JP2005058617A - Blood flow pump - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small, lightweight and durable blood flow pump capable of preventing a thrombus from being formed. <P>SOLUTION: The blood flow pump comprises an impeller, a casing for housing the impeller rotatably and having an inlet and an outlet, a magnetic driving means for rotating the impeller by acting magnetically on a magnet of the impeller from the outside of the casing, a magnetic attraction force adjusting means for adjusting the attraction force of the magnetic action, a rotation number controlling means for controlling the rotation number of the magnetic driving means, and a pair of pivot bearings for supporting pivots on the both ends of the rotation shaft of the impeller. The blood flow pump is characterized by that the distance between bearing surfaces of the both pivot bearings are longer than the length of the rotation shaft of the impeller, and the rotation number of the impeller is controlled at a predetermined number by the rotation number controlling means. With such a configuration, the impeller floats in the blood flow within the casing, and the pivots on the both ends of the rotation shaft of the impeller and the bearing surfaces of the both pivot bearings are kept in a non-contact state. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a blood flow pump for use in an oxygenator or the like, and more particularly to a centrifugal pump that can be operated with a rotating shaft in a non-contact state with respect to a bearing by controlling the number of rotations.

In recent years, research on replacing or complementing a dysfunctional heart with an artificial heart has been vigorously conducted in and out of the past decades.
An important issue in the practical application of an artificial heart is, first, prevention of thrombus generated in the artificial heart. In addition, improvement of the durability of the artificial heart is an indispensable issue for its practical use.

Initially, the research and development of artificial hearts was centered on bellows pumps and other pulsating pumps that have a pulsating function similar to that of the human heart. Due to its excellent durability, a continuous flow centrifugal pump that does not cause pulsation has been attracting attention.
That is, the centrifugal pump is easy to reduce in size and weight, so it is suitable for use as a complementary means for the original heart placed in parallel with the original heart Even so, various research and developments on centrifugal pumps are in progress due to the fact that the living body can be handled by physiological factors of the human body.

  For example, in Japan, an extracorporeal auxiliary circulation pump using a centrifugal pump passes the legal examination and is in the stage of practical use. Since this extracorporeal auxiliary circulation pump is assumed to be used within one month, the occurrence of thrombus is not a problem.

On the other hand, in the United States, research on practical application of a pulsatile pump is active, and in 2001, seven clinical cases of complete implantation of an artificial heart using a pulsatile pump were reported.
However, it is difficult to reduce the size of the pulsating artificial heart. For this reason, the pulsating artificial heart is limited to application to adults weighing 85 kg or more. On the other hand, a continuous flow artificial heart using a centrifugal pump is easy to miniaturize, and has an advantage that the physique limitation of a patient to be applied is not particularly required.
Japanese Patent Laid-Open No. 9-47506 Japanese Patent Laid-Open No. 10-03364 Japanese Patent Laid-Open No. 10-015056

As described above, many prior arts have been disclosed for a continuous flow blood pump using a centrifugal pump, but it is difficult to say that the following two points as conditions that can be used over a long period of time have been solved. Is the current situation.
That is,
(A) Suppression of thrombus generation.
(B) How to maintain stable rotation by preventing wear at the bearing portion of the rotating shaft.
The present invention has been made in view of the above-described conventional situation.
In the blood flow pump,
An impeller, a casing having a suction portion and a discharge portion, which rotatably stores the impeller, a magnetic driving means for rotating the impeller magnetically acting from outside the casing on a magnet provided in the impeller, and the magnetic action A magnetic attraction force adjusting means for adjusting the attraction force; a rotational speed control means for the magnetic drive means; and a pair of pivot bearings for supporting pivots at both ends of the rotating shaft of the impeller. The distance between them is set to be larger than the rotation shaft length of the impeller, and the rotation speed of the impeller is controlled to a predetermined number by the rotation speed control means so that the impeller floats in the blood flow in the casing and pivots at both ends of the rotation shaft of the impeller And the bearing surfaces of the two pivot bearings are maintained in a non-contact state to solve the above-described conventional problems.

  In the blood flow pump, the difference between the distance between the bearing surfaces of both pivot bearings and the rotation shaft length of the impeller may be set to 0.4 mm or more.

