A reinforced automobile bumper mounting bracket structure

Abstract

The present invention belongs to the technical field of bumper mounting brackets, and specifically is a reinforced automobile bumper mounting bracket structure, comprising a main cross beam, a top fixing plate and a bottom fixing plate are respectively arranged at the top and bottom of the rear side of the main cross beam, bolt holes are penetrated on the surfaces of the top fixing plate and the bottom fixing plate, and a plurality of telescopic mechanisms are arranged at the rear side of the main cross beam, and the telescopic mechanism comprises a cylinder; the present invention can make the mounting bracket move forward and away from the engine compartment through the setting of the telescopic mechanism, at this time, the bumper is buckled on the mounting bracket from the front, and then nuts and bolts are installed at the top fixing plate and the bottom fixing plate, because the main cross beam is completely located in the open space on the front side of the engine compartment, even if the nut falls, it will fall to the ground and will not fall into the engine compartment, so it is convenient to pick it up, and only the top angle or the bottom angle needs to be bent, so that the buckle of the bumper can enter the bending part of the fender, which is less difficult and more convenient to install the bumper.

Description

Technical Field

[0001] The invention belongs to the technical field of bumper mounting brackets, and in particular relates to a reinforced automobile bumper mounting bracket structure. Background Art

[0002] The bumper is one of the parts that decorate the appearance of the car. It is an important part of the overall shape of the car. Its appearance should be coordinated with the shape of the whole car and be integrated into one. The middle part of the bumper is installed through the mounting bracket in the middle of the front side of the engine compartment. The bumper is fixed and installed by bolts and nuts. The two sides are fitted with the fenders through snap-fitting. The buckles on both sides of the bumper are completely inserted into the inward curved part of the front end of the fender to limit the buckles, and finally the installation of the bumper is completed.

[0003] When installing the bumper on the mounting bracket, since the existing mounting bracket is located on the front side of the engine inside the engine compartment, when fixing the bumper with bolts and nuts, since the mounting bracket is close to the engine and the parts inside the engine compartment occupy more space, when the nut is initially screwed onto the bolt, it is inconvenient to tighten the nut, and the nut is easy to fall to the bottom inside the engine compartment and is not easy to pick up.

[0004] Therefore, it is necessary to invent a reinforced automobile bumper mounting bracket structure to solve the above problems. Summary of the invention

[0005] In view of the above problems, the present invention provides a reinforced automobile bumper mounting bracket structure to solve the problems raised in the above background technology.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solutions: a reinforced automobile bumper mounting bracket structure, comprising a main cross beam, a top fixing plate and a bottom fixing plate are respectively arranged at the top and bottom of the rear side of the main cross beam, bolt holes are penetrated through the surfaces of the top fixing plate and the bottom fixing plate, a plurality of telescopic mechanisms are arranged at the rear side of the main cross beam, the telescopic mechanism comprises a cylinder, a connecting rod is arranged inside the cylinder, the connecting rod is fixedly connected to the main cross beam, a cross bar is inserted in the middle of the connecting rod, a lower extension tube is arranged at the bottom of the connecting rod on the front side of the cross bar, an insertion rod is inserted at the bottom of the lower extension tube, a lifting plate is arranged at the bottom inside the rear end of the cylinder, the thickness of the front end of the lifting plate gradually decreases forward, an insertion hole is opened at the maximum thickness of the lifting plate, the insertion hole penetrates downwardly through the cylinder, and the diameter of the insertion hole and the insertion rod are the same.

[0007] Preferably, the top and bottom of the front end of the cylinder are fixedly connected with right-angle plates, the top of the horizontal section of the right-angle plate is hinged with a mounting rod, a torsion spring is provided at the hinge, the mounting rod can be deflected from a vertical state toward the cylinder, and a fixing cap is fixedly connected to the top of the mounting rod, and the inner wall of the fixing cap is a hexagonal prism structure with half of it cut off along a direction parallel to the front side of the cylinder.

