Method for installing a group of bottle containers for a station and auxiliary installation device therefor

Abstract

The application discloses a kind of installation method and auxiliary installation device of station bottle type container group, it is related to the technical field of installation equipment of station bottle type container group, first one end of container is fixed on first vertical plate, the other end is supported using support mechanism, and the installation of second vertical plate is realized using the cooperation of positioning shaft, long shaft sleeve, short shaft sleeve and bottle mouth flange step structure, not only need not to hoist container all the way, can significantly reduce installation difficulty and security risk, also can increase the installation quantity of station bottle type container group, and can guarantee the installation precision of container bottle mouth flange and second vertical plate, installation mode is also more simple and easy to operate, can significantly improve the installation efficiency when installing multiple containers.

Description

Technical Field

[0001] This invention relates to the field of installation equipment technology for station-use bottle container groups, and in particular to an installation method for station-use bottle container groups and its auxiliary installation device. Background Technology

[0002] As hydrogen refueling stations serve as refueling points for hydrogen, the increasing popularity and quantity of hydrogen fuel cell vehicles has become a pressing issue. The construction of hydrogen refueling stations inevitably requires hydrogen storage containers, which will greatly promote the development of such containers. Currently, the number of bottle-type containers loaded in hydrogen storage container groups is diversified and varied. When the number of bottle-type containers is large, installation becomes a challenge.

[0003] Traditional station-mounted bottle container assemblies typically contain only one or two containers. During installation, one end of the container is first fixed to an upright plate, then a lifting device is used to hoist the container horizontally. Finally, the upright plate is pushed into place at the other end of the container, completing the installation. This installation method has certain limitations. First, when installing multiple containers, it requires careful consideration of the different capacities of the containers.

[0004] First, it is difficult to hoist the container simultaneously; second, when installing the second upright plate, the container needs to be hoisted throughout the entire process, with no support at the bottom, making it difficult to align the container end with the positioning holes on the upright plate, and the installation of the upright plate is very difficult; third, the container is prone to shaking during hoisting, posing a very significant safety hazard. Summary of the Invention

[0005] The purpose of this invention is to provide an installation method and auxiliary installation device for station-use bottle container groups, so as to solve the problems existing in the prior art, increase the number of station-use bottle container groups installed, significantly reduce installation difficulty and safety hazards, and improve installation efficiency.

[0006] To achieve the above objectives, the present invention provides the following solution: The present invention provides an installation method for a station-use bottle container assembly, comprising the following steps:

[0007] 1) Fix the first upright plate to the first pier base;

[0008] 2) Hoist multiple containers in sequence, fix one end of the container to the first upright plate, and place the other end on the support mechanism;

[0009] 3) The bottle mouths of the containers are all threaded with positioning shafts. Two positioning shafts are fitted with two bushings of different lengths. The outer diameter of the bushings is the same as the size of the stepped structure of the bottle mouth flange. The ends of the bushings abut against the stepped structure of the bottle mouth flange. The second vertical plate is hoisted and moved toward the bottle mouth. After one positioning hole in the second vertical plate is fitted into the long bushing, the positioning shaft containing the short bushing is hoisted so that the short bushing enters the other positioning hole of the second vertical plate. The bottle mouth flanges on the two containers with bushings are pre-connected to the second vertical plate with bolts so that the second vertical plate is located at the front end of the stepped structure of the bottle mouth flange.

[0010] 4) Fit the bushings onto the positioning shafts of the remaining containers, and hoist the positioning shafts one by one, pushing the bushings from the positioning holes on the second vertical plate to the stepped structure of the bottle mouth flange.

[0011] 5) Move the second vertical plate along the bushing so that the stepped structure on the bottle neck flange of all containers enters the positioning hole of the second vertical plate, fix the second vertical plate to the bottle neck flange, and remove the positioning shaft and bushing.

[0012] 6) Lift up the second upright plate, place the second pier under the second upright plate, and place the second upright plate on the second pier.

[0013] 7) Remove the support frame, crossbeams, and positioning tie rods to complete the installation of the station-use bottle container assembly.

