JPH06288458A - Ball screw with removable screw shaft, and reactor control rod driving device utilizing this ball screw - Google Patents

Abstract

PURPOSE:To provide an internal circulating type ball screw preventing the fall of balls from a ball nut at the time of fitting/removing the ball nut to/from a screw shaft. CONSTITUTION:A ball circulating cap 5 is fitted into the ball circulating cap fitting hole 4 of a ball nut 2. The width of the opening part 8 of the ball circulating cap 5 is made narrower than the diameter of a ball 3, over the whole length. Ball retainers 6 are engagedly fixed to the inner surface of the ball nut 2. Each ball retainer 6 is provided with a hole part 12 into which the opening part side of the ball circulating cap 5 is fitted, and a spiral slot 13 formed in the same lead as a screw shaft 1 and the thread grooves 1a, 2a of the ball nut 2 and opened onto the screw shaft 1 side so as to be communicated with a ball circulating groove 7. The width of the screw shaft 1 side opening part of this spiral slot 13 is made narrower than the diameter of the ball 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an internal circulation type ball screw in which a ball circulation cap having a circulation passage communicating with a screw groove of a ball nut is provided on an inner surface of the ball nut. The present invention relates to a ball screw in which a ball does not drop from a ball nut when separated from a screw shaft. The present invention also relates to a control rod drive device for a nuclear reactor, which uses the above ball screw.

[0002]

15 and 16 show a conventional internal circulation type ball screw, in which 1 is a screw shaft, 2 is a ball nut, and 3 is a ball. The screw shaft 1 is a round rod-shaped shaft having a finite length, and a series of spiral screw grooves 1a on which the balls 3 roll are formed on the outer surface of the shaft to extend in the axial direction. The lead has a concave shape with a curvature adapted to the diameter of the ball 3 and has the same lead. Ball nut 2
Is formed in a hollow cylindrical shape, and a series of spiral thread grooves 2a on which the balls 3 roll are formed on the inner surface thereof in the same concave shape as the screw shaft 1 and with the same lead. The screw groove 2a of the ball nut 2 and the screw groove 1a of the screw shaft 1 face each other to form a passage for the ball 3. Then, the ball nut 2 is screwed into the screw shaft 1, and each of the screw grooves 1
A ball 3 is rotatably mounted between a and 2a.

Further, the ball nut 2 is formed with an oblong penetrating ball-circulation cap mounting hole 4 provided between the adjacent screw grooves 2a. The ball circulation cap mounting hole 4 has an opening 8b that opens to the screw shaft 1 side, and the ball 3 rolling along the screw groove 1a of the screw shaft 1 is inserted into the adjacent screw groove 1a and the screw groove 1a. A ball circulation cap 5b having a ball circulation groove 7b that guides it so as to ride over the peak portion 1b is fixed. Ball 3
The ball grooves closed by the screw grooves 1a and 2a and the ball circulation cap 5b are arranged so as to fill the inside of the closed ball passage for endless circulation.

As shown in FIGS. 17 to 19, an inner curved surface 9b facing the screw shaft 1 is provided on the ball circulation cap 5b.
The inner curved surface 9b has an opening 8b opening toward the screw shaft 1 side, and a closed long S-shaped ball circulation groove 7b is formed on the outer peripheral surface. The width of the opening 8b has a U-shaped cross section that is slightly larger than the diameter of the ball 3 over the entire length. In addition, 11b is an entrance / exit of the ball 3.

[0005]

By the way, the screw shaft and the ball nut must be separated from each other, as in the case of performing additional machining on the shaft end of the screw shaft or the ball nut, or when replacing the inserted ball. When it does not happen, in order to prevent the ball from falling off the ball nut, as shown in FIG. 20 or FIG.
A method of attaching and detaching is known. FIG. 20 shows a method of attaching and detaching the ball nut 2 by bringing the end surface of the temporary shaft 50 having an outer diameter smaller than the root groove diameter of the screw shaft 1 by 0.2 to 0.4 mm into contact with the end surface of the screw groove portion of the screw shaft 1. . FIG. 21 shows that when the end surface of the temporary shaft 50 cannot be directly contacted with the end surface of the screw groove of the screw shaft 1, a tape 54 or the like is wound around the shaft end machining portion of the screw shaft 1 so that the outer diameter of the temporary shaft 50 becomes the same. A method of attaching and detaching the ball nut 2 will be described. Reference numeral 52 is a stopper for preventing the ball nut 2 from falling off the temporary shaft 50.

