An impact-resistant tunnel support anchor

Abstract

The invention discloses an impact-resistant tunnel support anchor rod, which at least includes a hollow anchor rod, including a hollow rod body, an external threaded section formed on the rear end of the outer wall of the hollow rod body; a telescopic assembly, which is provided with two, and the front end of the hollow rod body is provided with two telescopic assemblies in sequence from front to back, and the telescopic assembly includes a rotatable flat thread component, and a radial telescopic clamp arm driven by the flat thread component to radially extend and retract, and the radial telescopic clamp arm is provided with at least three and evenly spaced in the circumferential direction; abutment side plates, which are evenly distributed on the outer wall of the hollow rod body in the circumferential direction, and the front and rear ends of the abutment side plates are respectively connected to the radial telescopic clamp arms of the two telescopic assemblies; an adjustment assembly, including a central main rod, a sliding front rod, a forward push assembly, a swing assembly, and a rear drive. In the support anchor rod described in this case, the abutment side plates can be fixed to the inner wall of the hole in a longer range, and have a better impact resistance effect.

Description

Technical Field

[0001] The present application relates to the field of mechanical technology, and in particular to an impact-resistant tunnel support anchor rod. Background Art

[0002] With the rapid economic development and the strong demand for the development of underground space, underground construction, national defense projects, mining projects, and deep nuclear waste burial are in the ascendant. Underground engineering development continues to move deeper and has even reached a depth of less than 4,000 meters underground. In the face of the strong demand for the development of underground space, it is particularly important to ensure the safety, stability and sustainability of underground space utilization. In tunnel construction, anchor rods are commonly used components for tunnel support, and there are many types. Due to the dynamic interaction between the anchor rod support system and the surrounding rock, especially the seismic interaction between the anchor rod body and the borehole, the anchor rod support system often collapses under strong earthquake disasters, causing serious losses.

[0003] The anchor rods in the prior art include mechanical anchor rods, grouting anchor rods, etc. The mechanical anchor rods are fixed by the expansion end at the front end thereof, and the grouting anchor rods are fixed by injecting slurry and consolidating. However, the front end fixing part of the mechanical anchor rod in the prior art is a structure similar to an expansion screw, and the contact surface between the expansion structure and the hole is very small. Once the rock formation around the hole is deformed, the consolidation force is lost. Summary of the invention

[0004] An impact-resistant tunnel support anchor rod, comprising at least

[0005] The hollow anchor rod comprises a hollow rod body and an external threaded section formed on the rear end of the outer wall of the hollow rod body;

[0006] There are two telescopic assemblies, and the front end of the hollow rod body is sequentially provided with two telescopic assemblies from front to back, and the telescopic assembly includes a rotatable plane thread component and a radial telescopic clamping arm driven by the plane thread component to radially extend and retract, and the radial telescopic clamping arm is provided with at least three and is evenly spaced in the circumferential direction;

[0007] Further:

[0008] The plane thread assembly includes a limiting convex ring formed on the inner wall of the hollow rod, a rotating ring rotatably arranged on the limiting convex ring, a limiting ring groove formed on the outer wall of the rotating ring for cooperating with the limiting convex ring, driven locking teeth uniformly distributed on the rear end surface of the rotating ring in the circumferential direction, and a plane thread formed on the front end surface of the rotating ring;

[0009] The radial telescopic clamping arm comprises side through holes uniformly distributed on the hollow rod body in the circumferential direction, a guide base arranged on the inner wall of the hollow rod body and corresponding to the side through holes one by one, a guide insert formed at the rear of the guide base, a radial insertion rod inserted in the side through hole, a guide slot formed on the front end surface of the radial insertion rod and matched with the guide insert, and a driven plug meshed with a plane thread is also provided at the rear end of the radial insertion rod;

[0010] The front and rear ends of the abutting side plates are respectively connected to the radial insertion rods of the two telescopic assemblies.

[0011] As a further implementation:

[0012] The rear end surface of the rotating ring of the telescopic assembly located at the rear is also provided with a guide cone groove which is narrow at the front and wide at the rear.

