Ball screw

Abstract

Provided is a ball screw in which the occurrence of adhesive wear can be reduced and stable driving can be maintained even in severe environments for lubricating, such as an underwater environment or an environment constantly exposed to water. The ball screw includes: a stainless steel screw shaft (21) having an outer circumferential screw groove (21a) formed in the outer circumferential surface; a stainless steel nut (22) having an inner circumferential screw groove (22a) formed in the inner circumferential surface; a plurality of balls (23) accommodated in a rolling path formed with the outer circumferential screw groove (21a) and the inner circumferential screw groove (22a); and a circulation part for circulating the plurality of balls (23) in the rolling path. The plurality of balls (23) includes a plurality of load balls (23a) and spacer balls (23b) disposed between adjacent load balls (23a). The load balls (23a) are made of ceramic. The spacer balls (23b) are made of stainless steel having a hardness lower than the material constituting the load balls. The diameter of the spacer balls (23b) is smaller than the diameter of the load balls (23a).

Description

Ball screw

[0001] The present invention relates to a ball screw that is particularly suitable for use in an underwater environment or an environment constantly exposed to water, and guides a reciprocating object in its moving direction.

[0002] Ball screws are generally used in various applications such as machine tools and transport machinery. Therefore, ball screws made of various materials have been proposed depending on the usage environment of the device. For example, Patent Document 1 proposes a ball screw that is suitable for use in a gas phase including under reduced pressure. The ball screw described in Patent Document 1 has a screw shaft and a nut formed of metal, while the balls are formed of ceramic.

[0003] However, in the ball screw described in Patent Document 1, when the balls roll in the same direction in the thread groove under an external load, the rolling direction is reversed and they rub against each other at the contact portion of adjacent hard ceramic balls, so there is a concern that the balls may be damaged. Also, at the entrance and exit portions of the circulation parts, the balls collide with each other, so there is a risk that the ceramic balls, which are brittle materials, may crack during high-speed rotation.

[0004] By the way, ball screws used in an underwater environment or an environment where seawater splashes are often made of stainless steel, which is a material resistant to rust. Specifically, by making the materials of the screw shaft, nut, circulation parts such as return tubes, and balls that make up the ball screw stainless steel, the effect of preventing rust can be obtained.

[0005] However, even if lubricating oil is applied to and sealed in the ball screw at the start of operation, in underwater environments or environments where water directly falls on the ball screw, the lubricating oil is quickly removed from the screw groove surface, resulting in poor lubrication. This poor lubrication causes adhesion between the ball and the screw groove of the screw shaft and nut, accelerating a rapid increase in torque and abnormal wear. This is because, when the ball screw is subjected to an external load, the screw groove of the screw shaft and nut is pressed towards the ball, causing the ball to roll on the screw groove surface. In particular, when the screw shaft, nut and ball are made of the same type of metal, the contact surface between the screw groove and the ball becomes a "metallic" relationship, increasing the risk of adhesive wear. It is known that "metallic" adhesion is more likely to occur when stainless steel is in contact with stainless steel than when ordinary steel is in contact with stainless steel.

[0006] Patent Document 2 proposes a ball screw in which the ball material is a Ni-Cr-Al-Mo alloy, and the shaft and nut material is a Ni-Cr-Al-Mo-Nb alloy or precipitation-hardening stainless steel, with the aim of obtaining excellent wear resistance and corrosion resistance even under harsh conditions such as underwater. However, even in the ball screw described in Patent Document 2, since the shaft, nut, and ball materials are all metals, it is difficult to sufficiently prevent adhesion of the contact surfaces.

[0007] Furthermore, Patent Document 3 discloses a ball screw in which steel balls made of bearing steel and ceramic balls made of silicon nitride are arranged alternately in a predetermined ratio adjacent to each other on a screw shaft and nut made of chromium-molybdenum steel. In the ball screw of Patent Document 3, the diameter of the ceramic balls is set to be smaller than the diameter of the steel balls so that the contact stress acting on the contact surfaces between the ceramic balls and both screw grooves is equal to the contact stress acting on the contact surfaces between the steel balls and both screw grooves.