In the above method, the temporary shaft 5 is attached to the shaft end of the screw shaft 1 while being careful to align the centers of the screw shaft 1 and the temporary shaft 50.
The ball nut 2 had to be attached and detached while pressing 0, and the ball 3 could be scratched or dropped if unfamiliar. Further, when the ball nut 2 is incorporated into the temporary shaft 50, the stopper 52 is loosened, the ball nut 2 falls off from the temporary shaft 50, and the ball 3 does not drop out, and the ball nut 2 itself may be damaged. there were.
Further, in applications where the ball nut 2 needs to be attached to and detached from the screw shaft 1, when the ball nut 2 is pulled out from the screw shaft 1, it is necessary to prevent the balls from falling off.

The present invention has been made to solve the above problems, and a first object of the present invention is to fit a ball circulation cap having a circulation passage communicating with the thread groove of the ball nut on the inner surface thereof to the ball nut. An object of the present invention is to provide a ball screw of an internal circulation type ball screw which is fixed and fixed so that the ball does not drop out from the ball nut when the ball nut is separated from the screw shaft. A second object of the present invention is to provide a control rod driving device for a nuclear reactor, which uses the above ball screw.

[0008]

According to the present invention, there is provided a screw shaft having a thread groove on an outer surface, a ball nut having a thread groove corresponding to the thread groove of the screw shaft on an inner surface, and the screw shaft and the ball nut. The ball having a plurality of freely rollable balls loaded between the two thread grooves, and an opening portion that fits into a ball circulation cap mounting hole provided in the ball nut and opens toward the screw shaft side. In a ball screw of internal circulation type having a ball circulation cap having a ball circulation groove communicating with the screw groove of the nut, in the ball circulation cap, the width of the opening of the ball circulation cap is larger than the diameter of the ball over the entire length. A narrow ball retainer is formed on the inner surface of the ball nut, which has the same lead as the thread groove of the ball nut, opens on the screw shaft side, and has a spiral slot communicating with the ball circulation groove. The width of the screw shaft-side opening of the spiral slot is narrower than the diameter of the ball, screw shaft has solved the above problems by detachable ball screw.

[0009]

The ball circulation cap has a shape to individually enclose the ball, and the width of the screw shaft side opening of the spiral slot of the ball retainer is narrower than the diameter of the ball. The balls do not fall out of the ball nut when separated.

Further, when an engaging groove is provided on the inner surface of the ball nut and the ball retainer is axially fixed by a retaining ring fitted in the engaging groove, a screw for attaching to the ball nut is thin. A ball retainer can also be used if the holes cannot be formed. Further, when the ball circulation cap is manufactured from the self-lubricating composite material, the ball screw can be widely used even in the application field where it is difficult to supply grease or lubricating oil.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. In the drawings, members and the like that are the same as or correspond to those of the conventional ball screw are designated by the same reference numerals. 1 to 10
Shows a first embodiment of the present invention.

As shown in FIGS. 1 to 3, the ball screw of the present invention comprises a screw shaft 1, a ball nut 2, a plurality of balls 3, a ball circulation cap 5 and a ball retainer 6. On the outer surface of the screw shaft 1, there is formed a thread groove 1a having a concave shape with a curvature adapted to the diameter of the ball 3 and having the same lead. The inner surface of the ball nut 2 has the same concave shape as the screw shaft 1 and has the same lead, and has a thread groove 1a of the screw shaft 1.
The thread groove 2a corresponding to is formed. Further, the ball nut 2 is formed with two elliptical penetrating ball circulation cap mounting holes 4 provided between the adjacent screw grooves 2a, and these two ball circulation cap mounting holes 4 are formed. Each have an opening 8 that opens to the screw shaft 1 side, and balls 3 that roll along the screw groove 1 a of the screw shaft 1
A ball circulation cap 5 having a ball circulation groove 7 for guiding to cross over the ridge 1b between the screw grooves is fixed to the adjacent screw groove 1a. The spiral slot 13 of the ball retainer 6 communicates with the ball circulation groove 7. The balls 3 are arranged in the spiral slot 13 so as to fill the ball passage closed by the screw grooves 1a and 2a and the ball circulation groove 7 and endlessly circulate. 16 is a mounting screw 1
8 is a screw insertion hole formed in the ball nut 2 for fixing the ball retainer 6 to the inner surface of the ball nut 2.