[0013] Abutment side plates are evenly distributed on the outer wall of the hollow rod in the circumferential direction, and the front and rear ends of the abutment side plates are respectively connected to the radial telescopic clamping arms of the two telescopic assemblies;

[0014] The adjusting assembly is inserted into the hollow rod body to drive the two telescopic assemblies to rotate;

[0015] As a further implementation:

[0016] The adjustment assembly includes a central main rod, a sliding front rod, a front push assembly, a swing assembly, and a rear drive member;

[0017] The central main rod comprises a built-in middle tube of a hollow cylindrical structure, an axial key hole formed on the inner wall of the built-in middle tube, a limiting ring groove formed on the inner wall of the built-in middle tube and located behind the axial key hole, a narrow-diameter front tube formed at the front end of the built-in middle tube, and a spring retaining ring formed at the outer edge of the front end of the narrow-diameter front tube;

[0018] The sliding front rod comprises a sliding sleeve slidably mounted on a spring retaining ring, a sliding retaining ring formed at the rear end of the sliding sleeve, a front rod compression spring arranged between the sliding sleeve and the narrow diameter front tube, and a front end convex ring formed at the front end of the sliding sleeve.

[0019] The forward push assembly includes a rotating core rod rotatably inserted into the inner wall of the built-in middle tube, a forward push convex ring formed on the side of the rotating core rod and matched with the limiting ring groove, an external threaded rod formed at the front end of the rotating core rod, an internal threaded tube screwed on the outer wall of the external threaded rod, and a sliding key provided on the outer wall of the internal threaded tube and slidably inserted into the axial key hole;

[0020] The swing assembly includes a front end block arranged in front of the front end convex ring, a transverse jack formed in the front end block in the transverse direction and connected to the front end convex ring, and the swing assembly also includes:

[0021] There are two swing mandrels, symmetrically distributed at the left and right ends of the horizontal insertion hole;

[0022] There are two swinging levers, which are rotatably sleeved on the corresponding swinging core shafts, and the inner side of the swinging lever is also provided with a driving lock tooth;

[0023] A return torsion spring is wound around the swing core shaft to drive the swing lever to swing inwards for storage;

[0024] A pressure swing arm is arranged inside the rear end of the swinging swing rod, and the inner side of the pressure swing arm abuts and cooperates with the front end of the internal threaded tube;

[0025] The swing assembly is used to drive the telescopic assembly located in the front to rotate;

[0026] The rear driving member is used to drive the telescopic assembly located at the rear to rotate.

[0027] As a further implementation:

[0028] The inner side of the compressed swing arm is inclined rearward.

[0029] As a further implementation:

[0030] The front end of the front end convex ring is symmetrically provided with inclined cutouts that abut against the compressed swing arm.

[0031] As a further implementation:

[0032] The rear driving member is a rear locking tooth evenly distributed on the outer wall of the built-in middle tube in the circumferential direction.

[0033] The pad assembly comprises a bowl-shaped pad which is sleeved on the external thread section and abuts against the tunnel surface, and a fixing nut which is screwed on the external thread section 12 .

[0034] Beneficial Effects

[0035] A support anchor described in this case, the abutment side plate can be fixed to the inner wall of the hole over a longer range, and has a better impact resistance effect: the hollow anchor is inserted into the pre-drilled hole, and then the telescopic assembly is controlled by the adjustment assembly to extend radially outward, driving the abutment side plate to extend outward and clamp the hole. The abutment side plate has a considerable length in the axial direction and can provide clamping force to the inner wall of the hole along its entire length direction.

[0036] A support anchor rod described in this case, the telescopic assembly acts on the driven plug and the radial insertion rod to make them slide radially through the flat thread of the flat thread component, thereby realizing the radial telescopic effect of the radial telescopic clamp arm.

[0037] The support anchor described in this case can be easily adjusted from the outside of the hole to the inside, so that the two telescopic assemblies can move synchronously:

[0038] The rear driving member is used to connect the driven locking teeth of the telescopic assembly located at the rear, and the forward pushing assembly drives the swinging assembly to expand and move forward, so that the swinging assembly is finally connected with the driven locking teeth of the telescopic assembly located at the front, and then the radial expansion of the telescopic assembly can be achieved by rotating the adjusting assembly. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of an embodiment of the support anchor rod.

[0040] Figure 2 It is a schematic diagram of an embodiment of the hollow anchor rod.

[0041] Figure 3 yes Figure 1 Enlarged schematic diagram of part A in the middle.

