JPH10157663A - Mandrel for rubber crawler belt - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a rubber crawler belt from coming off, to improve workability, and to prevent damage to an inside rubber surface of a rubber crawler belt and a flange part crack of an idler wheel by forming a mandrel inside into a curved shape curving from a mandrel engaging part engaged with the idler wheel to left and right blade part end parts. SOLUTION: In the crawler longitudinal direction of a rubber crawler belt 1, multiple mandrels 3 are buried, and a lower rolling wheel and an idler wheel 5 are engaged with an engaging part 3G in the mandrel 3 so as to be rolled. The mandrel 3 is formed into a curved shape curving from the engaging part 3G, with which the idler wheel 5 is engaged, to the end part of a blade part 3a. When the crawler is turned or moved backward on the slanting ground or on the rough ground, the idler wheel 5 cannot be moved toward the end part of the blade part 3a as the whole of the blade part 3a is formed into a curved shape, even when the idler wheel 5 almost comes off from the engaging part 3G in the mandrel 3. In this way, the rubber crawler belt 1 is prevented from coming off, and a repair time is not needed, so that workability is improved, while no defect such as damage of the rubber surface on the rubber crawler belt 1 inside is caused and durability is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rubber track used for a tracked vehicle of a construction machine such as a hydraulic shovel or a bulldozer, and more particularly to a rubber track core which has been improved to prevent the rubber track from coming off. Related to the shape.

[0002]

2. Description of the Related Art As shown in FIG. 13, a conventional core metal of a rubber crawler belt has a core metal 10 embedded in a rubber crawler belt 12. As shown in FIG.
The lower rolling wheel and the idler wheel 5 are engaged with the engaging portion 10a of the cored bar 10. A core wire 11 is embedded around the core metal 10. By the way, construction machines such as hydraulic excavators and bulldozers rotate the starter wheel clockwise to turn on an inclined ground during reverse running, or the rubber crawler belt 12 tilts when running backward on uneven terrain having large irregularities. The lower rolling wheel is released from the engaging portion 10a of the metal core 10. At this time,
The lower roller cannot get over the engagement portion 10a again and return to the original engagement position. For this reason, when the vehicle travels in the reverse direction in a state in which the lower rolling wheel is released, the idler wheel 5 is also released after the lower rolling wheel is released. There is a problem that the rubber crawler belt 12 is detached from the vehicle body due to the detachment of the lower rolling wheel and the idler wheel 5. As a measure for preventing the crawler belt from coming off used in the construction machine, there is a method in which the flange of the idler wheel 5 shown in FIG. There is also a track frame (not shown) in which a guard is attached to prevent the lower wheel from coming off.

[0003] As a prior art of the core metal of the rubber crawler belt, for example, according to Japanese Patent Application Laid-Open No. 4-39180, a circumferential tensile body (core metal) is buried in an endless belt-shaped main body made of an elastic body. Left and right engagement protrusions that engage with the drive wheels are protruded on the inner peripheral surface at regular intervals in the circumferential direction, lugs are integrally formed on the outer peripheral surface at regular intervals in the circumferential direction, and the main body has a flat portion at the center in the left and right direction, A crawler for snow that is curved upward and outward from the left and right sides of the flat part and is divided into left and right wing parts that are narrower than the flat part, wherein the left and right engagement protrusions are located on the left and right sides of the flat part. Reinforcing ribs are integrally formed with the main body, and are formed integrally with the left and right engaging projections and the inner peripheral surfaces of the left and right wings. The lugs are substantially the same from the flat portion to the substantially side ends of the left and right wings. It is described as being characterized by being formed at a height. In addition, as the prior art of other rubber track core metal,
Japanese Utility Model Laid-Open No. 1-150182 and Japanese Utility Model Laid-Open No. 1-12888
No. 0 has been filed.