  Further, in any one of the blood flow pumps, the casing and impeller may be made of titanium or a titanium alloy, and the surface roughness of titanium or titanium alloy at the blood contact portion in the casing or impeller may be polished to 0.5 microns or less. is there.

  Further, in any one of the above blood flow pumps, the suction portion is constituted by a pipe made of titanium or a titanium alloy having a predetermined angle with respect to the rotation axis of the impeller and connected to the upper portion of the casing, The surface roughness may be polished to 0.5 microns or less.

  Furthermore, in any one of the blood flow pumps described above, the bearing surfaces of a pair of pivot bearings that support the pivots at both ends of the impeller rotation shaft may be formed of an ultra-polymer material.

  Alternatively, in any one of the above-described blood flow pumps, the pivots at both ends of the rotation shaft of the impeller may be formed of a ceramic material.

  In any of the above-described configurations, the curvature of the bearing surface of the pivot bearing portion may be set to be larger than the curvature of the pivot of the rotation shaft of the impeller.

  The present invention can obtain a small and lightweight blood flow pump that can suppress thrombus and can withstand long-term use by the configuration and action disclosed above.

  Hereinafter, embodiments of the present invention will be described. The rotational speed of the impeller needs to be set in accordance with the required blood flow rate, but the rotational speed causes the impeller to float, that is, the pivot shafts at both ends of the impeller's rotational shaft are not on the bearing surface of the pivot bearing. It is also closely related to the contact state, and it is not easy to set the number of revolutions that realizes the impeller to rise in conjunction with securing a predetermined blood flow rate. Therefore, in this embodiment, a desired number of revolutions is set by an index created based on data that can be found in advance through experiments. The index in this case also means an index written in a computer program incorporated as firmware in the pump drive control means.

  By using pure titanium or a titanium alloy such as Ti4A16V in a required part of the pump constituent material, the biocompatibility is improved, and the surface roughness of the surface in contact with blood inside the pump is 0.5 μm or less, preferably 0 Polishing to 2 μm or less to prevent thrombus formation over a long period of time.

  In the present invention, the pivot shaft of the impeller rotating shaft and the bearing surface of the pivot bearing are in a non-contact state when the impeller is lifted, but contact is unavoidable at the start and end of rotation. In order to prevent wear during this contact, the bearing surface is made of a polymer material such as polyethylene, and the impeller rotating shaft including the pivot portion is made of ceramic such as a high-purity alumina sintered material.

  In order for the impeller to rise, a clearance is required between the impeller's rotational shaft length and the distance between the pair of bearing surfaces. However, it is difficult to prevent thrombus due to blood retention that makes the clearance value too small. . On the other hand, if the clearance is too large, the shaft support by the bearing will not be stable. According to experiments, the clearance value C is in the range of 0.4 mm ≦ C ≦ 1 mm, which is most preferable from the viewpoints of thrombus prevention effect, stable impeller floating, prevention of deviation from the pivot shaft bearing, and the like. Be evaluated.

According to the experimental data, the relationship between the rotation speed of the impeller, the clearance, and the behavior of the impeller in the casing is generally as follows. The larger the clearance value, the higher the impeller rises over a wider rotation speed range. Can be maintained.
A: When the clearance is 0.4 mm (a) 1000 revolutions / minute: The impeller does not float, so the lower pivot shaft and the bearing surface of the rotating shaft are in contact with each other, and the upper pivot shaft is not in contact with the bearing surface. It is in.
(B) 1400 revolutions / minute: The impeller is levitated and the upper and lower pivot shafts are not in contact with the bearing surface.
(C) 1600 revolutions / minute: The impeller is floating, and the upper and lower pivot shafts are not in contact with the bearing surface. However, the distance between the lower end of the impeller and the bottom of the casing is greater than that in (b), but the distance has not yet reached the clearance value of 0.4 mm.
(D) 1800 revolutions / minute: The impeller is lifted from the position (c), and the pivot shaft at the upper end is in contact with the bearing surface. Therefore, although the distance between the lower end pivot shaft and the bearing surface is theoretically a clearance value of 0.4 mm, the upper end pivot shaft and the bearing surface are in a state where contact and non-contact are repeated in practice. The distance between the pivot shaft and the bearing surface is not always maintained at 0.4 mm.