[0008] Preferably, a U-shaped hoop is movably connected to the outside of the fixing cap, the center of the bottom of the U-shaped hoop is arched upward, and a magnetic block is provided on the front side of the bottom of the U-shaped hoop, the top fixing plate and the bottom fixing plate are both made of steel that can be adsorbed by magnets, and the two ends of the U-shaped hoop are rotatably connected to the fixing cap through a pin shaft, the pin shaft passes through the side wall of the fixing cap, and a push plate is fixedly connected to the top of one end of the pin shaft located inside the fixing cap, and the height of the push plate gradually increases forward.

[0009] Preferably, the U-shaped hoop is made of stainless steel, and the inner wall of the fixing cap is smoothly arranged.

[0010] Preferably, a sliding plate is slidably connected between two adjacent connecting rods, and a clamping rod is arranged on the front side of the sliding plate. When the connecting rod moves backward and drives the insertion rod to align with the insertion hole, the front end of the clamping rod is clamped on the rear side surface of the main beam.

[0011] Preferably, both ends of the sliding plate are respectively inserted into corresponding connecting rods, and a sliding groove is provided on the connecting rod for the sliding plate to slide toward the front end of the connecting rod. A clamping spring is provided on the front side of the sliding plate, and the clamping spring is fixedly connected to the front end of the sliding groove.

[0012] Preferably, a ball is embedded in the bottom of the insertion rod, and the inclination angle of the line from the bottom of the ball to the rearmost part of the bottom of the insertion rod is the same as the inclination degree of the top surface of the front end of the lifting plate.

[0013] Preferably, the two sides of the main cross beam are curved, and the middle part of the main cross beam is arched forward, so that the thickness gradually decreases from the middle to the top and the bottom respectively.

[0014] Technical effects and advantages of the present invention:

[0015] 1. The present invention can make the mounting bracket move forward and away from the engine compartment at the beginning by setting the telescopic mechanism. At this time, the bumper is buckled on the mounting bracket from the front, and then nuts and bolts are installed at the top fixing plate and the bottom fixing plate to fix the bumper on the main cross beam. Since the main cross beam is completely located in the open space in front of the engine compartment, even if the nut falls, it will fall on the ground instead of falling into the engine compartment, so it is convenient to pick it up. Moreover, only the top corner or the bottom corner is bent, so that the buckle of the bumper can enter the bending part of the fender, which is less difficult and more convenient to install the bumper.

[0016] 2. The present invention allows the nut to enter the inner side of the fixing cap laterally backwards, and the shape of the hexagonal prism on the outer surface of the nut just matches the shape of the inner wall of the fixing cap, thereby preventing the nut from loosening. Furthermore, since the mounting rod is hinged, when the nut cannot be directly inserted into the inner side of the fixing cap after being tightened, the fixing cap can be pushed backwards without affecting the connecting rod driving the insertion rod to reach the insertion hole. When the insertion rod is inserted into the insertion hole, the main crossbeam is pushed into place, and the rotation of the nut can be adjusted again until the fixing cap deflected backwards can correspond to the nut and be sleeved on the nut, without the need to adjust the nut midway when the main crossbeam is not pushed into place.

[0017] 3. The present invention provides a U-shaped hoop. After the nut gradually moves into the interior of the fixing cap, the nut will push the push plate backward to drive the pin shaft to rotate, and finally the U-shaped hoop will rotate more than 90 degrees from a vertical state, so that the magnetic block on the U-shaped hoop is aligned with the corresponding top fixing plate or bottom fixing plate. When the vehicle has a large acceleration, due to the presence of the U-shaped hoop, the U-shaped hoop is put on the nut from the front, which can prevent the fixing cap from completely separating from the nut backward, thereby improving the stability of the clamping nut. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0019] Figure 1 It is a three-dimensional schematic diagram of the reinforced automobile bumper mounting bracket structure of the present invention;

[0020] Figure 2 The present invention Figure 1 A magnified view of part A;

[0021] Figure 3 The present invention Figure 1 A schematic diagram of one of the angles;

[0022] Figure 4 The present invention Figure 3 A magnified view of part A;

[0023] Figure 5 is a side sectional view of the cylinder in the present invention;

[0024] Figure 6 It is a schematic diagram of the buckle at the bottom corner of the bumper of the present invention after entering the front end of the fender (side view, the curved line in the figure is a simplified diagram of the fender).