[0042] Figure 4 It is a schematic diagram of an embodiment of a flat thread assembly.

[0043] Figure 5 Schematic diagram of an embodiment of an adjustment assembly.

[0044] Figure 6 yes Figure 5 Part B is an enlarged schematic diagram of an embodiment.

[0045] Figure 7 yes Figure 5 Part B is an enlarged schematic diagram of another embodiment.

[0046] Figure 8 yes Figure 5 Part B is an enlarged schematic diagram of another embodiment.

[0047] Fig. 9 yes Figure 5 An enlarged schematic diagram of an embodiment of part C in the middle.

[0048] Fig.10 yes Figure 5 Part C in the middle is an enlarged schematic diagram of another embodiment.

[0049] Fig.11 1 is a top view of an embodiment of a swing assembly.

[0050] In the figure:

[0051] 1. Hollow anchor rod, 11. Hollow rod body, 12. External thread section;

[0052] 2. Telescopic assembly;

[0053] 21. plane thread assembly, 21a. limiting convex ring, 21b. rotating ring, 21c. limiting ring groove, 21d. driven lock tooth, 21e. plane thread, 21f. guide cone groove;

[0054] 22. radial telescopic clamp arm, 22a. side through hole, 22b. guide base, 22c. guide insert, 22d. radial insert rod, 22e. guide slot, 22f. driven plug;

[0055] 3. Abut the side panels,

[0056] 4. Adjustment assembly;

[0057] 41. Central main rod, 41a. Built-in middle tube, 41b. Axial key hole, 41c. Limit ring groove, 41d. Narrow front tube, 41e. Spring retaining ring;

[0058] 42. Sliding front rod, 42a. Sliding sleeve, 42b. Sliding retaining ring, 42c. Front rod compression spring, 42d. Front end convex ring, 42e. Inclined cutout;

[0059] 43. Forward push assembly, 43a. Rotating core rod, 43b. Forward push convex ring, 43c. External thread rod, 43d. Internal thread tube, 43e. Sliding key;

[0060] 44. Swing assembly, 44a. Front end block, 44b. Transverse socket, 44c. Swing mandrel, 44d. Swing lever, 44e. Drive lock tooth, 44f. Reset torsion spring, 44g. Pressure swing arm;

[0061] 45. Rear drive parts;

[0062] 5. Gasket assembly, 51. Bowl-shaped gasket, 52. Fixing nut. DETAILED DESCRIPTION

[0063] An impact-resistant tunnel support anchor rod, comprising at least

[0064] The hollow anchor rod 1 comprises a hollow rod body 11 and an external threaded section 12 formed on the rear end of the outer wall of the hollow rod body 11;

[0065] The telescopic assemblies 2 are provided with two, and the front end of the hollow rod body 11 is provided with two telescopic assemblies 2 in sequence from front to back, and the telescopic assembly 2 includes a rotatable plane thread component 21, and a radial telescopic clamp arm 22 driven by the plane thread component 21 to radially extend and retract, and the radial telescopic clamp arm 22 is provided with at least three and is evenly spaced in the circumferential direction;

[0066] Further:

[0067] The plane thread assembly 21 includes a limiting protruding ring 21a formed on the inner wall of the hollow rod body 11, a rotating ring 21b rotatably provided on the limiting protruding ring 21a, a limiting ring groove 21c formed on the outer wall of the rotating ring 21b for cooperating with the limiting protruding ring 21a, driven locking teeth 21d uniformly distributed on the rear end surface of the rotating ring 21b in the circumferential direction, and a plane thread 21e formed on the front end surface of the rotating ring 21b;

[0068] The radial telescopic clamp arm 22 includes side through holes 22a uniformly distributed on the hollow rod body 11 in the circumferential direction, a guide base 22b arranged on the inner wall of the hollow rod body 11 and corresponding to the side through holes 22a, a guide insert 22c formed at the rear of the guide base 22b, a radial plug rod 22d inserted in the side through hole 22a, a guide slot 22e formed on the front end surface of the radial plug rod 22d and matched with the guide insert 22c, and a driven plug 22f engaged with the plane thread 21e is also provided at the rear end of the radial plug rod 22d;

[0069] The front and rear ends of the abutting side plates 3 are respectively connected to the radial insertion rods 22d of the two telescopic assemblies 2.

