Composite vibration well cementation tool

Abstract

The invention relates to a composite vibration well cementation tool, and belongs to the technical field of casing running well cementation tools in oil and gas well engineering. Comprising an eccentric vibration mechanism arranged in an outer cylinder, the eccentric vibration mechanism comprises a first center shaft, and an eccentric block is fixedly installed on the first center shaft and used for generating periodic radial mechanical vibration when the first center shaft rotates; the well cementation tool further comprises a power mechanism used for providing power for the first center shaft, an axial hydraulic pulse vibration mechanism used for generating pulse vibration and a rotational flow mechanism capable of changing the annular fluid flow state, the eccentric vibration mechanism of the well cementation tool can generate large eccentric force, and the generated radial vibration transmission range is large; and a synergistic effect is formed between the axial pulse vibration mechanism and the rotational flow mechanism, so that the overall well cementation quality is greatly improved.

Description

Technical Field

[0001] The invention relates to a composite vibration cementing tool, belonging to the technical field of casing cementing tools in oil and gas well engineering. Background Art

[0002] Cementing is an indispensable and important part of the drilling process. The quality of cementing directly affects the normal operation of drilling and completion, the production and life of oil and gas wells. Cementing mainly includes casing and cementing. According to the vibration principle, the existing vibration cementing tools can be mainly divided into hydraulic pulse type, mechanical type, audio type, air pulse type, motor vibration type, etc., but the vibration types of existing vibration cementing tools are relatively single. For example, the invention patent document with publication number CN105089547A discloses a downhole vibration cementing tool, including a motor vibration component, which can achieve downhole vibration in the cement slurry waiting stage. The utility model patent document with publication number CN205532466U discloses a water wheel vibration cementing tool, which generates eccentric vibration through the rotation of the impeller, drives the casing to generate radial mechanical vibration, and realizes vibration cementing. This cementing tool has a single vibration mode, weak vibration intensity, and a small energy transfer range. The vibration acts on the casing string or fluid only in a single dimension, which is limited to improving the cementing quality near the tool, but has no obvious improvement on the overall cementing quality. Therefore, a composite cementing tool that can vibrate in multiple dimensions is needed.

[0003] The invention patent document with the current publication number CN114687670A discloses a hydraulic composite vibration cementing tool based on dynamic and static control valves. The cementing tool includes a vibration module casing installed between the guide head and the casing to form a fluid channel; an eccentric vibration module installed in the vibration module casing, and the eccentric vibration module is configured to drive the hydraulic composite vibration cementing tool to produce periodic radial vibration when the fluid flows through the vibration module casing; a pulse fluctuation generating module installed in the vibration module casing behind the eccentric vibration module, and the pulse fluctuation generating module is configured to drive the hydraulic composite vibration cementing tool to produce periodic axial vibration when the fluid flows through the vibration module casing. The cementing device can improve the uniformity of cement slurry under the action of radial and axial dimensions. When the cement slurry passes through the guide blades wrapped by the eccentric turbine, it drives the eccentric turbine to produce rotational motion, thereby driving the cementing tool to move. However, since the eccentric block of the cementing tool is connected to the guide vane, the thickness of the eccentric block is limited by the limited space, the eccentric force generated is small, the vibration energy is low, and the radial vibration transmission range is limited. There is a gap between the guide vane of the cementing tool and the inner wall of the casing, and the fluid will flow out through the gap first. Only when the fluid flow reaches a certain value, the guide vane providing power will rotate. If the eccentric turbine cannot rotate or the speed is low when the fluid flow is small, the device cannot produce an effect, and the applicability of the cementing device is limited. On the other hand, the pulse wave generating module of the cementing tool realizes periodic opening and closing of the moving valve and the static valve of the disc switch to achieve periodic axial vibration. If the distance between the moving valve and the static valve is too large, the vibration effect will be limited. If the distance is too close, the solid phase particles in the fluid will block the moving valve and make the moving valve unable to rotate. Therefore, there are high requirements for the distance between the moving valve and the static valve, and the applicability is poor. Summary of the invention

[0004] The purpose of the present invention is to provide a composite vibration cementing tool to solve the problem that the existing pulse vibration tool has a limited radial vibration range, a single vibration mode, poor adaptability and leads to low overall cementing quality.

