Screw shaft, ball screw device, manufacturing method for screw shaft, and assembly method for ball screw device

Abstract

To provide a screw shaft capable of preventing a ball from being arranged on a circuit outer groove surface.SOLUTION: A screw shaft of the present disclosure has a screw shaft body having a cylindrical outer peripheral surface, a spiral groove surface formed on the outer peripheral surface, and at least one recess part recessed radially inward from the outer peripheral surface and accommodating a piece. The groove surface has one end and the other end connected to the same recess part, and has an outer peripheral raceway surface along which balls circulated by the piece arranged in the recess part roll, and a circuit outer groove surface that is the remainder of the groove surface. A groove width of the circuit outer groove surface is smaller than a groove width of the outer peripheral raceway surface.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to a screw shaft, a ball screw device, a method for manufacturing a screw shaft, and a method for assembling a ball screw device. [Background technology]

[0002] The ball screw device includes a nut, a screw shaft penetrating the nut, a plurality of balls arranged between the nut and the screw shaft, and a circulation part. An inner peripheral raceway surface is formed on the inner peripheral surface of the nut. An outer peripheral raceway surface facing the inner peripheral raceway surface is formed on the outer peripheral surface of the screw shaft. A spiral track is formed between the inner peripheral raceway surface and the outer peripheral raceway surface. A plurality of balls are arranged on the track and move in a spiral direction along the track. The circulation part returns the balls that have moved from one end of the track to the other end of the track to the one end of the track. One example of the circulation part is a top that returns the balls by one lead. In addition, in the following Patent Document 1, a recess recessed radially inward is formed on the outer peripheral surface of the screw shaft. A top is inserted into the recess and attached to the screw shaft.

[0003] Moreover, the following Patent Document 2 discloses forming a spiral groove surface on the outer circumferential surface of a screw shaft by rolling. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2004-225770 A [Patent Document 2] JP 2005-66608 A Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, when a spiral groove surface is formed on the outer peripheral surface of a screw shaft, and then a recess is formed by cutting out a part of the groove surface, a part of the spiral groove surface becomes an outer peripheral raceway surface. Meanwhile, the remaining part of the spiral groove surface is provided between the outer peripheral raceway surfaces. This remaining part of the spiral groove surface is a groove surface in which balls cannot circulate (hereinafter referred to as an "outside-circuit groove surface"). If a ball is mistakenly placed on the outside-circuit groove surface during assembly of a ball screw device, the relative rotation between the nut and the screw shaft is locked (restricted), and the ball screw device will not function.

[0006] The present disclosure has been made in consideration of the above, and aims to provide a screw shaft, a ball screw device, a method for manufacturing a screw shaft, and a method for assembling a ball screw device that suppresses the placement of balls on an outer-circuit groove surface when assembling a ball screw device. [Means for solving the problem]

[0007] In order to achieve the above object, a threaded shaft according to one embodiment of the present disclosure has a threaded shaft body having a cylindrical outer peripheral surface, a spiral groove surface formed on the outer peripheral surface, and at least one recess recessed radially inward from the outer peripheral surface for accommodating a piece. The groove surface has one end and the other end connected to the same recess, and has an outer peripheral raceway surface on which balls circulated by the piece placed in the recess roll, and an outer circuit groove surface which is the remaining part of the groove surface. The groove width of the outer circuit groove surface is smaller than the groove width of the outer peripheral raceway surface.

[0008] According to the present disclosure, the amount (volume) of balls accommodated on the outer-circuit groove surface is smaller than the amount (volume) of balls accommodated on the outer-circuit groove surface. That is, the balls arranged on the outer-circuit groove surface protrude radially outward from the balls arranged on the outer-circuit groove surface. If a ball is arranged on the outer-circuit groove surface by mistake during assembly of the ball screw device, the ball arranged on the outer-circuit groove surface will be caught on the end surface of the nut when the screw shaft is inserted into the inside of the nut. Therefore, the worker can notice that a ball is arranged on the outer-circuit groove surface, or the assembly device can detect it with a sensor or the like. Hereinafter, the expression "the worker notices" includes detection by a sensor or the like. In addition, if the ball arranged on the outer-circuit groove surface is inserted into the inside of the nut, the force required to rotate the screw shaft will be large. This also allows the worker to notice that the ball is arranged on the outer-circuit groove surface. From the above, it is possible to suppress the ball screw device from being assembled with a ball arranged on the outer-circuit groove surface.

