A subgrade pavement deflection detection device

Abstract

This invention discloses a roadbed and pavement deflection testing device, including a testing vehicle and a support frame mounted on the testing vehicle, as well as two sets of Beckman beam bodies, respectively disposed on both sides of the support frame; an extension and retraction mechanism mounted on the support frame; and two front-end self-testing mechanisms. This roadbed and pavement deflection testing device allows operators to unfold the two sets of Beckman beam bodies using the extension and retraction mechanisms and adjust them to a specified width for testing when measuring the deflection of the roadbed and pavement. In conjunction with the movement of the testing vehicle, the front-end self-testing mechanisms can position the detection head at the front end of the Beckman beam body 4cm in front of the rear axle detection point and automatically detach the testing equipment from the detection head at the front end of the Beckman beam body, simplifying the operation steps, reducing the adjustment time, and thus greatly improving the adjustment speed of the deflection testing device.

Description

Technical Field

[0001] This invention relates to the field of roadbed and pavement technology, specifically to a roadbed and pavement deflection detection device. Background Technology

[0002] The roadbed is the foundation of the track or pavement. It is an earthwork structure formed by excavation or filling. The main function of the roadbed is to provide the necessary conditions for the laying of the track or pavement and the operation of trains or vehicles. It also bears the static and dynamic loads of the track, locomotives, rolling stock, pavement, and traffic loads, while transferring and dispersing the loads deep into the foundation. In the longitudinal section, the roadbed must ensure the required elevation of the line. In the horizontal plane, the roadbed connects with bridges and tunnels to form a complete and continuous line. In civil engineering, the roadbed occupies an important position in terms of construction quantity, land area, and investment. Deflection is one of the important technical indicators for highway design and highway engineering quality evaluation. It reflects the overall strength of each layer of the pavement. If the deflection value is too large, the deformation will be greater, and the pavement layers will be more prone to cracking. The pavement deflection value is generally obtained through deflection testing.

[0003] Existing deflection testing devices generally include deflectometers, with the commonly used ones being the Beckman deflectometer and the falling weight deflectometer. The Beckman deflectometer consists of a Beckman beam, a dial indicator, and a frame, made of aluminum alloy. Its lever ratio (the ratio of the front arm to the rear arm) is typically 2:1. Specifically, to improve the accuracy of the data during deflection testing, two deflectometers are often used simultaneously. In practice, these two deflectometers must first be placed parallel to each other on the left and right sides of the test vehicle at predetermined ground positions corresponding to the tire clearance of the rear wheels. When the measurement point changes, the two deflectometers need to be manually moved. After moving them, the deflectometers on both sides need to be repositioned. This process is cumbersome and time-consuming, significantly reducing operator efficiency. Therefore, we propose an easily adjustable roadbed and pavement deflection testing device. Summary of the Invention

[0004] The purpose of this invention is to provide a roadbed and pavement deflection detection device to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a roadbed and pavement deflection testing device, comprising a testing vehicle and a support frame mounted on the testing vehicle, and further comprising two sets of Beckman beam bodies, each set on one side of the support frame; an extension and retraction mechanism mounted on the support frame; and two front-end self-testing mechanisms, each set positioned near the rear axle of the Beckman beam body. This allows operators to extend the two sets of Beckman beam bodies and adjust them to a specified width for testing when measuring roadbed and pavement deflection. In conjunction with the movement of the testing vehicle, the front-end self-testing mechanisms can position the detection head at the front of the Beckman beam body 4cm in front of the rear axle detection point, and automatically detach the testing equipment from the detection head at the front of the Beckman beam body. This simplifies the operation, reduces the adjustment time, and significantly improves the adjustment speed of the deflection testing device.

[0006] Preferably, the deployment and retraction mechanism includes a lifting assembly mounted on a support frame, the lifting assembly being connected to the two Beckman beam bodies, and the lifting assembly being used to drive the two Beckman beam bodies to move up and down synchronously.

[0007] The lifting assembly is provided with a fixed frame, and the fixed frame is provided with a displacement assembly. The displacement assembly is used to drive the two Beckman beam bodies to move synchronously to both sides.