[0070] Thanks to the above improved technical solution, the telescopic assembly 2 is rotated through the planar thread 21e of the planar thread component 21, acting on the driven plug 22f and the radial insertion rod 22d to make them slide radially, thereby realizing the radial telescopic effect of the radial telescopic clamp arm 22.

[0071] As a further implementation:

[0072] The rear end surface of the rotating ring 21b of the telescopic assembly 2 located at the rear is also provided with a guide cone groove 21f which is narrow at the front and wide at the rear.

[0073] The abutting side plates 3 are evenly distributed on the outer wall of the hollow rod body 11 in the circumferential direction, and the front and rear ends of the abutting side plates 3 are respectively connected to the radial telescopic clamping arms 22 of the two telescopic assemblies 2;

[0074] The adjusting assembly 4 is inserted into the hollow rod body 11 to drive the two telescopic assemblies 2 to rotate;

[0075] As a further implementation:

[0076] The adjustment assembly 4 includes a central main rod 41, a sliding front rod 42, a front push assembly 43, a swing assembly 44, and a rear drive member 45;

[0077] The central main rod 41 includes a built-in middle tube 41a of a hollow cylindrical structure, an axial key hole 41b formed on the inner wall of the built-in middle tube 41a, a limiting ring groove 41c formed on the inner wall of the built-in middle tube 41a and located behind the axial key hole 41b, a narrow-diameter front tube 41d formed at the front end of the built-in middle tube 41a, and a spring retaining ring 41e formed on the outer edge of the front end of the narrow-diameter front tube 41d.

[0078] The sliding front rod 42 includes a sliding sleeve 42a slidably mounted on a spring retaining ring 41e, a sliding retaining ring 42b formed at the rear end of the sliding sleeve 42a, a front rod compression spring 42c arranged between the sliding sleeve 42a and the narrow diameter front tube 41d, and a front end convex ring 42d formed at the front end of the sliding sleeve 42a.

[0079] The forward push assembly 43 includes a rotating core rod 43a rotatably inserted into the inner wall of the built-in middle tube 41a, a forward push convex ring 43b formed on the side of the rotating core rod 43a and matched with the limiting ring groove 41c, an external threaded rod 43c formed at the front end of the rotating core rod 43a, an internal threaded tube 43d screwed on the outer wall of the external threaded rod 43c, and a sliding key 43e provided on the outer wall of the internal threaded tube 43d and slidably inserted into the axial key hole 41b;

[0080] The swing assembly 44 includes a front end block 44a disposed in front of the front end convex ring 42d, a transverse insertion hole 44b formed in the front end block 44a in the transverse direction and connected to the front end convex ring 42d, and the swing assembly 44 also includes:

[0081] There are two swing core shafts 44c, which are symmetrically distributed at the left and right ends of the horizontal insertion hole 44b;

[0082] There are two swinging levers 44d, which are rotatably sleeved on the corresponding swinging core shafts 44c, and the inner side of the swinging levers 44d is also provided with a driving lock tooth 44e;

[0083] The return torsion spring 44f is wound around the swing core shaft 44c, driving the swing lever 44d to swing inwards and be stored;

[0084] A pressure swing arm 44g is arranged inside the rear end of the swinging swing rod 44d, and the inner side of the pressure swing arm 44g abuts against the front end of the internal threaded tube 43d;

[0085] The swing assembly 44 is used to drive the telescopic assembly 2 located in the front to rotate;

[0086] The rear driving member 45 is used to drive the telescopic assembly 2 located at the rear to rotate.

[0087] As a further implementation:

[0088] The inner side of the pressure-receiving swing arm 44g is inclined rearward.

[0089] As a further implementation:

[0090] The front end of the front end convex ring 42d is symmetrically provided with inclined cutouts 42e that abut against the pressure-bearing swing arm 44g.

[0091] As a further implementation:

[0092] The rear driving member 45 is a rear locking tooth evenly distributed on the outer wall of the built-in middle tube 41 a in the circumferential direction.

[0093] The pad assembly 5 comprises a bowl-shaped pad 51 which is sleeved on the external thread section 12 and abuts against the tunnel surface, and a fixing nut 52 which is screwed on the external thread section 12 .