[0008] Japan National Patent No. 1-69961 Publication Japanese Patent No. 3075731 Publication Japanese Patent No. 4829436

[0009] However, in the ball screw described in Patent Document 3, the material of the steel ball is bearing steel, which is susceptible to rust, and therefore, when used underwater, it is not possible to suppress the occurrence of rust. Also, the diameter of the steel ball is slightly larger than that of the ceramic ball, and the structure is such that both the steel ball and the ceramic ball bear the load when an external load is applied. Here, Patent Document 3 describes SCM415H and SCM420H as the materials for the shaft and nut, indicating that both the steel ball, shaft, and nut are made of "steel". For this reason, if the lubrication condition is poor, it is not possible to prevent adhesion between the steel ball that receives the external load and the screw grooves of the steel shaft and nut.

[0010] This invention has been made in view of the above problems, and aims to provide a ball screw that can reduce the occurrence of adhesive wear and maintain stable operation even in environments with severe lubrication requirements, such as underwater environments or environments that are constantly exposed to water.

[0011] The above object of the present invention is achieved by the following configuration: (1) A ball screw comprising: a stainless steel screw shaft having a helical outer screw groove formed on its outer circumference; a stainless steel nut disposed around the screw shaft and having a helical inner screw groove formed on its inner circumference opposite to the outer screw groove; a plurality of balls housed in a rolling path formed by the opposing outer screw groove and the inner screw groove; and a circulation part for circulating the plurality of balls in the rolling path, wherein the plurality of balls comprises a plurality of load balls and spacer balls disposed between adjacent load balls, the load balls are made of ceramic, the spacer balls are made of stainless steel with lower hardness than the material constituting the load balls, and the diameter of the spacer balls is smaller than the diameter of the load balls.

[0012] According to the ball screw of the present invention, the load ball is made of ceramic, and the screw shaft and nut are made of stainless steel, forming them from dissimilar materials. Furthermore, since there are spacer balls that have a smaller diameter and lower hardness than the load ball, adhesive wear at the contact surfaces between the load ball, spacer balls and the screw shaft and nut can be prevented even in environments with severe lubrication requirements.

[0013] Figure 1 is a perspective view showing a ball screw according to an embodiment of the present invention. Figure 2 is a simplified schematic diagram showing a cross-section of the ball screw shown in Figure 1 along the dashed line D.

[0014] The ball screw according to the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to the embodiments described below, and can be modified as appropriate without departing from the spirit of the invention.

[0015] Figure 1 is a perspective view showing a ball screw according to an embodiment of the present invention. The ball screw 10 comprises a screw shaft 21, a nut 22, a plurality of balls 23, and one or more circulation sections 27. The screw shaft 21 is cylindrical in shape and extends in the axial direction, and a helical outer circumferential screw groove 21a having a predetermined lead is formed on its outer circumferential surface. The nut 22 is substantially cylindrical in shape, and its inner diameter is larger than the outer diameter of the screw shaft 21. The nut 22 is fitted around the screw shaft 21 with a predetermined radial gap. On the inner circumferential surface of the nut 22, a helical inner circumferential screw groove 22a is formed, having a lead equal to that of the outer circumferential screw groove 21a of the screw shaft 21 and facing the outer circumferential screw groove 21a. A rolling path 25 with a substantially circular cross-section is formed in the region where the outer circumferential screw groove 21a of the screw shaft 21 and the inner circumferential screw groove 22a of the nut 22 face each other.

[0016] Multiple balls 23 are rotatably housed within the rolling path 25. The circulation section 27 is cylindrical and has a return path 26 for circulating the multiple balls 23. One end of the return path 26 is connected to the rolling path 25, and the other end of the return path 26 is connected to a rolling path 25 that is at least one turn away from the aforementioned rolling path 25. In other words, the circulation section 27 circulates the balls 23 by scooping up the balls 23 in the rolling path 25 on the front side in the direction of travel of the nut 22 and returning them to the rolling path 25 on the rear side in the direction of travel of the nut 22.

[0017] Figure 2 is a simplified schematic diagram showing a cross-section along the dashed line D of the ball screw shown in Figure 1. Note that the circulation section 27 is not shown in Figure 2. As shown in Figure 2, in this embodiment, the ball 23 has a plurality of load balls 23a and spacer balls 23b arranged between the plurality of load balls 23a. The load balls 23a are made of ceramic. The spacer balls 23b are made of stainless steel, which has a lower hardness than the material that makes up the load balls 23a. The diameter of the spacer balls 23b is formed to be smaller than the diameter of the load balls 23a. Specifically, the diameter of the spacer balls 23b is set so that even if the nut 22 is subjected to an external load and the load balls 23a undergo elastic deformation between the screw shaft 21 and the nut 22, the spacer balls 23b will not be subjected to any load.