[0004]

However, FIG.
In the shape of the metal core 10 shown in FIG. 5, the idler wheel 5 or the like that has climbed over the engaging portion 10a cannot return to the engaging portion 10a again. In addition, even if the flange of the idler wheel 5 is raised, it is not possible to completely prevent the rubber crawler belt from coming off due to a lateral load when turning or running on an inclined or uneven terrain. Further, even if a guard is attached to the track frame, the rubber track cannot be completely prevented from coming off, and there is a problem that the rubber track is damaged by the guard when the rubber track comes off. Furthermore, when the rubber crawler belt comes off, the grease of the idle wheel cushion device (not shown) for adjusting the tension of the idle wheel 5 is removed, the idle wheel 5 is set in a free state, and the engagement with the engaging portion 10a of the cored bar 10 is performed again. And it takes a lot of work time to repair. In addition, when the rubber crawler belt comes off, defects such as damage to the rubber surface inside the rubber crawler belt and loss of the engaging portions of the cored bar and the flanges of the rolling wheels such as idler wheels, starting wheels, and lower rolling wheels are caused. There is a problem that it occurs and durability decreases.

In the rubber crawler cores described in JP-A-4-39180, JP-A-1-150182, and JP-A-1-162880, the rolling wheels are detached outward from the left and right engaging projections. Then, it cannot return to the original position. These prior arts have the same structure as the engaging portion 10a of the cored bar 10 shown in FIG.
Crawler slippage cannot be completely prevented.

The present invention focuses on the problems of the prior art described above, and the inside of the core metal of the rubber crawler belt is curved from the engaging portion toward the left and right wings, so that the idler wheel, the starting wheel and the lower wheel are provided. Even if the rolling wheel such as is disengaged from the engaging portion, the rubber crawler core is constituted so that the rolling wheel cannot move outward from the curved surface and returns to the original engaging portion, An object of the present invention is to provide a core metal core of a rubber crawler belt which completely prevents the crawler belt from being detached, improves workability, and does not cause damage to a rubber surface inside the rubber crawler belt due to the detachment of the rubber crawler belt or loss of a flange of an idler wheel. .

[0007]

In order to achieve the above object, a first embodiment of a rubber crawler core according to the present invention is provided.
The present invention provides a core metal having an engaging portion for engaging an idler wheel at predetermined intervals in parallel in the longitudinal direction of a crawler, and wing portions formed on the left and right outer sides of the engaging portion, and the core metal. In a rubber track of an endless track band in which the starting wheel and the idler wheel are wound by covering and adhering, the inside of the cored bar 3 is separated from the cored bar engaging portion 3G engaged with the idler wheel 5 with the right and left. It is formed in a curved shape toward the end of the wing portion 3a. According to the above configuration, when the core metal of the rubber crawler is turned on an inclined ground or uneven ground, and when traveling backward, a lateral load is applied to the rubber crawler and the lower rolling wheel is disengaged from the engaging portion 3G of the core metal 3, Further, even if the rubber crawler belt slips off to the idler wheel by traveling backward, the idler wheel cannot move toward the end of the blade because the entire wing portion of the core metal is curved. As described above, even when the rubber crawler belt comes off, the idler wheel cannot move toward the end of the wing portion. Therefore, when the vehicle runs backward from the portion 5a directly below the idler wheel as shown in FIG. The restraining force is lost, and the recoil spring biasing the idler wheel returns to the original position of the engaging portion 3G and engages, so that the rubber crawler belt does not come off. This eliminates the need for working time for removing the grease of the conventional idler wheel cushion device due to the detachment of the rubber crawler belt and reengaging the idler wheel from the free state with the engaging portion of the cored bar. Accordingly, since the rubber crawler belt does not come off, the workability is improved, and the durability is improved without any trouble such as damage to the rubber surface inside the rubber crawler belt.