[0008] Each group of Beckman beam bodies consists of three Beckman beam units. The support frame is equipped with two extension and retraction components that cooperate with each group of Beckman beam bodies. The extension and retraction components are used to drive the three adjacent Beckman beam units to unfold, thereby ensuring normal testing and use.

[0009] Preferably, the lifting assembly includes an electric actuator mounted on a support frame, with a lifting bend at the end of the electric actuator, a fixed frame connected to the lifting bend, a Y-shaped seat on the support frame, and a sliding frame at the end of the Y-shaped seat that cooperates with the lifting bend.

[0010] Preferably, the displacement assembly includes two sets of support plates respectively disposed on both sides of the fixed frame. A bidirectional lead screw is rotatably disposed between any two sets of support plates, and a guide post is disposed between the other two sets. Both sides of the bidirectional lead screw are threadedly connected to connecting frames. The ends of the connecting frames are sleeved in the guide post. The connecting frames are provided with a placement seat for placing the Beckman beam unit.

[0011] Preferably, the unfolding assembly includes fixed seats at both ends of the Beckman beam unit within the placement seat, each fixed seat having a rotating shaft rotatably mounted therein, the rotating shafts being respectively mounted on two other Beckman beam units in the same group, each rotating shaft having a torsion spring mounted on its outer side, the two ends of the torsion springs being connected to the fixed seat and the rotating shaft respectively, and each rotating shaft having a positioning plate at its end, with rollers mounted on the positioning plate.

[0012] The support frame is equipped with a control component, which allows the Beckman beam units not placed in the placement seat to unfold from both sides.

[0013] Preferably, the control component includes multiple mounting cylinders respectively disposed on the support frame. The outer side of each mounting cylinder has an irregularly shaped slot, which is used in conjunction with rollers to unfold the Beckman beam units not disposed in the placement seat from both sides when the electric push rod drives the placement seat to descend.

[0014] Preferably, a straight slot is provided on the Beckman beam unit at the front end. The front self-testing mechanism includes an adjustment seat set in the straight slot. The top of the adjustment seat is made of flexible steel. A fastening bolt is provided in the adjustment seat. An adjusting nut is provided on the fastening bolt. The fastening bolt passes through the straight slot.

[0015] The adjustment seat is equipped with an adsorption detection component, which enables the detection device to automatically detach from the front end of the Beckman beam body when the detection head on the front end of the Beckman beam is positioned 4cm in front of the rear axle detection point.

[0016] Preferably, the adsorption detection assembly includes an electromagnet mounted on an adjustment seat, an adsorption block mounted on the electromagnet for use with it, a positioning frame mounted on the adsorption block, and a detection element mounted on the positioning frame. The detection element allows the detection head on the front-end Beckman beam unit to be positioned 4cm in front of the rear axle detection point. An adjustment slot is provided on the fixing frame.

[0017] Preferably, the detection element includes a fixing plate disposed on the positioning frame, a pressure detector disposed on the fixing plate, a spring disposed on the fixing plate, and a detection plate disposed at the end of the spring.

[0018] Preferably, the initial height of the irregular slot on the Beckman beam unit located at the rear end is different.

[0019] The present invention has at least the following beneficial effects:

[0020] 1. When workers are measuring the deflection of the roadbed and pavement, the unfolding mechanism can be used to unfold two sets of Beckman beam bodies and adjust them to the specified width for testing. With the movement of the testing vehicle, the front-end self-testing mechanism can be used to place the testing head at the front end of the Beckman beam body 4cm in front of the rear axle testing point, which simplifies the operation steps, reduces the time required for adjustment, and thus greatly improves the adjustment rate of the deflection testing device.

