Mandrel and assembly method for motor type CRDM assembly

Abstract

The present invention discloses a mandrel and an assembly method for a motor-type CRDM assembly, wherein the mandrel comprises a mandrel body, and an electric heating device is further provided on the mandrel body, and the electric heating device is used to heat the side of the mandrel body. The assembly method is a motor-type CRDM assembly method based on the mandrel. The mandrel and the assembly method provided by the present solution are used to reduce the cavitation between the outer side of the spacer and the stator winding, which is beneficial to improving the electromagnetic efficiency of the CRDM.

Description

Technical Field

[0001] The present invention relates to the technical field of control rod drive mechanisms, and in particular to a core rod for assembling a motor type CRDM and an assembling method thereof. Background Art

[0002] CRDM is the abbreviation of "control rod drive mechanism", which is the servo mechanism of the reactor control and protection system. It can drive the control rod assembly up and down in the core according to instructions, maintain the control rod assembly at a specified height, release the control rod assembly when the power is cut off, and quickly insert it into the core, thereby completing the functions of reactor startup, power adjustment, power maintenance, normal shutdown and accident shutdown.

[0003] At present, most CRDMs in marine nuclear reactors are motor-type CRDMs. However, motor-type CRDMs in the prior art have the disadvantages of high failure rate and inconvenience in use. In the prior art, regarding the optimization design of motor-type CRDM, there is a technical solution with application number: CN201721371157.0 and patent name: A shielded motor-type CRDM. The technical solution includes a technical feature spacer, which plays a shielding role in the CRDM and can solve the following problems: avoid the degradation of CRDM insulation performance in high temperature and high humidity environments, and various complex problems caused by mildew of electrical materials, and provide strong guarantees for the reliable operation of reactors on water and in high temperature and high humidity environments.

[0004] Since the spacer has a thin-walled characteristic, in order to ensure the installation accuracy of the spacer and improve the installation efficiency, a technical solution such as application number CN201810709587.1 and patent name "A method for installing a spacer in a motor-type CRDM" has appeared in the prior art.

[0005] According to the characteristics of the spacer, further optimizing the assembly of the motor-type CRDM to facilitate the performance of the motor-type CRDM is an important direction for technical personnel to study and explore. Summary of the invention

[0006] In view of the above-mentioned characteristics of the spacer, further optimizing the assembly of the motor-type CRDM to facilitate the performance of the motor-type CRDM is an important direction of research and exploration for technical personnel in this field. The present invention provides a core rod and an assembly method for assembling a motor-type CRDM. The core rod and the assembly method provided by this scheme are used to reduce the air pockets between the outer side of the spacer and the stator winding, which is beneficial to improving the electromagnetic efficiency of the CRDM.

[0007] The core rod and assembly method for motor-type CRDM assembly provided by the present invention solve the problem through the following technical points: the core rod for motor-type CRDM assembly includes a core rod body, and the core rod body is also provided with an electric heating device, and the electric heating device is used to heat the side of the core rod body.

[0008] In the prior art, the use of spacers added to motor-type CRDM can effectively solve the problem of moisture in motor-type CRDM. In order to reduce the electromagnetic efficiency caused by the use of spacers, in the prior art, the influence of spacers on the electromagnetic efficiency of motor-type CRDM can be effectively avoided by selecting the material of the spacers. Further optimizing the structural design and material selection of motor-type CRDM to facilitate the electromagnetic efficiency of the motor-type CRDM mentioned above is a technical problem that needs to be solved urgently by those skilled in the art.

[0009] In response to the above-mentioned problem about electromagnetic efficiency, this solution provides a specific form of mandrel for spacer installation. When the mandrel is used, the motor type CRDM is assembled in the following way:

[0010] S1. Sleeve the spacer to be assembled on the core rod body to obtain a combination consisting of the core rod body and the spacer; S2. Insert the combination into the motor type CRDM from one end of the motor type CRDM so that the spacer is in its installation position in the motor type CRDM; S3. Heat the spacer with an electric heating device, and the heating temperature is such that the temperature of the spacer is between 250°C and 300°C; S4. Complete the welding of the two ends of the spacer with the end cover and the base of the motor type CRDM, and the welding is such that both ends of the spacer are connected to the end cover or the base through a ring weld; S5. Use the threading hole on the electrical connector on the motor type CRDM to vacuum the air cavity between the spacer and the stator winding, and after the vacuum treatment is completed, the threading hole is potted and sealed. The design of this scheme takes into account the situation that when the motor type CRDM is working, the thermal expansion of the stator winding is less than the thermal expansion of the spacer. In the above step S3, the electric heating device is used to heat the spacer to its working temperature, so that it is convenient to directly reflect the matching condition with the stator winding in the working state through its thermal expansion, and it is beneficial to change the design size of the spacer during manufacturing so that the gap between the stator winding and the outer side of the spacer is as small as possible to reduce the influence of the cavitation introduced by the gap on the electromagnetic efficiency; in step S4, since the spacer is heated in step S3, the heated end of the spacer has better flexibility, and when the spacer is welded by laser welding or argon arc welding, the spacer can be further improved through the corresponding constraint tooling to optimize the end welding quality; in the above step S5, since the spacer is heated in step S3, the spacer expands after heating, and the gap between it and the stator winding is smaller than that in the cold state, so it is beneficial to obtain a higher vacuum degree to improve the gap between the spacer and the stator winding when the spacer is working, and finally achieve the purpose of benefiting the electromagnetic efficiency of the motor type CRDM.

[0011] Further technical solutions are:

[0012] As a specific mandrel arrangement scheme, the arrangement is as follows: the mandrel body comprises a mandrel and an outer sleeve, the outer sleeve is mounted on the mandrel and the outer sleeve serves as a surface layer on the side of the mandrel body;

[0013] The electric heating device includes a cylindrical heating layer, which is arranged on the inner side of the outer sleeve and is coaxial with the outer sleeve. In this solution, the outer sleeve is used as a component directly outside the core rod to constrain the inner side of the spacer, and the wall of the outer sleeve is used as an intermediate spacer between the heating layer and the spacer. The use of this solution not only facilitates obtaining a smoother outer side of the core rod to protect the inner wall of the spacer, but also facilitates uniform heating of the spacer and obtains thermal deformation closer to the working state. The heating layer is a direct heating component on the electric heating device, and is arranged in a cylindrical shape and coaxial with the outer sleeve, which is also intended to allow the spacer to be heated evenly.

[0014] As a specific implementation scheme for making the heating layer generate heat relatively uniformly at all places, it is set as follows: the heating layer includes a cylindrical substrate and a heating wire embedded in the substrate, the heating wire is a spiral coaxial with the substrate, and the heating wire extends from one end of the substrate to the other end. In this scheme, for ease of manufacturing, the substrate is made of insulating potting material.

[0015] In order to accurately obtain the temperature of the spacer, and as a technical solution to achieve temperature acquisition without affecting the spacer: it also includes a temperature sensor arranged on the inner side of the heating layer for measuring the temperature of the inner surface of the heating layer, and the outer side of the substrate is in contact with the inner side of the outer sleeve. In this solution, the temperature sensor is arranged on the inner side of the heating layer, and the temperature value of the spacer can be indirectly obtained through the temperature change on the inner side of the heating layer. The setting position of the temperature sensor does not affect the corresponding operation of the spacer.

[0016] More specifically, the temperature sensor is fixed on the core shaft. Preferably, for the convenience of wiring, the core shaft is configured as a hollow shaft, the center hole of the hollow shaft is used as a threading hole, and a radial hole communicating with the center hole is provided on the core shaft as a mounting hole for the temperature sensor.

[0017] As a technical solution that can not only complete the connection between the core shaft and the outer sleeve, but also allow the outer sleeve to freely extend to the end when it is deformed by heat, so as to reduce or avoid the influence of the deformation of the outer sleeve on the shape or deformation of the spacer: the core shaft is in the shape of a stepped shaft, a partial section of the core shaft is a connecting section with an external thread on the outside, a partial section of the core shaft is a positioning section with a smooth outer wall, the inner side of the outer sleeve is provided with an internal thread threadedly connected to the connecting section, and the partial section inside the outer sleeve can slide with the positioning section. The sliding fit means that it can not only realize radial constraint on the outer sleeve, but also free axial expansion and contraction when thermal expansion occurs. For those skilled in the art, the above sliding fit can be adopted in a transition fit manner.