[0009] The outer-circuit groove surface of the screw shaft is located between the two outer peripheral raceway surfaces and has a closed groove surface whose one end and the other end are connected to different recesses. The groove width of the closed groove surface may be smaller than the groove width of the outer peripheral raceway surface. Alternatively, the outer-circuit groove surface of the screw shaft has an open groove surface whose only one end is connected to the recess. The groove width of the open groove surface may be smaller than the groove width of the outer peripheral raceway surface.

[0010] A ball screw device according to an aspect of the present disclosure includes the above-mentioned screw shaft, a nut that is inserted through the screw shaft and has an inner raceway surface formed on its inner peripheral surface, and a plurality of balls that are arranged in a raceway between the outer raceway surface and the inner raceway surface. In a cross section taken in an axial direction parallel to the screw shaft, imaginary lines of the balls when arranged on the circuit outer groove surface are at least partially arranged radially outward from the inner raceway surface.

[0011] According to the present disclosure, if a ball is mistakenly placed on the outer-circuit groove surface during assembly of the ball screw device, the ball placed on the outer-circuit groove surface may get caught on the end surface of the nut, or the force required to insert the screw shaft into the nut may become large. Therefore, it is possible to prevent the ball screw device from being assembled with a ball placed on the outer-circuit groove surface.

[0012] In addition, a manufacturing method of a screw shaft according to one embodiment of the present disclosure includes a preparation step of preparing a material for the screw shaft, a groove surface forming step of forming a spiral groove surface on the outer peripheral surface of the material, a recess forming step of forming a recess for accommodating a piece on the outer peripheral surface of the material, a heat treatment step of heat treating the material, and an outer peripheral raceway surface forming step of grinding a portion of the groove surface, one end and the other end of which connect to the same recess, to form an outer peripheral raceway surface.

[0013] According to the present disclosure, the portion of the groove surface that is not ground becomes an outer-circuit groove surface that is smaller than the groove width of the outer peripheral raceway surface. And, with this screw shaft, it is possible to prevent a ball screw device from being assembled with balls disposed on the outer-circuit groove surface.

[0014] A method for assembling a ball screw device according to an aspect of the present disclosure includes a temporary assembly step of preparing the screw shaft and a cylindrical temporary nut, inserting the screw shaft into the temporary nut from one opening of the temporary nut, and arranging the balls between the inner peripheral surface of the temporary nut and the outer peripheral raceway surface, a facing step of opposing an end face of the temporary nut to an end face of the nut, a screw shaft moving step of moving the screw shaft into the nut while rotating the screw shaft and the nut relatively, and an assembly completion step of removing the temporary nut from the nut to complete the assembly. The inner diameter of the temporary nut is equal to or larger than the outer diameter when the balls are arranged on the outer peripheral raceway surface and is smaller than the outer diameter when the balls are arranged on the circuit outer groove surface.

[0015] According to the temporary assembly process, even if a ball is mistakenly placed on the outer-circuit groove surface, when the screw shaft is inserted into the temporary nut, the ball placed on the outer-circuit groove surface will get caught on the end surface of the nut. Also, even if the ball placed on the outer-circuit groove surface is inserted into the temporary nut, the force required to insert the screw shaft will be large. In other words, at the time of the temporary assembly process, the worker or the like will notice that the ball is placed on the outer-circuit groove surface. Similarly, at the time of the screw shaft moving process, the ball placed on the outer-circuit groove surface will get caught on the end surface of the nut, or the force required to insert the screw shaft into the nut will be large. From the above, the worker or the like will notice that the ball is placed on the outer-circuit groove surface many times. Therefore, it is suppressed that the ball screw device is assembled with the ball placed on the outer-circuit groove surface.

[0016] In the method for assembling a ball screw device, an inner diameter of the temporary nut may be the same as an outer diameter when the balls are disposed on the outer circumferential raceway surface. Effect of the Invention

[0017] According to the present disclosure, it is possible to prevent the ball screw device from being assembled in a state in which the balls are disposed on the outer-circuit groove surface. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional view of a ball screw device according to an embodiment. [Diagram 2] FIG. 2 is a schematic diagram of the outer circumferential surface of the screw shaft according to the embodiment cut in the axial direction and developed into a planar shape. [Diagram 3] FIG. 3 is a cross-sectional view of the ball screw device taken along line III-III in FIG. [Figure 4] FIG. 4 is a flow diagram showing a method for manufacturing the screw shaft according to the embodiment. [Diagram 5] FIG. 5 is a cross-sectional view of the outer circumferential raceway surface and the circuit outer groove surface after the outer circumferential raceway surface forming step of the embodiment. [Figure 6] FIG. 6 is a flow chart showing a method of assembling the ball screw device according to the embodiment. [Figure 7] FIG. 7 is a diagram showing a temporary assembly process in which balls are placed on the outer-circuit groove surface of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the following modes for carrying out the invention (hereinafter, referred to as embodiments). Furthermore, the components in the following embodiments include those that a person skilled in the art can easily imagine, those that are substantially the same, and those that are within the so-called equivalent range. Furthermore, the components disclosed in the following embodiments can be appropriately combined.