[0094] This case also discloses an anchor bolt fixing method, comprising the following steps:

[0095] Step 1, punching holes:

[0096] Use punching equipment to drill holes at marked locations in the laneway;

[0097] Step 2: Insert the hollow anchor rod 1:

[0098] Insert the hollow anchor rod 1, the telescopic assembly 2, and the abutting side plate 3 into the hole;

[0099] Step 3, abut against the side plate 3 for fixing:

[0100] Step 3.1, internal threaded tube 43d moves:

[0101] Insert the adjustment assembly 4 into the hollow rod body 1 and push it forward until the rear cable abuts against the driven locking tooth 21d of the telescopic assembly 2 at the rear;

[0102] Then, the rotating core rod 43a, the forward push ring 43b, and the external threaded rod 43c are rotated to drive the internal threaded tube 43d and the sliding key 43e to move along the axial key hole 41b;

[0103] Step 3.2, the swing assembly 44 is opened:

[0104] The internal threaded tube 43d moves forward and squeezes the compressed swing arm 44g, driving the compressed swing arm 44g and the swinging swing rod 44d to make them turn outwards to overcome the damping of the return torsion spring 44f, until the swinging swing rod 44d and the driving locking tooth 44e abut against the bottom of the horizontal insertion hole 44b;

[0105] Step 3.3, the swing assembly 44 moves forward to lock the telescopic assembly 2 located in the front:

[0106] The internal threaded tube 43d continues to move forward, driving the swing assembly 44, the sliding sleeve 42a, the sliding retaining ring 42b, the front end convex ring 42d, and the inclined cutout 42e to overcome the damping of the front rod compression spring 42c and move forward;

[0107] Until the swing assembly 44 is locked with the driven locking tooth 21d of the telescopic assembly 2 located in the front;

[0108] Step 3.4, open the abutting side panel 3:

[0109] The rotating adjustment assembly 4 drives the two telescopic assemblies 2 to open synchronously through the swing assembly 44 and the rear drive member 45:

[0110] The driven lock teeth 21d, the rotating ring 21b, and the plane thread 21e rotate to drive the driven plug 22f, the radial plug rod 22d, and the guide slot 22e to move outward;

[0111] This in turn drives the abutting side plate 3 to open outwards and abut and fix in the hole.

[0112] It should be noted that the method further includes a grouting and bonding step, in which after the adjustment assembly 4 is withdrawn, grout is injected through the hollow rod body 1 of the hollow anchor rod 1 and fixed.

[0113] It should be noted that the damping of the front rod compression spring 42c under normal conditions is much greater than the torque of the return torsion spring 44f, and the former is at least 5 times the latter. Therefore, when the internal threaded tube 43d moves forward, it always drives the swing assembly 44 to swing first and then pushes the swing assembly 44 forward.