[0018] As a technical solution that facilitates the outer sleeve to constrain the position of the spacer on its axis, can provide end constraints for both ends of the outer sleeve, and can provide axial deformation space for the heating layer to reduce or avoid the deformation of the heating layer on the outer sleeve: the outer sleeve includes a constant diameter section and a variable diameter section, the variable diameter section is in a trumpet shape, and the end of the variable diameter section with a smaller diameter is butted against one end of the constant diameter section;

[0019] The matching point of the connecting section on the outer sleeve is located at one end of the equal diameter section, the matching point of the positioning section is located at the other end of the equal diameter section, and the outer diameter of the positioning section is equal to the inner diameter of the section on the outer sleeve used to match the positioning section.

[0020] Of the connecting section and the positioning section, the connecting section is located on the end of the outer sleeve provided with the reducing section;

[0021] The heating layer is located between the two ends of the equal-diameter section, and the length of the heating layer is smaller than the spacing between the connecting section and the positioning section.

[0022] In order to facilitate the connection between the core shaft and the corresponding assembly equipment, it is configured as follows: a fixing frame for fixing the heating layer and the core shaft is fixed on the core shaft, at least one end of the core shaft protrudes relative to the outer sleeve, and also includes a connecting seat arranged on the protruding section of the core shaft relative to the outer sleeve.

[0023] The present solution also discloses an assembly method for a motor-type CRDM, which uses the mandrel described in any one of the above items to complete the motor-type CRDM assembly, including the following steps performed in sequence:

[0024] S1, putting the spacer to be assembled on the mandrel body to obtain a combination of the mandrel body and the spacer;

[0025] S2, inserting the assembly into the motor-type CRDM from one end of the motor-type CRDM, so that the spacer is in its installation position in the motor-type CRDM;

[0026] S3, heating the spacer by an electric heating device, and the heating temperature is such that the temperature of the spacer is between 250° C. and 300° C.;

[0027] S4, completing the welding of the two ends of the spacer with the end cover and the base of the motor-type CRDM, and the welding is: making both ends of the spacer connected to the end cover or the base through the girth weld;

[0028] S5. Use the threading holes on the electrical connector of the motor-type CRDM to evacuate the air pockets between the spacer and the stator winding. After the evacuation is completed, the threading holes are sealed by potting.

[0029] The present invention has the following beneficial effects:

[0030] In response to the above-mentioned problem about electromagnetic efficiency, this solution provides a specific form of core rod and assembly method for spacer installation. When the core rod is used, the motor type CRDM is assembled in the following way:

[0031] S1. Sleeve the spacer to be assembled on the core rod body to obtain a combination consisting of the core rod body and the spacer; S2. Insert the combination into the motor type CRDM from one end of the motor type CRDM so that the spacer is in its installation position in the motor type CRDM; S3. Heat the spacer with an electric heating device, and the heating temperature is such that the temperature of the spacer is between 250°C and 300°C; S4. Complete the welding of the two ends of the spacer with the end cover and the base of the motor type CRDM, and the welding is such that both ends of the spacer are connected to the end cover or the base through a ring weld; S5. Use the threading hole on the electrical connector on the motor type CRDM to vacuum the air cavity between the spacer and the stator winding, and after the vacuum treatment is completed, the threading hole is potted and sealed. The design of this scheme takes into account the situation that when the motor type CRDM is working, the thermal expansion of the stator winding is less than the thermal expansion of the spacer. In the above step S3, the electric heating device is used to heat the spacer to its working temperature, so that it is convenient to directly reflect the matching condition with the stator winding in the working state through its thermal expansion, and it is beneficial to change the design size of the spacer during manufacturing so that the gap between the stator winding and the outer side of the spacer is as small as possible to reduce the influence of the cavitation introduced by the gap on the electromagnetic efficiency; in step S4, since the spacer is heated in step S3, the heated end of the spacer has better flexibility, and when the spacer is welded by laser welding or argon arc welding, the spacer can be further improved through the corresponding constraint tooling to optimize the end welding quality; in the above step S5, since the spacer is heated in step S3, the spacer expands after heating, and the gap between it and the stator winding is smaller than that in the cold state, so it is beneficial to obtain a higher vacuum degree to improve the gap between the spacer and the stator winding when the spacer is working, and finally achieve the purpose of benefiting the electromagnetic efficiency of the motor type CRDM. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a front view of a specific embodiment of the mandrel for assembling a motor-type CRDM according to the present invention;

[0033] Figure 2 yes Figure 1 A cross-sectional view of the structure shown.