[0014] Preferably, in step 2), the container is installed layer by layer on the first upright plate from bottom to top, and in step 4), the bushing is installed layer by layer on the second upright plate.

[0015] Preferably, in step 4), the length of the bushing fitted on the positioning shaft of the remaining container is less than that of the short bushing in step 3).

[0016] Preferably, in step 6), the second upright plate is lifted using jacks, and in steps 3) and 4), a crane is used for hoisting.

[0017] The present invention also provides an auxiliary installation device for a station-use bottle container assembly based on the above-described installation method for station-use bottle container assemblies, comprising a first pier, a second pier, and a support mechanism. The first pier is used to fix a first upright plate, and the second pier is used to support a second upright plate. The first upright plate and the second upright plate are respectively used to connect the bottom flange and the top flange of the container. The auxiliary installation device further includes a positioning shaft threadedly connected to the top of the container and a bushing that can be fitted onto the positioning shaft. The outer diameter of the bushing is the same as the diameter of the stepped structure of the top flange.

[0018] Preferably, the support mechanism includes a support frame spaced apart and a crossbeam fixed on the support frame. The crossbeam is parallel to and directly opposite the surface of the first upright plate, and the crossbeam is provided with a support portion for supporting the container.

[0019] Preferably, the support plate is a plurality of triangular pads spaced apart on the crossbeam.

[0020] Preferably, the auxiliary installation device further includes several parallel positioning rods, one end of which is vertically fixed to the first pier and the other end is fixed to the support frame.

[0021] Preferably, the end of the positioning shaft is provided with an external thread, and a recessed step is provided at the end of the external thread, the length of the external thread being less than the length of the thread at the mouth of the container.

[0022] Preferably, the positioning shaft is further provided with a through hole that radially penetrates its middle portion, and the through hole is spaced apart from the recessed step.

[0023] Compared with the prior art, the present invention has the following technical effects:

[0024] 1. The present invention provides a support mechanism formed by a support frame and a crossbeam, which can support the container layer by layer. When installing the second upright plate, it is not necessary to hoist the container throughout the process, which can significantly reduce the installation difficulty and safety hazards, and can also increase the number of station-use bottle container groups installed.

[0025] 2. In this invention, by connecting a positioning shaft to the container bottle mouth and using long bushings, short bushings and universal bushings to position and install the second vertical plate, not only can the installation accuracy of the container bottle mouth flange and the second vertical plate be guaranteed, but the installation method is also simpler and easier to implement. It can significantly improve the installation efficiency when installing multiple containers. In addition, bushings of different lengths can all protect the sealing end face of the bottle mouth after being fitted, preventing the bottle mouth end face from being bumped during the installation process.

[0026] 3. In this invention, the length of the external thread on the positioning shaft is less than the length of the bottle mouth thread, and a recessed step is provided on one side of the external thread, which can prevent damage to the bottle mouth sealing step surface and the bottle mouth end face due to thread clearance during the adjustment and guidance process of installing the second vertical plate.

[0027] 4. In this invention, all components are connected by bolts, facilitating disassembly and reuse. Furthermore, the crossbeams can be increased to accommodate different quantities and layers of bottle containers, allowing for easy adjustment and meeting installation requirements for various numbers and layers of bottle containers. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of the installation of a traditional station-use bottle container assembly.

[0030] Figure 2 This is a schematic diagram of the auxiliary installation device in this invention;

[0031] Figure 3 This is a schematic diagram of the positioning shaft.

[0032] Figure 4 This is a schematic diagram of the long bushing structure;

[0033] Figure 5 A schematic diagram of the structure in which the first upright plate is installed on the first pier.

[0034] Figure 6 This is a structural diagram showing the installation of a single-layer container.

[0035] Figure 7 for Figure 6 A magnified view of a portion of the image;

[0036] Figure 8 A schematic diagram of the structure when installing multi-layer containers on the first vertical plate;

[0037] Figure 9 This is a schematic diagram of the mating structure of the second vertical plate, the long bushing, and the short bushing.

[0038] Figure 10 This is a schematic diagram showing the structure after all the universal bushings have been installed.

