JP2003329101A - Ball screw - Google Patents

Abstract

[PROBLEMS] To extend the life of a relatively large ball screw used in an electric injection molding machine or a press machine, which is subjected to a high-speed rotation and a high load. SOLUTION: The ball screw of the present invention includes a retaining piece 2 having two concave surfaces 23 each facing a ball 9.
1 are disposed between adjacent balls 9, respectively, and the radius of curvature Rg of the cross section along the axis of at least one of the male screw groove 3 a or the female screw groove 5 is set to 5 of the ball diameter D.
It is characterized by a configuration of 2.5% or more and 55% or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ball screw, and more particularly, to a ball screw used in an electric injection molding machine, a press machine or the like, which has a relatively large size, a high speed rotation, and a high load.

[0002]

2. Description of the Related Art In recent years, ball screws have been widely used as a substitute for hydraulic cylinders for converting rotary motion of an electric injection molding machine, electric press machine or the like into straight motion, and smoothly performing straight motion without friction loss. There is. A ball screw is freely rotatable between a screw shaft having an external thread groove formed on the outer peripheral surface thereof, a nut having an internal thread groove having the same pitch as the external thread groove formed on the inner peripheral surface, and the male thread groove and the female thread groove. The nut is formed with a circulation passage that communicates one end side and the other end side of the female thread groove with each other.

When the screw shaft and the nut are rotated relative to each other, the balls roll forward in the female screw groove while rolling in the rolling passage formed by the male screw groove and the female screw groove to reach one end of the female screw groove. The reached ball circulates from the one end side to the other end side of the female thread groove through the circulation passage, is supplied again between the male thread groove and the female thread groove, and rolls in the rolling path. There is.

The balls rolling between the male screw groove and the female screw groove are rotating in the same direction, and when the balls come into direct contact with each other, on the surface of the ball, which is the contact portion, in the opposite directions. Because of the motion, relative slip at a speed twice as high as the rolling speed occurs, a phenomenon called ball competition occurs and the ball surface is prematurely worn or frictional heat causes seizure of the ball or thread groove. Occasionally, problems such as occurrence may occur. The contact point between the ball and the screw shaft or the ball and nut becomes a very small ellipse called a contact ellipse through the grease or the lubricant, and the surface pressure at the contact point becomes extremely high.

The conventional ball screw is manufactured by using a special material so as to withstand the high surface pressure. That is, for the screw shaft and the nut, steel for carburizing bearings such as SCM420 and steel for induction hardening such as SAE4150 are used.
The one that is heat-treated to adjust the surface hardness to HRC 56-63 is used. In addition, the ball has SU
A high carbon chrome bearing steel such as J2 is used and is hardened to have a surface hardness of HRC60 or more.

A ball screw used in an electric injection molding machine, an electric press machine or the like is used with a short stroke to which a high load is momentarily applied. It is used under severe conditions. Therefore, the oil film on the ball rolling surface is scraped off, and it is difficult for the lubricant to enter the contact surface between the screw groove and the ball.
There is a problem that the oil film formation tends to be insufficient, and the rolling surfaces of the screw shaft, the nut, and the ball are likely to be worn or peeled due to surface damage. In particular, in the contact surface between the balls where relative sliding at a speed twice as high as the rolling speed occurs, there was some damage caused by the competition of the balls.

Further, deformation of the machine base due to the action of a high load, misalignment at the time of mounting, or the like makes the above-described ball competition even more remarkable and further shortens the life.

[0008] As a means for canceling the ball bidding,
Japanese Unexamined Patent Publication No. 2000-291770 proposes a ball screw of a type in which a spacer ball having a diameter of about several μm to several μm is interposed between load balls supporting a load. . The interposition of the spacer balls can eliminate the competition between the balls, but the spacer balls cannot receive the load, and the number of the balls that receive the load is substantially reduced.
There is a problem that the allowable load of the ball screw is reduced.