[0005] To achieve the above object, the solution of the present invention includes:

[0006] A composite vibration cementing tool of the present invention comprises an eccentric vibration mechanism arranged inside an outer cylinder, wherein the eccentric vibration mechanism comprises a first center shaft, on which an eccentric block is fixedly mounted for generating periodic radial mechanical vibration when the first center shaft rotates, and the cementing tool also comprises a power mechanism for providing power to the first center shaft.

[0007] Beneficial effects: The composite vibration cementing tool of the present invention is installed at the bottom of the casing and is lowered into the well bottom with the casing. It can replace the guide shoe and there is no need to change the existing casing lowering process. During the cementing process, the fluid generates downward power through the power mechanism of the composite vibration cementing tool, and the casing column generates periodic radial mechanical vibration through the eccentric block. The eccentric block is set as a whole block, and the angle can be set as required. It can generate a larger eccentric force at a lower rotation speed, and the generated periodic radial mechanical vibration has a wider transmission range. At the same time, the power mechanism and the eccentric vibration mechanism are set separately, and the power mechanism has a variety of options and has strong applicability.

[0008] Furthermore, in order to prevent the eccentric block from moving downward, limit plates fixed to the outer cylinder and used to limit the position of the eccentric block are respectively provided in the upper and lower directions of the eccentric block, and the upper and lower limit plates are both provided with fluid channels.

[0009] Furthermore, the power mechanism includes a second center shaft coaxially connected to the first center shaft and transmitting torque to the first center shaft, and blades are rotatably connected to the second center shaft, and the blades are used to rotate when fluid passes through to generate power to transmit torque to the first center shaft.

[0010] Beneficial effects: The power mechanism of the present invention generates power through the rotation of blades, and uses the second central axis to transmit torque downward to provide torque for the eccentric vibration mechanism.

[0011] Furthermore, the cementing tool also includes an axial hydraulic pulse vibration mechanism, which includes a third center shaft coaxially connected to the first center shaft and used to transmit torque, and a rotary switch for generating periodic axial hydraulic pulse vibration is fixed at the lower end of the third center shaft.

[0012] Beneficial effect: the eccentric vibration mechanism causes the casing string to produce periodic radial mechanical vibration, and the axial hydraulic pulse vibration mechanism produces periodic axial hydraulic pulse vibration, which acts on the casing string and the fluid in multiple dimensions. The vibration effects are superimposed on each other, which can significantly improve the uniformity of the annular cement slurry distribution and improve the bonding quality of the first and second interfaces of cementing, thereby improving the cementing quality. At the same time, it also solves the problem that the vibration energy of a single vibration cementing tool is weak and the effect of improving the cementing quality is not obvious.

[0013] Furthermore, a corresponding guide hole is provided at the upper end of the rotary switch, and the rotary switch includes a fixed sleeve and a rotating sleeve for being installed below the third center axis and rotating with the third center axis. A first through hole penetrating the side wall is provided on the side wall of the fixed sleeve, and a second through hole connected to the normally open water channel and penetrating its own side wall is provided on the side wall of the rotating sleeve. When the rotating sleeve rotates with the third center axis, there is an opening position in which the second through hole is connected to the first through hole, and a closing position in which the second through hole is staggered from the first through hole, so as to realize the opening and closing of the rotary switch, and the normally open water channel is connected to the guide hole.

[0014] Beneficial effects: The present invention realizes axial vibration through a rotary switch. The rotary switch is correspondingly provided with a plurality of holes. When the rotary sleeve rotates with the third center axis, there is an open position in which the second through hole is connected to the first through hole. Part of the fluid flows out from the lateral opening channel, and the other part of the fluid flows out from the normally open water channel. When the rotary switch is closed, the fluid flows out from the normally open water channel. The rotary switch is a closed space and continuously rotates through the third center axis, so the axial vibration effect can be ensured. The rotational torque is small and the solid particles in the fluid will not affect the rotation of the rotary switch.