[0025] In the figure: main cross beam 1, top fixing plate 2, bottom fixing plate 3, telescopic mechanism 4, cylinder 41, connecting rod 42, cross bar 43, lower extension tube 44, insertion rod 45, lifting plate 46, insertion hole 47, right angle plate 5, mounting rod 6, fixing cap 7, U-shaped hoop 8, magnetic block 9, pin 10, pushing plate 11, sliding plate 12, tightening rod 13, sliding groove 14, tightening spring 15, ball 16. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention;

[0027] In the description of the present invention, it should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate directions or positional relationships based on the attached Figure 1 The arrows in the figure point to the orientation or positional relationship shown in the main viewing direction, which is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, in the description of the present invention, the meaning of "multiple" is two or more, unless otherwise clearly and specifically defined.

[0028] The present invention provides Figure 1-6 The reinforced automobile bumper mounting bracket structure shown in the figure comprises a main cross beam 1, a top fixing plate 2 and a bottom fixing plate 3 are respectively arranged at the top and bottom of the rear side of the main cross beam 1, and bolt holes are penetrated on the surfaces of the top fixing plate 2 and the bottom fixing plate 3, and a plurality of telescopic mechanisms 4 are arranged at the rear side of the main cross beam 1, and the telescopic mechanisms 4 comprise a cylinder 41, and a connecting rod 42 is arranged inside the cylinder 41, and the connecting rod 42 is fixedly connected to the main cross beam 1, and a cross rod 43 is inserted in the middle of the connecting rod 42, and the cross rod 43 acts as an axis, so that the connecting rod 42 can rotate around the cross beam 1. The rod rotates, and the cross bar 43 can move forward or backward along the cylinder 41. During the movement, the height of the cross bar 43 remains unchanged. At the same time, a lower extension tube 44 is provided at the bottom of the connecting rod 42 on the front side of the cross bar 43, and an insertion rod 45 is inserted at the bottom of the lower extension tube 44. A lifting plate 46 is provided at the bottom of the inner side of the rear end of the cylinder 41. The thickness of the front end of the lifting plate 46 gradually decreases forward. An insertion hole 47 is opened at the maximum thickness of the lifting plate 46. The insertion hole 47 penetrates downward through the cylinder 41, and the diameters of the insertion hole 47 and the insertion rod 45 are the same.

[0029] During installation, due to the presence of the telescopic mechanism 4, the mounting bracket can be initially moved forward and away from the engine compartment. At this time, the bumper is buckled on the main cross beam 1 from the front, and then nuts and bolts are installed on the top fixing plate 2 and the bottom fixing plate 3 to fix the bumper on the main cross beam 1. Since the main cross beam 1 is completely located in the open space in front of the engine compartment, even if the nut falls, it will fall to the ground instead of falling into the engine compartment, so it is convenient to pick up. Then the fixed bumper is pushed backward, that is, towards the engine compartment. Due to the front end of the fenders on both sides of the vehicle, The bumper is tilted backward from the bottom to the top, and the top and bottom ends of the two side edges of the bumper are provided with protruding buckles. The two side edges are deformed simultaneously to make the buckles snap into the inner bend of the front end of the fender. The bending force is large and it is not easy to operate. Therefore, when the present invention is close to the fender, due to the existence of the cross bar 43 and the lifting plate 46, the cross bar 43 can make the front end of the connecting rod 42 deflect downward. Therefore, when the bumper is pushed to move backward, the buckles at the bottom of the side edges of the bumper will reach the fender first. At this time, the two bottom corners of the bumper can be bent to make the corresponding buckles pass through the bend of the fender first (such as Figure 6 The bumper is then pushed to move, and when the clips at the top of the bumper side reach the fender, the two top corners of the bumper can be bent so that the corresponding clips pass through the bend of the fender first, and then the clips can be inserted into the fender in a sequence. Compared with directly bending the two side edges of the bumper, only bending the top angle or the bottom angle is less difficult, making it more convenient to install the bumper. Finally, when the insertion rod 45 moves backward along the front end of the lifting plate 46, the thickness of the front end of the lifting plate 46 gradually decreases forward, that is, the front end of the front end of the lifting plate 46 gradually becomes higher backward, so that the insertion rod and the lower extension tube 44 rise, so that the connecting rod 42 is in a horizontal state, and the front end of the connecting rod 42 rises, driving the main cross beam 1 to become higher, so that the clips on both side edges of the bumper are completely fitted to the fender, and the insertion rod 45 is inserted into the insertion hole 47 to complete the locking of the connecting rod 42. During the driving of the vehicle, the main cross beam 1 will not be affected by the inertia during acceleration and deceleration to drive the connecting rod 42 to move.

