <NUM> in contact with the left surface <NUM>. Though not illustrated in <FIG>, likewise, the other fourth wheel <NUM> is attached to the rear surface of the other lower auxiliary member 51b so as to roll on the right surface <NUM> of the support column <NUM> in contact therewith.

The conveyance device <NUM> comprises a second adjustment mechanism 70B for adjusting the position of the second wheel <NUM>. Specifically, the second adjustment mechanism 70B has a bracket 71b affixed to the lower main member 51a and machine bolts 72b. Furthermore, for example, an affixation part 52a of the second wheel <NUM> can be affixed to the lower main member 51a by bolts 52b in slots provided in the affixation part 52a. When the second wheel <NUM> becomes worn, the position of the second wheel <NUM> can be adjusted with respect to the lower member <NUM> along the slots by loosening the bolts 52b and tightening the machine bolts 72b. Once the position has been adjusted, the bolts 52b can be tightened.

Referring to <FIG>, the linking members <NUM> link the upper member <NUM> and the lower member <NUM> to each other. Specifically, the linking members <NUM> extend diagonally and linearly so as to link the upper main member 41a and the lower main member 51a in a side view. From another point of view, the linking members <NUM> extend diagonally and linearly so as to pass through (overlap) the first wheel <NUM> and the second wheel <NUM> in the side view. The pair of linking members <NUM> link the left and right ends of the upper main member 41a to the left and right ends of the lower main member 51a, respectively.

As shown in <FIG>, the lifting device <NUM> has a substantially inverted Z-shaped structure in a left side view (i.e., substantially Z-shaped in a right-side view) formed by the upper member <NUM>, the linking members <NUM>, and the lower member <NUM>, as described above. Due to this Z-shaped structure, the weight of the loading platform <NUM> and the workpiece W can be distributed to the upper rear first wheel <NUM> and the lower front second wheel <NUM> by the shortest distance by the linear linking members <NUM>, and the lifting device <NUM> can be stabilized in the left-right directions by the third and fourth wheels <NUM>, <NUM>.

Referring to <FIG> and <FIG>, the lifting device <NUM> is driven by a gear G which is rotated by a motor M affixed to the lifting device <NUM> and a linear guide member L which is affixed to the rear surface <NUM> of the support column <NUM> and which meshes with the gear G. Note that in <FIG>, in order to facilitate understanding, several constituent elements such as the motor M are not shown. For example, the motor M can be a servomotor, and can be affixed to a constituent element of the lifting device <NUM> (for example, the linking members <NUM>, etc.). The linear guide member L can have a plurality of pins PN arranged in a straight line and a pair of plates PL for supporting the plurality of pins PN therebetween. Alternatively, the linear guide member L may be, for example, a rack comprising a plurality of teeth. The rotation of the motor M (i.e., the height of the lifting device <NUM> and the loading platform <NUM>) can be controlled by, for example, the local controller of the conveyance device <NUM>.

Referring to <FIG>, the conveyance device <NUM> comprises a safety device <NUM> which detects inclination of the lifting device <NUM> when an upward force is exerted on the loading platform <NUM>. For example, the safety device <NUM> can be affixed to a constituent element (for example, below the motor M, etc.) of the lifting device <NUM>.

<FIG> is an enlarged perspective view showing section A of <FIG>, and shows the safety device <NUM> as viewed from below and behind. The safety device <NUM> can be, for example, a limit switch which detects rotation, or can include a rotation lever <NUM>. Normally, the tip of the rotation lever <NUM> is slightly separated from the plates PL affixed to, for example, the rear surface <NUM> of the support column <NUM>.