[0034] The reference numerals in the figure are: 1. outer sleeve, 2. heating layer, 3. temperature sensor, 4. reducing section, 5. connecting seat, 6. core shaft, 7. connecting section, 8. positioning section, 9. fixing frame. DETAILED DESCRIPTION

[0035] The present invention will be further described in detail below in conjunction with the embodiments, but the structure of the present invention is not limited to the following embodiments.

[0036] Embodiment 1:

[0037] like Figure 1 and Figure 2 As shown, the mandrel used for assembling the motor-type CRDM includes a mandrel body, and the mandrel body is also provided with an electric heating device, and the electric heating device is used to heat the side of the mandrel body.

[0038] In the prior art, the use of spacers added to motor-type CRDM can effectively solve the problem of moisture in motor-type CRDM. In order to reduce the electromagnetic efficiency caused by the use of spacers, in the prior art, the influence of spacers on the electromagnetic efficiency of motor-type CRDM can be effectively avoided by selecting the material of the spacers. Further optimizing the structural design and material selection of motor-type CRDM to facilitate the electromagnetic efficiency of the motor-type CRDM mentioned above is a technical problem that needs to be solved urgently by those skilled in the art.

[0039] In response to the above-mentioned problem about electromagnetic efficiency, this solution provides a specific form of mandrel for spacer installation. When the mandrel is used, the motor type CRDM is assembled in the following way:

[0040] S1. Sleeve the spacer to be assembled on the core rod body to obtain a combination consisting of the core rod body and the spacer; S2. Insert the combination into the motor type CRDM from one end of the motor type CRDM so that the spacer is in its installation position in the motor type CRDM; S3. Heat the spacer with an electric heating device, and the heating temperature is such that the temperature of the spacer is between 250°C and 300°C; S4. Complete the welding of the two ends of the spacer with the end cover and the base of the motor type CRDM, and the welding is such that both ends of the spacer are connected to the end cover or the base through a ring weld; S5. Use the threading hole on the electrical connector on the motor type CRDM to vacuum the air cavity between the spacer and the stator winding, and after the vacuum treatment is completed, the threading hole is potted and sealed. The design of this scheme takes into account the situation that when the motor type CRDM is working, the thermal expansion of the stator winding is less than the thermal expansion of the spacer. In the above step S3, the electric heating device is used to heat the spacer to its working temperature, so that it is convenient to directly reflect the matching condition with the stator winding in the working state through its thermal expansion, and it is beneficial to change the design size of the spacer during manufacturing so that the gap between the stator winding and the outer side of the spacer is as small as possible to reduce the influence of the cavitation introduced by the gap on the electromagnetic efficiency; in step S4, since the spacer is heated in step S3, the heated end of the spacer has better flexibility, and when the spacer is welded by laser welding or argon arc welding, the spacer can be further improved through the corresponding constraint tooling to optimize the end welding quality; in the above step S5, since the spacer is heated in step S3, the spacer expands after heating, and the gap between it and the stator winding is smaller than that in the cold state, so it is beneficial to obtain a higher vacuum degree to improve the gap between the spacer and the stator winding when the spacer is working, and finally achieve the purpose of benefiting the electromagnetic efficiency of the motor type CRDM.

[0041] Embodiment 2:

[0042] like Figure 1 and Figure 2 As shown, this embodiment is further limited on the basis of the embodiment 1: as a specific mandrel setting scheme, it is set as follows: the mandrel body includes a mandrel 6 and an outer sleeve 1, the outer sleeve 1 is installed on the mandrel 6 and the outer sleeve 1 serves as the surface layer of the side of the mandrel body;

[0043] The electric heating device includes a cylindrical heating layer 2, which is arranged on the inner side of the outer sleeve 1, and the heating layer 2 is coaxial with the outer sleeve 1. In this solution, the outer sleeve 1 is used as a component directly used to constrain the inner side of the spacer on the outside of the core rod, and the wall of the outer sleeve 1 is used as an intermediate spacer between the heating layer 2 and the spacer. The use of this solution is not only convenient for obtaining a smoother outer side of the core rod to protect the inner wall of the spacer, but also convenient for the spacer to be heated evenly and obtain thermal deformation closer to the working state. The heating layer 2 is a direct heating component on the electric heating device, and is arranged in a cylindrical shape and coaxial with the outer sleeve 1, which is also intended to allow the spacer to be heated evenly.