According to a second aspect of the present invention, in the configuration of the first aspect, the inner shape of the metal core 3 is such that a front end of the idler wheel 5 is engaged from a point M at a rear end of the starting wheel 6 behind the vehicle body. From the point E of the core bar engaging portion 3G, which is inside the arc S having a radius L1 up to the point E of the core bar engaging portion 3G to be engaged with the front end of the idler wheel 5, Curve H towards the end of 3a
The wing 3a is formed in a linear shape, and the distance L2 from the point M at the rear end of the starting wheel 6 to the point C intersecting the curved H line is equal to the wing 3a.
Are formed so as to be gradually reduced toward the end of the. According to the above configuration, when the core metal of the rubber crawler is turned on a slope or uneven ground, or when traveling backward, a lateral load is applied to the rubber crawler and the lower rolling wheel is released from the engaging portion 3G of the core metal 3. . At this time, the lower wheel cannot get over the engagement portion 3G again and return to the original engagement position. For this reason, when the vehicle travels in the reverse direction in a state in which the lower rolling wheel is released, the idler wheel 5 is also released after the lower rolling wheel is released. Even when the idler wheel 5 is about to be derailed, the distance L from the point M at the rear end of the starting wheel to the point C at which the curved H line is intersected is obtained.
2 is formed so as to gradually decrease toward the end of the wing 3a, so that the idler wheel cannot move toward the end of the wing. As described above, even when the rubber crawler belt is coming off, the idler wheel cannot move toward the end of the wing portion. Therefore, as shown in FIG. The restraining force is lost, and the recoil spring biasing the idler wheel returns to the original position of the engaging portion 3G and engages, so that the rubber crawler belt does not come off. Accordingly, since the rubber crawler belt does not come off, the workability is improved, and the durability is improved without any trouble such as damage to the rubber surface inside the rubber crawler belt.

According to a third aspect of the present invention, in the configuration of the first aspect, the inner shape of the cored bar 3 is engaged with a front end of the idler wheel 5 from a point M at a rear end of the starting wheel 6 behind the vehicle body. An arc R having a radius L3 smaller than the radius L1 is formed inside an arc S having a radius L1 up to the point E of the cored bar engaging portion 3G. According to the above-described configuration, a lateral load is applied to the rubber crawler belt 3 during turning on a sloped or uneven ground, or when traveling backward, so that the engagement portion 3G
The lower wheel derails from. At this time, the lower wheel cannot get over the engagement portion 3G again and return to the original engagement position. For this reason, when the vehicle travels in the reverse direction in a state in which the lower rolling wheel is released, the idler wheel 5 is also released after the lower rolling wheel is released. Even when the idler wheel 5 is coming off, an arc R having a radius L3 up to a point E of the cored bar engaging portion with which the front end of the idler wheel 5 engages is formed. Can not move towards the end of the. As described above, even when the rubber crawler belt comes off, the idler wheel cannot move toward the end of the wing portion. Therefore, when the vehicle runs backward from the portion 5a directly below the idler wheel as shown in FIG. The restraining force is lost, and the recoil spring biasing the idler wheel returns to the original position of the engaging portion 3G and engages, so that the rubber crawler belt does not come off. Accordingly, since the rubber crawler belt does not come off, the workability is improved, and the durability is improved without any trouble such as damage to the rubber surface inside the rubber crawler belt. And
If the inside of the cored bar is formed by an arc R, manufacture is easy.

According to a fourth aspect of the present invention, in the core metal for a rubber crawler according to any one of the first to third aspects, the core metal 3 has a trapezoidal cross section, and the outer end of the idler wheel 5 is engaged. It is formed in a trapezoidal shape that gradually decreases from the mating core bar engaging portion 3G toward the ends of the left and right wing portions 3a. According to the above configuration, the core metal 3 has a trapezoidal cross section, and the left and right wing portions 3a,
Since it is formed in a trapezoidal shape that gradually decreases toward the end of 3a, the adjacent metal cores 3, 3 do not overlap with each other. Therefore, in addition to the effects of the first to third inventions, even if the inside of the cored bar 3 is curved, the rubber crawler belt can wind the idler wheel.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A rubber crawler core according to the present invention will be described below with reference to FIGS. First, a large number of metal cores 3 are buried in the crawler longitudinal direction of the rubber crawler belt 1 shown in FIG. In FIG. 1, the lower rolling wheel 4 engages with the engaging portion 3G of the cored bar 3 and rolls. However, the idler wheel 5 shown in FIG. 3 also engages with the engaging portion 3G and rolls. It has become. 2 is a core wire. As shown in FIG.
In addition, an idler wheel 5, a lower rolling wheel 4, and an upper rolling wheel 7 are rotatably mounted. A starting wheel 6 for transmitting a driving force from a driving device (not shown) is provided. This idler wheel 5,
The lower crawler wheel 4, the upper crawler wheel 7, and the starting wheel 6 are wound around the rubber crawler belt 1 to form an endless rubber crawler belt. This idler wheel 5
L between the point P at the front end of the vehicle and the point M at the rear end of the starting wheel 6
The rubber track 1 is adjusted by a cushion device such as a recoil spring that adjusts the idle wheel (not shown) to adjust the tension to 1.