[0030] Refer to the instruction manual Figure 1-4 The top and bottom of the front end of the cylinder 41 are fixedly connected with a right-angle plate 5, and the top of the horizontal section of the right-angle plate 5 is hinged with a mounting rod 6, and a torsion spring is provided at the hinge. The mounting rod 6 can be deflected from a vertical state toward the cylinder 41, and a fixing cap 7 is fixedly connected to the top of the mounting rod 6. The inner wall of the fixing cap 7 is a hexagonal prism structure with half of it cut off along a direction parallel to the front side of the cylinder 41.

[0031] When the nut is installed on the bolt, there is a possibility that the nut will loosen due to the vibration of the vehicle after the vehicle has traveled a certain mileage. Therefore, after the main cross beam 1 is pushed backward, the nut can be moved horizontally backward to the inside of the fixing cap 7. Since the inner wall of the fixing cap 7 is a hexagonal prism structure with half of it cut off along a line parallel to the front side of the cylinder 41, when the nut reaches the inside of the fixing cap 7, the shape of the hexagonal prism on the outer surface of the nut just matches the shape of the inner wall of the fixing cap 7, which can prevent the nut from loosening. In addition, since the mounting rod 6 is hinged, when the nut cannot be directly inserted into the inside of the fixing cap 7 after being tightened, the fixing cap 7 can be pushed backward without affecting the connecting rod 42 driving the insertion rod 45 to reach the insertion hole 47. When the insertion rod 45 is inserted into the insertion hole 47, the main cross beam 1 is pushed into place at this time, and the nut can be adjusted to rotate until the fixing cap 7 deflected backward can correspond to the nut and be sleeved on the nut. There is no need to adjust the nut midway when the main cross beam 1 is not pushed into place.

[0032] Refer to the instruction manual Figure 1-4 A U-shaped hoop 8 is movably connected to the outside of the fixing cap 7. The center of the bottom of the U-shaped hoop 8 is arched upward, and a magnetic block 9 is arranged on the front side of the bottom of the U-shaped hoop 8. The top fixing plate 2 and the bottom fixing plate 3 are both made of steel that can be adsorbed by magnets. Both ends of the U-shaped hoop 8 are rotatably connected to the fixing cap 7 through a pin 10. The pin 10 passes through the side wall of the fixing cap 7, and a push plate 11 is fixedly connected to the top of one end of the pin 10 located inside the fixing cap 7, and the height of the push plate 11 gradually increases forward.

[0033] Since the connecting rod 42 is configured as a torsion spring type hinge structure, when acceleration occurs during vehicle driving, the fixing cap 7 drives the top end of the connecting rod 42 to deflect backward due to the inertial force generated by the acceleration. Therefore, a U-shaped hoop 8 is provided. After the nut gradually moves into the fixing cap 7, the nut will push the push plate 11 backward and drive the pin 10 to rotate, and finally the U-shaped hoop 8 is rotated more than 90 degrees from the vertical state, so that the magnetic block 9 on the U-shaped hoop 8 is aligned with the corresponding top fixing plate 2 or bottom fixing plate 3. When the vehicle has a large acceleration, due to the presence of the U-shaped hoop 8, the U-shaped hoop 8 is put on the nut from the front, which can prevent the fixing cap 7 from completely disengaging from the nut backward, thereby improving the stability of the clamping nut.