As a ball screw which does not significantly reduce the allowable load and prevents the balls from competing with each other, for example, JP-A-11-315835 and JP-A-2000-199.
Japanese Patent Laid-Open No. 556, 2001-21018, and Japanese Patent Laid-Open No. 2001-124172 disclose ball screws in which a retaining piece having two concave surfaces facing each other is interposed. There is.

By arranging the balls in contact with the concave surface of the retaining piece, a much larger number of load balls can be arranged as compared with a ball screw using spacer balls, and the allowable load capacity can be increased. Can be suppressed. Further, there is an advantage that balls do not compete with each other and the lubricant can be retained in the concave surface of the retaining piece, so that poor lubrication can be significantly reduced.

In order to reduce the surface pressure at the contact ellipse which is the contact point between the ball and the screw shaft or the ball and the nut,
There is also proposed a so-called Gothic arch-shaped screw groove with a smaller radius of curvature. For example, JP-A-200
In JP-A-0-39052, the contact surface pressure between the male thread groove and the ball is reduced by making the radius of curvature of the male thread groove of the screw shaft smaller than the radius of curvature of the female thread groove of the nut.
A ball screw is disclosed which has a longer life by relaxing the condition of a screw shaft that is more easily damaged than a nut.

[0012]

However, when the ball screw is used under a high load condition, a large shearing force acts on the inside of the material just below the contact ellipse, and the ball is auctioned with the retaining piece. However, although the radius of curvature of the male screw groove of the screw shaft is reduced to reduce the contact surface pressure between the male screw groove and the ball, there is a problem that the screw shaft or the nut is separated. Therefore, if the balls are prevented from competing against each other through the retaining piece and the radius of curvature of the male thread groove is reduced to reduce the contact surface pressure, it is difficult to operate under high-speed, high-load, short-stroke reciprocating motion. However, there is room for further improvement in order to achieve a long life of the ball screw.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a ball screw which has a longer life under severe operating conditions.

[0014]

The ball screw according to claim 1 of the present invention has a screw shaft having a spiral male thread groove formed on the outer peripheral surface and a spiral female thread groove formed on the inner peripheral surface. A ball screw comprising a nut and a plurality of balls rollably arranged in a rolling path formed by the male screw groove and the female screw groove, each of which faces the ball.
Retaining pieces having concave surfaces are respectively arranged between the adjacent balls, and the radius of curvature of the cross section along the axis of at least one of the male screw groove or the female screw groove is equal to the diameter of the ball. It is characterized by being 52.5% or more and 55% or less.

According to the ball screw having the above structure, the retaining pieces having two concave surfaces facing the balls are arranged between the adjacent balls.
For example, the balls roll and circulate in the rolling path together with the retaining piece while contacting the Gothic arch-shaped concave surface of the retaining piece with extremely low friction, thereby preventing the balls from competing with each other. Therefore, it is possible to prevent malfunction due to competition, noise and abnormal noise, and damage due to friction of the balls. Further, a much larger number of load balls can be arranged as compared with a ball screw using spacer balls, and the life can be extended without reducing the allowable load capacity.

Further, since the radius of curvature of the cross section along the axis of at least one of the male thread groove and the female thread groove is set to 52.5% or more and 55% or less of the diameter of the ball,
It is possible to suppress the generation of abnormal tissue due to rolling fatigue and prevent peeling due to the abnormal tissue.

A ball screw according to a second aspect of the present invention is the ball screw described above, wherein the male screw groove and the female screw groove are formed in a Gothic arch shape in cross section.

According to the ball screw having the above structure, since the cross-sectional shape of the thread groove is a Gothic arch shape, the radius of curvature of the thread groove is reduced in order to reduce the area of the contact ellipse and the slip at the contact point. Larger than usual,
It is possible to prevent the gap between the edge of the screw groove and the ball from becoming excessively large, and it is possible to maintain the positioning accuracy between the screw shaft and the nut in a good state.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the ball screw of the present invention will be described below with reference to FIGS. 1 is a plan view showing an embodiment of the ball screw of the present invention, and FIG.
3 is an enlarged sectional view of the retaining piece, FIG. 4 is an enlarged sectional view taken along the axial direction of the ball screw, and FIG. 5 is a radius of curvature of the male screw groove of the screw shaft / ball. It is a correlation diagram which shows the relationship between a diameter and a life ratio.