FIG. 4 shows a state where the lower crawler wheel 4 and the idler wheel 5 are wound around the rubber crawler belt 1. The metal core 3 is formed in a trapezoidal shape in a side view (or cross section), and is embedded at equal intervals around the entire circumference of the rubber crawler belt 1. The idler wheel 5 is connected to an adjusting rod 5a for adjusting the tension. The adjustment rod 5a is connected to a recoil spring (not shown). Due to this recoil spring force, a reaction force acts on the load applied from the rubber crawler belt 1 to the idler wheel 5 in the F direction, and the idler wheel 5
The self-alignment is performed so as to engage with the engaging portion 3G. The adjusting rod 5a urges the idler wheel 5 by a pressure applied from a grease chamber (not shown), and the grease chamber is filled with grease so that the idler wheel 5 moves leftward as shown in FIG. Adjustment is possible. Therefore, the idler wheel 5 and the lower rolling wheel 4 do not come off from the engaging portion 3G of the metal core 3 of the rubber crawler belt 1 in a state where the rubber wheel 1 is normally operated by the recoil spring force. However, construction machines such as hydraulic excavators and bulldozers equipped with conventional rubber crawler belts often work on sloping or uneven terrain, and the vehicle body is tilted during turning or reverse running, causing a large lateral load to act on the rubber crawler belt 1. At this time, it cannot be resisted by the recoil spring force.
As a result, the idler wheel 5 and the lower rolling wheel 4 come off.

Next, the core metal of the rubber track according to the present invention will be described in detail with reference to FIG. 3 shows a core metal 3 on the left side of the rubber crawler belt 1 shown in FIG. The right portion has the same shape and is omitted here. An engaging portion 3 </ b> G with which the flange of the idler wheel 5 is engaged is formed on the cored bar 3. When the lower rolling wheel separates from the engaging portion 3G during reverse running and then spreads to the idler wheel 5, the flange of the idler wheel 5 separates from the engaging portion 3G of the cored bar 3 in the W direction and the rubber crawler belt 1 comes off. . The first shape of the wing portion 3a of the metal core 3 will be described. Here, the distance L1 from the rear end of the starting wheel 6 to the front end of the idler wheel 5 shown in FIG.
The description will be made with L1 shown in FIG. The radius from the point M at the rear end of the starting wheel 6 to the point E of the core bar engaging portion 3G with which the front end of the idler wheel 5 engages is defined as a radius L1. As described above, the idler wheel 5 that is disengaged from the cored bar engaging portion 3G can move along the arc S line having the radius L1 starting from the point E of the cored bar engaging portion 3G around the point M. It is. However, the first shape of the metal core of the present invention, which forms the wing portion 3a of the metal core 3 that curves inward from the arc S line, is a metal core engaging portion 3G with which the front end of the idler wheel 5 engages. And a distance L2 from the point M at the rear end of the starting wheel to a point C intersecting with the curved H line is formed at the end of the wing 3a. It is formed so as to gradually decrease toward the portion. Thus, the idler wheel 5 cannot derail along the curved H line inside the arc S line. Core 3
The second shape of the wing portion 3a will be described. As described above, the arc R line having the radius L3 (smaller than the radius L1) up to the point E of the cored bar engaging portion with which the front end of the idler wheel 5 engages inside the arc S line having the radius L1. Wings 3a to be formed
Even in the case described above, the idler wheel 5 cannot derail along the arc R line located inside the arc S line in the same manner as described above.

Next, the Z view of FIG. 3 will be described with reference to FIG. Although the lower right traveling body is omitted in FIG. 5, the left and right lower traveling bodies have the same configuration, and therefore, the lower left traveling body will be described. The center frame 10 is integrally fixed to the track frame 8. This track frame 8
The idler wheel 5 rotatably attached to the front side of the rubber track 1
Is engaged with the engaging portion 3G of the core metal 3 buried in the core 3. A drive device 9 such as a hydraulic motor is attached to the rear side of the track frame 8. The driving device 9 is provided with a starting wheel 6 that is driven to rotate. The driving force from the starting wheel 6 is engaged with the metal core 3 embedded in the rubber crawler belt 1. Such a configuration is conventionally used. This idler wheel 5
As shown in FIG. 4, the spring is biased by an unillustrated recoil spring force via an adjusting rod 5a. As shown in FIGS. 5 and 6, when the rubber crawler belt 1 has the idler wheel 5 and the starting wheel 6 wound thereon, the distance L1 between the outer end of the idler wheel 5 and the outer end of the starting wheel 6 is smaller than the distance L1. Is held. In this manner, the idler wheel 5 and the starting wheel 6 are maintained at the distance L1, and can run and turn with the rubber crawler belt 1 wound thereon.