[0018] In the ball screw 10 configured as described above, the screw shaft 21 rotates relative to the nut 22, and the multiple balls 23 circulate within an infinite circulation path consisting of a rolling path 25 and a return path 26. As a result, the nut 22 moves linearly in the axial direction of the screw shaft 21.

[0019] The ball screw 10 according to this embodiment has a load ball 23a made of ceramic and a spacer ball 23b made of stainless steel, so that rust can be suppressed even when used in an underwater environment or an environment that is constantly exposed to water. Furthermore, because the load ball 23a is made of ceramic, it is possible to prevent it from becoming "matching" with the stainless steel screw shaft and nut. The spacer ball is made of the same type of stainless steel as the shaft and nut, but since it is not subjected to external load, it does not become "matching". Therefore, even in environments with severe lubrication requirements, such as underwater environments or environments that are constantly exposed to water, it is possible to prevent adhesive wear between the load ball 23a and the outer circumference screw groove 21a of the screw shaft 21 and the inner circumference screw groove 22a of the nut 22.

[0020] Furthermore, if all balls were to be designated as load balls 23a, there is a concern that cracks may occur due to collisions between the balls because ceramic has high hardness. In particular, at the contact points where load balls 23a are in contact with each other, the surfaces of adjacent load balls 23a rub against each other in opposite directions as they roll within the rolling path 25, thus increasing the load applied to the load balls 23a.

[0021] In contrast, in this embodiment, spacer balls 23b are arranged between adjacent load balls 23a. Therefore, the probability of collisions between load balls 23a with high hardness can be reduced. Furthermore, since the spacer balls 23b are made of stainless steel, which has lower hardness than the material that makes up the load balls 23a, even if the load balls 23a and spacer balls 23b collide, damage to the load balls 23a can be reduced. Moreover, in this embodiment, since the diameter of the spacer balls 23b is smaller than the diameter of the load balls 23a, the spacer balls 23b can rotate freely within the rolling path 25. For example, when load balls 23a and spacer balls 23b are arranged alternately as shown in Figure 2, the spacer balls 23b can rotate between the load balls 23a in the opposite direction to the rotation direction of the load balls 23a, without being affected by the rotation direction of the screw shaft 21 and the nut 22. Therefore, when the screw shaft 21 of the nut 22 moves or stops in the axial direction, or moves in the reverse direction, even if the load balls 23a collide with each other or with the spacer balls 23b, wear and adhesion between the load balls and the spacer balls can be suppressed.

[0022] Note that the load balls 23a and spacer balls 23b do not need to be arranged perfectly alternately within the rolling path 25. For example, the number of load balls 23a can be greater or less than the number of spacer balls 23b, depending on the total length of the rolling path 25 and the return path 26, and the number of load balls 23a and spacer balls 23b.

[0023] Even if the number of load balls 23a is greater than the number of spacer balls 23b, and the load balls 23a are partially arranged next to each other, it is possible to prevent adhesive wear between the load balls 23a and the outer screw groove 21a of the screw shaft 21 and the inner screw groove 22a of the nut 22. Furthermore, compared to the case where all are load balls 23a, the probability of the load balls colliding with each other and being damaged can be significantly reduced. Moreover, even if the number of spacer balls 23b is greater than the number of load balls 23a, and the spacer balls 23b are partially arranged next to each other, the spacer balls are not subjected to external load, so it is possible to prevent adhesive wear and damage caused by collisions between the load balls 23a.

[0024] As long as the diameter of the spacer ball 23b is smaller than the diameter of the load ball 23a, the ratio of their diameters is not particularly limited. When the ball screw 10 is used in the usual way, when a predetermined load is applied to the nut 22, the load ball 23a is pressed between the inner circumferential thread groove 22a of the nut 22 and the outer circumferential thread groove 21a of the screw shaft 21, causing some deformation. In this embodiment, it is preferable that the diameter of the spacer ball 23b is smaller than the shortest diameter of the deformed load ball 23a, so that even when the load ball 23a is deformed due to the applied load, no load is applied to the spacer ball 23b. In other words, it is preferable that the diameter of the spacer ball 23b is designed to be smaller than the distance between the inner circumferential thread groove 22a and the outer circumferential thread groove 21a when the load ball 23a is subjected to a load. As a result, load-bearing contact does not occur between the stainless steel spacer ball 23b and the outer screw groove 21a and inner screw groove 22a, thus preventing adhesive wear between the spacer ball 23b and the nut 22 and screw shaft 21. Furthermore, since the spacer ball 23b can always rotate freely, it does not interfere with the rotation of the load ball 23a, and stable drive can be maintained.