[0021] 2. After adjustment, the present invention automatically detaches the detection equipment (such as temperature detector, pressure detector, wheel pressure detector, etc.) from the detection head at the front end of the Beckman beam body, thereby minimizing the influence of external factors on the detection head and ensuring the reliability of the detection data. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the overall structure of the present invention;

[0023] Figure 2 For the present invention Figure 1 Schematic diagram of partial cross-section;

[0024] Figure 3 For the present invention Figure 2 Another structural diagram;

[0025] Figure 4 For the present invention Figure 3 Schematic diagram of partial cross-section;

[0026] Figure 5 This is a schematic diagram of the Beckman beam body and the front-end self-testing mechanism of the present invention;

[0027] Figure 6 For the present invention Figure 5 Schematic diagram of partial cross-section;

[0028] Figure 7 This is a schematic diagram of the front-end self-testing mechanism of the present invention;

[0029] Figure 8 For the present invention Figure 7 Another structural diagram;

[0030] Figure 9 This is a schematic diagram of the irregular groove structure in Embodiment 2 of the present invention;

[0031] Figure 10 This is a magnified structural diagram of region A of the present invention.

[0032] In the diagram: 1. Inspection vehicle; 2. Inspection component; 21. Fixing plate; 22. Pressure detector; 23. Spring; 24. Inspection plate; 3. Spreading and retracting mechanism; 31. Fixing frame; 4. Lifting assembly; 41. Electric actuator; 42. Lifting bending frame; 43. Y-shaped seat; 44. Sliding frame; 5. Displacement assembly; 51. Support plate; 52. Two-way lead screw; 53. Guide column; 54. Connecting frame; 55. Placement seat; 6. Spreading and retracting assembly; 61. 62. Fixed base; 63. Rotating shaft; 64. Torsion spring; 65. Positioning plate; 7. Roller; 8. Control component; 91. Mounting cylinder; 10. Irregular groove; 11. Front-end self-testing mechanism; 12. Straight groove; 13. Adjusting seat; 14. Fastening bolt; 15. Adjusting nut; 16. Adsorption detection assembly; 17. Electromagnet; 18. Adsorption block; 19. Positioning frame; 10. Support frame; 11. Beckman beam body; 12. Beckman beam unit. Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example

[0034] Please see Figure 1-8 The present invention provides a technical solution:

[0035] A roadbed and pavement deflection testing device includes a testing vehicle 1 and a support frame 10 mounted on the testing vehicle 1, and also includes...

[0036] The Beckman beam body 11 is provided in two sets, which are respectively set on both sides of the support frame 10.

[0037] The unfolding mechanism 3 is installed on the support frame 10. The two sets of Beckman beam bodies 11 can be unfolded and adjusted to a specified width for testing using the unfolding mechanism 3.

[0038] The deployment and retraction mechanism 3 includes a lifting assembly 4 mounted on the support frame 10. The lifting assembly 4 is connected to two Beckman beam bodies 11, and the lifting assembly 4 drives the two Beckman beam bodies 11 to move up and down synchronously.

[0039] The lifting assembly 4 includes an electric push rod 41 mounted on the support frame 10. The end of the electric push rod 41 is provided with a lifting bend frame 42. The fixed frame 31 is connected to the lifting bend frame 42. The support frame 10 is provided with a Y-shaped seat 43. The end of the Y-shaped seat 43 is provided with a sliding frame 44 that cooperates with the lifting bend frame 42. The electric push rod 41 drives the lifting bend frame 42 to move downward so that it slides in the sliding frame 44 without interfering with the bottom position of the support frame 10, ensuring the normal descent of the fixed frame 31. After the position is appropriate, the detection head and the support point (set on the Beckman beam unit 12 located in the placement seat 55, and the ratio of the front arm to the rear arm is still 2:1) land on the ground, and the placement seat 55 does not contact the Beckman beam body 11, ensuring the normal operation of the detection.

[0040] A fixed frame 31 is provided on the lifting assembly 4, and a displacement assembly 5 is provided on the fixed frame 31. The displacement assembly 5 is used to drive the two Beckman beam bodies 11 to move synchronously to both sides.