As shown in FIGS. 1 to 4, the ball screw 1 of the present embodiment has a cylindrical shape in which a male screw groove 3a is formed on the outer peripheral surface and a female screw groove 5 is formed on the inner peripheral surface. Nut 7 and a plurality of balls 9 interposed between the male screw groove 3a and the female screw groove 5.

As shown in FIG. 4, the screw shaft 3 has a nut 7
For guiding along the axial direction of the screw shaft 3 and having a diameter D of 5 of the ball 9 over the entire length of the outer peripheral surface.
A so-called Gothic arch-shaped male screw groove 3a is formed in a spiral shape in which arcs having a radius of curvature Rg of 2.5% or more and 55% or less intersect at an intermediate portion. Screw shaft 3
Is steel for carburized bearings such as SCM420 and SAE4150
Induction hardening steel such as
The hardness is adjusted to about RC56 to 63. Further, the pitch of the male screw groove 3a can be set to an arbitrary pitch in accordance with the specifications of a device (not shown) in which the ball screw 1 is incorporated.

The nut 7 linearly moves along the axial direction of the screw shaft 3 and has a cylindrical shape, and a flange 11 for fixing to a table (not shown) or the like of the apparatus is formed at one end. The outer peripheral surface is partially cut away to form the flat surface portion 1.
3 is formed. The nut 7 is manufactured by using steel for carburizing bearings such as SCM420 and steel for induction hardening such as SAE4150, and heat treated to have a surface hardness of HRC56 to HRC56 to 6.
Hardened to about 3.

On the inner peripheral surface of the nut 7, an arc having a curvature radius Rg of 52.5% or more and 55% or less of the diameter D of the female thread groove 5 having the same shape and pitch as the male thread groove 3a, that is, the ball 9 is formed. A so-called Gothic arch-shaped internal thread groove 5 is formed by intersecting each other at an intermediate portion.

In the female thread groove 5, a tube 15 made of steel pipe, which is a circulation path for communicating one end side with the other end side, is fixed to the flat surface portion 13 by a tube retainer 17. Then, the ball 9 is transferred through the tube 15 to circulate the ball 9 from one end side to the other end side of the female thread groove 5. Dust seals 19 made of plastic are arranged at both ends of the nut 7 to prevent foreign matter from entering the nut 7 from the outside.

The ball 9 is for rolling the nut 7 smoothly without friction loss by rolling, and is free to roll in the rolling path constituted by the male screw groove 3a and the female screw groove 5. A plurality of them are arranged. The balls 9 are made of high carbon chrome bearing steel such as SUJ2 and are hardened to have a surface hardness of HRC60 or more.

As shown in FIG. 3, the retaining piece 2
Reference numeral 1 is for preventing direct contact between the balls 9, and a plurality of balls 9 are interposed between the balls 9.
The retaining piece 21 is manufactured, for example, by injection-molding polyamide, fluororesin, or polyethylene impregnated with lubricating oil, which itself has a lubricating action.

The retaining piece 21 is manufactured in a disk shape having concave spherical surfaces 23 having a radius of curvature R larger than the radius r (D / 2) of the ball 9 at both ends. This makes it possible to arrange a large number of balls 9 by reducing the thickness t of the central portion with respect to the total length L and reduce the contact area between the balls 9 and the retaining piece 21 to reduce sliding resistance. It is configured to be minimal.

The shape of the concave spherical surface 23 is not limited to the spherical shape, and is a so-called Gothic arch-shaped concave surface formed by intersecting two arcs at the intermediate portion or a conical concave surface. It may be. Further, a through hole may be provided between both concave surfaces, or a lubricant may be retained in the through hole to reduce the contact resistance with the ball 9.