[0020] Fig. 1 is a cross-sectional view of a ball screw device according to an embodiment. Fig. 2 is a schematic diagram of an outer peripheral surface of a screw shaft according to an embodiment cut in the axial direction and developed into a planar shape. In Fig. 2, the closed groove surface 16 is dotted to make it easier to understand. Fig. 3 is a cross-sectional view of the ball screw device cut along line III-III in Fig. 2.

[0021] As shown in Fig. 1, the ball screw device 1 of the embodiment includes a nut 2, a screw shaft 3, balls 4 (not shown in Fig. 1, see Fig. 3), and a number of blocks 5. Hereinafter, a direction parallel to a center line O of the screw shaft 3 will be referred to as an axial direction. Also, the ball screw device 1 of the embodiment will be described with an example of use in which a rotational motion is transmitted to the screw shaft 3 to cause a linear motion of the nut 2, but the present disclosure may also be used in which a rotational motion is transmitted to the nut 2 to cause a linear motion of the screw shaft 3, and there is no particular restriction on this.

[0022] The nut 2 is formed in a cylindrical shape centered on the center line O. Therefore, the two end faces 2a of the nut 2 are circular when viewed from the axial direction. A spiral inner circumferential raceway surface 6 is formed on the inner circumferential surface of the nut 2. The inner circumferential raceway surface 6 extends continuously from one end to the other end of the inner circumferential surface of the nut 2 in the axial direction. Therefore, one end and the other end of the inner circumferential raceway surface 6 each serve as an opening 6a that cuts out the end faces 2a of the nut 2.

[0023] The screw shaft 3 includes a cylindrical shaft portion 10 and a screw shaft body 11. The shaft portion 10 is a portion that is connected to other components and transmits rotational motion. The screw shaft 3 is manufactured by processing a single metal material, and the shaft portion 10 and the screw shaft body 11 are integrated (inseparable).

[0024] 2, a spiral groove surface 13 and four recesses 14 are provided on a cylindrical outer peripheral surface 12 of the screw shaft body 11. The spiral groove surface 13 is provided on the entire outer peripheral surface 12 of the screw shaft body 11. The recesses 14 are recesses recessed radially inward from the outer peripheral surface 12, and the top 5 is housed in the recesses 14.

[0025] As shown in Fig. 1, the top 5 is a circulation device that returns the ball 4, which has moved about one lead, by about one lead. The top 5 has an S-shaped circulation groove surface 5a. Note that in the present disclosure, there is no particular restriction on the method of fixing the top 5 to the recess 14. Also, the shape of the circulation groove surface 5a of the top 5 may be linear, and there is no particular restriction in the present disclosure.

[0026] 2, the recesses 14 (links 5) are disposed at equal intervals in the circumferential direction. That is, the four recesses 14 are disposed in different directions as viewed from the center line O. As a result, the load acting on the screw shaft 3 from the nut 2 via the balls 4 is evenly distributed in the circumferential direction.

[0027] As shown in Fig. 1, the helical groove surface 13 is a helical groove surface that faces the inner peripheral raceway surface 6 in the radial direction and extends in the same direction as the inner peripheral raceway surface 6. Also, as shown in Fig. 2, the helical groove surface 13 is divided by four recesses 14. In other words, the helical groove surface 13 has four outer peripheral raceway surfaces 15, three closed groove surfaces 16, and two open groove surfaces 17.

[0028] The outer peripheral raceway surface 15 extends for approximately one revolution in the circumferential direction around the center line O. One end and the other end of the outer peripheral raceway surface 15 are connected to the same (common) recess 14. The closed groove surface 16 is disposed between the outer peripheral raceway surfaces 15. The closed groove surface 16 extends for approximately one-quarter of the revolution in the circumferential direction around the center line O. One end and the other end of the closed groove surface 16 are connected to different recesses 14. The open groove surface 17 is disposed at one end and the other end of the groove surface 13 in the axial direction. Only one end of the open groove surface 17 is connected to the recess 14. In this embodiment, the other end of the open groove surface 17 reaches the end face of the screw shaft body 11 and becomes an opening that cuts out the end face of the screw shaft body 11. Note that the other end of the open groove surface 17 of the present disclosure does not have to cut out the end face of the screw shaft body 11. In other words, the other end of the open groove surface 17 may have a shape that is cut outward in the radial direction before reaching the end face of the screw shaft body 11 .

