JP2012077914A - Ball screw mechanism - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ball screw mechanism enabling reduction in cost and suppression of wear.SOLUTION: Since internal thread grooves 2a, 2a are not connected to each other, it is unnecessary to form an internal thread groove on an entire inner peripheral face of a nut 2, so as to avoid tool wear etc. The inner peripheral face of the nut 2 on which the internal thread grooves 2a, 2a are not formed has no space into which balls 3 come, so as to also avoid a possibility of wrong insertion. When a trunnion hole 2c etc. is provided on the inner peripheral face of the nut 2 on which the internal thread grooves 2a, 2a are not formed, wear of a pin 5 engaged with the trunnion hole 2c and the trunnion hole 2c can be avoided.

Description

  The present invention relates to a ball screw mechanism that can be used for various applications such as an electric actuator.

  In recent years, labor saving of vehicles and the like has progressed, and for example, a system for operating a transmission, a parking brake, and the like of an automobile not by hand but by the power of an electric motor has been developed. An electric actuator used for such an application may use a ball screw mechanism in order to convert the rotational motion transmitted from the electric motor into the axial motion with high efficiency.

  Here, the normal ball screw mechanism consists of a screw shaft, a nut, and a ball. When the nut rotates relative to the screw shaft, the ball rolls along the rolling path in the nut, Although a smooth operation is performed, a circulation member for circulating the ball that has reached one end of the rolling path to the other end is required. As such a circulation member, a tube, a top, etc. are known.

  The circulation function of the ball screw mechanism is taken as an example of a general coma type (deflector type). The circulation action will be described. The ball rolls slightly on the rolling path formed between the screw shaft and the nut, and circulates. By being guided to the top, the screw shaft is overcome and returned to the original passage again. Such a circulation piece for controlling the ball circulation is mounted so as to be fitted into a hole penetrating in the radial direction in the nut. In addition, an S-shaped groove that forms a closed circuit over the crest of the screw shaft is formed on the circulation piece by connecting the ball groove staggered portion in the axial direction corresponding to about one lead of the screw shaft. Has been.

  By the way, in the case of a light load, the circulation of the ball screw mechanism is sufficient in one circuit. However, when the load applied to the ball screw mechanism is large to some extent, two or more such circulation circuits are provided, and the ball screw mechanism is rated. It is common practice to increase the load. However, in the ball screw mechanism, when a plurality of circulation circuits are provided, it is common to continuously form a female screw groove having a considerable length in the axial direction so that a plurality of circuits can be arranged on the nut. is there. Patent Document 1 discloses a nut manufacturing method in which a plurality of circulation circuits can be provided.

JP-A-8-39309 JP 2002-235829 A

However, there are the following problems in continuously forming the female thread groove having a considerable length in the axial direction as much as a plurality of circuits can be arranged on the nut.
(1) When a plurality of circulation circuits are formed using the technique disclosed in Patent Document 1, a portion having a spiral groove continuously formed on the inner periphery of the nut and another portion different therefrom are respectively used as circulation circuits. In other words, unused grooves that do not actually roll the ball are formed simultaneously in parts other than the circulation circuit. However, there is a problem in that cutting to such an unused groove makes wear of the blade tool severe and shortens the tool life.
(2) The unused groove where the ball does not actually roll is a closed space sandwiched between a plurality of circulation members. On the other hand, in the ball screw assembly, a ball may be accidentally inserted into this closed space for some reason, and this erroneously inserted ball is caught between the screw shaft and the nut to lock the ball screw operation. Therefore, it must be removed by inspection after completion, resulting in poor yield.
(3) As one form of use of the ball screw, there is a configuration in which a trunnion hole is provided in the nut, and a swingable link member having a pin engaged with the trunnion hole is assembled to transmit the axial motion of the nut. Are known. Here, the trunnion hole is generally machined as a through hole to the nut, and the trunnion hole is arranged at the center of a plurality of circulation paths so as to be advantageous in consideration of the acting load on the ball screw. There are many. In this case, if a continuous ball screw groove is formed on the inner diameter side of the nut, the effective contact surface between the pin and the hole inserted into the trunnion hole is reduced, and wear of the pin and the trunnion hole may be accelerated.

  On the other hand, Patent Document 2 states that by placing a padding such as resin in a non-circulating portion of a nut where the ball does not circulate during use, the ball is erroneously mixed into the non-circulating portion during assembly. A technique for avoiding malfunction by avoiding is disclosed. However, if the non-circulating part is filled with padding, it may fall off due to vibration or the like. Also, when the top and the padding are integrated to prevent dropping, the padding is not likely to drop off during operation, but the cost may increase.