[0015] Furthermore, the cementing tool also includes a cyclone mechanism for allowing fluid to enter the annulus, and the cyclone mechanism is installed at the lower end of the axial hydraulic pulse vibration mechanism.

[0016] Beneficial effect: The swirl mechanism at the bottom of the tool can make the fluid enter the annulus and spiral up in the annulus, changing the original flow state and effectively improving the cement slurry displacement efficiency, thereby significantly improving the cementing quality.

[0017] Furthermore, the swirl mechanism includes a straight through hole for connecting to a normally open water channel, and at least two evenly arranged lateral swirl flow holes, one end of the lateral swirl flow hole is used to connect to the straight water channel, and the other end spirally extends to the outer cylinder wall. The surface of the outer cylinder wall of the swirl mechanism extends in an arc shape, and the arc-shaped recesses are distributed in a clockwise direction or in a counterclockwise direction.

[0018] Beneficial effects: The lateral swirl water channel makes the fluid spiral upward in the annulus, changing the original flow state of the annular fluid and improving the cement slurry displacement efficiency.

[0019] Furthermore, in order to prevent the blade from moving downward, the upper and lower directions of the blade respectively include limit plates fixed to the outer cylinder for limiting the position of the power mechanism, and the upper and lower limit plates are both provided with fluid channels.

[0020] Furthermore, the eccentric block is a sector-shaped cylinder with a set angle.

[0021] Furthermore, a thrust bearing for limiting the downward movement of the eccentric block is arranged between the lower portion of the eccentric block and the lower limit plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a schematic diagram of the internal structure of a composite vibration cementing tool in an embodiment of the present invention;

[0023] Figure 2 is a schematic diagram of section A in a composite vibration cementing tool in an embodiment of the present invention;

[0024] Figure 3 is a schematic diagram of section B in a composite vibration cementing tool in an embodiment of the present invention;

[0025] Figure 4 is a schematic diagram of a C section of a composite vibration cementing tool in an embodiment of the present invention;

[0026] Figure 5 is a schematic diagram of the D section of the composite vibration cementing tool in an embodiment of the present invention;

[0027] In the figure: 1-power mechanism, 2-eccentric vibration mechanism, 3-axial hydraulic pulse vibration mechanism, 4-swirl mechanism, 5-double female joint, 6-male and female joint;

[0028] 101-outer cylinder, 102-bearing, 103-upper limit plate, 104-blade, 105-second central axis, 106-thrust bearing, 107-lower limit plate, 108-thread;

[0029] 201-first central axis, 202-bearing, 203-upper limit plate, 204-outer cylinder, 205-eccentric block, 206-thrust bearing, 207-lower limit plate, 208-thread;

[0030] 301-third central axis, 302-flow guide hole, 303-rotary switch, 304-outer cylinder, 305-normally open water channel;

[0031] 401-lateral swirl flow hole, 402-straight through hole. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0033] Composite vibration cementing tool embodiment:

[0034] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0035] In the description of the present invention, it should be noted that the terms "upper", "lower", etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the system or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. The above words have no special meaning and cannot be understood as indicating or implying relative importance.

[0036] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "disposed" and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0037] like Figure 1 A composite vibration cementing tool shown includes a power mechanism 1, an eccentric vibration mechanism 2, an axial hydraulic pulse vibration mechanism 3 and a cyclone mechanism 4. The cementing tool is installed at the bottom of the casing, including an outer cylinder and a power mechanism 1, an eccentric vibration mechanism 2 and an axial hydraulic pulse vibration mechanism 3 which are sequentially assembled and installed inside the outer cylinder (including a power mechanism outer cylinder 101, an eccentric vibration mechanism outer cylinder 204 and an axial hydraulic pulse vibrator outer cylinder 304) along the axial direction of the outer cylinder; the power mechanism includes a second central shaft 105 which transmits torque to the eccentric vibration mechanism and the axial hydraulic pulse vibration mechanism after power is generated; the eccentric vibration mechanism is connected to the second central shaft in a transmission manner and is used to generate radial periodic mechanical vibration under the drive of the power mechanism; the axial hydraulic pulse vibration mechanism is used to generate periodic axial pulse vibration under the drive of the power mechanism.