[0025] If the number of load balls 23a is significantly greater than the number of spacer balls 23b, the number of adjacent load balls 23a increases, making it difficult to suppress damage due to collisions. Therefore, the number of load balls 23a is preferably 67% or less, and more preferably 50% or less, of the total number of load balls 23a and spacer balls 23b. On the other hand, if the number of load balls 23a is too small, the load on the load balls 23a becomes high when using the ball screw, making it difficult to maintain stable drive. Therefore, the number of load balls 23a is preferably 33% or more, and more preferably 50% or more, of the total number of load balls 23a and spacer balls 23b.

[0026] Furthermore, in order to obtain the maximum effect of the present invention, it is even more preferable that the number of load balls 23a be 50% of the total number of load balls 23a and spacer balls 23b. That is, it is particularly preferable that the ball 23 has load balls 23a and the same number of spacer balls 23b as the load balls 23a, and that the load balls 23a and spacer balls 23b are arranged alternately within the rolling path 25.

[0027] Next, the materials and characteristics of the components constituting the ball screw 10 according to this embodiment will be described.

[0028] <Load Balls> (Vickers hardness of load balls: Hv770 or higher) In this embodiment, the load balls 23a are made of a material including ceramic, and their hardness is not particularly limited as long as it is harder than that of the spacer balls 23b. However, if the Vickers hardness of the load balls 23a is Hv770 or higher, it is considered that the strength of the load balls 23a of the ball screw 10 is sufficient. Therefore, the Vickers hardness of the load balls 23a is preferably Hv770 or higher, more preferably Hv1000 or higher, and even more preferably Hv1400 or higher.

[0029] On the other hand, there is no particular limit to the upper limit of the Vickers hardness of the load ball 23a, but if the hardness of the load ball 23a becomes too high, its brittleness deteriorates and the load ball 23a becomes prone to breakage. Therefore, it is preferable that the Vickers hardness of the load ball 23a be Hv2500 or less. The Vickers hardness of the load ball 23a can be measured in accordance with the "Vickers hardness test - test method" described in JIS Z 2244:2009.

[0030] (Material of the load ball) The load ball 23a is made of a material containing ceramic. Specifically, the ceramic is Si ３ N ４ Examples include SiC. ３ N ４ Ceramic balls made of SiC have a surface Vickers hardness of, for example, Hv1400 or higher, so the load ball 23a is Si ３ N ４ If the material is made of a ceramic containing at least one of the two materials, and SiC, the desired hardness for the load ball 23a can be obtained.

[0031] <Spacer Ball> (Vickers hardness of spacer ball: Hv760 or less) In this embodiment, the spacer ball 23b is made of stainless steel and is not particularly limited in hardness as long as its hardness is lower than that of the load ball 23a. However, if the Vickers hardness of the spacer ball 23b is Hv760 or less, it is possible to suppress damage to the load ball 23a due to contact or collision with the load ball 23a. Therefore, it is preferable that the Vickers hardness of the spacer ball 23b is Hv760 or less.

[0032] On the other hand, there is no particular limit to the lower limit of the Vickers hardness of the spacer ball 23b, but for a stainless steel spacer ball 23b, it is practically desirable that the Vickers hardness be Hv350 or higher. Therefore, it is preferable that the Vickers hardness of the spacer ball 23b be Hv350 or higher. The Vickers hardness of the spacer ball 23b can be measured in the same manner as the Vickers hardness of the load ball 23a.

[0033] (Material of spacer balls) The spacer balls 23b are made of stainless steel because of their high corrosion resistance and the fact that they can be made to be harder than the load balls 23a. However, considering the ease of availability, it is preferable to use spacer balls 23b made of martensitic stainless steel. It is also possible to use precipitation-hardening stainless steel from the viewpoint of machinability.

[0034] <Screw shaft and nut> (Material of screw shaft and nut) Precipitation-hardening stainless steel has the characteristics of high rust resistance and ease of processing. Therefore, it is preferable that both the screw shaft 21 and the nut 22 be made of precipitation-hardening stainless steel, and for example, it is preferable to use SUS630 or SUS631.