  The present invention has been made in view of the problems of the prior art, and an object of the present invention is to provide a ball screw mechanism that can reduce the manufacturing cost and suppress wear and malfunction.

The ball screw mechanism according to the first aspect of the present invention comprises:
A screw shaft having a male screw groove formed on the outer peripheral surface;
A nut disposed so as to surround the screw shaft and having an internal thread formed on an inner peripheral surface thereof;
A plurality of balls arranged to freely roll along a rolling path formed between opposing screw grooves;
A circulation member having a circulation groove for returning the ball from one end of the rolling path to the other end;
The female screw grooves are provided in a plurality of strips and separated in the axial direction in a non-connected state.

The ball screw mechanism of the second invention is
A screw shaft having a male screw groove formed on the outer peripheral surface;
A nut disposed so as to surround the screw shaft and having an internal thread formed on an inner peripheral surface thereof;
A plurality of balls arranged to freely roll along a rolling path formed between opposing screw grooves;
A circulation member for returning the ball from one end of the rolling path to the other end;
The nut has a discharge portion for discharging the ball to the circulation member, the inner and outer circumferences communicating with each other, and a supply portion for supplying the ball from the circulation portion. The axial direction outside is narrower than between the discharge part and the supply part.

  According to the ball screw mechanism of the first aspect of the present invention, since the female screw grooves are provided in a plurality of strips and separated in the axial direction in a non-connected state, the female screw grooves are formed on the entire inner peripheral surface of the nut. There is no need to do so, and tool wear and the like can be avoided. Further, there is no room for the ball to enter the inner peripheral surface of the nut in which the female screw groove is not formed, and the possibility of erroneous insertion can be avoided. Furthermore, if a trunnion hole or the like is provided on the inner peripheral surface of the nut in which the female screw groove is not formed, wear of the pin and trunnion hole engaged therewith can be avoided. Such a female screw groove can be formed by a machining method in which cutting is started from a hole formed in the nut for attaching the circulation member, and the cutting is finished at the hole. According to such a machining method, since it is not necessary to form a female thread groove on the entire nut, the machining time can be suppressed even if the rotation speed of the tool is lowered, and the rotation speed of the tool can be slowed down. Control can be easily performed with high accuracy.

  The circulation member is preferably provided in the same number as the number of the female screw grooves, but a plurality of circulation paths may be provided in a single circulation member.

  It is preferable that the female screw groove is formed by cutting a plurality of the nuts at the same time, because the processing efficiency is improved.

  It is preferable that a plurality of female thread grooves are ground on the nut at the same time because the processing efficiency is improved.

  According to the ball screw mechanism of the second aspect of the present invention, the nut is communicated between the inner and outer circumferences, and the ball is supplied from the circulation portion to a discharge portion for discharging the ball to the circulation member. The female screw groove is narrower on the outer side in the axial direction than between the discharge portion and the supply portion, so that the discharge portion is a non-circulating region where the ball does not roll. Since the ball cannot enter the space surrounded by the male screw groove on the outer side in the axial direction from the supply unit, the ball is not assembled in a state where it is mistakenly mixed. Here, “narrow” means that the axial cross-sectional area of the male screw groove and the female screw groove is narrower outside the discharge part and the supply part in the axial direction, and the female screw groove Includes a state in which the film is shallow or not formed. The “axially outer side” may be one side instead of both sides.

  When the circulating member is a tube, the nut has a screw hole for attaching a fixture for fixing the tube, and the female screw groove is formed at a place other than the screw hole. When tapping is formed with a tap, burrs and the like are suppressed from remaining in the female screw groove.

  In the female thread groove, if the surface treatment is performed only between the discharge portion and the supply portion, wear when the ball rolls can be suppressed. Surface treatment includes, but is not limited to, grinding treatment.