[0038] Specifically, the cementing tool in this embodiment is connected to the casing through a double female joint 5. Specifically, the double female joint 5 connects the casing and the power mechanism 1. The power mechanism 1 provides power and transmits torque downward through the center shaft. In this embodiment, the cementing tool is assembled together by the power mechanism 1, the eccentric vibration mechanism 2, and the axial hydraulic pulse vibration mechanism 3. The power mechanism and the eccentric vibration mechanism, and the eccentric vibration mechanism and the outer cylinder of the axial hydraulic pulse vibration mechanism are connected through male and female joints 6. In other embodiments, any structure that can connect the various mechanisms can be used.

[0039] The double female connector 5 connects the sleeve and the outer cylinder 101 of the power mechanism 1, as shown in FIG. Figure 1In this embodiment, the second central axis 105 of the power mechanism 1 is rotatably connected with a blade 104 for providing power. The upper and lower directions of the blade 104 respectively include limit plates fixed to the outer cylinder 101 for limiting the position of the power mechanism. The second central axis passes through the upper limit plate 103 and the lower limit plate 107 through the bearing 102. The upper and lower limit plates are both provided with fluid channels. A thrust bearing 106 for limiting the downward movement of the power mechanism is also provided between the lower portion of the blade and the lower limit plate.

[0040] The second central axis 105 passes through the upper limit plate 103 and the lower limit plate 107 through the bearing 102. The thrust bearing 106 prevents the power mechanism from moving downward. The upper limit plate 103 and the lower limit plate 107 define the position of the power mechanism. The entire device is located in the outer cylinder 101. The fluid passes through the blades 104, causing the blades to rotate, driving the second central axis 105 to rotate. A thread 108 is provided at the lower end of the second central axis 105. Figure 1 In other embodiments, other power mechanisms may be used to drive the second central shaft 105 to rotate, thereby transmitting torque downward.

[0041] In other embodiments, the blades used to provide power may be a turbine structure or a screw structure, as long as any mechanism that can drive the second central shaft 105 to rotate and transmit torque downwards will do.

[0042] like Figure 2 As shown, the upper limit plate 103 and the lower limit plate 107 are fixed in the outer cylinder 101, the bearing 102 defines the second central axis position, and both the upper limit plate 103 and the lower limit plate 107 are provided with channels, through which the fluid flows into and out of the power mechanism 1.

[0043] The eccentric vibration mechanism 2 is connected to the lower part of the power mechanism 1 through the male and female joints 6. Figure 1 As shown. The eccentric vibration mechanism 2 of this embodiment includes a first center shaft 201 coaxially connected to the second center shaft for transmitting torque, wherein the first center shaft 201 of this embodiment is connected to the second center shaft 105 through a thread 108 to transmit torque. An eccentric block 205 for causing the sleeve to generate radial periodic mechanical vibration is fixed to the first center shaft 201, and the eccentric block includes limit plates (including an upper limit plate 203 and a lower limit plate 207) fixed to the outer cylinder 204 in the upper and lower directions for limiting the position of the eccentric block, and a thrust bearing 206 for limiting the downward movement of the eccentric block is also provided below the eccentric block.