[0035] <Circulation Section> (Material of the Circulation Section) The circulation section 27 is a component for circulating the balls 23 between the outer circumferential thread groove 21a of the screw shaft 21 of the ball screw 10 and the inner circumferential thread groove 22a of the nut 22, between other circumferential grooves. The ball screw according to this embodiment is suitable for use in water or environments where it is exposed to water, and the circulation section 27 is required to have high rust resistance. Furthermore, since the balls 23 can move freely within the circulation section 27, no load is applied to the balls 23 within the circulation section 27, so even if the circulation section 27 and the spacer balls 23b are made of the same material, adhesion will not occur. However, since the balls 23 move at high speed in the return path 26, the section also needs to have enough strength to withstand collisions of the balls 23. Therefore, the circulation section 27 is preferably made of, for example, stainless steel or resin, and as the resin material, it is preferable to use POM (Polyoxymethylene: polyacetal), PTFE (Polytetrafluoroethylene: polytetrafluoroethylene), PP (Polypropylene: polypropylene), PC (polycarbonate: polycarbonate), or mPPE (modified Polyphenylene ether: modified polyphenylene ether), which have weather resistance and water resistance.

[0036] Furthermore, when the ball screw 10 according to this embodiment is used in an unlubricated state underwater, the material of the circulation section 27 can be selected according to the environment. For example, when the ball screw 10 is used in an underwater environment close to the boiling point of water, it is preferable that the circulation section 27 has high water resistance and heat resistance. It is also preferable to use a material that is less prone to dimensional changes due to water absorption when the ball screw 10 is used underwater. From these points of view, it is more preferable that the circulation section 27 be made of stainless steel. Moreover, it is preferable to construct the circulation section 27 from austenitic stainless steel such as SUS304 because it is easy to obtain and process as a material.

[0037] Ball screws used in machine tools and the like in a gas phase, rather than underwater, generally require precise positioning and are configured to have no axial play between the screw shaft 21 and the nut 22. Therefore, the ball screw is assembled with preload so that there is virtually no gap between the ball 23 and the screw shaft 21 and nut 22. In contrast, the ball screw 10 according to this embodiment is intended for use in water or environments where it may be submerged in water, and is often used, for example, for transporting objects, so precise positioning is not required. Therefore, when the ball screw 10 is not in use, that is, when no load is applied to the ball screw 10, there may be a gap between the load ball 23a and the outer screw groove 21a, and between the load ball 23a and the inner screw groove. However, similar to ball screws used in a gas phase, the ball screw may be configured to have preload applied to the load ball 23a when no load is applied to the ball screw itself. Even in this case, the spacer ball 23b only needs to be set to have a smaller diameter than the load ball 23a so that it does not receive preload load.

[0038] As described above, the following matters are disclosed in this specification:

[0039] (1) A ball screw having a screw shaft made of stainless steel with a spiral outer thread groove formed on its outer peripheral surface, a nut made of stainless steel disposed around the screw shaft and having a spiral inner thread groove formed on its inner peripheral surface facing the outer thread groove, a plurality of balls accommodated in a rolling path formed by the opposing outer thread groove and inner thread groove, and a circulation part for circulating the plurality of balls in the rolling path, wherein the plurality of balls include a plurality of load balls and spacer balls disposed between adjacent load balls, the load balls are made of ceramic, the spacer balls are made of stainless steel having a lower hardness than the material constituting the load balls, and the diameter of the spacer balls is smaller than the diameter of the load balls.

[0040] According to this structure, in an environment where lubrication is severe, such as an underwater environment or an environment constantly exposed to water, it is possible to obtain a ball screw that can reduce the occurrence of seizure wear and maintain stable driving.

[0041] (2) The ball screw according to (1), wherein the diameter of the spacer ball is designed such that no load is applied to the spacer ball when a load is applied to the load ball during use of the ball screw.

[0042] According to this structure, since no load-bearing contact occurs between the stainless steel spacer balls and the outer and inner thread grooves, it is possible to prevent the occurrence of seizure wear between the spacer balls and the nut and screw shaft. Also, since the spacer balls can always rotate freely, stable driving can be maintained without interfering with the rotation of the load balls.

[0043] (3) The ball screw according to (1) or (2), wherein the plurality of balls include the plurality of load balls and the same number of spacer balls as the load balls.

[0044] According to this structure, the locations where the load balls are adjacent to each other can be reduced, and damage caused by collisions can be suppressed. Also, since the number of spacer balls does not become too large, an excessive load on the load balls can be suppressed, and stable driving can be maintained.

[0045] (4) The ball screw according to (3), wherein the load balls and the spacer balls are alternately arranged.

[0046] According to this structure, since the load balls do not collide with each other, damage caused by collisions can be prevented.