[0029] As shown in Fig. 3, the outer circumferential raceway surface 15 has a gothic arc shape. Also, the closed groove surface 16 and the open groove surface 17 (not shown in Fig. 3) also have a gothic arc shape. Note that in the present disclosure, the outer circumferential raceway surface 15, the closed groove surface 16, and the open groove surface 17 may be circular arcs.

[0030] A raceway 8 is formed between the outer circumferential raceway surface 15 and the inner circumferential raceway surface 6. That is, balls 4 are arranged in this raceway 8. As a result, the nut 2 is rotatably supported on the screw shaft 3 via the balls 4. When a rotational motion is transmitted to the shaft portion 10 to drive the ball screw device 1, the balls 4 that have rolled from one end of the outer circumferential raceway surface 15 to the other end enter the circulation groove surface 5a of the top 5 and return to one end of the outer circumferential raceway surface 15.

[0031] As shown in FIG. 3, no balls 4 are arranged between the closed groove surface 16 and the inner raceway surface 6. Although not specifically shown, no balls 4 are arranged between the open groove surface 17 and the inner raceway surface 6 either. In other words, the area between the closed groove surface 16 and the inner raceway surface 6 does not form a raceway 8. Similarly, the area between the open groove surface 17 and the inner raceway surface 6 does not form a raceway 8. As described above, the closed groove surface 16 and the open groove surface 17 are the remainder of the spiral groove surface 13. Hereinafter, the closed groove surface 16 and the open groove surface 17 will be collectively referred to as the outer-circuit groove surface 18.

[0032] 1, one end and the other end of the closed groove surface 16 are not continuous with the circulating groove surface 5a of the block 5. In other words, one end and the other end of the closed groove surface 16 are a groove surface that is closed by the block 5. For this reason, if the balls 4 are mistakenly placed between the closed groove surface 16 and the inner circumferential raceway surface 6, the balls 4 will not roll, and the nut 2 will be locked with respect to the screw shaft 3 (will not rotate relative to the screw shaft 3).

[0033] As shown in Fig. 3, the groove shape of the closed groove surface 16 is smaller than that of the outer peripheral raceway surface 15. In other words, the groove width H16 of the closed groove surface 16 is smaller than the groove width H15 of the outer peripheral raceway surface 15. Therefore, the amount (volume) of the balls 4 accommodated in the closed groove surface 16 is smaller than the amount (volume) of the balls 4 accommodated in the outer peripheral raceway surface 15. For this reason, for example, when the balls 4 are arranged in the closed groove surface 16, the protruding amount K16 of the balls 4 from the closed groove surface 16 is larger than the protruding amount K15 of the balls 4 arranged in the outer peripheral raceway surface 15. In other words, a part of the imaginary line of the balls 4 is arranged radially outward from the inner peripheral raceway surface 6.

[0034] The groove shape of the open groove surface 17 is the same as that of the closed groove surface 16. That is, the groove width of the open groove surface 17 is the same as the groove width H16 of the closed groove surface 16, and is smaller than the groove width H15 of the outer peripheral raceway surface 15. For this reason, for example, when the ball 4 is arranged on the open groove surface 17, the protruding amount of the ball 4 on the open groove surface 17 is the same as the protruding amount K16 of the ball 4 on the closed groove surface 16. That is, a part of the imaginary line of the ball 4 arranged on the open groove surface 17 is arranged radially outward from the inner peripheral raceway surface 6. In addition, when the ball 4 is arranged between the open groove surface 17 and the inner peripheral raceway surface 6, the ball 4 falls off from the other end (opening) of the open groove surface 17. Next, a manufacturing method of the screw shaft 3 will be described.

[0035] Fig. 4 is a flow diagram showing a method for manufacturing a screw shaft of an embodiment. Fig. 5 is a cross-sectional view of the outer peripheral raceway surface and the circuit outer groove surface after the outer peripheral raceway surface forming step of the embodiment. As shown in Fig. 4, the method for manufacturing a screw shaft of this embodiment includes a preparation step S1, a groove surface forming step S2, a recess forming step S3, a heat treatment step S4, and an outer peripheral raceway surface forming step S5.

[0036] The preparation step S1 is a step of preparing a material 50 for the screw shaft 3. The material 50 is an integrated material having a cylindrical material body 51 and a shaft portion 10. The material body 51 is a portion that will become the screw shaft body 11 by processing. The shaft portion 10 has already been processed, but the present disclosure is not limited to this. The material of the material 50 is a steel material, but the present disclosure is not limited to this and any metal material may be used.