As shown in FIGS. 4 to 6, the ball circulation cap 5 includes an inner curved surface 9 facing the screw shaft 1 side.
The inner curved surface 9 has an opening 8 that opens to the screw shaft 1 side, and a closed long S-shaped ball circulation groove 7 is formed on the outer peripheral surface. The width of this opening 8 is the maximum width portion 10 of the ball circulation groove 7 over the entire length, that is, the ball 3
Therefore, the ball circulation cap 5 can individually enclose the ball 3. The ball insertion / removal portions 11 at both ends of the ball circulation groove 7 are aligned with the spiral slots 13 of the ball retainer 6.

As shown in FIGS. 7 and 8, a ball retainer 6 having a predetermined length and having a wall thickness disposed in an annular space between the outer surface of the screw shaft 1 and the inner surface of the ball nut 2.
Is composed of a pair of retainers 6a and 6b. The material of the ball retainer 6 (6a, 6b) is metal or synthetic resin. The left end surface 14 of the retainer 6a on the right side in the figure and the right end surface 15 of the retainer 6b on the left side in the figure work together to form a hole 12 into which the opening side of the ball circulation cap 5 is fitted, and the screw shaft 1.
Also, a spiral slot 13 having the same lead as the screw grooves 1a and 2a of the ball nut 2 is formed. The spiral slot 13 is aligned with the ball circulation groove 7 of the ball circulation cap 5 fitted in the hole 12. Further, as shown in FIGS. 9 and 10, the width of the outer peripheral side opening of the spiral slot 13 is slightly larger than the diameter of the ball 3, and the width of the screw shaft 1 side opening is smaller than the diameter of the ball 3. In addition, 17 is a screw hole for fixing the ball retainer 6 (6a, 6b) to the inner surface of the ball nut 2 using the mounting screw 18. This ball retainer 6
(6a, 6b) can be molded by machining, but can also be molded by using a mold or a mold.

Next, the order of assembling the ball screw will be described. The ball circulation cap 5 is fitted into one of the two ball circulation cap mounting holes 4 provided in the ball nut 2. The ball circulation cap mounting hole 4 has a stepped portion 4a.
Are formed. By bringing the step portion 22 formed on the ball circulation cap 5 into contact with the stepped portion 4a, the inner curved surface 9 of the ball circulation cap 5 and the inner surface of the ball retainer 6 (6a, 6b) are aligned with each other. It is set. The retainer 6a on the right side in the figure and the retainer 6 on the left side in the figure
Insert b from the left and right into the inner surface of the ball nut 2. The left end surface 14 of the right retainer 6a and the right end surface 15 of the left retainer 6b are positioned so as to form a hole 12 in conformity with the opening side of the ball circulation cap 5, and then the mounting screw is attached. 18 is screwed into the screw hole 17 of the ball retainer 6 through the screw insertion hole 16 provided in the ball nut 2, and the ball retainer 6 (6a, 6b) is fixed to the inner surface of the ball nut 2. Although FIG. 2 shows an embodiment in which the mounting screw 18 is inserted from the outside, the mounting screw is inserted from the inside into an insertion hole provided in the ball retainer 6 (6a, 6b) to form a screw formed on the ball nut. The ball retainer 6 (6a, 6b) may be fixed by screwing it into the hole. The other ball retainer 6 (6a, 6b) is similarly fixed to the inner surface of the ball nut 2.

Attach both ball retainers 6 to the ball nut 2
After fixing to the inner surface of, the ball circulation cap 5 is removed from the ball circulation cap mounting hole 4. A predetermined number of balls 3 each having a size capable of applying a predetermined amount of axial clearance or preload as a ball screw are inserted into the spiral slot 13 of the ball retainer 6 from the ball circulation cap mounting hole 4.
Further, a predetermined number of balls 3 are inserted in the ball circulation groove 7 formed in the ball circulation cap 5. In this case, if grease is applied to the ball inlet / outlet portions 11 at both ends of the ball circulation groove 7, the ball 3 will not fall out of the ball circulation groove 7. Next, the ball circulation cap 5 with the balls 3 inserted therein is fitted into the ball circulation cap mounting hole 4. Then, the space above the ball circulation cap 5 in the ball circulation cap mounting hole 4 is filled with a metal adhesive so that the ball circulation cap 5 does not pop out (not shown). The ball nut 2 holding the ball 3 is attached to the screw shaft 1, and the assembly is completed.