[0034] Refer to the instruction manual Figure 1-4 The U-shaped hoop 8 is made of stainless steel, and the inner wall of the fixing cap 7 is smoothly arranged.

[0035] The U-shaped hoop 8 is made of stainless steel to prevent the U-shaped hoop 8 from rusting, and the inner wall of the fixing cap 7 is set to be smooth, that is, the inner wall of the fixing cap 7 is polished to reduce the friction of the inner wall of the fixing cap 7, thereby reducing the wear of the fixing cap 7 on the nut.

[0036] Refer to the instruction manual Figure 1-4 A sliding plate 12 is slidably connected between two adjacent connecting rods 42, and a clamping rod 13 is arranged on the front side of the sliding plate 12. When the connecting rod 42 moves backward and drives the insertion rod 45 to align with the insertion hole 47, the front end of the clamping rod 13 is clamped on the rear side surface of the main cross beam 1, and the two ends of the sliding plate 12 are respectively inserted into the corresponding connecting rods 42, and a sliding groove 14 is opened on the connecting rod 42 for the sliding plate 12 to slide toward the front end of the connecting rod 42, and a clamping spring 15 is arranged on the front side of the sliding plate 12, and the clamping spring 15 is fixedly connected to the front end of the sliding groove 14.

[0037] When the main cross beam 1 drives the connecting rod 42 to move backward, the sliding plate 12 is blocked by the cylinder 41, so that when the connecting rod 42 continues to move into the cylinder 41, the sliding plate 12 slides on the connecting rod 42, and the sliding plate 12 approaches the front end of the connecting rod 42, compressing the clamping spring 15. When the main cross beam 1 moves into place, the clamping rod 13 just presses against the rear side of the main cross beam 1 at the center of the two adjacent connecting rods 42, increasing the strength of the main cross beam 1. At the same time, due to the restoring force of the clamping spring 15, the connecting rod 42 can be pushed forward in the reverse direction, so that the connecting rod 42 drives the insertion rod 45 to have a forward force, and the insertion rod 45 and the hole wall of the insertion hole 47 have a squeezing force, which increases the friction between the insertion rod 45 and the hole wall. When the vehicle vibrates, the insertion rod 45 will not move upward, thereby preventing the insertion rod 45 from moving upward and completely disengaging from the limiting effect of the insertion hole 47, ensuring the stability of the insertion rod 45 when the vehicle vibrates.

[0038] Refer to the instruction manual Figure 5 A ball 16 is embedded in the bottom of the insertion rod 45 , and the inclination angle of the line from the bottom of the ball 16 to the rearmost part of the bottom of the insertion rod 45 is the same as the inclination degree of the top surface of the front end of the lifting plate 46 .

[0039] Since the inclination angle of the line from the bottom of the ball 16 to the rearmost bottom of the insertion rod 45 is the same as the inclination degree of the top surface of the front end of the lifting plate 46, it can be ensured that when the ball 16 moves on the inclined surface at the front end of the lifting plate 46 and the horizontal plane at the top of the lifting plate 46, it can reduce friction and reduce the wear between the insertion rod 45 and the lifting plate 46.

[0040] Refer to the instruction manual Figure 1 The two sides of the main cross beam 1 are curved, and the middle part of the main cross beam 1 is arched forward, so that its thickness gradually decreases from the middle to the top and the bottom respectively.

[0041] The two sides of the main cross beam 1 are curved, which can facilitate bending the bumper so that the buckles on both sides of the bumper can be inserted into the fender. At the same time, the middle part of the main cross beam 1 is thick, which can ensure the strength of the main cross beam 1. If the bumper is set to fully match and fit the main cross beam 1, the arched position in the middle part of the main cross beam 1 can prevent the bumper from moving downward.