It is a side view of the ball screw mechanism which is this Embodiment. It is sectional drawing which cut | disconnected the ball screw mechanism of FIG. 1 by the II-II line | wire, and looked at the arrow direction. FIG. 3 is a cross-sectional view of the ball screw mechanism of FIG. 1 taken along line III-III and viewed in the direction of the arrow. FIG. 4 is a cross-sectional view of a nut shown in a state where a screw shaft, a piece and a ball are removed in the ball screw mechanism shown in FIG. 3. It is a figure which shows the state of the internal peripheral surface process of the nut. It is a figure which shows the state of the internal peripheral surface process of the nut 2 concerning a modification. It is sectional drawing of the ball screw mechanism concerning this Embodiment to which a trunnion is applied. It is sectional drawing of the ball screw mechanism concerning the comparative example to which a trunnion is applied. It is a figure which shows the state of the internal peripheral surface processing of the nut 2 concerning another embodiment. It is an axial sectional view of a ball screw mechanism according to another embodiment. It is a top view of the nut concerning this Embodiment. It is the figure which cut | disconnected the nut in the position shown by the XII-XII line | wire in FIG. 10, and was seen in the arrow direction.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a side view of the ball screw mechanism according to the present embodiment, and FIG. 2 is a cross-sectional view of the ball screw mechanism of FIG. FIG. 3 is a cross-sectional view of the ball screw mechanism of FIG. 1 taken along line III-III and viewed in the direction of the arrow. 4 is a cross-sectional view of the nut shown with the screw shaft, the top, and the ball removed in the ball screw mechanism shown in FIG.

  In FIG. 1, a screw shaft 1 connected to a motor (not shown) is supported so as to be rotatable only. A male screw groove 1 a is formed on the outer peripheral surface of the screw shaft 1. A substantially cylindrical nut 2 connected to a driven member (not shown) and supported so as to be movable only in the axial direction is arranged so as to surround the screw shaft 1 and has two female screws on the inner peripheral surface. Grooves 2a and 2a are formed. A plurality of balls 3 are arranged so as to roll freely in two spiral rolling paths formed between the opposing screw grooves.

  As shown in FIG. 4, the two female screw grooves 2 a and 2 a formed on the inner peripheral surface of the nut 2 are each wound around 360 degrees and are not connected to each other in the axial direction of the nut 2. It has a lead angle equal to the lead angle of the male screw groove 1a of the screw shaft 1, and has a relationship of being connected as a single female screw groove when extended by the lead angle.

  As shown in FIG. 3, the nut 2 is provided with two top holes 2b aligned in the axial direction so as to penetrate in the radial direction. A cylindrical piece 4 is disposed in each hole 2b. The top 4 which is a circulation member has an S-shaped circulation groove 4 a formed on the inner surface when assembled to the nut 2. The circulation groove 4a has a shape connected to both ends of the female screw groove 2a. That is, both ends of the female thread groove 2a are connected via the hole 2b.

  The operation of the present embodiment will be described. When power from an electric motor (not shown) is transmitted to the screw shaft 1, the balls 3 roll on the rolling path and circulate through the circulation path 4 a of the top 4. The rotational movement is efficiently converted into the axial movement of the nut 2, and a driven member (not shown) can be moved in the axial direction.

  Here, the manufacturing method of the nut 2 is demonstrated. FIG. 5 is a diagram illustrating a state of the inner peripheral surface processing of the nut 2. In FIG. 5, a tool T having a cutting edge tip Tp attached to the outer periphery of the tip is inserted into the nut 2 in which the holes 2b and 2b for the top are formed. The tool T rotates around the X axis and is driven to move in the axial direction according to the rotation. When the tool T rotates, the cutting edge tip Tp revolves around the X axis.

  First, when the tool T is positioned so that the cutting edge tip Tp is placed in the hole 2b (right side in the drawing) and then the tool T is rotated around the X axis, the cutting edge tip Tp is cut from the wall surface of the hole 2b. Starting, the inner peripheral surface of the nut 2 is cut along the revolution track until the tool T makes one revolution. At this time, since the tool T moves in the axial direction in accordance with the rotation, the female thread groove 2a having a predetermined lead angle is formed. When the tool T rotates nearly 360 degrees, the cutting edge tip Tp enters the hole 2b, so that the tool T is stopped. If cutting cannot be performed once, the tool T is returned to the starting point and rotated again while increasing the cutting amount.

  When the machining of one female thread groove 2a is completed, the tool T is brought closer to the axis of the nut 2, the tool T is further fed out, and moved again radially outward of the nut 2, and placed in the left hole 2b in the drawing. The tool T is positioned in Thereafter, the female thread groove 2a is turned by the same method.

  According to the processing method of the present embodiment, the rotational speed of the tool T may be relatively low, so that the internal thread groove 2a formed in the nut 2 can be easily controlled to the minimum necessary. In addition, the load on the cutting edge tip Tp is applied only when the internal thread groove 2a is processed, and unnecessary internal thread grooves that are not used for rolling of the ball 3 are not cut, so that the life of the tool T is extended.