The order of removing the ball 3 from the ball nut 2 will be described. Remove the ball nut 2 from the screw shaft 1. After removing the ball circulation cap 5 from the ball circulation cap mounting hole 4, the ball circulation cap mounting hole 4 is turned downward. Due to the weight of the ball, the ball 3 slips out of the ball circulation cap mounting hole 4 while rolling along the thread groove 2a of the ball nut 2. Therefore, it is not necessary to remove the ball retainer 6 from the ball nut 2.

The ball nut 2 is attached to the screw shaft 1,
The state in which the balls 3 are rotatably loaded in the screw groove 1a of the screw shaft 1 and the screw groove 2a of the ball nut 2 is as shown in FIG. In FIG. 10, the ball nut 2 is the screw shaft 1
The ball 3 is held in a spiral slot 13 formed by the left end surface 14 of the right retainer 6a of the ball retainer 6 and the right end surface 15 of the left retainer 6b which are shown together. In this case, it goes without saying that the ball 3 in the ball circulation groove 7 formed in the ball circulation cap 5 is enclosed by the opening 8 (not shown).

FIG. 11 shows a second embodiment of the present invention. In this embodiment, a locking groove 19 into which a C-shaped retaining ring 20 is fitted is formed on the inner surface of the ball nut 2, and the ball retainer 6 (6a, 6b) is fixed to the inner surface of the ball nut 2. That is, the C-shaped retaining ring 20 having a shape as shown in FIG. 12 and having a thickness that fits in the locking groove 19 is slightly reduced in diameter by utilizing the spring action of the retaining ring 20. Locking groove 19 provided on the inner surface of the ball nut 2
To fix the ball retainer 6 (6a, 6b) to the inner surface of the ball nut 2 in the axial direction. Reference numeral 21 is a hole for reducing the retaining ring 20 and attaching / detaching it to / from the locking groove 19. In addition, the hole 12 is formed in each of the ball retainers 6a and 6b.
There may be an axial slit (not shown) aligned with. The slit is formed by machining an annular ball retainer. Ball retainer 6 (6a,
6b) elastically expands and presses against the inner surface of the ball nut 2. The outer diameter of the ball retainer 6 (6a, 6b) is
It is desirable that the diameter is substantially the same as the inner diameter of the ball nut 2. However, since the slit is provided and the slit expands elastically, the ball retainer 6
Even if an error occurs in the outer diameter of (6a, 6b), the ball retainer 6 (6a, 6b) itself absorbs the error. When the ball retainer 6 (6a, 6b) is made of resin, the outer diameter may be formed to be slightly larger than the inner diameter of the ball nut 2 instead of forming the slit. The ball retainer 6 (6a, 6b) is lightly press-fitted into the ball nut 2 to prevent relative displacement. Other configurations are the same as those in the first embodiment, and detailed description thereof will be omitted. The reference numerals shown in FIG. 11 correspond to the same members described in the first embodiment.

In this embodiment, the ball retainer 6 (6a,
6b), the ball retainer 6 (6
It is not necessary to provide the screw hole 17 for fixing a, 6b) and the screw insertion hole 16 in the ball nut 2. In particular, the annular space between the screw shaft 1 and the inner surface of the ball nut 2 is small, and the ball retainer 6 (6a, 6a) disposed in this space is small.
This is suitable when the wall thickness of b) becomes thin and the screw hole 17 cannot be threaded. Further, the retainer 6b on the left side of the ball liner 6 in the right row and the retainer 6 on the right side of the ball liner 6 in the left row
It is also possible to reduce the number of the locking grooves 19 by making a a integral type.

The ball circulating cap 5 shown in FIG. 4 is usually made of metal or synthetic resin. And, like the above-mentioned ball retainer 6 (6a, 6b),
It is formed by machining, or is formed by a mold or a die. However, the ball circulation cap 5 may be made of a self-lubricating composite material containing a solid lubricant. Examples of self-lubricating composite materials include those obtained by impregnating a base material such as metal with a solid lubricant such as tungsten disulfide or molybdenum disulfide, or burning a solid lubricant and a metal such as copper, tin or tantalum. There is one tied up. Therefore, it is not necessary to supply lubricating oil such as grease or oil to the ball screw, and it is not necessary to provide the ball nut 2 with a grease nipple or a lubricating oil supply passage.