[0044] Specifically, the upper limit plate 203 and the lower limit plate 207 are fixed on the outer cylinder 204, and the eccentric block 205 is fixed on the first central shaft 201. The upper limit plate 203 and the lower limit plate 207 are penetrated by the bearing 202. The thrust bearing 206 prevents the eccentric block 205 from moving downward. The upper limit plate 203 and the lower limit plate 207 define the position of the eccentric block 205. The entire eccentric vibration device is placed in the outer cylinder 204. The first central shaft 201 transmits the torque of the power mechanism 1, drives the eccentric block 206 to rotate, generates periodic radial mechanical vibration, and thus drives the sleeve to generate radial vibration. The vibration energy is transmitted upward through the sleeve. A thread 208 is set at the bottom of the first central shaft 201. Figure 1 As shown. Figure 3 As shown, the eccentric block 205 in this embodiment is set as a fan-shaped column at a certain angle. As the first central axis 201 rotates, eccentric vibration is generated. The fluid enters the eccentric vibration mechanism 2 through the upper limit plate 203, passes through the middle annular area, and flows out from the lower limit plate 207.

[0045] The axial hydraulic pulse vibration mechanism 3 is connected to the lower part of the eccentric vibration mechanism 2 through the male and female joints 6. The axial hydraulic pulse vibration mechanism includes a third central shaft 301 coaxially connected to the first central shaft for transmitting torque, wherein the upper end of the third central shaft 301 is connected to the first central shaft 201 through a thread 208, and a rotary switch 303 for generating periodic axial hydraulic pulse vibration is fixedly provided at the lower end of the third central shaft 301. Driven by the third central shaft 301, the rotary switch 303 rotates to realize periodic opening and closing, and the fluid periodically flows out of the rotary switch, thereby generating periodic axial hydraulic pulse vibration.

[0046] The upper end of the rotary switch 303 is provided with a corresponding diversion hole 302. The rotary switch includes a fixed sleeve and a rotating sleeve for being installed below the third central axis and rotating with the third central axis. The side wall of the fixed sleeve is provided with a first through hole penetrating the side wall. The side wall of the rotating sleeve is provided with a second through hole connected to the normally open water channel and penetrating its own side wall. When the rotating sleeve rotates with the third central axis, there is an opening position where the second through hole is connected to the first through hole, and a closing position where the second through hole is staggered with the first through hole, so as to realize the opening and closing of the rotary switch, and the normally open water channel 305 is connected to the diversion hole. A plurality of first through holes and second through holes can be provided.

[0047] A normally open water channel 305 is provided inside the rotary switch 303, and a portion of the fluid can flow in from the flow guide hole 302 and flow out from the normally open water channel 305. Figure 4As shown, the rotary switch 303 is provided with corresponding through holes around the inside and outside, and rotates under the drive of the central shaft 301. When the guide holes on the inside and outside correspond to each other, the rotary switch 303 is opened, and part of the fluid flows out from the rotary switch 303. Otherwise, the rotary switch 303 is closed, and the fluid only flows out from the normally open waterway 305. The periodic opening and closing of the rotary switch causes the fluid to generate periodic axial pulse vibration, and the vibration energy is transmitted from bottom to top in the annular fluid.

[0048] The swirl mechanism 4 is fixedly mounted on the lower end of the axial hydraulic pulse vibration mechanism 3. Figure 5 As shown, the swirl mechanism includes a straight through hole 402 for connecting with a normally open water channel, and at least two evenly arranged lateral swirl flow holes 401, one end of the lateral swirl flow hole is used to connect with the straight water channel, and the other end spirally extends to the outer cylinder wall. The surface of the outer cylinder wall of the swirl mechanism extends in an arc shape, and the arc-shaped recesses are distributed in a clockwise direction or in a counterclockwise direction.

[0049] Specifically, the lower end of the cyclone mechanism 4 is designed in an arc shape, and the inside is provided with a lateral cyclone flow hole 401 and a straight through hole 402 with the same rotation direction. Among them, in order to improve the efficiency of the fluid flowing through the through hole, multiple lateral cyclone waterways can be evenly arranged, and the straight through hole can be arranged at the central axis position of the cyclone mechanism. A part of the fluid flowing out of the axial hydraulic pulse vibration mechanism 3 enters the annulus through the lateral cyclone flow hole 401, and spirally rises in the annulus to change the original flow state of the annulus fluid, and another part of the fluid enters the annulus through the straight through hole.