B: When the clearance is 1.00 mm (a) 1000 revolutions / minute: The impeller does not float, so the lower pivot shaft and the bearing surface of the rotating shaft are in contact, and the upper pivot shaft is not in contact with the bearing surface. It is in.
(B) 1400 revolutions / minute: The impeller is levitated and the upper and lower pivot shafts are not in contact with the bearing surface.
(C) 1600 revolutions / minute: The impeller is floating, and the upper and lower pivot shafts are not in contact with the bearing surface. However, although the distance between the lower end of the impeller and the bottom of the casing is greater than that in (b), the distance has not yet reached the clearance value of 1.00 mm.
(D) 1800 revolutions / minute: The impeller is still floating, and the upper and lower pivot shafts are not in contact with the bearing surface. However, the distance between the lower end of the impeller and the bottom of the casing is further increased as compared with the above (c), but the distance has not yet reached the clearance value of 1.00 mm.
(E) 1800 revolutions / minute: The impeller is further raised from the position (d), and the upper end pivot shaft is in contact with the bearing surface.
Therefore, the distance between the lower end pivot shaft and the bearing surface is theoretically a clearance value of 1.00 mm, but in reality, the upper end pivot shaft and the bearing surface are repeatedly in contact and non-contact. The distance between the pivot shaft and the bearing surface is not always maintained at 1.00 mm.

As described above, the impeller can be lifted only by controlling the rotation speed of the impeller, that is, only by setting the rotation speed of the magnetic drive means to a predetermined value, no special means for floating the impeller is required. The rotational speed control means of the magnetic drive means differs depending on the rotational drive means (electric motor) provided in the magnetic drive means. However, when an AC motor such as a synchronous motor or an induction motor is employed, an inverter circuit is used for a DC brushless motor. Is the pulse width modulation (Pulse
It is composed of a small and lightweight circuit such as rotation speed control by switching such as Width Modulation or phase locked loop.

The impeller in the casing is rotated by being coupled with the magnetic drive means outside the casing through the so-called magnet coupling through the bottom wall of the casing, and the magnetic attraction force between the impeller and the magnetic drive means is constant. is there.
Since the impeller is lifted against this magnetic attractive force, a magnetic attractive force adjusting means for adjusting the balance between the magnetic attractive force and the floating force may be provided to control the behavior of the impeller in the casing. desirable. The impeller can be made to move up and down in the casing by controlling the number of rotations to prevent blood retention, but this up-and-down behavior can be more easily controlled by adjusting the balance.

  The pipe as the suction portion attached to the upper portion of the casing has a bent portion from the anatomical point of view of the pump in vivo or from the hydrodynamic and physiological viewpoint when blood flows into the casing. A structure in which the inlet inlet portion does not physically damage the body tissue is adopted.

  When the radius of curvature of the bearing surface of the pivot bearing is set to be approximately twice or more than the radius of curvature of the pivot of the impeller's rotating shaft, the pivot shaft of the impeller swings around the center of the bearing under predetermined conditions in the pivot bearing. Because of this, the lower part of the impeller can also swing (swing), and blood retention at the bottom of the casing is eliminated.

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a longitudinal sectional view showing an embodiment of a blood flow pump.
In the figure, 1 is a conical impeller, 2 is a casing for rotatably housing the impeller, 3 is magnetic drive means provided below the casing 2, 4 is interposed between the casing 2 and the magnetic drive means 3. It is the mounted magnetic attraction force adjusting means.
The impeller 1, the casing 2, and the outer portion 38 of the magnetic drive device 3 are made of biocompatible material such as pure titanium or titanium alloy Ti4A16V. Furthermore, the front side of the impeller 1, the inner wall of the casing 2, etc. The contacted portion has a polished surface with a roughness of 0.2 μm or less so as to suppress adhesion of blood kosaka.

  A blade 13 is provided on the side portion 11 of the impeller 1, and blood flowing from the suction portion 24 of the casing 2 as the impeller 1 rotates is discharged from the discharge portion 25. A plurality of permanent magnets 14 are embedded in the bottom 12 of the impeller 1, and the impeller 1 is rotated in cooperation with the magnetic driving means 3.