By using the ball circulating cap 5 made of the self-lubricating composite material, the balls 3 passing through the ball circulating groove 7 of the ball circulating cap 5 are brought into sliding contact with the ball circulating groove 7 to thereby achieve solid lubrication. The agent is received little by little to form a film on the surface of the ball, and a solid lubricant is supplied to the thread groove 1a of the screw shaft 1 and the thread groove 2a on the inner surface of the ball nut 2 while circulating through the infinite ball circulation path. In the thread grooves 1a and 2a, the coating film of the solid lubricant formed with the ball 3 as a medium has an extremely small amount and is very thin. However, since the solid lubricant is supplied as the balls 3 circulate, the durability and operability of the ball screw can be favorably maintained for a long period of time.

In the above embodiments, the number of the ball circulation caps 5 is two, and the outer shape is oval when viewed from the direction perpendicular to the axis of the screw shaft 1. Needless to say, is not limited to the embodiment. Further, the method for mounting the ball circulation cap 5 on the ball nut 2 is as follows.
In the embodiment, the ball nut 2 is inserted from the outside and stopped at the stepped portion 4a of the ball circulation cap mounting hole 4. However, the conventional method of inserting from the inside of the ball nut 2 or another method is used. A method of combining and fixing the articles may be used.

Further, in the present embodiment, as the preloading method for the ball screw, the oversized ball system by the ball diameter is exemplified, but either the thread groove 1a of the screw shaft 1 or the thread groove 2a of the ball nut 2 is lead. A ball of a double nut type in which two ball nuts 2 are attached to the screw shaft 1 to provide a preload, or a preload is applied by a spacer or a spring sandwiched between the two ball nuts 2. The present invention can be applied to screws.

13 and 14 show a third embodiment of the present invention. This embodiment relates to a drive device including a ball screw. Hereinafter, a case where this drive device is a control rod drive device in a nuclear reactor will be described.

FIG. 13 is a partial cross-sectional view of the nuclear reactor 100. Reference numeral 101 is a reactor core, 102 is a reactor vessel, 103 is a reactor vessel lid, 104 is a control rod driving device, and 105 is a control rod. The control rod 105 is normally inserted in the core 101, and is raised and lowered by the control rod driving device 104. This controls the reactor power. The control rod drive device 104 and the control rod 105 are connected by a drive shaft 106.

FIG. 14 shows the control rod drive device 104 in detail. The control rod drive device 104 includes the reactor vessel lid 1
It is provided integrally with 03. Control rod drive device 104
Is the motor rotor 10 to which the ball nut 107 is fixed.
8, bearings 109 and 109 that rotatably support the motor rotor 108, and a motor armature 110 that applies a rotational force to the motor rotor 108. Reference numeral 111 indicates a pressure-resistant case for the motor armature 110, and reference numeral 1
Reference numeral 12 indicates a drive shaft housing.

The drive shaft 106 has a screw groove 106 'on its outer surface and functions as a screw shaft of a ball screw. Further, the drive shaft 106 is restrained against rotation (not shown). Therefore, when the reactor vessel lid 103 is lowered and the drive shaft 106 and the ball nut 107 are screwed together to drive the motor, the control rod 105 and the drive shaft 106 move up and down. This allows
The vertical position of the control rod 105, that is, the reactor power is controlled.

In the nuclear reactor, the fuel is exchanged by removing the reactor vessel lid 103 at every refueling cycle (1 to 4 years). At this time, the control rod 105 must be left inserted in the core 101. The conventional drive device is an electromagnet type or a roller nut type, but there are problems that the outer diameter of the drive device becomes large and that the transmission efficiency decreases. If a ball screw is used, a transmission efficiency of about 80% can be obtained, the structure can be simplified, and the outer diameter can be reduced, but the above problem can be solved, but when the reactor vessel lid is removed, Another problem was that the balls that make up the ball screw fall into the core.

The drive device according to the present invention prevents the balls from falling off while maintaining the advantages of the ball screw, such as size reduction and high transmission efficiency, as they are.
Even if the drive shaft 106 connected to the shaft 5, that is, the screw shaft is separated from the ball nut 107, the balls of the ball screw do not drop off.

[0031]

As described above, according to the present invention,
The ball circulation groove of the ball circulation cap independently encloses the ball, and the spiral slot of the ball retainer communicating with the ball circulation groove also holds the ball. Therefore, when the ball nut is separated from the screw shaft, the ball does not drop out from the ball nut. The present invention is particularly suitable for applications in which the ball nut needs to be attached to and detached from the screw shaft.