[0044] As a specific implementation scheme for making the heating layer 2 generate heat relatively uniformly everywhere, it is set as follows: the heating layer 2 includes a cylindrical substrate and a heating wire embedded in the substrate, the heating wire is a spiral coaxial with the substrate, and the heating wire extends from one end of the substrate to the other end. In this scheme, for ease of manufacturing, the substrate is made of insulating potting material.

[0045] In order to accurately obtain the temperature of the spacer, and as a technical solution to achieve temperature acquisition without affecting the spacer: it also includes a temperature sensor 3 arranged on the inner side of the heating layer 2 for measuring the temperature of the inner surface of the heating layer 2, and the outer side of the substrate is in contact with the inner side of the outer sleeve 1. In this solution, the temperature sensor 3 is arranged on the inner side of the heating layer 2, and the temperature value of the spacer can be indirectly obtained through the temperature change on the inner side of the heating layer 2. The setting position of the temperature sensor 3 does not affect the corresponding operation of the spacer.

[0046] More specifically, the temperature sensor 3 is fixed on the core shaft 6. Preferably, in order to facilitate wiring, the core shaft 6 is configured as a hollow shaft, the center hole of the hollow shaft is used as a threading hole, and a radial hole communicating with the center hole is provided on the core shaft 6 as a mounting hole for the temperature sensor 3.

[0047] As a technical solution that can not only complete the connection between the core shaft 6 and the outer sleeve 1, but also allow the outer sleeve 1 to freely extend to the end when it is deformed by heat, so as to reduce or avoid the influence of the deformation of the outer sleeve 1 on the shape or deformation of the spacer: the core shaft 6 is in the shape of a stepped shaft, and a partial section of the core shaft 6 is a connecting section 7 with an external thread on the outside, and a partial section of the core shaft 6 is a positioning section 8 with a smooth outer wall. The inner side of the outer sleeve 1 is provided with an internal thread threadedly connected to the connecting section 7, and the partial section inside the outer sleeve 1 can slide with the positioning section 8. The sliding fit means that it can not only realize radial constraint on the outer sleeve 1, but also free axial expansion and contraction when thermal expansion occurs. As a person skilled in the art, the above sliding fit can be adopted in a transition fit manner.

[0048] As a technical solution that facilitates the outer sleeve 1 to constrain the spacer sleeve on its axis, can provide end constraints for both ends of the outer sleeve 1, and can provide axial deformation space for the heating layer 2 to reduce or avoid the deformation of the heating layer 2 on the outer sleeve 1: the outer sleeve 1 includes a constant diameter section and a variable diameter section 4, the variable diameter section 4 is in a trumpet shape, and the end of the variable diameter section 4 with a smaller diameter is butted against one end of the constant diameter section;

[0049] The fitting point of the connecting section 7 on the outer sleeve 1 is located at one end of the equal diameter section, the fitting point of the positioning section 8 is located at the other end of the equal diameter section, and the outer diameter of the positioning section 8 is equal to the inner diameter of the section on the outer sleeve 1 used to fit with the positioning section 8.

[0050] Of the connecting section 7 and the positioning section 8, the connecting section 7 is located on the end of the outer sleeve 1 provided with the reducing section 4;

[0051] The heating layer 2 is located between the two ends of the equal-diameter section, and the length of the heating layer 2 is smaller than the distance between the connecting section 7 and the positioning section 8 .

[0052] In order to facilitate the connection between the core shaft 6 and the corresponding assembly equipment, it is configured as follows: a fixing frame 9 for fixing the heating layer 2 and the core shaft 6 is also fixed on the core shaft 6, and at least one end of the core shaft 6 protrudes relative to the outer sleeve 1, and also includes a connecting seat 5 arranged on the protruding section of the core shaft 6 relative to the outer sleeve 1.

[0053] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific embodiments of the present invention are limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, other embodiments obtained without departing from the technical solution of the present invention should be included in the protection scope of the present invention.