The diameter dimension of the retaining piece 21 (d
s) is set to be smaller than the diameter D of the ball 9 and the diameter of the ball 9 when the ball 9 is elastically deformed by the maximum load acting on the ball 9, and the retaining piece 21 has the male screw groove 3a and the female screw groove 5 When passing through the tube 15 which is a rolling path and a circulating path constituted by, the rolling path, the tube 15 and their connecting portions do not interfere with each other, so that they can smoothly circulate. Specifically, the diameter is 0.5 to 0.9 times the diameter dimension D of the ball 9 (ds = 0.5D to 0.
9D) is preferable.

If the clearance between the ball 9 and the retaining piece 21 arranged in the rolling path is too large, the retaining piece 21 will fall down and cannot perform its function. Since the frictional force between the piece 21 and the ball 9 becomes large and causes a malfunction, the gap is set to be appropriate.

Specifically, assuming that all the balls 9 and the retaining pieces 21 arranged in the rolling path are gathered together on one side, between the ball 9 at the head and the retaining piece 21 at the tail. Assuming that the total clearance S1 is the total clearance S1, the total clearance S1 is larger than 0 (S1> 0), and one retaining piece 21 at the end is removed, the first ball 9 and the last ball 9 (S2) is smaller than 0.8 times the diameter (ds) of the retaining piece 21 (S2 <0.
8 × ds), the number of balls 9 and the number of retaining pieces 21 are set. This prevents the retaining piece 21 from falling more than about 60 ° in the rolling path,
The function can be exhibited well.

The operation of the ball screw of this embodiment will be described. As shown in FIGS. 1 and 2, when the screw shaft 3 is rotated by a motor (not shown), the ball screw 1 of this embodiment has a nut 7 screwed through a plurality of balls 9.
Moves in the axial direction of the screw shaft 3. At this time, since the male screw groove 3a and the female screw groove 5 rotate relatively in opposite directions, the ball 9 rolls with respect to the male screw groove 3a and the female screw groove 5 to move forward in the female screw groove 5, The ball 9 that has reached one end side of 5 rolls in the tube 15 and is supplied to the other end side of the female thread groove 5 and circulates again.

In the ball screw 1, in addition to the rolling passage consisting of the male screw groove 3a and the female screw groove 5 being continuous in a spiral shape, the ball screw 1 has a moving direction at a connecting portion between the rolling passage and the tube 15 which is a circulation passage. And the load acting on the ball 9 at the connecting portion changes suddenly from the unloaded state to the unloaded state, or vice versa.
Behavior is complicated.

Since the male screw groove 3a and the female screw groove 5 have a Gothic arch shape, the contact state between the male screw groove 3a and the female screw groove 5 and the ball 9 changes depending on the load.
Rolling while changing to point contact, 3 point contact or 4 point contact. Further, since the rolling path is spirally continuous, the rolling does not occur in an ideal rolling state, but the male screw groove 3
A slip is generated between a and the female screw groove 5 and the ball 9 by spin motion.

The contact point between the male screw groove 3a and the female screw groove 5 and the ball 9 has an elliptical shape of a small area called a contact ellipse through grease or lubricating oil, and the contact ellipse has a very high surface. There is pressure. Further, in the ball screw 1 on which a high load is applied, the contact ellipse becomes large, so that the differential slip in the contact ellipse is large.

In the ball screw 1 used under the condition that a high load is intermittently applied, such as an injection molding machine or a press machine, the balls 9 tend to compete with each other, which is peculiar to the ball screw, and the wear of the balls 9 is accelerated. There is. However, since the retaining piece 21 is interposed between the balls 9, direct contact between the balls 9 is prevented. Further, the relative sliding speed between the retaining piece 21 and the ball 9 is
The speed is half that when they directly contact each other.

Therefore, the friction between the retaining piece 21 and the ball 9 is reduced, and the lubricity of the material of the retaining piece 21 and the lubricity of the grease retained in the gap between the retaining piece 21 and the ball 9 are reduced. The wear of the retaining piece 21 and the ball 9 is prevented. As a result, there is no malfunction due to competition between the balls 9, noise is not generated, sound quality is not deteriorated, and the balls can be smoothly circulated in the rolling path, and smooth and quiet operation can be performed for a long period of time. .