[0037] The groove surface forming step S2 is a step of forming a spiral groove surface 13 on the cylindrical outer peripheral surface 52 of the material body 51. The groove surface 13 is formed by rolling, but in the present disclosure, it may be a turning process, and is not particularly limited. In addition, the rolling process (forming the spiral groove surface 13) is performed on the entire outer peripheral surface 52 of the material body 51. As a result, a continuous spiral groove surface 13 is formed from one end to the other end of the outer peripheral surface 52 of the material body 51 in the axial direction.

[0038] The recess forming step S3 is a step of forming recesses 14 on the outer peripheral surface 52 of the material body 51. The recesses 14 are formed by cutting. As a result, four recesses 14 are formed on the outer peripheral surface 52 of the material body 51. Furthermore, the groove surface 13 is divided into a plurality of parts by forming the recesses 14. That is, a rolling groove surface 15A, a closed groove surface 16, and an open groove surface 17 are formed. The rolling groove surface 15A is a portion whose one end and the other end are connected to the same recess 14. Furthermore, the rolling groove surface 15A, the closed groove surface 16, and the open groove surface 17 each have the same groove shape (cross-sectional shape).

[0039] The heat treatment step S4 is a step of quenching and tempering the outer peripheral surface 52 of the material body 51. In the present disclosure, a carburizing treatment may be performed before quenching. This allows the screw shaft 3 to have the desired hardness and toughness. In the present disclosure, the heat treatment method is not particularly limited.

[0040] The outer circumferential raceway surface forming step S5 is a step of grinding the rolling groove surface 15A with a grindstone 80. As a result, the rolling groove surface 15A is ground to form the outer circumferential raceway surface 15, and the screw shaft 3 is completed. As shown in FIG. 5, the grinding allowance L is 0.5 mm or less. According to the above-mentioned manufacturing method of the screw shaft 3, the circuit outer groove surface 18 (closed groove surface 16 and open groove surface 17) is formed with a groove width H16 (see FIG. 3) smaller than the groove width H15 (see FIG. 3) of the outer circumferential raceway surface 15. Next, a method of assembling the ball screw device 1 will be described.

[0041] Fig. 6 is a flow diagram showing a method for assembling the ball screw device of the embodiment. As shown in Fig. 6, the method for assembling the ball screw device 1 includes a provisional assembly process S11, a facing process S12, a screw shaft moving process S13, and an assembly completion process S14.

[0042] In the temporary assembly process S11, first, the screw shaft 3 and a cylindrical temporary nut 70 are prepared. The screw shaft 3 is prepared with the block 5 assembled in the recess 14. The inner diameter r of the temporary nut 70 is the same as the outer diameter R15 (see FIG. 3) in the state where the balls 4 are arranged on the outer peripheral raceway surface 15 (the state where the screw shaft 3 and the balls 4 are combined). Note that "same" here includes not only completely same things, but also things that are recognized as substantially "same" due to tolerances (the range of general manufacturing errors).

[0043] Next, the screw shaft 3 is inserted into the temporary nut 70 from one opening of the temporary nut 70. Here, the screw shaft 3 is inserted to such an extent that a part of the outer circumferential raceway surface 15 of the screw shaft 3 is covered by the temporary nut 70. Then, a plurality of balls 4 are inserted into the open portion of the outer circumferential raceway surface 15, and the balls 4 fill the outer circumferential raceway surface 15 and the circulation groove surface 5a.

[0044] Next, the screw shaft 3 is further inserted into the temporary nut 70. According to this, the balls 4 arranged on the outer circumferential raceway surface 15 and the circulation groove surface 5a are covered on the outer circumferential side by the temporary nut 70, and do not fall off from the outer circumferential raceway surface 15 and the circulation groove surface 5a. Then, this is repeated until all the outer circumferential raceway surfaces 15 and the circulation groove surface 5a are filled with the balls 4. In addition, in this embodiment, a tapered surface 74 is provided on one end face 73 of the temporary nut 70. According to this, when the screw shaft 3 is inserted into the temporary nut 70, the balls 4 arranged on the outer circumferential raceway surface 15 and the circulation groove surface 5a are guided radially inward by the tapered surface 74. In other words, the balls 4 are pressed against the outer circumferential raceway surface 15 and the circulation groove surface 5a, and are less likely to get caught on the end face 73. Therefore, the work of inserting the screw shaft 3 into the temporary nut 70 becomes easier.