[0039] Figure 11 A structural diagram showing the use of jacks to lift the second upright plate and the upcoming installation of the second pier.

[0040] Among them, 1. First upright plate; 2. First support; 3. Container; 4. Second upright plate; 5. Positioning shaft; 6. Long bushing; 7. Short bushing; 8. General bushing; 9. Bottle neck flange; 10. Step structure; 11. Sealing end face; 12. Second support; 13. Jack; 14. Support frame; 15. Crossbeam; 16. Triangular pad; 17. Positioning tie rod; 18. External thread; 19. Recessed step; 20. Through hole. Detailed Implementation

[0041] 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.

[0042] The purpose of this invention is to provide an installation method and auxiliary installation device for station-use bottle container groups, so as to solve the problems existing in the prior art, increase the number of station-use bottle container groups installed, significantly reduce installation difficulty and safety hazards, and improve installation efficiency.

[0043] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0044] Example 1:

[0045] like Figures 2 to 11 As shown, this embodiment provides an installation method for a station-use bottle container group, including the following steps:

[0046] 1) Fix the first upright plate 1 on the first pier 2; wherein the first upright plate 1 is a common component of the station bottle container 3 group, and several positioning holes are evenly arranged on the first upright plate 1, and several through holes are evenly arranged around each positioning hole. The through holes are used to insert bolts to bolt the first upright plate 1 to the bottom flange of the container 3.

[0047] 2) Use the lifting device to lift multiple containers 3 in sequence, fix one end of the container 3 on the first upright plate 1, and place the other end on the support mechanism. Use the first upright plate 1 and the support components to effectively support the container 3. When installing the second upright plate 4 later, it is not necessary to use the lifting mechanism to lift the container 3 throughout the entire process.

[0048] 3) A positioning shaft 5 is threaded onto the bottle mouth of container 3. It should be noted that the positioning shaft 5 can also be fixed at the bottle mouth of container 3 before container 3 is fixed onto the first upright plate 1. Two bushings of different lengths are fitted onto the two positioning shafts 5. The outer diameter of the bushings is the same as the size of the stepped structure 10 of the bottle mouth flange 9. The ends of the bushings abut against the end face of the stepped structure 10 of the bottle mouth flange 9. Then, the second upright plate 4 is vertically lifted and moved toward the bottle mouth of container 3 using a hoisting mechanism, in preparation for installing the second upright plate 4 at the bottle mouth of container 3. The structure of the second upright plate 4 is similar to that of the first upright plate 1. The positioning hole on the second upright plate 4 is slightly larger than the outer diameter of the stepped structure 10 on the bottle mouth flange 9, ensuring that the stepped structure 10 can enter the positioning hole without causing wobbling.

[0049] During installation, first insert the positioning hole on the second upright plate 4 into the long bushing 6, then use another hoisting mechanism to hoist the positioning shaft 5 where the short bushing 7 is located, so that the short bushing 7 enters the other positioning hole of the second upright plate 4. Remove the two hoisting mechanisms, and use bolts to pre-connect the bottle mouth flanges 9 on the two containers 3 with bushings to the second upright plate 4, so that the second upright plate 4 is located at the front end of the stepped structure 10 of the bottle mouth flange 9, thus completing the pre-installation of the second upright plate 4.

[0050] 4) Fit a bushing onto the positioning shaft 5 of the remaining container 3 (hereinafter referred to as the bushing fitted onto the positioning shaft 5 of the remaining container 3 as the universal bushing 8). Since there may be a positional deviation between the bottle mouth flange 9 and the positioning hole on the second vertical plate 4 after the container 3 is placed on the support mechanism, in this step, the positioning shaft 5 is lifted one by one using the lifting device, and the universal bushing 8 is pushed one by one from the positioning hole on the second vertical plate 4 to the stepped structure 10 of the bottle mouth flange 9. After all the universal bushings 8 are installed, the second vertical plate 4 is located on the long bushing 6, the short bushing 7, the universal bushing 8, and the front end of the stepped structure 10 of the bottle mouth flange 9.