[0042] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A reinforced automobile bumper mounting bracket structure, comprising a main cross beam (1), a top fixing plate (2) and a bottom fixing plate (3) are respectively arranged at the top and bottom of the rear side of the main cross beam (1), and bolt holes are penetrated through the surfaces of the top fixing plate (2) and the bottom fixing plate (3), characterized in that: A plurality of telescopic mechanisms (4) are arranged at the rear side of the main cross beam (1), and the telescopic mechanisms (4) include a cylinder (41), a connecting rod (42) is arranged inside the cylinder (41), the connecting rod (42) is fixedly connected to the main cross beam (1), a cross bar (43) is inserted in the middle of the connecting rod (42), a lower extension tube (44) is arranged at the bottom of the connecting rod (42) in front of the cross bar (43), an insertion rod (45) is inserted at the bottom of the lower extension tube (44), a lifting plate (46) is arranged at the bottom of the inner side of the rear end of the cylinder (41), the thickness of the front end of the lifting plate (46) gradually decreases forward, an insertion hole (47) is opened at the maximum thickness of the lifting plate (46), the insertion hole (47) penetrates downwardly through the cylinder (41), and the diameter of the insertion hole (47) and the insertion rod (45) are the same; The top and bottom of the front end of the cylinder (41) are fixedly connected to a right-angle plate (5); the top of the horizontal section of the right-angle plate (5) is hinged with a mounting rod (6); a torsion spring is provided at the hinge; the mounting rod (6) can be deflected from a vertical state toward the cylinder (41); the top of the mounting rod (6) is fixedly connected to a fixing cap (7); the inner wall of the fixing cap (7) is a shape of a hexagonal column structure with half of the structure cut off along a line parallel to the front side of the cylinder (41).

2. The reinforced automobile bumper mounting bracket structure according to claim 1, characterized in that: A U-shaped hoop (8) is movably connected to the outside of the fixing cap (7), the center of the bottom of the U-shaped hoop (8) is arched upward, and a magnetic block (9) is arranged on the front side of the bottom of the U-shaped hoop (8), the top fixing plate (2) and the bottom fixing plate (3) are both made of steel that can be adsorbed by magnets, and the two ends of the U-shaped hoop (8) are rotatably connected to the fixing cap (7) through a pin shaft (10), the pin shaft (10) passes through the side wall of the fixing cap (7), and a push plate (11) is fixedly connected to the top of one end of the pin shaft (10) located inside the fixing cap (7), and the height of the push plate (11) gradually increases forward.

3. The reinforced automobile bumper mounting bracket structure according to claim 2, characterized in that: The U-shaped hoop (8) is made of stainless steel, and the inner wall of the fixing cap (7) is smoothly arranged.

4. The reinforced automobile bumper mounting bracket structure according to claim 1, characterized in that: A sliding plate (12) is slidably connected between two adjacent connecting rods (42), and a clamping rod (13) is arranged on the front side of the sliding plate (12). When the connecting rod (42) moves backward and drives the insertion rod (45) to align with the insertion hole (47), the front end of the clamping rod (13) is clamped against the rear side surface of the main cross beam (1).

5. The reinforced automobile bumper mounting bracket structure according to claim 4, characterized in that: The two ends of the sliding plate (12) are respectively inserted into the corresponding connecting rod (42), and the connecting rod (42) is provided with a sliding groove (14) for the sliding plate (12) to slide toward the front end of the connecting rod (42), and a clamping spring (15) is arranged on the front side of the sliding plate (12), and the clamping spring (15) is fixedly connected to the front end of the sliding groove (14).

6. The reinforced automobile bumper mounting bracket structure according to claim 1, characterized in that: A ball (16) is embedded in the bottom of the insertion rod (45), and the inclination angle of the line connecting the bottom of the ball (16) to the rearmost part of the bottom of the insertion rod (45) is the same as the inclination degree of the top surface of the front end of the lifting plate (46).

7. The reinforced automobile bumper mounting bracket structure according to claim 1, characterized in that: Both sides of the main cross beam (1) are curved, and the middle part of the main cross beam (1) is arched forward, so that its thickness gradually decreases from the middle part to the top and bottom respectively.

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