According to the second aspect of the present invention, the ball retainer can be used even when the wall thickness is thin and the screw hole for attaching to the ball nut cannot be formed.

According to the ball screw of the third aspect, by using the self-lubricating composite material as the material of the ball circulation cap, oil-free and lubrication-free use can be achieved. For this reason, the electronics-related, nuclear-related,
Low temperature in a wide range of fields such as precision equipment, aviation and space,
The ball screw can be used in severe environments such as high temperature, water, and vacuum.

According to the fourth aspect of the invention, even in the case of refueling the nuclear reactor, the control rod driving device can be removed while the control rod is inserted in the nuclear reactor. In other words, a ball screw can be used to ensure high transmission efficiency, and even if the control rod, that is, the screw shaft is pulled out from the ball nut, the ball is retained inside the ball nut, so that high transmission is achieved. A highly efficient and highly reliable drive device can be obtained.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing a first embodiment of a ball screw of the present invention.

FIG. 2 is a right side view including a partial cross section of FIG.

FIG. 3 is a front view of the ball nut including a partial cross section.

FIG. 4 is a perspective view of a ball circulation cap that constitutes the ball screw of the present invention.

5 is a plan view of the ball circulation cap of FIG. 4. FIG.

6 is a cross-sectional view taken along the line I-I of FIG.

FIG. 7 is a front view of the ball retainer.

FIG. 8 is a right side view of FIG.

FIG. 9 is a cross-sectional view illustrating a state in which a ball nut is attached to a screw shaft, and balls are rotatably loaded in a screw groove of the screw shaft and a screw groove of the ball nut.

FIG. 10 is a cross-sectional view illustrating a state in which the spiral slot of the ball retainer holds the ball when the ball nut is removed from the screw shaft.

FIG. 11 is a vertical sectional view showing a second embodiment of the ball screw of the present invention.

FIG. 12 is a front view of a retaining ring.

FIG. 13 is a cross-sectional view of a nuclear reactor having a control rod driving device using a ball screw according to the present invention.

14 is an enlarged cross-sectional view of the control rod drive device of FIG.

FIG. 15 is a cross-sectional view of a conventional ball screw.

16 is a perspective view of the ball nut of FIG.

FIG. 17 is a perspective view of a ball circulation cap that constitutes a conventional ball screw.

18 is a plan view of the ball circulation cap of FIG.

19 is a sectional view taken along the line II-II of FIG.

FIG. 20 is an example of a conventional method for attaching and detaching a ball nut to and from a screw shaft.

FIG. 21 is another example of a conventional method for attaching and detaching a ball nut to and from a screw shaft.

[Explanation of symbols]

 1 Screw Shaft 2 Ball Nut 1a, 2a Screw Groove 3 Ball 4 Ball Circulation Cap Mounting Hole 5 Ball Circulation Cap 6 Ball Retainer 7 Ball Circulation Groove 8 Opening 12 Hole 13 Spiral Slot 19 Locking Groove 20 Retaining Ring 100 Atom Furnace 104 Control rod drive device 105 Control rod 106 Drive shaft (screw shaft)

Claims (4)

[Claims] 1. A screw shaft having a thread groove on the outer surface, a ball nut having a thread groove corresponding to the thread groove of the screw shaft on the inner surface, and a screw shaft mounted between the screw shaft and the ball nut. A plurality of balls that can roll freely, and a ball that fits in a ball circulation cap mounting hole provided in the ball nut and has an opening that opens to the screw shaft side and communicates with the thread groove of the ball nut. An internal circulation type ball screw having a ball circulation cap having a circulation groove, wherein the width of the opening of the ball circulation cap is narrower than the diameter of the ball over the entire length, and the inner surface of the ball nut is Has a ball retainer, which has the same lead as the thread groove of the ball nut, opens toward the screw shaft side, and has a spiral slot communicating with the ball circulation groove. The width of the side opening being narrower than the diameter of the ball, screw shaft detachable ball screw. 2. The ball screw according to claim 1, wherein an engaging groove is provided on an inner surface of the ball nut, and the ball retainer is axially fixed by a retaining ring fitted in the engaging groove. 3. The ball screw according to claim 1, wherein the ball circulation cap is made of a self-lubricating composite material. 4. A control of a nuclear reactor comprising a ball screw according to claim 1, wherein the screw shaft is connected to a control rod of a nuclear reactor and the ball nut is rotationally driven. Rod drive.