  FIG. 6 is a diagram illustrating a state of the inner peripheral surface processing of the nut 2 according to the modification. In FIG. 6, inside the nut 2 in which the top holes 2b and 2b are formed, the outer periphery of the tip and a position away from the outer periphery in the axial direction by a distance equal to the distance between the holes 2b and 2b (that is, the tool T ′). The tool T ′ with the cutting edge tips Tp ′ and Tp ′ attached thereto is inserted. The tool T ′ rotates around the X axis and is driven to move in the axial direction according to the rotation. When the tool T ′ rotates, the cutting edge tips Tp ′ and TP ′ revolve around the X axis.

  First, when the tool T ′ is positioned so that the respective cutting edge tips Tp ′ and Tp ′ are placed in the respective holes 2b and 2b, and then the tool T ′ is rotated around the X axis, each cutting edge tip Tp ′ is obtained. , Tp ′ starts cutting simultaneously from the wall surfaces of the holes 2b and 2b, and cuts the inner peripheral surface of the nut 2 along the revolution track until the tool T ′ makes one revolution. At this time, since the tool T 'moves in the axial direction according to the rotation, the female thread grooves 2a and 2a having a predetermined lead angle are formed at the same time. When the tool T 'rotates near 360 degrees, the cutting edge tips Tp' and Tp 'enter the holes 2b and 2b, so that the tool T' is stopped. In this manner, by providing the two cutting edge tips Tp ′ and Tp ′, the two female thread grooves 2 a and 2 a can be formed by one turning, so that the processing efficiency is increased. Needless to say, in the case of a nut having three or more female thread grooves, three or more cutting edge tips may be provided.

  Further, there is no room for the ball 3 to enter the inner peripheral surface of the nut 2 where the female screw groove 2a is not formed, and the possibility of erroneous insertion of the ball can be avoided.

  Further, a ball mechanism provided with a trunnion will be described. FIG. 7 is a cross-sectional view of the ball screw mechanism according to the present embodiment to which the trunnion is applied, and FIG. 8 is a cross-sectional view of the ball screw mechanism according to a comparative example to which the trunnion is applied. In FIG. 7, the nut 2 is formed with trunnion holes 2c and 2c penetrating oppositely to the center thereof. The pins 5 and 5 are inserted through the trunnion holes 2c and 2c. Link members 6 and 6 are swingably disposed around the pins 5 and 5. When the nut 2 moves in the axial direction, the pins 5 and 5 are pulled or pushed in the axial direction accordingly to drive a driven member (not shown).

  In the ball screw mechanism of the comparative example shown in FIG. 8, trunnion holes 12c "and 12c" penetrating through the nut 12 "are formed in the nut 12" formed with the female thread groove 12a over the entire inner periphery. Pins 5 and 5 are inserted into the trunnion holes 12c ″ and 12c ″. Link members 6 and 6 are swingably disposed around the pins 5 and 5. The nut 12 ″ is an axis. When moving in the direction, the pins 5 and 5 are pulled or pushed in the axial direction accordingly to drive the driven member.

  Here, in the ball screw mechanism of the comparative example, since the female thread groove 12a ″ is formed on the entire inner peripheral surface of the nut 12 ″, the trunnion hole 12c ″ is always formed at a position where the female thread groove 12a ″ is located. It becomes. That is, in the case of the trunnion hole 12c ″ penetrating the bottom of the female screw groove 12a ″, the effective contact area on the wall surface of the trunnion hole 12c ″ is reduced. Therefore, when the pin 5 is inserted, the trunnion hole 12c ″ and the pin 5 There is a risk that the surface pressure becomes high and large wear occurs. On the other hand, according to the present embodiment, if the trunnion hole 2c is provided between the pair of female screw grooves 2a and 2a, the trunnion hole 2c penetrates the inner peripheral surface without the female screw groove. Therefore, when the pin 5 is inserted, the surface pressure between the trunnion hole 2c and the pin 5 can be kept low, thereby suppressing the occurrence of wear.

  FIG. 9 is a diagram showing a state of inner peripheral surface processing of the nut 2 according to another embodiment. In FIG. 9, inside the nut 2 in which the holes 2b and 2b for the top are formed, the outer periphery of the tip and a position away from the outer periphery in the axial direction by a distance equal to the distance between the holes 2b and 2b (that is, the grinding wheel G The grinding wheel G provided with the grinding portions Gp and Gp is inserted, respectively, so as to have the same phase around the rotation axis. The grinding wheel G rotates around the X axis and is driven to move in the axial direction according to the rotation. When the grinding wheel G rotates, the grinding parts Gp and Gp revolve around the X axis.