[0041] The displacement assembly 5 includes two sets of support plates 51 respectively set on both sides of the fixed frame 31. A bidirectional screw 52 is rotatably arranged between any two sets of support plates 51, and a guide post 53 is arranged between the other two sets. Both sides of the bidirectional screw 52 are threadedly connected to the connecting frame 54. The ends of the connecting frame 54 are sleeved in the guide post 53. The connecting frame 54 is provided with a placement seat 55 for placing the Beckman beam unit 12. As shown in the figure, this solution selects to set the bidirectional screw 52 on the side near the Y-shaped seat 43, and the end of the bidirectional screw 52 is provided with a knob for the operator to rotate. In the initial state, the Beckman beam body 11 is located on both sides of the support frame 10 to ensure that the Beckman beam body 11 can be unfolded normally. When the Beckman beam unit 12 is unfolded, the bidirectional screw 52 can be rotated by rotating the knob, thereby moving the connecting frame 54 and the placement seat 55 from both sides inward.

[0042] Each Beckman beam body 11 consists of three Beckman beam units 12. The Beckman beam unit 12 placed in the placement seat 55 does not rotate. The Beckman beam unit 12 at the front end unfolds from the outside, and the Beckman beam unit 12 at the rear end unfolds from the inside. This unfolding method is because the detection head is set below the Beckman beam unit 12 at the front end. If it rotates inward, it will interfere with the connecting frame 54 and the fixing frame 31. The detection part that cooperates with the dial indicator is set above the Beckman beam unit 12 at the rear end. When it rotates, there will be no interference. The two Beckman beam units 12 at the rear end rotate with the end away from the support frame 10 as the origin of rotation, thus ensuring that there is no interference. The support frame 10 is equipped with two unfolding and retracting components 6 that cooperate with each Beckman beam body 11. The unfolding and retracting components 6 are used to drive the three adjacent Beckman beam units 12 to unfold, thus ensuring normal detection and use.

[0043] The unfolding assembly 6 includes fixed seats 61 at both ends of the Beckman beam unit 12 disposed in the placement seat 55. Each fixed seat 61 is rotatably provided with a rotating shaft 62. The rotating shaft 62 is respectively disposed on the other two Beckman beam units 12 in the same assembly. Each rotating shaft 62 is provided with a torsion spring 63 on its outer side. The two ends of the torsion spring 63 are respectively connected to the fixed seat 61 and the rotating shaft 62. Each rotating shaft 62 is provided with a positioning plate 64 at its end. Rollers 65 are provided on the positioning plate 64.

[0044] The support frame 10 is equipped with a control component 7, which allows the Beckman beam unit 12, which is not located in the placement seat 55, to unfold from both sides.

[0045] The control component 7 includes multiple mounting cylinders 71 respectively mounted on the support frame 10. The mounting cylinder 71 has an irregularly shaped slot 72 on its outer side. The irregularly shaped slot 72 is used in conjunction with the roller 65 to unfold the Beckman beam unit 12 that is not set in the placement seat 55 from both sides when the electric push rod 41 drives the placement seat 55 to descend. When unfolded, the two Beckman beam units 12 located at the rear end will rotate outward from the inside of the support frame 10. Therefore, in this scheme, the distance between the two support frames 10 must meet the requirement that the two Beckman beam units 12 unfold synchronously.

[0046] Two front-end self-testing mechanisms 8 are provided. The two front-end self-testing mechanisms 8 are respectively located on the Beckman beam body 11 near the rear axle of the calibration vehicle. In conjunction with the movement of the testing vehicle 1, the front-end self-testing mechanism 8 can be used to place the detection head at the front end of the Beckman beam body 11 4cm in front of the detection point on the rear axle, and automatically detach the detection device from the detection head at the front end of the Beckman beam body 11. The preferred detection position in this solution is 4cm, but it is not limited and can be adjusted by selecting 3-5cm.

[0047] A straight slot 81 is provided on the Beckman beam unit 12 located at the front end. The front self-testing mechanism 8 includes an adjusting seat 82 set in the straight slot 81. The top of the adjusting seat 82 is made of flexible steel. A fastening bolt 83 is set in the adjusting seat 82. An adjusting nut 84 is set on the fastening bolt 83. The fastening bolt 83 passes through the straight slot 81. During adjustment, according to the rear axle radius of the standard vehicle, plus 3-5cm of the self-selected test position, the position of the fastening bolt 83 is adjusted in the straight slot 81 to adjust the position of the test piece 2, ensuring normal bending test. Generally, multiple measurement points on a certain highway only need to be adjusted once, because the standard vehicle does not change and the test position does not change. When adjusting the position, a tool is used to pass through the adjusting slot on the positioning frame 93. Moreover, this solution adopts the method of double adjusting nuts 84 to increase the axial preload of the adjusting seat 82, thereby ensuring that it will not loosen during the test.