Pivot shafts 16 and 17 are formed at the upper and lower ends of the rotating shaft 15 of the impeller 1 and are loosely fitted to the upper pivot bearing 26 and the lower pivot bearing 27, respectively.
As shown in FIG. 2, when the impeller 1 is stationary, a predetermined clearance C is formed between the upper end of the pivot shaft 16 and the bearing surface of the upper pivot bearing 26. In this embodiment, C = 0.6 mm. Is set. That is, the length of the rotating shaft 15 of the impeller 1 is 0.6 mm shorter than the distance between the bearing surfaces of the upper pivot bearing 26 and the lower pivot bearing 27. According to the experimental results, the effect of thrombus prevention is drastically reduced when the value of the clearance C is less than 0.4 mm.

  In this embodiment, as shown in FIG. 3, the radius of curvature R of the bearing surfaces of the pivot bearings 26 and 27 is set to be twice the radius of curvature r of the pivot shafts 16 and 17, so This makes it easy to move.

  Moreover, although the pivot shafts 16 and 17 of the rotating shaft 15 are formed of a ceramic material, only the surface may be covered with ceramics. On the other hand, the bearing surfaces of the upper pivot bearing 26 and the lower pivot bearing 27 are formed of a polymer material such as polyethylene.

Further, in FIG. 1, reference numeral 24 denotes a pipe as a suction portion fixed to the upper portion of the conical casing 2, which is composed of a horizontal base portion 24 a and an opening portion 24 b bent from now on. Projecting substantially parallel to the inner wall surface of the inclined surface portion 21 and the inclined surface portion 11 of the impeller 1, the blood flow sucked from the suction portion 24 thereby causes the rotation shaft 15, the inner wall surface of the inclined surface portion 21, the inclined surface portion 11 of the impeller 1, etc. To prevent hemolysis. Reference numeral 25 denotes a pipe serving as a discharge portion attached horizontally to the slope portion 21 of the casing 2.
The pipes constituting the suction part 24 and the discharge part 25 are made of pure titanium or Ti4A16V, which is a titanium alloy, and have a polished surface with a roughness of 0.2 μm or less on the inner side in contact with blood. Kosaka's adhesion is suppressed.

  In FIG. 1, the magnetic drive means 3 provided below the casing 2 includes a coupling magnet 31 and a rotary drive means (electric motor) 32 that rotates the coupling magnet 31 about the rotation axis of the impeller 1. It has. The coupling magnet 31 is provided so as to face the plurality of permanent magnets 14 of the impeller 1, the two magnets are magnetically coupled, and the impeller 1 rotates with the rotation of the permanent magnet 14 as the coupling magnet 31 rotates. To do. A plurality of coupling magnets 31 are arranged symmetrically around the shaft 33 and are fixed to the upper portion of the rotor 35 supported by the shaft 33 via bearings 34. The rotor 35 is a rotor similar to that of a DC brushless motor, 36 is a winding, and 37 is a stator. In addition, g is a space formed between the bottom plate 23 and the coupling magnet 31.

There are various rotational speed control means depending on the type of the rotation drive means (electric motor) 32, but if the rotation drive means (electric motor) 32 is a DC motor, pulse width modulation (Pulse
When an AC motor is employed by switching such as (Width Modulation), the rotational speed is controlled by an inverter circuit, and the rotational speed control means is provided integrally with or separately from the rotational drive means (electric motor) 32.

  Further, in the embodiment shown in FIG. 1, a magnetic attractive force adjusting means 4 is provided between the casing 2 and the magnetic driving means 3. In the figure, the magnetic attractive force adjusting means 4 is a spacer that is detachably interposed between the support plate 41 of the casing 2 and the outer portion 38 of the magnetic drive means 3 or between the support plate 41 and the bottom surface of the casing 2. 42 and the support plate. By attaching and detaching the spacer 42, the distance between the permanent magnet 14 of the impeller 1 and the coupling magnet 31 of the magnetic drive means 3 is adjusted to adjust the magnetic attractive force between the two. The magnetic attractive force between the two may be adjusted by adjusting the exciting current corresponding to the coupling magnet 31 made of an electromagnet, regardless of the spacer.

  When the blood flow pump according to the present application is used as an implantable complete heart or auxiliary heart, the overall system includes a blood flow pump implanted in the body, a percutaneous power transmission path, an external power source, An example of an animal experiment as an auxiliary heart used in combination with a living heart will be described below.