As shown in FIG. 3, the retaining piece 2
Since the thickness t of the concave spherical surface 23 formed in the central portion of 1 is small, a large number of balls 9 can be arranged as compared with a ball screw using spacer balls. This allows
Ball screw 1 that can withstand a large load and has greater rigidity
Can be manufactured. The radius of curvature R of the concave spherical surface 23 is
Since the radius is set to be larger than the radius r of the ball 9, the contact area between the ball 9 and the retaining piece 21 becomes small and the sliding resistance becomes relatively small, and the grease is not filled in the gap between the ball 9 and the concave spherical surface 23. It becomes easy to invade and drive resistance decreases.

Outer diameter dimension ds of the retaining piece 21
Has an outer diameter of 0.5 to 0.9 times the diameter dimension D of the ball 9, so that when the retaining piece 21 passes through the rolling path and the tube 15 together with the ball 9, the rolling piece It smoothly circulates without interfering with the moving path, the tube 15, and the connecting portion thereof, and suppresses torque fluctuations and wear of the retaining piece 21.

Assuming that the rolling path, the clearance between the ball 9 and the retaining piece 21 in the tube 15 is the total clearance S1> 0, and that one retaining piece 21 at the end is removed, Since the clearance S2 between the ball 9 and the rearmost ball 9 is set to 0.8 <ds ×, the retaining piece 21 does not fall down in the rolling path, and good operation is maintained. You can

The inventor of the present invention eliminates the competition between the balls 9 by rolling the retaining piece 21 between the rolling balls 9, but the fatigue damage occurs under the high-speed rotation and high-load use conditions. As a result of research over a long period of time, it was found that in the damage mode, in addition to the surface-originating type delamination caused by poor lubrication, which has been conventionally considered, there is an internal origin type delamination. That is, it was found that a large shearing force acts on the inside of the material just below the contact ellipse, and an abnormal structure is generated due to rolling fatigue inside the material, which is one of the causes of peeling.

The abnormal structure is a structure accompanied by diffusion of carbon contained in cementite in the metal structure due to rolling fatigue, and is called a white structure. The mechanism of abnormal tissue generation is not clear at this time, but slips due to changes in the load state, differential slips and spin slips in the contact ellipse are large, and they tend to occur under high load conditions, causing mechanochemical reactions and It is thought that the cause is hydrogen penetrating into.

As shown in FIG. 4, in the ball screw 1 of this embodiment, the radius of curvature Rg of at least one of the male screw groove 3 a and the female screw groove 5 is 52.
It is 5% or more and 55% or less. Therefore, since the radius of curvature Rg is larger than the radius of curvature that has been conventionally used, the area of the contact ellipse is reduced, the slippage at the contact point is small, the generation of abnormal tissue is suppressed, and the life is extended. it can.

If the radius of curvature Rg of the thread groove is less than 52.5%, the effect of reducing slippage is small and the effect of suppressing abnormal tissue is small. When the radius of curvature Rg of the thread groove exceeds 55%, the area of the contact ellipse becomes too small,
The surface pressure increases and the life of the ball screw 1 decreases. Moreover, the rigidity of the ball screw 1 is reduced, which adversely affects the positioning accuracy. Considering the effect of suppressing abnormal tissue and the surface pressure of the contact ellipse, the radius of curvature Rg of the thread groove is 52.5% or more and 55% or less of the diameter D of the ball 9, preferably 53.
% And 54% or less is preferable.

[0045]

EXAMPLES Examples of the ball screw of the present invention and comparative examples for comparison with the examples will be described. That is, Examples 1 to 8 performed to confirm the effect of the ball screw of the present invention, and Comparative Example 1 performed to compare with each Example.
-The test of Comparative Example 5 will be described.