FIG. 7 is a view of the rubber crawler belt 1 viewed from the non-ground surface side, and FIG. 8 is a sectional view taken along the line AA of FIG. 7 and 8
As shown in the figure, the core metals 3 and 3 are embedded at equal intervals in the longitudinal direction of the rubber crawler belt 1. The metal core 3 is formed with an engaging portion 3G with which the flanges of the idler wheel 5 and the lower wheel 4 engage. Further, the core metal 3 is separated from the engaging portion 3G by the left and right wing portions 3a, 3a.
The cross section, which is gradually reduced toward the end of a, is formed in a trapezoidal shape. The left and right wing portions 3a, 3a of the cored bar 3 shown in FIG. 8 are substantially curved. The curved shape of the metal core 3 will be described in detail with reference to FIG. FIG. 9 of the BB cross-sectional view of FIG.
As shown in FIG. 1, the metal cores 3, 3 embedded in the rubber crawler belt 1 are formed in a trapezoidal shape.

Next, the operation of FIGS. 1 to 9 will be described with reference to FIGS. 10 to 12. As shown in FIG. 10, the idler wheel 5 and the lower roller 4 are engaged with the cored bar 3 so that the rubber crawler belt 1 can be driven. Since the idler wheel 5 and the lower wheel 4 are attached to the track frame 8, when a large lateral load in the direction F1 shown in FIG. 11 is applied during reverse running, it cannot be resisted by the recoil spring force. When the lower rolling wheel 4 separates from the cored bar 3 and travels further backward, the releasing wheel spreads to the idler wheel 5 and the same wheel 5 is removed. The wing portion 3a of the core metal 3 of the rubber crawler belt of the present invention has a radius L1 from the point M at the rear end of the starting wheel 6 to the point E of the core metal engaging portion 3G with which the front end of the idler wheel 5 engages. E of the metal core engaging portion 3G with which the front end of the idler wheel 5 engages inside the arc S
From the point toward the end of the wing portion 3a, and the distance L2 from the point M at the rear end of the starting wheel to the point C intersecting the curved H line is the end of the wing portion 3a. Are formed so as to gradually decrease toward. According to the metal core of the rubber crawler belt of the present invention, even when the idler wheel 5 is disengaged from the engaging portion 3G of the metal core 3 shown in FIG. Since the entire wing 3a is curved, the idler wheel 5 cannot move toward the end of the wing 3a. As described above, even when the rubber crawler belt comes off, the idler wheel cannot move toward the end of the wing portion. Therefore, when the vehicle runs backward from the portion 5a directly below the idler wheel as shown in FIG. The restraining force is lost, and the recoil spring biasing the idler wheel returns to the original position of the engaging portion 3G and engages, so that the rubber crawler belt does not come off. This eliminates the need for repair time to remove the grease from the conventional idler cushion device when the rubber crawler belt comes off, leave the idler wheels in a free state, and then re-engage them with the core metal engaging portions, improving workability. At the same time, there is no problem such as damage to the rubber surface inside the rubber crawler belt, and the durability is improved.

Further, the wing portion 3a of another core metal 3 of the present invention is
A radius smaller than the inner radius of the circular arc S from the point M at the rear end of the starting wheel 6 to the point E of the cored bar engaging portion 3G with which the front end of the idler wheel 5 engages. If the idler wheel 5 is disengaged from the engaging portion 3G of the metal core 3 shown in FIG.
Since the entire wing 3a is curved, the idler wheel 5 cannot move toward the end of the wing 3a. As described above, even when the rubber crawler belt comes off, the idler wheel cannot move toward the end of the wing portion. Therefore, when the vehicle runs backward from the portion 5a directly below the idler wheel as shown in FIG. The restraining force is lost, and the recoil spring biasing the idler wheel returns to the original position of the engaging portion 3G and engages, so that the rubber crawler belt does not come off. The manufacture is easy by making the wing part 3a of the metal core 3 into an arc shape.