[0048] The adjustment seat 82 is equipped with an adsorption detection component 9. The adsorption detection component 9 is used to automatically detach the detection device from the front end of the Beckman beam body 11 when the detection head on the front end of the Beckman beam body 12 is placed 4cm in front of the rear axle detection point.

[0049] The adsorption detection assembly 9 includes an electromagnet 91 set on the adjustment seat 82, an adsorption block 92 used in conjunction with the electromagnet 91, a positioning frame 93 set on the adsorption block 92, and a detection element 2 set on the positioning frame 93. The detection element 2 can be used to position the detection head on the front Benkelman beam unit 12 at a position 4cm in front of the rear axle detection point. An adjustment slot is opened on the fixed frame 31.

[0050] The detection component 2 includes a fixed plate 21 set on the positioning frame 93. A pressure detector 22 is set on the fixed plate 21. A spring 23 is also set on the fixed plate 21. A detection plate 24 is set at the end of the spring 23. When the detection vehicle 1 is moving, when the detection plate 24 contacts the rear axle, the spring 23 is compressed. The pressure detector 22 will detect the pressure and then control the detection vehicle 1 to stop. At the same time, the electromagnet 91 is turned off, so that the adsorption block 92 and the positioning frame 93 and the detection equipment on them are separated, reducing their impact on the Beckman beam body 11. The fixed plate 21 is equipped with detection equipment such as a temperature detector, a pressure detector 22, and a wheel pressure detector, which are used to measure the parameters required before detection. These are not shown in the figure. The dial indicator on the Beckman beam body 12 at the rear end is installed and adjusted by the staff.

[0051] In use, the staff drives the electric actuator 41 to work via remote control or other means, unfolding the Beckman beam body 11 without touching the ground, turning the knob to adjust the spacing, and then driving the testing carriage 1 to work. After the pressure sensor detects the signal, the testing carriage 1 stops, and the fixing frame 31 disengages. The standard carriage moves according to regulations, and after the data is measured, the next position measurement is performed until the measurement is completed and then stops. First, the knob is turned to adjust the spacing, and then the electric actuator 41 drives the Beckman beam to retract for the next use. Example

[0052] Please see Figure 1-10 The present invention provides a technical solution:

[0053] The difference between this solution and embodiment 1 is that the initial height of the irregular slot 72 on the Beckman beam unit 12 located at the rear end is different, so that the two Beckman beam units 12 located at the rear end rotate asynchronously. During the unfolding process, they form a misalignment, which can reduce the distance required for the Beckman beam unit 12 located at the rear end to rotate, and reduce the width of the support frame 10 and the size of the deflection detection device.