Referring to <FIG>, when an upward force is exerted on the loading platform <NUM> (for example, when the bottom surface of the loading platform <NUM> collides with the pallet stocker <NUM>), since the lifting device <NUM> has a Z-shaped structure as described above, a moment is generated in the direction in which the first wheel <NUM> and the second wheel <NUM> are separated from the support column <NUM>, and the lifting device <NUM> is slightly inclined counterclockwise in <FIG>. With reference to <FIG>, due to this inclination, the tip of the rotation lever <NUM> comes into contact with the plate PL, and the safety device <NUM> detects inclination of the lifting device <NUM>. For example, the safety device <NUM> can communicate with the local controller of the conveyance device <NUM>. When the local controller receives a signal from the safety device <NUM> indicating that inclination of the lifting device <NUM> has been detected, the local controller may execute a predetermined measure (for example, stopping the movement of the conveyance device <NUM> or notifying that inclination has been detected). Note that the safety device <NUM> can have any of various configurations as long as it is capable of detecting inclination of the lifting device <NUM>, and is not limited to the above configuration.

Referring to <FIG>, the loading platform <NUM> is moved in the up-down directions by the lifting device <NUM>. The loading platform <NUM> is arranged in front of the second wheel <NUM> and is connected to the front lower sides of the linking members <NUM>. A slide device <NUM> for moving the product left and right along the Z-axis is provided on the loading platform <NUM>. The slide device <NUM> comprises a base member <NUM>, an intermediate member <NUM>, and a workpiece holding part <NUM>. The base member <NUM> is affixed to the loading platform <NUM>. The intermediate member <NUM> is configured so as to move left and right on the base member <NUM>, and the workpiece holding part <NUM> is configured so as to move left and right on the intermediate member <NUM>. The workpiece holding part <NUM> is configured so as to hold a pallet P. Due to this configuration, the workpiece holding part <NUM> can move the pallet P left and right along the Z-axis, and can convey the pallet P between the processing machines 1A, 1B, the pallet stocker <NUM>, and the stations 3A, 3B (refer to <FIG>). The intermediate member <NUM> and the workpiece holding part <NUM> can be driven by, for example, an unillustrated motor, and the positions thereof in the Z-axis (i.e., the position of the pallet P in the Z-axis) can be controlled by the local controller of the conveyance device <NUM>.

In the conveyance device <NUM> described above, the first wheel <NUM> and the second wheel <NUM> of the lifting device <NUM> are arranged so as to roll on the rear surface <NUM> and the front surface <NUM> of the support column <NUM>, respectively. Thus, the first wheel <NUM> and the second wheel <NUM> are arranged outside of the support column <NUM>, and thus, the structure can be simplified and accessibility to the first wheel <NUM> and the second wheel <NUM>, for example, during assembly and maintenance can be improved. Thus, a conveyance device having suitable maintainability with a simple structure can be provided.

Furthermore, in the conveyance device <NUM>, the support column <NUM> is a square column having a front surface <NUM>, a rear surface <NUM>, a right surface <NUM>, and a left surface <NUM>, the upper member <NUM> has an upper main member 41a which is arranged behind the support column <NUM> and which supports the first wheel <NUM>, and a pair of upper auxiliary members 41b which protrude forward from the upper main member 41a and which support the pair of third wheels <NUM>, the upper member <NUM> is open toward the front, the lower member <NUM> has a lower main member 51a which is arranged in front of the support column <NUM> and which supports the second wheel <NUM>, and a pair of lower auxiliary members 51b which protrude rearward from the lower main member 51a and which support the pair of fourth wheels <NUM>, the lower member <NUM> is open toward the rear, and the linking members <NUM>, in a side view, extend diagonally and linearly so as to link the upper main member 41a and the lower main member 51a. Such lifting device <NUM> has, in a right-side view, as described above, a substantially Z-shaped structure formed by the upper member <NUM>, the linking members <NUM>, and the lower member <NUM>. In this Z-shaped structure, the upper rear upper main member 41a, which supports the first wheel <NUM>, and the lower front lower main member 51a, which supports the second wheel <NUM>, are diagonally and linearly linked by the linking members <NUM>, and thus, the weight of the loading platform <NUM> and the workpiece W can be transmitted to the upper rear first wheel <NUM> by the shortest distance. Thus, the force can be efficiently dispersed in the front and rear. Furthermore, stability in the left-right directions can be improved by the third and fourth wheels <NUM>, <NUM>. As described above, the Z-shaped structure of the lifting device <NUM> can efficiently achieve both the distribution of the force in the front-rear directions and stability in the left-right directions.