[0045] The facing step S12 is a step of facing (contacting) the other end face 71 of the temporary nut 70 to the end face 2a of the nut 2. An annular fitting portion 72 is formed on the end face 71 of the temporary nut 70 in this embodiment. Therefore, when the end face 2a of the nut 2 and the end face 71 of the temporary nut 70 are faced to each other, the nut 2 is fitted into the fitting portion 72. This causes the center of the nut 2 and the center of the temporary nut 70 to coincide.

[0046] The screw shaft moving step S13 is a step of moving the screw shaft body 11 of the screw shaft 3 from the temporary nut 70 into the inside of the nut 2. Specifically, the screw shaft 3 is pushed (moved) into the inside of the nut 2 while rotating the nut 2 and the screw shaft 3 relative to each other. As a result, the balls 4 arranged on the outer peripheral raceway surface 15 enter the inside of the inner peripheral raceway surface 6 from the openings 6a (see FIG. 1) of the end face 2a of the nut 2. Note that the balls 4 in the circulation groove surface 5a are accommodated in the circulation groove surface 5a and therefore are not caught by the threads of the inner peripheral raceway surface 6. As a result, the entire screw shaft body 11 of the screw shaft 3 moves into the inside of the nut 2.

[0047] In the assembly completion step S14, the temporary nut 70 is removed from the nut 2 to complete the assembly. In this way, the ball screw device 1 is completed.

[0048] Next, the effect of the ball screw device 1 of the embodiment will be described. In the temporary assembly process S11, the ball 4 may be erroneously placed on the closed groove surface 16. According to this embodiment, as shown in Fig. 3, the protrusion amount K16 of the imaginary line of the ball 4 placed on the closed groove surface 16 is larger than the protrusion amount K15 of the ball 4 placed on the outer circumferential raceway surface 15. In other words, the outer diameter R16 when the ball 4 is placed on the closed groove surface 16 is larger than the outer diameter R15 when the ball 4 is placed on the outer circumferential raceway surface 15.

[0049] 7 is a diagram showing a temporary assembly process in the case where a ball is arranged on the outer-circuit groove surface of the embodiment. As shown in FIG. 7, in the temporary assembly process S11, when the screw shaft 3 is inserted into the temporary nut 70, the ball 4 arranged on the closed groove surface 16 is caught on one end surface 73 (tapered surface 74) of the temporary nut 70. In other words, even if a worker or the like applies a force to move the screw shaft 3 into the temporary nut 70, the screw shaft 3 does not move. For this reason, the worker or the like notices that the ball 4 is arranged on the closed groove surface 16.

[0050] Also, in the screw shaft moving step S13, when the screw shaft 3 is moved inside the nut 2, the ball 4 does not enter the opening 6a of the inner peripheral raceway surface 6 of the nut 2, but gets caught on the end face 2a of the nut 2. This also allows the worker or the like to notice that the ball 4 is disposed on the closed groove surface 16.

[0051] Furthermore, when the nut 2 is inserted into the nut 2 with the ball 4 disposed on the closed groove surface 16, the ball 4 is crushed between the closed groove surface 16 and the inner peripheral raceway surface 6 and does not roll. In other words, a large force is required to rotate the screw shaft 3. Therefore, the worker or the like notices that the ball 4 is disposed on the closed groove surface 16.

[0052] Similarly, when the ball 4 is inserted into the temporary nut 70 with the ball 4 disposed on the closed groove surface 16, the ball 4 is crushed between the closed groove surface 16 and the inner peripheral surface of the temporary nut 70 and is difficult to roll. The force required to move the screw shaft 3 increases. Therefore, the worker or the like notices that the ball is disposed on the closed groove surface 16.

[0053] In the above, the case where the ball 4 is arranged on the closed groove surface 16 has been described as an example, but the same state occurs when the ball 4 is arranged on the open groove surface 17. That is, the ball 4 arranged on the open groove surface 17 may get caught on the end surface 73 of the temporary nut 70, or the force moving the screw shaft 3 may increase. Therefore, the worker or the like may notice that the ball 4 is arranged on the open groove surface 17.

[0054] As described above, according to this embodiment, the worker or the like is made aware that the balls 4 are disposed on the outer-circuit groove surface 18 (the closed groove surface 16 and the open groove surface 17) during the assembly work. This prevents the ball screw device 1 from being assembled in a state in which the balls 4 are disposed on the outer-circuit groove surface 18. Furthermore, in this embodiment, the worker is made aware that the balls 4 are disposed on the outer-circuit groove surface 18 a total of four times. This reliably prevents the ball screw device 1 from being assembled in a state in which the balls 4 are disposed on the outer-circuit groove surface 18.