[0047] (5) The ball screw according to any one of (1) to (4), wherein the Vickers hardness of the load balls is 770 Hv or more.

[0048] According to this structure, sufficient strength as the load balls of the ball screw can be obtained.

[0049] (6) The ball screw according to any one of (1) to (5), wherein the Vickers hardness of the spacer balls is 760 Hv or less.

[0050] According to this structure, it is possible to suppress damage to the load balls caused by the spacer balls coming into contact with or colliding with the load balls.

[0051] (7) The load balls are made of a ceramic containing at least one of Si ３ N ４ and SiC, and the ball screw according to any one of (1) to (6).

[0052]

[0052] According to this structure, a desired hardness as the load balls can be obtained.

[0053] (8) The ball screw according to any one of (1) to (7), wherein both the screw shaft and the nut are made of precipitation hardening stainless steel.

[0054] According to this structure, the screw shaft and the nut have excellent rust resistance and can be easily processed.

[0055] (9) The ball screw according to any one of (1) to (8), characterized in that the spacer ball is made of martensitic stainless steel.

[0056] According to this structure, the spacer balls have the desired hardness and are readily available.

[0057] (10) The ball screw according to any one of (1) to (9), characterized in that the circulation part is made of stainless steel.

[0058] This structure allows the circulation section to have high strength, water resistance, and heat resistance, and to suppress dimensional changes caused by water absorption.

[0059] (11) The ball screw according to any one of (1) to (10), characterized in that, when no load is applied to the ball screw, there is a gap between the load ball and the outer circumference screw groove, and between the load ball and the inner circumference screw groove.

[0060] This structure makes it possible to obtain a ball screw that can be suitably used underwater or in environments constantly exposed to water, and that does not require precise positioning.

[0061] Although various embodiments have been described above with reference to the drawings, it goes without saying that the present invention is not limited to these examples. It is clear to those skilled in the art that various modifications or alterations can be conceived within the scope of the claims, and these will naturally also fall within the technical scope of the present invention. Furthermore, the components of the above embodiments may be combined in any way without departing from the spirit of the invention.

[0062] This application is based on a Japanese patent application (Patent Application No. 2024-193088) filed on November 1, 2024, the contents of which are incorporated by reference within this application.

[0063] 10 Ball screw 21 Screw shaft 21a Outer circumference thread groove 22 Nut 22a Inner circumference thread groove 23 Ball 23a Load ball 23b Spacer ball 25 Rolling path 26 Return path 27 Circulation section

Claims

1. A ball screw comprising: a stainless steel screw shaft having a helical outer thread groove formed on its outer circumference; a stainless steel nut disposed around the screw shaft and having a helical inner thread groove formed on its inner circumference opposite to the outer thread groove; a plurality of balls housed in a rolling path formed by the opposing outer and inner thread grooves; and a circulation section for circulating the plurality of balls in the rolling path, wherein the plurality of balls comprises a plurality of load balls and spacer balls disposed between adjacent load balls, the load balls are made of ceramic, the spacer balls are made of stainless steel with lower hardness than the material constituting the load balls, and the diameter of the spacer balls is smaller than the diameter of the load balls.

2. The ball screw according to claim 1, characterized in that, when the ball screw is in use, the diameter of the spacer ball is designed such that no load is applied to the spacer ball when a load is applied to the load ball.

3. The ball screw according to claim 1, characterized in that the plurality of balls comprises the plurality of load balls and the same number of spacer balls as the load balls.

4. The ball screw according to claim 3, characterized in that the load balls and the spacer balls are arranged alternately.

5. The ball screw according to claim 1, characterized in that the Vickers hardness of the load ball is Hv770 or higher.

6. The ball screw according to claim 1, characterized in that the Vickers hardness of the spacer ball is Hv760 or less.

7. The load ball is Si ３ N ４ A ball screw according to any one of claims 1 to 6, characterized in that it is made of a ceramic containing at least one of and SiC.

8. The ball screw according to any one of claims 1 to 6, characterized in that both the screw shaft and the nut are made of precipitation-hardening stainless steel.

9. The ball screw according to any one of claims 1 to 6, characterized in that the spacer ball is made of martensitic stainless steel.

10. The ball screw according to any one of claims 1 to 6, characterized in that the circulation section is made of stainless steel.

11. The ball screw according to any one of claims 1 to 6, characterized in that, when no load is applied to the ball screw, there is a gap between the load ball and the outer circumference screw groove, and between the load ball and the inner circumference screw groove.

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