  The blood flow pump is implanted to bypass the living heart (exactly the ventricle). In this way, the burden on the affected living heart is reduced for a predetermined period to recover the heart disease during that period, which is about 6 months for dilated cardiomyopathy and ischemic cardiomyopathy. About 2 years. Implantation is performed with a pair of pumps, one (right pump) between the right ventricle and the pulmonary artery of the living heart and the other (left pump) between the left ventricle and the aorta.

  Due to the bypass implantation of the blood flow pump, a pressure difference of about 0-50 mmHg for the right pump and about 0-100 mmHg for the left pump is repeatedly received 70-100 times per minute. Here, the pressure difference is a pressure difference between the suction part 24 and the discharge part 25 of the pump, and the fact that this pressure difference occurs 70-100 times per minute means that the pressure difference occurs as many times as the number of beats of the living heart. Means.

  Thus, a blood flow pump that is a centrifugal pump that generates a continuous flow generates a pulsatile flow as long as the living heart is operating. That is, the centrifugal pump maintains a large flow rate (corresponding to the systole of the heart) when the pressure difference is low, but pulsates a small flow rate (corresponding to the diastole of the heart) when the pressure difference is high.

FIG. 4 is a graph showing the position of the impeller 1 in the casing 2 in relation to the impeller rotational speed, blood flow rate, and pressure difference based on the experimental data. The Y axis shows the pressure difference value, and the X axis shows the blood flow rate. ing.
In this graph, reference numerals 101, 102, 103, 104, 105, 106, 107, and 108 indicate positions of the impeller 1 in the casing 2 when the impeller rotational speed is 1200 RPM, 1400 RPM, 1600 RPM, 1800 RPM, 2000 RPM, 2200 RPM, 2400 RPM, and 2600 RPM, respectively. Is shown. In the left region of the oblique line L in the graph, the upper contact, that is, the upper pivot shaft 16 of the impeller 1 is in contact with the bearing surface of the upper pivot bearing 26 in the range above the upper dotted line L1. In the range below the lower dotted line L2, the bottom contact, that is, the lower end pivot shaft 17 is in contact with the bearing surface of the lower pivot bearing 27.
In the region between the upper dotted line L1 and the lower dotted line L2, the impeller 1 is in a floating state.

Based on the graph, it is possible to obtain the impeller rotational speed necessary for maintaining the floating or top contact of the impeller 1 at the required blood flow rate. Now, when the blood flow rate requires a little over 5 liters per minute, the impeller 1 is in a floating state at an impeller rotational speed of 1600 RPM. For example, when the rotational speed is increased to 1800 RPM, the impeller 1 shows a top contact state. As described above, regarding the relationship between the pressure difference and the flow rate, the flow rate decreases as the pressure difference increases, and conversely, the flow rate increases as the pressure difference decreases. When the impeller rotational speed is 1600 RPM, a pressure difference of about 50 mmHg is required to obtain a flow rate of more than 5 liters per minute. When the pressure difference is less than this, a predetermined flow rate cannot be obtained unless the rotational speed is increased. Thus, using the above graph as an index, it is possible to know the rotational speed at which the floating state or the top contact state can be maintained, and by inputting this rotational speed to the rotational speed control means, the impeller is brought into a floating or top contact state. It can be maintained to achieve thrombus prevention and other objectives of the present invention. As shown in the graph of FIG. 5, the thrombus prevention effect is particularly remarkable. FIG. 5A is a graph comparing the adhesion of platelets on the bottom surface of the impeller 1 between the bottom contact and the top contact.
The adhesion of platelets to the bottom surface of the impeller 1 in the case of the top contact is reduced by 27.9% compared to the case of the bottom contact.
FIG. 5B is a graph comparing platelet adhesion on the bottom surface of the casing 2 between the bottom contact and the top contact.
The adhesion of platelets to the bottom surface of the casing 2 in the case of the top contact is reduced by 35.2% compared to the case of the bottom contact.