[0055] As described above, the ball screw device 1 of the embodiment includes a screw shaft 3, a nut 2 that is inserted through the screw shaft 3 and has an inner circumferential raceway surface 6 formed on its inner circumferential surface, and a plurality of balls 4 arranged in a raceway 8 between the outer circumferential raceway surface 15 and the inner circumferential raceway surface 6. The screw shaft 3 has a screw shaft body 11 having a cylindrical outer circumferential surface 12, a spiral groove surface 13 formed on the outer circumferential surface 12, and at least one recess 14 that is recessed radially inward from the outer circumferential surface 12 and accommodates a top 5. The groove surface 13 has an outer circumferential raceway surface 15 on which the balls 4 circulated by the top 5 arranged in the recess 14 roll, and an outer-circuit groove surface 18 that is the remaining part of the groove surface 13. The groove width H16 of the outer-circuit groove surface 18 is smaller than the groove width H15 of the outer circumferential raceway surface 15. In a cross-sectional view taken in an axial direction parallel to the screw shaft 3, at least a portion of the imaginary line of the ball 4 when disposed on the outer-circuit groove surface 18 is disposed radially outward of the inner peripheral raceway surface 6. In detail, the outer-circuit groove surface 18 is located between the two outer peripheral raceway surfaces 15, and has a closed groove surface 16 having one end and the other end connected to different recesses 14, and an open groove surface 17 having only one end connected to the recess 14. The groove widths of the closed groove surface 16 and the open groove surface 17 are each smaller than the groove width H15 of the outer peripheral raceway surface 15.

[0056] According to the embodiment, if the ball 4 is erroneously placed on the outer-circuit groove surface 18 (closed groove surface 16, open groove surface 17) during assembly of the ball screw device 1, an operator or the like is configured to notice it. This prevents the ball screw device from being assembled in a state where the ball is placed on the outer-circuit groove surface 18 (closed groove surface 16, open groove surface 17).

[0057] In addition, the manufacturing method of the screw shaft 3 includes a preparation step S1 of preparing a material 50 of the screw shaft 3, a groove surface forming step S2 of forming a spiral groove surface 13 on the outer peripheral surface 52 of the material 50, a recess forming step S3 of forming a recess 14 for accommodating a block 5 on the outer peripheral surface 52 of the material 50, a heat treatment step S4 of heat treating the material 50, and an outer peripheral raceway surface forming step S5 of grinding a portion of the groove surface 13, one end of which connects to the same recess 14 (rolling groove surface 15A), to form an outer peripheral raceway surface 15.

[0058] According to this manufacturing method, the portion of the groove surface 13 that is not ground becomes the outer-circuit groove surface 18 (the closed groove surface 16, the open groove surface 17). According to this screw shaft 3, it is possible to prevent the ball screw device from being assembled in a state in which the balls 4 are arranged on the outer-circuit groove surface 18 (the closed groove surface 16, the open groove surface 17).

[0059] The assembly method of the ball screw device 1 of the embodiment includes a temporary assembly process S11 in which the screw shaft 3 and the cylindrical temporary nut 70 are prepared, the screw shaft 3 is inserted into the temporary nut 70 from one opening of the temporary nut 70, and the balls 4 are arranged between the inner peripheral surface of the temporary nut 70 and the outer peripheral raceway surface 15, a facing process S12 in which the end face 71 of the temporary nut 70 and the end face 2a of the nut 2 are opposed to each other, a screw shaft moving process S13 in which the screw shaft 3 is moved into the nut 2 while rotating the screw shaft 3 and the nut 2 relatively, and an assembly completion process S14 in which the temporary nut 70 is removed from the nut 2 and the assembly is completed. The inner diameter r of the temporary nut 70 is the same as the outer diameter R15 when the balls 4 are arranged on the outer peripheral raceway surface 15.

[0060] According to this assembly method, not only in the screw shaft moving step S13 but also in the temporary assembly step S11, the worker or the like notices that the balls 4 are arranged on the outer-circuit groove surface 18 (the closed groove surface 16 and the open groove surface 17). This reliably prevents the ball screw device from being assembled in a state where the balls are arranged on the outer-circuit groove surface.

[0061] Although the embodiment has been described above, the present disclosure is not limited to the examples shown in the embodiment. For example, the temporary nut 70 in the embodiment has the fitting portion 72, but the present disclosure may be a temporary nut 70 that does not have the fitting portion 72. Also, the present disclosure may assemble the ball screw device 1 without using the temporary nut 70.