  First, when the grinding wheel G is positioned so that the grinding parts Gp and Gp are placed in the respective holes 2b and 2b, and then the grinding wheel G is rotated around the X axis, the grinding parts Gp and Gp are Grinding is simultaneously started from the wall surfaces of the holes 2b and 2b, and the inner peripheral surface of the nut 2 is ground along the revolution track until the grinding wheel G rotates once. At this time, since the grinding wheel G moves in the axial direction in accordance with the rotation, the female thread grooves 2a and 2a having a predetermined lead angle are simultaneously formed. When the grinding wheel G rotates nearly 360 degrees, the grinding parts Gp and Gp enter the holes 2b and 2b, so that the grinding wheel G is stopped. By repeating this, deeper thread grooves 2a and 2a can be formed. In addition, you may combine with the cutting process shown in FIG. Thus, by providing the two grinding portions Gp and Gp, the two female thread grooves 2a and 2a can be formed by one rotation of grinding, so that the processing efficiency is increased. Needless to say, in the case of a nut having three or more female thread grooves, three or more grinding portions may be provided.

  FIG. 10 is an axial sectional view of a ball screw mechanism according to another embodiment. FIG. 11 is a top view of the nut according to the present embodiment, and FIG. 12 is a view in which the nut is cut at the position indicated by the line XII-XII in FIG.

  In FIG. 10, the ball screw mechanism is opposed to a screw shaft 11 having a male screw groove 11a formed on the outer peripheral surface and a nut 12 disposed so as to surround the screw shaft 11 and having a female screw groove 12a formed on the inner peripheral surface. A plurality of balls 13 arranged so as to roll along a rolling path formed between both screw grooves 11a and 12a, and a tube 14 which is a circulating member for returning the balls 13 from one end of the rolling path to the other end. And have.

  A mounting plate 15, which is a fixture for fixing the tube 14 to the nut 12, is attached to the nut 12 with a bolt 16. That is, the nut 12 has a pair of insertion holes formed so that the inner and outer circumferences of the nut 12 communicate with each other so that both ends of the tube 14 can be inserted (one of which is a discharge portion provided at one end of the rolling path, and the other is a rolling member). A supply section 12b provided at the other end of the runway and a pair of screw holes 12c for screwing the bolts 16 are formed. The female screw groove 12a is formed between the pair of insertion holes 12b, and the outer side of the insertion hole 12b in the axial direction is gradually rounded up and shallow, that is, inclined toward the inner peripheral surface.

  In order to machine the female thread groove 12a in this way, for example, while rotating the tool shown in FIG. 5 in a spiral manner, the rotational axis X is gradually moved radially outward from the outer side of the one insertion hole 12b in the axial direction. The rotation axis X is fixed when it reaches one insertion hole 12b, is moved in a spiral shape in that state, and the rotation axis X is gradually radiused from the time it reaches the other insertion hole 12b. It can be formed by moving inward in the direction. Thereafter, drilling of the insertion hole 12b, drilling and tapping of the screw hole 12c, and grinding of the female screw groove 12a are performed.

  In the present embodiment, when power from an electric motor (not shown) is transmitted to the screw shaft 11, the rolling path defined between the male screw groove 11a and the female screw groove 12a between the pair of insertion holes 12b. , And the ball 13 circulating through the tube 14 efficiently converts the rotational movement into the axial movement of the nut 12 and can move a driven member (not shown) in the axial direction.

  According to the present embodiment, the female screw groove 12a of the nut 12 is shallower on the outer side in the axial direction than between the pair of insertion holes 12b, so that the pair of non-circulating regions where the ball 13 does not roll. Since the ball 13 cannot enter the space surrounded by the male screw groove 11a on the outer side in the axial direction from the insertion hole 12b, the ball 13 is not assembled in a state where it is mistakenly mixed. The internal thread groove 12a may be ground only between the pair of insertion holes 12b, which are regions where the balls 13 roll.