[0050] The composite vibration cementing tool in this embodiment can replace the guide shoe, be installed at the bottom of the casing, and be lowered into the bottom of the well with the casing, without changing the existing casing lowering process. During the cementing process, the fluid passes through the multi-dimensional composite vibration cementing tool, causing the casing string to produce periodic radial mechanical vibrations, and the fluid to produce periodic axial hydraulic pulse vibrations, and the vibration effects are superimposed on each other; the lateral vortex waterway at the bottom of the tool causes the fluid to spiral up in the annulus, changing the original flow state and effectively improving the cement slurry displacement efficiency; the vibration generated by the tool of the present invention acts on the casing string and the fluid in multiple dimensions, which can significantly improve the uniformity of the annular cement slurry distribution, improve the bonding quality of the first and second interfaces of cementing, thereby improving the cementing quality, and at the same time solves the problem that the vibration energy of a single vibration cementing tool is weak and the effect on improving cementing quality is not obvious.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite vibration cementing tool, comprising an eccentric vibration mechanism arranged inside an outer cylinder, characterized in that: The eccentric vibration mechanism includes a first central shaft, on which an eccentric block is fixedly mounted for generating periodic radial mechanical vibration when the first central shaft rotates. The cementing tool also includes a power mechanism for providing power to the first central shaft.

2. The composite vibration cementing tool according to claim 1, characterized in that: Limiting plates fixed on the outer cylinder and used to limit the position of the eccentric block are respectively arranged in the upper and lower directions of the eccentric block, and the upper and lower limiting plates are both provided with fluid channels.

3. The composite vibration cementing tool according to claim 1, characterized in that: The power mechanism includes a second central shaft coaxially connected to the first central shaft and transmitting torque to the first central shaft. The second central shaft is rotatably connected with blades, which are used to rotate when fluid passes through to generate power to transmit torque to the first central shaft.

4. The composite vibration cementing tool according to claim 1, characterized in that: The cementing tool also includes an axial hydraulic pulse vibration mechanism, which includes a third center shaft coaxially connected to the first center shaft and used to transmit torque, and a rotary switch for generating periodic axial hydraulic pulse vibration is fixed at the lower end of the third center shaft.

5. The composite vibration cementing tool according to claim 4, characterized in that: A corresponding guide hole is provided at the upper end of the rotary switch, and the rotary switch includes a fixed sleeve and a rotating sleeve for being installed below the third central axis and rotating with the third central axis. A first through hole penetrating the side wall is provided on the side wall of the fixed sleeve, and a second through hole connected to the normally open water channel and penetrating its own side wall is provided on the side wall of the rotating sleeve. When the rotating sleeve rotates with the third central axis, there is an opening position in which the second through hole is connected to the first through hole, and a closing position in which the second through hole is staggered from the first through hole, so as to realize the opening and closing of the rotary switch, and the normally open water channel is connected to the guide hole.

6. The composite vibration cementing tool according to claim 5, characterized in that: The cementing tool also includes a cyclone mechanism for allowing fluid to enter the annulus, and the cyclone mechanism is installed at the lower end of the axial hydraulic pulse vibration mechanism.

7. The composite vibration cementing tool according to claim 6, characterized in that: The swirl mechanism includes a straight through hole for connecting with a normally open water channel, and at least two evenly arranged lateral swirl flow holes, one end of the lateral swirl flow hole is used to connect with the straight water channel, and the other end spirally extends to the outer cylinder wall. The surface of the outer cylinder wall of the swirl mechanism extends in an arc shape, and the arc-shaped recesses are distributed in a clockwise direction or in a counterclockwise direction.

8. The composite vibration cementing tool according to claim 3, characterized in that: The upper and lower directions of the blades respectively include limit plates fixed on the outer cylinder and used to limit the position of the power mechanism, and the upper and lower limit plates are both provided with fluid channels.

9. The composite vibration cementing tool according to claim 2, characterized in that: The eccentric block is a sector-shaped cylinder with a set angle.

10. The composite vibration cementing tool according to claim 2, characterized in that: A thrust bearing for limiting the downward movement of the eccentric block is arranged between the lower limit plate and the eccentric block.

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