Claims

1. An impact-resistant tunnel support anchor, characterized in that: At least include The hollow anchor rod (1) comprises a hollow rod body (11) and an external threaded section (12) formed on the rear end of the outer wall of the hollow rod body (11); The telescopic assemblies (2) are provided with two of them. The front end of the hollow rod body (11) is provided with two telescopic assemblies (2) in sequence from front to back. The telescopic assembly (2) comprises a rotatably arranged planar threaded component (21), and a radially telescopic clamping arm (22) driven by the planar threaded component (21) to radially telescope. The radially telescopic clamping arm (22) is provided with at least three of them and is evenly spaced in the circumferential direction. Abutment side plates (3) are evenly distributed on the outer wall of the hollow rod body (11) in the circumferential direction, and the front and rear ends of the abutment side plates (3) are respectively connected to the radial telescopic clamping arms (22) of the two telescopic assemblies (2); The adjusting assembly (4) is inserted into the hollow rod body (11) to drive the two telescopic assemblies (2) to rotate; A backing plate assembly (5) comprises a bowl-shaped backing plate (51) sleeved on the external thread section (12) and abutting against the tunnel surface, and a fixing nut (52) screwed on the external thread section (12); The planar thread assembly (21) comprises a limiting convex ring (21a) formed on the inner wall of the hollow rod body (11), a rotating circular ring (21b) rotatably provided on the limiting convex ring (21a), a limiting ring groove (21c) formed on the outer wall of the rotating circular ring (21b) for cooperating with the limiting convex ring (21a), driven locking teeth (21d) uniformly distributed circumferentially on the rear end surface of the rotating circular ring (21b), and a planar thread (21e) formed on the front end surface of the rotating circular ring (21b); The radial telescopic clamp arm (22) comprises side through holes (22a) uniformly distributed on the hollow rod body (11) in the circumferential direction, a guide base (22b) arranged on the inner wall of the hollow rod body (11) and corresponding to the side through holes (22a), a guide insert (22c) formed at the rear of the guide base (22b), a radial insert rod (22d) inserted in the side through hole (22a), and a guide slot (22e) formed on the front end surface of the radial insert rod (22d) and cooperating with the guide insert (22c), and the rear end of the radial insert rod (22d) is also provided with a driven plug (22f) meshing with the plane thread (21e); The front and rear ends of the abutting side plates (3) are respectively connected to the radial insertion rods (22d) of the two telescopic assemblies (2); The adjustment assembly (4) comprises a central main rod (41), a sliding front rod (42), a front push assembly (43), a swing assembly (44), and a rear drive member (45); The central main rod (41) comprises a built-in middle tube (41a) of a hollow cylindrical structure, an axial key hole (41b) formed on the inner wall of the built-in middle tube (41a), a limiting ring groove (41c) formed on the inner wall of the built-in middle tube (41a) and located behind the axial key hole (41b), a narrow-diameter front tube (41d) formed on the front end of the built-in middle tube (41a), and a spring retaining ring (41e) formed on the outer edge of the front end of the narrow-diameter front tube (41d); The sliding front rod (42) comprises a sliding sleeve (42a) slidably mounted on a spring retaining ring (41e), a sliding retaining ring (42b) formed at the rear end of the sliding sleeve (42a), a front rod compression spring (42c) disposed between the sliding sleeve (42a) and the narrow diameter front tube (41d), and a front end convex ring (42d) formed at the front end of the sliding sleeve (42a); The forward push component (43) comprises a rotating core rod (43a) rotatably inserted into the inner wall of the built-in middle tube (41a), a forward push convex ring (43b) formed on the side of the rotating core rod (43a) and matched with the limiting ring groove (41c), an externally threaded rod (43c) formed at the front end of the rotating core rod (43a), an internally threaded tube (43d) screwed on the outer wall of the externally threaded rod (43c), and a sliding key (43e) provided on the outer wall of the internally threaded tube (43d) and slidably inserted into the axial key hole (41b); The swing assembly (44) comprises a front end block (44a) arranged in front of the front end convex ring (42d), and a transverse insertion hole (44b) formed in the front end block (44a) in the transverse direction and connected to the front end convex ring (42d). The swing assembly (44) further comprises: Two swing mandrels (44c) are provided and symmetrically distributed at the left and right ends of the horizontal insertion hole (44b); Two swinging swing rods (44d) are provided, and the swinging swing rods (44d) are rotatably sleeved on corresponding swinging core shafts (44c), and the inner side of the swinging swing rods (44d) is also provided with driving lock teeth (44e); A return torsion spring (44f) is wound around the swing core shaft (44c) and drives the swing lever (44d) to swing inwards for storage; A pressure-bearing swing arm (44g) is arranged on the inner side of the rear end of the swinging swing rod (44d), and the inner side of the pressure-bearing swing arm (44g) is in abutment with the front end of the internally threaded tube (43d); The swing assembly (44) is used to drive the telescopic assembly (2) located in the front to rotate; The rear driving member (45) is used to drive the telescopic assembly (2) located at the rear to rotate.

2. The impact-resistant tunnel support anchor according to claim 1, characterized in that: The rear end surface of the rotating ring (21b) of the telescopic assembly (2) located at the rear is also provided with a guide cone groove (21f) which is narrow at the front and wide at the rear.

3. The impact-resistant tunnel support anchor according to claim 2, characterized in that: The inner side of the pressure-bearing swing arm (44g) is inclined rearward.

4. The impact-resistant tunnel support anchor according to claim 3 is characterized in that: The front end of the front end convex ring (42d) is symmetrically provided with inclined cutouts (42e) that abut against the pressure-bearing swing arm (44g).

5. The impact-resistant tunnel support anchor according to claim 4, characterized in that: The rear driving member (45) is a rear locking tooth evenly distributed in the circumferential direction on the outer wall of the built-in middle tube (41a).

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