[0051] 5) Then move the second vertical plate 4 along the bushing so that the stepped structure 10 on the bottle mouth flange 9 of all containers 3 enters the positioning hole of the second vertical plate 4, fix the second vertical plate 4 to the bottle mouth flange 9, remove the positioning shaft 5, long bushing 6, short bushing 7 and general bushing 8 to complete the installation of the second vertical plate 4.

[0052] 6) After the second upright plate 4 is installed, its bottom end is flush with the bottom end of the first upright plate 1, and the bottom end of the second upright plate 4 is suspended. At this time, use a lifting machine to lift the second upright plate 4 so that the container 3 is separated from the support mechanism. Place the second pier 12 under the second upright plate 4 and place the second upright plate 4 on the second pier 12. In this step, the second pier 12 is at the same height as the first pier 2. The height to which the second upright plate 4 is lifted does not need to be too high, just enough to ensure that the second pier 12 is placed at the bottom of the second upright plate 4.

[0053] 7) Remove the support frame 14, crossbeam 15, and positioning tie rod 17 to complete the installation of 3 sets of station-use bottle containers.

[0054] Furthermore, in step 2), the container 3 is installed layer by layer on the first upright plate 1 from bottom to top, and the support mechanism can also support the container 3 layer by layer from bottom to top; in step 4), the universal bushing 8 is installed layer by layer on the second upright plate 4, preferably from top to bottom.

[0055] Furthermore, in step 4), the length of the universal bushing 8 fitted onto the positioning shaft 5 of the remaining container 3 is shorter than that of the short bushing 7 in step 3). Typically, the thickness of the first upright plate 1 and the second upright plate 4 is approximately 20mm. The long bushing 6 is 260mm in size, the short bushing 7 is 230mm in size, and the universal bushing 8 is 50mm in size. The long bushing 6 and the short bushing 7 primarily facilitate the fitting of the second upright plate 4 into the bushings, while the smaller size of the universal bushing 8 is mainly for ease of installation. All three types of bushings can protect the sealing end face 11 and the sealing step surface of the bottle mouth from impacts. Those skilled in the art will understand that the sealing step surface is located on the inner wall of the bottle mouth, outside the bottle mouth thread.

[0056] Furthermore, in step 6), the second upright plate 4 is lifted using jack 13, and in steps 3) and 4), a crane is used for hoisting.

[0057] Example 2:

[0058] like Figures 2 to 11 As shown, this embodiment provides an auxiliary installation device for the station-use bottle container 3-group based on the installation method of the station-use bottle container 3-group described in Embodiment 1. The device includes a first support 2, a second support 12, and a support mechanism. The first support 2 is used to fix the first upright plate 1, and the second support 12 is used to support the second upright plate 4. The first upright plate 1 and the second upright plate 4 are respectively used to connect the bottle bottom flange and the bottle mouth flange 9 of the container 3. The auxiliary installation device also includes a positioning shaft 5 that is threadedly connected to the bottle mouth of the container 3 and a bushing that can be sleeved on the positioning shaft 5. The outer diameter of the bushing is the same as the diameter of the stepped structure 10 of the bottle mouth flange 9. Furthermore, one end of the bushing is provided with a concave structure. When the bushing abuts against the stepped structure 10, the concave structure can avoid and protect the sealing end face 11 of the bottle mouth.

[0059] Furthermore, in this embodiment, the support mechanism includes spaced-apart support frames 14 and crossbeams 15 fixed on the support frames 14. The crossbeams 15 are parallel to and directly opposite the surface of the first upright plate 1, and the crossbeams 15 are provided with support portions for supporting the container 3. As the container 3 is installed layer by layer from bottom to top, the crossbeams 15 are also installed layer by layer.

[0060] Furthermore, the support plate consists of several triangular pads 16 spaced apart on the crossbeam 15.

[0061] Furthermore, the auxiliary installation device also includes several parallel positioning rods 17, preferably two rods. One end of the positioning rod 17 is vertically fixed to the first pier 2, and the other end is fixed to the support frame 14. At the same time, after the first upright plate 1 is fixed to the first pier 2, its surface is parallel to the side of the first pier 2. After the two positioning rods 17 are connected, the line connecting the two positioning rods 17, the first pier 2, and the other end of the positioning rod 17 forms a rectangle.