For the test, JIS 1192 number 25 ×
A ball screw of 10 × 500-C5 is used and subjected to a ball screw durability life tester made by NSK under the following test conditions, the test is interrupted at regular intervals to confirm the presence or absence of peeling of the screw shaft, and until peeling occurs. The test time was the life time.

The screw shaft and the nut were made by carburizing and quenching using SCM420 steel for carburized bearings. Further, the shapes of the male screw groove and the female screw groove were manufactured as a Gothic arch shape having a radius of curvature shown in Table 1 with respect to the diameter D of the ball. The ball had a diameter of 4.76 mm (3/16 inch), and a steel ball manufactured by SUJ2 was used. In addition, in Examples 1 to 6, both the screw shaft and the nut are thread grooves having a radius of curvature within the range defined in the present embodiment. In Example 7 and Example 8, the male screw groove of the screw shaft has a radius of curvature in the range defined in the present embodiment, but the radius of curvature of the female screw groove of the nut is smaller than the range defined in the present embodiment. ing.

[0048]

[Table 1]

The retaining piece was made of nylon, the concave surface facing the ball was conical, and the outer diameter dimension was 0.8 times the diameter of the ball. Also, the total gap S1
Is larger than 0 (S1> 0), and assuming that one retaining piece at the end is removed, the gap S2 between the leading ball and the trailing ball is equal to the outer diameter ds of the retaining piece. Smaller than 0.8 times (S2 <0.
8 × ds). In Comparative Example 5, in order to confirm the effect of the retaining piece, the retaining piece was not used and a full ball screw in which balls were densely arranged was used. As the lubricating grease, a grease obtained by adding an extreme pressure agent and a rust preventive agent to a mineral oil type grease was used in order to easily cause a mechanochemical reaction.

Test conditions Part number: NSK ball screw 25 × 10 × 500-C
5 (ball diameter 3/16 inch) Tester: NSK ball screw durability life tester Test load: Axial load 5800N (P / C =
0.5) Rotational speed: 500 r. p. m. Stroke: 40 mm Lubricant: Mineral oil grease

The test results are shown in Table 1, and FIG. 5 shows the test results as a correlation between the radius of curvature / ball diameter of the male screw groove of the screw shaft and the life ratio. The life ratios shown in Table 1 and FIG. 5 are based on the life time of Comparative Example 10 (1.0),
It is expressed as a ratio to this.

In the ball screws of Examples 1 to 6, the radius of curvature of the screw shaft and the thread groove of the nut are within the range defined by this embodiment. Since the slip at the contact point between the ball and the male thread groove and the ball and the female thread groove is small,
The generation of abnormal tissues is suppressed, and even in Example 6 having the lowest life ratio, 1.7, and Example 3 and Example 4 showing the highest value.
It has reached 2.8, and it can be seen that it has improved significantly.

In Example 7 and Example 8, only the male screw groove of the screw shaft has a radius of curvature within the range specified in this embodiment,
The radius of curvature of the female thread groove of the nut is an example in which the radius of curvature is equal to or less than the range specified in the present embodiment. The life test conducted this time is a test condition in which fatigue damage to the screw shaft is likely to occur, but the slip at the contact point of the screw shaft having a radius of curvature within the specified range of this embodiment is small, and an abnormal structure is generated. Is suppressed and the life is improved. As a result, the radius of curvature of the thread groove of either one of the screw shaft and the nut that is most likely to cause rolling fatigue depending on the usage conditions of the ball screw is manufactured as the radius of curvature within the specified range of the present embodiment. This suggests that the life expectancy can be expected to be improved.