By the operation of the living heart, the pressure difference as described above is generated in the casing 2 in accordance with the pulsation of the living heart, thereby generating a pressure difference between the upper part and the lower part of the impeller. On the other hand, as described above, when the impeller 1 is stationary, a clearance of 0.6 mm is formed between the upper end of the pivot shaft 16 and the bearing surface of the upper pivot bearing 26. As a result, the impeller that rotates in the casing moves up and down even if the rotational speed does not vary. Further, as described above, the radius of curvature R of the bearing surfaces of the pivot bearings 26 and 27 is set to be twice the radius of curvature r of the pivot shafts 16 and 17, so that the pumping blood volume is the beat of the living heart. When a difference occurs in the amount of suction and discharge in synchronization with the movement, a swinging motion (tail swinging motion) occurs in the impeller.
Such an up-and-down movement of the impeller and an oscillating motion (tail-swing motion) fluidize the staying part of the blood in the casing and contribute to the prevention of thrombus generation.

In the experiment, when 6 pumps were implanted in the left ventricle and 5 pumps were implanted in the right ventricle and operated for 1 to 6 months, no thrombus was observed in the casing. In this experiment, the pulsation of the living heart is 70 to 100 times / min. Therefore, the above-mentioned vertical movement and swinging motion (tail swinging motion) of the impeller is also 70 to 100 times / min. It is thought that the fluctuation also occurred in the staying portion at 70 to 100 times / minute.
In addition to the above-described movement of the impeller, the thrombus is controlled by the blood flow at the bottom of the casing due to a gap formed between the lower end of the pivot shaft and the bearing surface of the lower pivot bearing by the impeller floating or the top contact. Has also contributed.

  As for the above-described vertical movement and swinging movement (tail swinging movement) of the impeller, the example of the pulsation of the living heart has been described. However, the magnetic attractive force between the impeller magnet and the coupling magnet is adjusted by the magnetic attractive force adjusting means. It can also be realized by continuously changing.

It is a longitudinal section concerning one example of a blood flow pump. It is sectional drawing which shows the clearance between an impeller rotating shaft (pivot shaft) and a bearing. It is an expanded sectional view of a pivot shaft and a pivot bearing. It is a graph which shows the position in the casing of the impeller in the relationship between an impeller rotation speed, a blood flow rate, and a pressure difference. It is a graph which shows the change of the adhesion rate of platelets by the position of an impeller.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Impeller 2 Casing 3 Magnetic drive means 4 Magnetic attraction force adjustment means 16 Pivot shaft (upper end)
17 Pivot shaft (lower end)
26 Pivot bearing (top)
27 Pivot bearing (lower part)

Claims (7)

An impeller, a casing having a suction portion and a discharge portion, which rotatably stores the impeller, a magnetic driving means for rotating the impeller magnetically acting from outside the casing on a magnet provided in the impeller, and the magnetic action A magnetic attraction force adjusting means for adjusting the attraction force; a rotational speed control means for the magnetic drive means; and a pair of pivot bearings for supporting pivots at both ends of the rotating shaft of the impeller. The distance between them is set to be larger than the rotation shaft length of the impeller, and the rotation speed of the impeller is controlled to a predetermined number by the rotation speed control means so that the impeller floats in the blood flow in the casing and pivots at both ends of the rotation shaft of the impeller And a blood flow pump that maintains the bearing surfaces of both the pivot bearings in a non-contact state. 2. The blood flow pump according to claim 1, wherein the difference between the distance between the bearing surfaces of both pivot bearings and the rotation shaft length of the impeller is set to 0.4 mm or more. The casing or impeller is made of titanium or a titanium alloy, and the surface roughness of titanium or titanium alloy at the blood contact portion in the casing or impeller is polished to 0.5 microns or less. Or blood flow pump as described. The suction part is composed of a pipe made of titanium or a titanium alloy having a bent part and connected to the upper part of the casing, and the surface roughness of the inner surface of the pipe is polished to 0.5 microns or less. The blood flow pump according to any one of claims 1 to 3. The blood flow pump according to any one of claims 1 to 4, wherein the bearing surfaces of the pair of pivot bearings that support both end pivots of the rotating shaft of the impeller are formed of an ultra high polymer material. 6. The blood flow pump according to claim 1, wherein both end pivots of the rotating shaft of the impeller are formed of a ceramic material. The blood flow pump according to any one of claims 1 to 6, wherein the curvature of the bearing surface of the pivot bearing portion is set larger than the curvature of the pivot of the rotating shaft of the impeller.

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