Claims

1. A mandrel for assembling a motor-type CRDM, comprising a mandrel body, characterized in that: The mandrel body is also provided with an electric heating device, and the electric heating device is used to heat the side of the mandrel body; The mandrel body comprises a mandrel (6) and an outer sleeve (1), wherein the outer sleeve (1) is mounted on the mandrel (6) and the outer sleeve (1) serves as a surface layer on the side of the mandrel body; The electric heating device comprises a cylindrical heating layer (2), the heating layer (2) being arranged on the inner side of an outer sleeve (1), and the heating layer (2) and the outer sleeve (1) being coaxial; the core shaft (6) is a hollow shaft.

2. The mandrel for assembling a motor-type CRDM according to claim 1, characterized in that: The heating layer (2) comprises a cylindrical substrate and a heating wire embedded in the substrate. The heating wire is in a spiral shape coaxial with the substrate and extends from one end of the substrate to the other end.

3. The mandrel for assembling a motor-type CRDM according to claim 2, characterized in that: It also includes a temperature sensor (3) arranged on the inner side of the heating layer (2) for measuring the inner surface temperature of the heating layer (2), and the outer side of the base body is in contact with the inner side of the outer sleeve (1).

4. The mandrel for assembling a motor-type CRDM according to claim 3, characterized in that: The temperature sensor (3) is fixed on the core shaft (6).

5. The mandrel for assembling a motor-type CRDM according to claim 1, characterized in that: The core shaft (6) is in the shape of a stepped shaft, a partial section of the core shaft (6) is a connecting section (7) with an external thread on the outside, a partial section of the core shaft (6) is a positioning section (8) with a smooth outer wall, an internal thread is provided on the inside of the outer sleeve (1) and is threadedly connected to the connecting section (7), and the partial section on the inside of the outer sleeve (1) can slidably cooperate with the positioning section (8).

6. The mandrel for assembling a motor-type CRDM according to claim 5, characterized in that: The outer sleeve (1) comprises a constant diameter section and a variable diameter section (4); the variable diameter section (4) is in a bell-mouth shape, and an end of the variable diameter section (4) with a smaller diameter is butted against one end of the constant diameter section; The mating point of the connecting section (7) on the outer sleeve (1) is located at one end of the equal-diameter section, the mating point of the positioning section (8) is located at the other end of the equal-diameter section, and the outer diameter of the positioning section (8) is equal to the inner diameter of the section on the outer sleeve (1) used to cooperate with the positioning section (8).

7. The mandrel for assembling a motor-type CRDM according to claim 6, characterized in that: Of the connecting section (7) and the positioning section (8), the connecting section (7) is located on one end of the outer sleeve (1) provided with the reducing section (4); The heating layer (2) is located between the two ends of the equal-diameter section, and the length of the heating layer (2) is smaller than the distance between the connecting section (7) and the positioning section (8).

8. The mandrel for assembling a motor-type CRDM according to any one of claims 2 to 7, characterized in that: A fixing frame (9) for fixing the heating layer (2) to the core shaft (6) is also fixed on the core shaft (6); at least one end of the core shaft (6) protrudes outward relative to the outer sleeve (1), and also includes a connecting seat (5) arranged on the protruding section of the core shaft (6) relative to the outer sleeve (1).

9. An assembly method for a motor-type CRDM assembly, characterized in that: The method uses the mandrel according to any one of claims 1 to 8 to complete the assembly of the motor-type CRDM, comprising the following steps performed in sequence: S1, putting the spacer to be assembled on the mandrel body to obtain a combination of the mandrel body and the spacer; S2, inserting the assembly into the motor-type CRDM from one end of the motor-type CRDM, so that the spacer is in its installation position in the motor-type CRDM; S3, heating the spacer by an electric heating device, and the heating temperature is such that the temperature of the spacer is between 250° C. and 300° C.; S4, completing the welding of the two ends of the spacer with the end cover and the base of the motor-type CRDM, and the welding is: making both ends of the spacer connected to the end cover or the base through the girth weld; S5. Use the threading holes on the electrical connector of the motor-type CRDM to evacuate the air pockets between the spacer and the stator winding. After the evacuation is completed, the threading holes are sealed by potting.

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