  Further, in the conventional nut, as shown by the dotted line in FIG. 12, the female screw groove (12a ′) having the same depth is formed across the both ends of the nut. There is a possibility of interference with the hole 12c, and thus, there is a problem that when the tapped hole 12c is tapped, a burr or the like is likely to occur, which falls to the rolling surface and easily wears. On the other hand, according to the present embodiment, the female screw groove 12a is not formed up to the end of the nut 12, and therefore interference with the screw hole 12c is avoided, so that occurrence of burrs or the like can be suppressed. Furthermore, since the grinding tool does not interfere with the screw hole 12c during the grinding of the female thread groove 12a, the wear and chipping of the tool can be suppressed.

  Two or more tubes 14 may be provided. In that case, with reference to a pair of insertion holes 12b through which both ends of each tube 14 are inserted, a female screw groove is formed between the tubes 14 and the outside in the axial direction. What is necessary is just to change the shape of 12a. A frame may be used instead of the tube 14. In such a case, the discharge unit and the supply unit may be a connected space.

  The present invention has been described above with reference to the embodiments. However, the present invention should not be construed as being limited to the above-described embodiments, and can be modified or improved as appropriate.

DESCRIPTION OF SYMBOLS 1, 11 Screw shaft 2, 12 Nut 3, 13 Ball 4 Top 5 Pin 6 Link 14 Tube T Cutting tool G Grinding wheel

The ball screw mechanism of the present invention is
A screw shaft having a male screw groove formed on the outer peripheral surface;
A nut disposed so as to surround the screw shaft and having an internal thread formed on an inner peripheral surface thereof;
A plurality of balls arranged to freely roll along a rolling path formed between opposing screw grooves;
A tube for returning the ball from one end of the rolling path to the other end;
The nut has a discharge portion for communicating the inner and outer circumferences to discharge the ball to the tube , and a supply portion for supplying the ball from the tube , and the discharge portion and the supply portion Is separated by one or more rounds in the inner circumference of the nut, and the female thread groove interferes with a screw hole for attaching a fixture for fixing the tube, on the outer side in the axial direction than between the discharge portion and the supply portion. It is characterized by being gradually rounded up and shallow so as not to.

According to the ball screw mechanism of the present invention, the nut includes a discharge portion for communicating the inner and outer periphery and discharging the ball to the tube , and a supply portion for supplying the ball from the tube. And the discharge portion and the supply portion are separated from each other by one or more rounds on the inner periphery of the nut, and the female screw groove fixes the tube on the outer side in the axial direction than between the discharge portion and the supply portion. Since it is gradually rounded up and shallow so as not to interfere with the screw hole for mounting the fixture, the ball is non-circulating in which the ball does not roll, and outside the supply unit in the axial direction, Since the ball cannot enter the space surrounded by the male screw groove, the ball is not assembled in a state where it is mistakenly mixed. The “axially outer side” may be one side instead of both sides.

Claims (7)

A screw shaft having a male screw groove formed on the outer peripheral surface;
A nut disposed so as to surround the screw shaft and having an internal thread formed on an inner peripheral surface thereof;
A plurality of balls arranged to freely roll along a rolling path formed between opposing screw grooves;
A circulation member having a circulation groove for returning the ball from one end of the rolling path to the other end;
The ball screw mechanism is characterized in that the female screw grooves are provided in a plurality of strips and are axially separated in a non-connected state.   The ball screw mechanism according to claim 1, wherein the circulation member is provided in the same number as the number of the female screw grooves.   The ball screw mechanism according to claim 1 or 2, wherein the female screw groove is formed by cutting a plurality of threads on the nut at the same time.   The ball screw mechanism according to claim 1, wherein the female screw groove is formed by grinding a plurality of the nuts simultaneously on the nut. A screw shaft having a male screw groove formed on the outer peripheral surface;
A nut disposed so as to surround the screw shaft and having an internal thread formed on an inner peripheral surface thereof;
A plurality of balls arranged to freely roll along a rolling path formed between opposing screw grooves;
A circulation member for returning the ball from one end of the rolling path to the other end;
The nut has a discharge portion for discharging the ball to the circulation member, the inner and outer circumferences communicating with each other, and a supply portion for supplying the ball from the circulation portion. A ball screw mechanism characterized in that the outer side in the axial direction is narrower than between the discharge portion and the supply portion.   The circulating member is a tube, the nut has a screw hole for attaching a fixture for fixing the tube, and the female screw groove is formed at a place other than the screw hole. The ball screw mechanism according to claim 5.   The ball screw mechanism according to claim 5 or 6, wherein a surface treatment is performed only between the discharge portion and the supply portion in the female screw groove.

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