Furthermore, in the conveyance device <NUM>, the lifting device <NUM> is driven by the gear G which is rotated by the motor M affixed to the lifting device <NUM> and the linear guide member L which is affixed to the rear surface <NUM> of the support column <NUM> and which meshes with the gear G. Since the linear guide member L is affixed to the rear surface <NUM> of the support column <NUM>, a structure which is compact as compared with, for example, a lifting device which is driven using a moving member such as a chain can be obtained.

Furthermore, the conveyance device <NUM> comprises a safety device <NUM> which detects inclination of the lifting device <NUM> when an upward force is exerted on the loading platform <NUM>. In the conveyance device <NUM>, as described above, when an upward force is exerted on the loading platform <NUM>, due to the Z-shaped structure, a moment is generated in the direction in which the first wheel <NUM> and the second wheel <NUM> are separated from the support column <NUM>. Thus, by detecting inclination of the lifting device <NUM> due to such a moment, safety can be improved.

Furthermore, the conveyance device <NUM> comprises a first adjustment mechanism 70A for adjusting the position of the first wheel <NUM> with respect to the upper member <NUM> and a second adjustment mechanism 70B for adjusting the position of the second wheel <NUM> with respect to the lower member <NUM>. The first wheel <NUM> and the second wheel <NUM> may wear with use. When the first wheel <NUM> and the second wheel <NUM> become worn, the posture of the lifting device <NUM> with respect to the support column <NUM> changes, which may cause the lifting device <NUM> to incline. In order to maintain a constant posture of the lifting device <NUM> with respect to the support column <NUM>, it is necessary to adjust at least one of the position of the first wheel <NUM> with respect to the upper member <NUM> and the position of the second wheel <NUM> with respect to the lower member <NUM>, and in the conveyance device <NUM>, these can be adjusted by the first adjustment mechanism 70A and the second adjustment mechanism 70B. Thus, a constant posture of the lifting device <NUM> with respect to the support column <NUM> can be maintained over the long term.

Furthermore, the conveyance device <NUM> comprises a reinforcing member <NUM> which links the carriage <NUM> and the rear surface <NUM> of the support column <NUM>. In the conveyance device <NUM>, since the lower member <NUM> of the lifting device <NUM> opens rearward, even if such a reinforcing member <NUM> is provided on the rear surface <NUM> of the support column <NUM>, there is no interference between the lifting device <NUM> and the reinforcing member <NUM>. Thus, the strength of the conveyance device <NUM> can be increased.

Though the embodiments of the conveyance device have been described, the present invention is not limited to the embodiments described above. A person skilled in the art would understand that various changes can be made to the embodiments described above, within the scope of the claims.

Claim 1:
A conveyance device (<NUM>) for carrying a product, the conveyance device (<NUM>) comprising:
(i) a carriage (<NUM>) which travels in horizontal forward and rearward directions,
(ii) a support column (<NUM>) which protrudes upwardly from the carriage,
(iii) a lifting device (<NUM>) which moves in the upward and downward directions along the support column, comprising:
an upper guide (<NUM>) having a first wheel (<NUM>) which rolls on a rear surface of the support column and an upper member having an upper main member (41a) which is arranged behind the support column and configured to support the first wheel (<NUM>),
a lower guide (<NUM>), arranged below the upper guide, having a second wheel which rolls on a front surface of the support column and a lower member (<NUM>) which supports the second wheel (<NUM>), and
a linking member (<NUM>) which links the upper member and the lower member to each other, and
(iv) a loading platform (<NUM>) which is moved in the upward and downward direction by the lifting device (<NUM>), which is arranged in front of the second wheel, and which is linked to a front side of the linking member,
wherein
the lower guide (<NUM>) has a pair of fourth wheels which roll on the left and right surfaces of the support column (<NUM>),
the lower member comprises:
a lower main member (51a) which is arranged in front of the support column and which supports the second wheel, and
a pair of lower auxiliary members (51b) which protrude rearwardly from the lower main member so as to face each other with the support column interposed therebetween, and which support the pair of fourth wheels,
the lower member is open toward the rear, and
the linking member (<NUM>), in a side view, extends diagonally and linearly so as to link the upper main member and the lower main member.