[0062] Furthermore, the end of the positioning shaft 5 is provided with an external thread 18, and a recessed step 19 is provided at the end of the external thread 18. The outer diameter of the external thread 18 is 0.2 mm smaller than the standard value to avoid seizing. In addition, the length of the external thread 18 is less than the length of the bottle mouth thread of the container 3, and the sum of the lengths of the external thread 18 and the recessed step 19 is not less than the sum of the depth of the bottle mouth thread and the bottle mouth sealing step surface of the container 3. This ensures that after the positioning shaft 5 is connected to the bottle mouth, the recessed step 19 is radially aligned with the sealing end face 11, preventing damage to the bottle mouth sealing step surface and the sealing end face 11 due to thread clearance during the adjustment and guidance process of installing the second vertical plate 4.

[0063] Furthermore, the positioning shaft 5 is also provided with a through hole 20 that radially penetrates its middle. The through hole 20 is spaced apart from the recessed step 19. The diameter of the through hole 20 is 20mm. It is mainly used to facilitate the insertion of a plug rod into the through hole 20 for screwing when installing and removing the spindle.

[0064] The manufacturing process of each component in the auxiliary installation device:

[0065] (1) First pier 2 and second pier 12

[0066] Eight rectangular steel pipes of Q345D material with dimensions of 250×150×10 were machined at designated positions on the side. 31 through holes are used to connect the positioning tie rod 17; two holes are opened on it to insert M24 nuts respectively, the nuts are welded and fixed, and the excess parts are ground to make the upper surface of the first pier 2 and the second pier 12 smooth and flat.

[0067] (2) Support frame 14

[0068] Support frame 14 includes uprights and crossbars, all made of Q345D material, using rectangular steel pipes with dimensions of 200×100×10 mm. The pipes are cut to the specified lengths according to the drawings and welded together as per the drawing positions. Four crossbars are machined at designated locations on each crossbar. 25 through holes are used to connect the crossbeam 15. Two holes are machined at designated positions on the lower side of the column. The through hole 31 is used to connect the positioning tie rod 17. Then, a 600×200×10 base plate is welded under the column. Finally, triangular pieces are welded to the lower side of the crossbar and the lower side of the column for reinforcement and to make it stably supported.

[0069] (3) Crossbeam 15

[0070] Using Q345D material, rectangular steel pipes with dimensions of 100×50×5 mm were cut to the specified length according to the drawings, and four were machined at the designated locations. 25 through holes are used to connect the support frame 14. Then, two rectangular steel pipes are spot welded together to form a crossbeam 15. Finally, triangular pads 16 made of nylon PA6 are processed according to the dimensions in the drawing and fixed to the crossbeam 15 with countersunk screws to form positioning grooves. The position must be accurate.

[0071] (4) Positioning rod 17

[0072] Select Q345D material, rectangular steel pipe with specifications of 100×50×5, cut to the specified length according to the drawing dimensions, and process two at the designated position at one end. 31 through holes are used to connect the support frame 14; the other end is welded to a piece of material with dimensions of 200×150×10, and four of them are machined at designated positions. The bottom plate with 31 through holes is fixed, and this end is used to connect to the pier base.

[0073] (5) Positioning axis 5

[0074] The machined parts are made of 45# steel and machined by turning. The thread size is 0.2mm smaller than the standard requirement. After the thread is machined, a recessed step 19 is machined on its rear side. The length of the recessed step 19 is approximately 5mm longer than the depth of the bottle neck sealing step surface. This prevents damage to the bottle neck sealing step surface and sealing end face 11 due to thread clearance during hoisting. Finally, a [missing information - likely a design element] is machined in the middle of the spindle. The 20 through holes facilitate the screwing of the insertion rod during installation and disassembly.

[0075] (6) Bushing

[0076] Made of Nylon 1010, the bushing consists of three specifications: long bushing 6, short bushing 7, and universal bushing 8, with dimensions of 260mm, 230mm, and 50mm respectively, ensuring precise positioning and adjustment during assembly.