On the other hand, in Comparative Examples 1 and 2, the radius of curvature of the thread groove of the screw shaft and the nut is smaller than the range specified in this embodiment. Therefore, the ball and the male thread groove,
The slip at the contact point between the ball and the female thread groove becomes large, abnormal tissues are easily generated, and peeling occurs, resulting in a short life. In Comparative Examples 3 and 4, the radius of curvature of the thread groove of the screw shaft and the nut is larger than the range specified in the present embodiment. Therefore, although the slip at the contact point between the ball and the male screw groove and between the ball and the female screw groove is small, the area of the contact ellipse is small, the surface pressure is high, rolling fatigue is likely to occur, and the life is shortened. In Comparative Example 5, both the radius of curvature of the screw shaft and the radius of curvature of the screw groove of the nut are within the range specified in the present embodiment, but since the retaining piece is not used, a competition between adjacent balls occurs. The ball surface is extremely worn and damaged, resulting in the shortest life.

The above test results are shown in FIG. 5 as a correlation between the radius of curvature / ball diameter of the thread groove of the screw shaft and the life ratio. The radius of curvature of the thread groove is 52.5% to 5 of the ball diameter D.
It can be seen that the life of the ball screw is greatly improved in the range of 5%. Particularly, in the range of 53% to 54%, a remarkable improvement is shown. From the above test results, it was possible to improve the life of the ball screw by optimizing the radius of curvature of at least one of the screw grooves of the screw shaft and the nut within the range specified in the present embodiment. .

[0056]

As described above, according to the ball screw of the first aspect of the present invention, the retaining pieces having the two concave surfaces facing the balls are respectively arranged between the adjacent balls. I chose Therefore, the ball circulates and rolls in the rolling path together with the retaining piece while contacting the Gothic arch-shaped concave surface of the retaining piece with extremely low friction, thereby preventing the balls from bidding and operating by bidding. Generation of defects, noise and abnormal sounds,
It is possible to prevent damage due to friction of the ball.

Further, a much larger number of load balls can be arranged as compared with a ball screw using spacer balls, and the life can be extended without reducing the allowable load capacity. In addition, since the radius of curvature of the cross section along the axis of at least one of the male thread groove and the female thread groove is set to 52.5% or more and 55% or less of the diameter of the ball, abnormal tissue due to rolling fatigue is generated. It is suppressed, and the occurrence of exfoliation due to the abnormal tissue can be prevented.

According to the ball screw of the second aspect of the present invention, since the cross-sectional shape of the thread groove is a Gothic arch shape, in order to reduce the area of the contact ellipse and the slip at the contact point, Even if the radius of curvature of the screw groove is set larger than usual, the clearance between the edge of the screw groove and the ball does not become excessive, and the positioning accuracy between the screw shaft and the nut can be maintained in a good state. .

[Brief description of drawings]

FIG. 1 is a plan view showing an embodiment of a ball screw of the present invention.

FIG. 2 is a vertical sectional view taken along the line AA in FIG.

FIG. 3 is an enlarged cross-sectional view of a retaining piece.

FIG. 4 is an enlarged view of a cross section along the axial direction of the ball screw.

FIG. 5 is a correlation diagram showing the relationship between the radius of curvature / ball diameter of the male thread groove of the screw shaft and the life ratio.

[Explanation of symbols]

1 ball screw 3 screw shaft 3a Male thread groove 5 female thread groove 7 nuts 9 balls 21 retaining piece 23 concave D ball diameter Rg radius of curvature of thread groove

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shigeru Okita             1-5-50 Kumei, Kugenuma, Fujisawa-shi, Kanagawa             Within NSK Ltd. F-term (reference) 3J062 AA25 AB22 AC07 BA17 BA26                       CD06 CD47

Claims (2)

[Claims] 1. A screw shaft having a spiral male screw groove formed on an outer peripheral surface, a nut having a spiral female screw groove formed on an inner peripheral surface, and a rolling member formed by the male screw groove and the female screw groove. A ball screw comprising a plurality of balls rollably arranged in a moving path, wherein retaining pieces having two concave surfaces facing the balls are arranged between the adjacent balls. A ball screw having a radius of curvature of a cross section along an axis of at least one of the male screw groove and the female screw groove is 52.5% or more and 55% or less of a diameter of the ball. 2. The ball screw according to claim 1, wherein the cross sections of the male screw groove and the female screw groove are formed in a Gothic arch shape.