[0054] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0055] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A roadbed and pavement deflection testing device, comprising a testing vehicle (1) and a support frame (10) mounted on the testing vehicle (1), characterized in that: It also includes a Beckman beam body (11), which is provided in two sets, respectively located on both sides of the support frame (10); The unfolding and retracting mechanism (3) is set on the support frame (10). The unfolding and retracting mechanism (3) can unfold the two sets of Beckman beam bodies (11) and adjust them to a specified width for testing. Two front-end self-testing mechanisms (8) are provided. The two front-end self-testing mechanisms (8) are respectively located on the Beckman beam body (11) near the rear axle of the calibration vehicle. In conjunction with the movement of the testing vehicle (1), the front-end self-testing mechanism (8) can place the detection head at the front end of the Beckman beam body (11) 4cm in front of the rear axle detection point and automatically detach the testing equipment from the detection head at the front end of the Beckman beam body (11). The unfolding and retracting mechanism (3) includes a lifting assembly (4) installed on the support frame (10). The lifting assembly (4) is connected to the two Beckman beam bodies (11). The lifting assembly (4) drives the two Beckman beam bodies (11) to move up and down synchronously. The lifting assembly (4) is provided with a fixed frame (31), and the fixed frame (31) is provided with a displacement assembly (5). The displacement assembly (5) is used to drive the two Beckman beam bodies (11) to move synchronously to both sides. Each group of the Beckman beam body (11) is composed of three Beckman beam units (12). The support frame (10) is provided with two unfolding and retracting components (6) that are used in conjunction with each group of the Beckman beam body (11). The unfolding and retracting components (6) are used to drive the three adjacent Beckman beam units (12) to unfold, thereby ensuring normal testing and use. The lifting assembly (4) includes an electric push rod (41) mounted on a support frame (10), with a lifting bend frame (42) at the end of the electric push rod (41), and the fixed frame (31) connected to the lifting bend frame (42). A Y-shaped seat (43) is mounted on the support frame (10), and a sliding frame (44) is mounted at the end of the Y-shaped seat (43) to cooperate with the lifting bend frame (42). The displacement assembly (5) includes two sets of support plates (51) respectively arranged on both sides of the fixed frame (31). A bidirectional screw rod (52) is rotatably arranged between any two sets of support plates (51), and a guide post (53) is arranged between the other two sets. A connecting frame (54) is threaded to both sides of the bidirectional screw rod (52). The ends of the connecting frame (54) are all sleeved in the guide post (53). A placement seat (55) for placing the Beckman beam unit (12) is provided on the connecting frame (54). The unfolding assembly (6) includes fixed seats (61) at both ends of the Beckman beam unit (12) in the placement seat (55). Each fixed seat (61) is rotatably provided with a rotating shaft (62). The rotating shaft (62) is respectively provided on the other two Beckman beam units (12) in the same group. Each rotating shaft (62) is provided with a torsion spring (63) on its outer side. The two ends of the torsion spring (63) are respectively connected to the fixed seat (61) and the rotating shaft (62). Each rotating shaft (62) is provided with a positioning plate (64) at its end. The positioning plate (64) is provided with a roller (65). The support frame (10) is provided with a control component (7); The control component (7) includes multiple mounting cylinders (71) respectively set on the support frame (10). The mounting cylinder (71) has an irregular groove (72) on its outer side. The irregular groove (72) is used in conjunction with the roller (65) to unfold the Beckman beam unit (12) not set in the placement seat (55) from both sides when the electric push rod (41) drives the placement seat (55) to descend.

2. The roadbed and pavement deflection detection device according to claim 1, characterized in that: A straight slot (81) is provided on the Beckman beam unit (12) located at the front end. The front self-testing mechanism (8) includes an adjustment seat (82) set in the straight slot (81). The top of the adjustment seat (82) is made of flexible steel. A fastening bolt (83) is provided in the adjustment seat (82). An adjusting nut (84) is provided on the fastening bolt (83). The fastening bolt (83) passes through the straight slot (81). The adjustment seat (82) is provided with an adsorption detection component (9). The adsorption detection component (9) is used to automatically detach the detection device from the front end of the Beckman beam body (11) when the detection head on the front end of the Beckman beam body (12) is placed 4cm in front of the rear axle detection point.

3. The roadbed and pavement deflection detection device according to claim 2, characterized in that: The adsorption detection component (9) includes an electromagnet (91) set on an adjustment seat (82), an adsorption block (92) used in conjunction with the electromagnet (91), a positioning frame (93) set on the adsorption block (92), a detection element (2) set on the positioning frame (93), and the detection element (2) can be used to position the detection head on the front Benkelman beam unit (12) 4cm in front of the rear axle detection point. An adjustment slot is opened on the fixing frame (31).

4. The roadbed and pavement deflection detection device according to claim 3, characterized in that: The detection component (2) includes a fixing plate (21) set on the positioning frame (93), a pressure detector (22) is set on the fixing plate (21), a spring (23) is also set on the fixing plate (21), and a detection plate (24) is set at the end of the spring (23).

5. The roadbed and pavement deflection detection device according to claim 4, characterized in that: The initial height of the irregular slot (72) on the Beckman beam unit (12) located at the rear end is different.

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