[0077] Any adaptive changes made according to actual needs are within the scope of protection of this invention.

[0078] Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A method for installing a station-use bottle-type container assembly, characterized in that, Includes the following steps: 1) Fix the first upright plate to the first pier base; 2) Hoist multiple containers in sequence, fix one end of the container to the first upright plate, and place the other end on the support mechanism; 3) The bottle mouth of the container is threaded with a positioning shaft. Two positioning shafts are fitted with two bushings of different lengths. The longer bushing is called the long bushing and the shorter bushing is called the short bushing. The outer diameter of the long bushing and the short bushing is the same as the size of the stepped structure of the bottle mouth flange. The ends of the long bushing and the short bushing can abut against the stepped structure of the bottle mouth flange. The second vertical plate is hoisted and moved towards the bottle mouth. After one positioning hole in the second vertical plate is fitted into the long bushing, the positioning shaft where the short bushing is located is hoisted so that the short bushing enters the other positioning hole of the second vertical plate. The bottle mouth flanges on the two containers with long bushings or short bushings are pre-connected to the second vertical plate with bolts so that the second vertical plate is located at the front end of the stepped structure of the bottle mouth flange. 4) Fit a universal bushing onto the positioning shaft of the remaining containers, and hoist the positioning shafts one by one, pushing the universal bushing from the positioning hole on the second vertical plate to the stepped structure of the bottle mouth flange. 5) Move the second vertical plate along the long bushing so that the stepped structure on the bottle neck flange of all containers enters the positioning hole of the second vertical plate, fix the second vertical plate to the bottle neck flange, and remove the positioning shaft and all bushings. 6) Lift up the second upright plate, place the second pier under the second upright plate, and place the second upright plate on the second pier. 7) Remove the support frame, crossbeams, and positioning tie rods to complete the installation of the station-use bottle container assembly.

2. The installation method of the station-use bottle container group according to claim 1, characterized in that, In step 2), the container is installed layer by layer on the first upright plate from bottom to top. In step 4), the universal bushing is installed layer by layer on the second upright plate.

3. The installation method of the station-use bottle container group according to claim 2, characterized in that, In step 4), the length of the bushing fitted on the positioning shaft of the remaining container is less than that of the short bushing in step 3).

4. The installation method of the station-use bottle container group according to claim 3, characterized in that, In step 6), the second upright plate is lifted using jacks. In steps 3) and 4), a crane is used for hoisting.

5. An auxiliary installation device for a station-use bottle-type container group, based on the installation method of the station-use bottle-type container group as described in any one of claims 1-4, characterized in that, The device includes a first support, a second support, and a support mechanism. The first support is used to fix the first upright plate, and the second support is used to support the second upright plate. The first upright plate and the second upright plate are used to connect the bottom flange and the top flange of the container, respectively. The auxiliary installation device also includes a positioning shaft that is threaded to the top of the container and a bushing that can be fitted onto the positioning shaft. The outer diameter of the bushing is the same as the diameter of the stepped structure of the top flange.

6. The auxiliary installation device for the station-use bottle container group according to claim 5, characterized in that, The support mechanism includes a support frame spaced apart and a crossbeam fixed on the support frame. The crossbeam is parallel to and directly opposite the surface of the first upright plate, and the crossbeam is provided with a support portion for supporting the container.

7. The auxiliary installation device for the station-use bottle container group according to claim 6, characterized in that, The supporting part consists of several triangular pads spaced apart on the crossbeam.

8. The auxiliary installation device for the station-use bottle container group according to claim 7, characterized in that, The auxiliary installation device also includes several parallel positioning rods, one end of which is vertically fixed to the first pier and the other end is fixed to the support frame.

9. The auxiliary installation device for station-use bottle container groups according to any one of claims 5-8, characterized in that, The end of the positioning shaft is provided with an external thread, and a recessed step is provided at the end of the external thread. The length of the external thread is less than the length of the thread at the mouth of the container.

10. The auxiliary installation device for the station-use bottle container group according to claim 9, characterized in that, The positioning shaft is also provided with a through hole that radially penetrates its middle portion, and the through hole is spaced apart from the recessed step.

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