[0062] Further, the inner diameter r of the temporary nut 70 of the embodiment is the same as the outer diameter R15 (see FIG. 3) in the state where the ball 4 is disposed on the outer peripheral raceway surface 15. However, in the present disclosure, it may be equal to or greater than the outer diameter R15 in the state where the ball 4 is disposed on the outer peripheral raceway surface 15 and less than the outer diameter R16 in the state where the ball 4 is disposed on the circuit outer groove surface 18 (R15 ≦ r < R16). If the inner diameter r is within this range, as in the embodiment, an operator or the like can be made aware that the ball 4 is disposed on the circuit outer groove surface 18.

[0063] Further, the circuit outer groove surface 18 of the embodiment has a closed groove surface 16 and an open groove surface 17. However, the circuit outer groove surface 18 of the present disclosure may include only one of the closed groove surface 16 and the open groove surface 17.

[0064] Further, in order to facilitate the operation of inserting the screw shaft 3 into the temporary nut 70, in the embodiment, the temporary nut 70 is provided with a tapered surface 74. However, in the present disclosure, a configuration other than the tapered surface 74 may be provided on the temporary nut 70, and it is not limited to the tapered surface 74. Further, the tapered surface 74 may not be provided on the end surface 73 of the temporary nut 70 of the present disclosure.

Explanation of Reference Numerals

[0065] 1 Ball screw device 2 Nut 3 Screw shaft 4 Ball 5 Disk 6 Inner peripheral raceway surface 8 Raceway 11 Screw shaft body 12 Outer peripheral surface 13 Groove surface 14 Recess 15 Outer peripheral raceway surface 16 Closed groove surface 17 Open groove surface 18 Circuit outer groove surface 50 Material 70 Temporary nut

Claims

1. A screw shaft main body having a cylindrical outer peripheral surface; A spiral groove surface formed on the outer circumferential surface; At least one recessed portion recessed radially inward from the outer circumferential surface and configured to accommodate a piece; having The groove surface is an outer circumferential track surface on which balls circulated by the pieces disposed in the recesses roll, the outer circumferential track surface having one end and the other end connected to the same recess; a circuit outer groove surface which is a remainder of the groove surface; having The groove width of the outer circuit groove surface is smaller than the groove width of the outer peripheral raceway surface. Screw shaft.

2. the outer-circuit groove surface is located between the two outer circumferential raceway surfaces, and has a closed groove surface, one end and the other end of which are connected to different recesses, The groove width of the closed groove surface is smaller than the groove width of the outer circumferential raceway surface. The screw shaft according to claim 1.

3. the outer-circuit groove surface has an open groove surface, only one end of which is connected to the recess, The groove width of the open groove surface is smaller than the groove width of the outer circumferential raceway surface. The screw shaft according to claim 1 or 2.

4. The screw shaft according to claim 1 , A nut that is inserted through the screw shaft and has an inner circumferential raceway formed on its inner circumferential surface; a plurality of balls disposed in a raceway between the outer circumferential raceway surface and the inner circumferential raceway surface; Equipped with In a cross section taken in an axial direction parallel to the screw axis, an imaginary line of the ball when disposed on the circuit outer groove surface is at least partially disposed radially outward from the inner peripheral raceway surface. Ball screw device.

5. A preparation process for preparing a material for the screw shaft; a groove surface forming step of forming a spiral groove surface on an outer peripheral surface of the material; a recess forming step of forming a recess in an outer peripheral surface of the material for accommodating a piece; A heat treatment step of heat treating the material; a peripheral raceway surface forming step of grinding a portion of the groove surface, one end and the other end of which are connected to the same recess, to form a peripheral raceway surface; Includes Manufacturing method of screw shaft.

6. A temporary assembly process of preparing the screw shaft and a cylindrical temporary nut according to claim 1, inserting the screw shaft into the temporary nut from one opening of the temporary nut, and disposing the ball between the inner peripheral surface of the temporary nut and the outer peripheral raceway surface; A facing process of facing an end surface of the temporary nut and an end surface of a nut; A screw shaft moving process in which the screw shaft is moved into the nut while rotating the screw shaft and the nut relatively; an assembly completion step of removing the temporary nut from the nut to complete the assembly; Including, The inner diameter of the temporary nut is equal to or larger than the outer diameter when the ball is disposed on the outer peripheral raceway surface, and is smaller than the outer diameter when the ball is disposed on the circuit outer groove surface. A method for assembling a ball screw device.

7. The inner diameter of the temporary nut is the same as the outer diameter when the balls are disposed on the outer circumferential raceway surface. A method for assembling the ball screw device according to claim 6.

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