Further, the metal core 3 of the present invention has a trapezoidal cross section, and sequentially from the metal core engaging portion 3G with which the outer end of the idler wheel 5 engages, toward the ends of the left and right wings 3a, 3a. Since it is formed in a trapezoidal shape to reduce the size, the adjacent metal cores 3 and 3 do not overlap each other. Thereby, even if the inside of the core metal 3 is curved, the rubber crawler belt can wind the idler wheel 5 and the like.

The core metal shape of the present invention is useful for preventing a rubber track of a construction machine from coming off. It goes without saying that the present invention can be applied to rubber tracks of agricultural and industrial machines other than construction machines.

[Brief description of the drawings]

FIG. 1 is a perspective view of a core metal of a rubber crawler belt of the present invention.

FIG. 2 is an explanatory diagram of the shape of a core metal wing.

FIG. 3 is an explanatory diagram of the lower traveling body.

FIG. 4 is a diagram illustrating a state in which a rubber crawler belt is wound around an idler wheel and a wheel.

FIG. 5 is a Z view of FIG. 3;

FIG. 6 is an explanatory diagram of a distance L1 between the outer end of the idler wheel and the outer end of the starting wheel.

FIG. 7 is an explanatory view of the core metal of the rubber crawler belt as viewed from the non-ground surface side.

FIG. 8 is a sectional view taken along the line AA of FIG. 7;

FIG. 9 is a sectional view taken along the line BB of FIG. 7;

FIG. 10 is an explanatory diagram of an engaged state between a core metal of a rubber crawler belt and an idler wheel.

FIG. 11 is an explanatory view when the idler wheel is released from the core metal of the rubber crawler belt.

FIG. 12 is an explanatory view of a state in which the idler wheel is again engaged with the core metal of the rubber crawler belt.

FIG. 13 is an explanatory view of a conventional metal core of a rubber crawler belt.

[Explanation of symbols]

1: Rubber track, 3: Core, 3a: Wings (core), 3G:
Engaging portion (core bar), 4 ... down-wheel, 5 ... idle wheel, 6 ... starting wheel, 7 ... up-wheel, 8 ... track frame.

Claims (4)

[Claims] 1. A metal core having an engaging portion for engaging an idler wheel at predetermined intervals in parallel in the longitudinal direction of a crawler, and wings formed on the left and right outer sides of the engaging portion, and the core. In a crawler belt of an endless track band formed by covering and bonding gold and winding a starting wheel and an idler wheel, the core metal (3) has a core metal engaged with an idler wheel (5). Left and right wings from the engagement part (3G)
(3a) A core metal core of a rubber track, which is formed in a curved shape toward an end portion. 2. The rubber crawler core according to claim 1, wherein the inner shape of the core metal is the starting wheel at the rear of the vehicle body.
(6) inside the arc S having a radius L1 from the point M at the rear end to the point E of the cored bar engaging portion (3G) with which the front end of the idler wheel (5) engages; Further, the idler wheel (5) is formed in a curved H line shape from the point E of the cored bar engaging portion (3G) with which the front end of the idler wheel (5G) is engaged toward the end of the wing portion (3a), and the starting is performed. Wheel (6)
A metal core for a rubber track, wherein a distance L2 from a point M at a rear end to a point C at which the curved line H intersects is gradually reduced toward an end of the wing (3a). 3. The core of a rubber crawler belt according to claim 1, wherein the inner shape of the core is the starting wheel at the rear of the vehicle body.
An arc S having a radius L1 from the point M at the rear end of (6) to the point E of the core bar engaging portion (3G) with which the front end of the idler wheel (5) engages.
And a radius L smaller than the radius L1.
3. A metal core of a rubber track, wherein an arc R is formed. 4. The rubber crawler core according to claim 1, wherein said core (3) has a trapezoidal cross section and an outer end of an idler wheel (5) is engaged. Core bar engaging part
(3G) A core metal core of a rubber crawler belt, which is formed in a trapezoidal shape that gradually decreases from the left and right wing portions (3a, 3a).