Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16H—GEARING
  • F16H25/00—Gearings comprising primarily only cams, cam-followers and screw-and-nut mechanisms
  • F16H25/18—Gearings comprising primarily only cams, cam-followers and screw-and-nut mechanisms for conveying or interconverting oscillating or reciprocating motions
  • F16H25/20—Screw mechanisms
  • F16H25/22—Screw mechanisms with balls, rollers, or similar members between the co-operating parts; Elements essential to the use of such members

Definitions

• This disclosure relates to a ball screw device.
• a ball screw device is a device that converts rotational motion into linear motion, and linear motion into rotational motion.
• a ball screw device comprises a screw shaft, a nut that passes through the screw shaft, multiple balls arranged between the screw shaft and the nut, and a circulation unit for circulating the balls.
• An example of a circulation unit is a ball that returns the balls approximately one lead.
• a mounting hole is formed on the outer surface of the screw shaft, and a ball is inserted into the mounting hole.
• An S-shaped groove surface is formed on the radially outer surface of the ball, connecting one end of the outer peripheral raceway surface to the other.
• the ball also has a tang for scooping up balls.
• other circulation units include an S-shaped groove surface formed directly on the outer surface of the screw shaft.
• the present disclosure has been made in light of the above, and aims to provide a ball screw device that can reliably scoop up balls even without a tang.
• a ball screw device comprises a screw shaft having an outer peripheral raceway surface formed on its outer peripheral surface, a nut having an inner peripheral raceway surface formed on its inner peripheral surface and inserted onto the screw shaft, and a plurality of balls arranged in a raceway formed between the outer peripheral raceway surface and the inner peripheral raceway surface.
• An S-shaped groove surface for circulating the balls is formed directly on the outer peripheral surface of the screw shaft.
• a corner that intersects with the outer peripheral raceway surface or the S-shaped groove surface is formed on the outer peripheral surface of the screw shaft.
• the diameter of the balls is defined as Dw.
• the radius of the arc forming a cross section perpendicular to the groove of the inner peripheral raceway surface of the nut is defined as Rn.
• the contact angle of the ball with respect to the inner peripheral raceway surface of the nut is defined as ⁇ .
• the angle on the cross section perpendicular to the groove that indicates the effective range of the inner peripheral raceway surface of the nut is defined as ⁇ 1.
• the diameter of a circle connecting the centers of the plurality of balls arranged in the raceway is defined as Dm.
• the diameter of the outer peripheral surface of the screw shaft is defined as Ds.
• equation (1) includes a correction coefficient k, taking into account the centrifugal force acting on the ball. Therefore, even if centrifugal force acts on the ball (even if a radially outward load due to centrifugal force acts), the ball can be reliably scooped up radially inward. In other words, a tang is not required.
• the correction coefficient k may be greater than 1.3.
• the S-shaped groove surface may be formed directly on the outer peripheral surface of the screw shaft.
• the surface hardness of the S-shaped groove surface is 56 HRC or higher.
• the S-shaped groove surface has been subjected to induction hardening.
• the S-shaped groove surface has been subjected to carburizing hardening.
• the S-shaped groove surface is less likely to be damaged.
• a mounting hole recessed radially inward is formed on the outer peripheral surface of the screw shaft.
• a top is attached to the mounting hole.
• the S-shaped groove surface may be formed on the radially outer surface of the top.
• the direction parallel to the central axis of the screw shaft is referred to as the axial direction.
• An imaginary line that is perpendicular to the central axis and connects the central axis to one end of the S-shaped groove surface is referred to as the first imaginary line.
• An imaginary line that is perpendicular to the central axis and connects the central axis to the other end of the S-shaped groove surface is referred to as the second imaginary line.
• the diameter of the outer peripheral surface of the screw shaft is 25 mm or less
• the angle formed between the first imaginary line and the second imaginary line when viewed from the axial direction is preferably 50° or more and 120° or less.
• the angle formed between the first imaginary line and the second imaginary line when viewed from the axial direction is preferably 30° or more and 90° or less.
• the bending angle of the ball is relatively small when it moves between the outer circumferential track surface and the S-shaped groove surface. Therefore, the ball moves smoothly between the outer circumferential track surface and the S-shaped groove surface.
• the S-shaped groove surface extends linearly when viewed from the outside in the radial direction, and has a pair of end portions that connect to both ends of the outer circumferential raceway surface, and an intermediate portion that connects the pair of end portions.
• the intermediate portion extends arc-shaped when viewed from the outside in the radial direction, and has a pair of R portions that connect to the end portions.
• the depth of the outer circumferential raceway surface may be greater than the depth of the inner circumferential raceway surface.
• the initial contact angle between the outer peripheral raceway surface and the ball is larger than the initial contact angle between the inner peripheral raceway surface and the ball.
• the radius of curvature of the arc forming the cross section perpendicular to the groove of the outer peripheral raceway surface is smaller than the radius of curvature of the arc forming the cross section perpendicular to the groove of the inner peripheral raceway surface.
• the outer peripheral surface (groove shoulder) of the screw shaft is located near Dm (the diameter of the circle connecting the centers of the multiple balls arranged in the raceway), and the depth of the outer peripheral raceway surface is relatively large.
• the balls are less likely to ride up onto the groove shoulder of the screw shaft.
• the inner peripheral surface of the nut is located away from Dm, and the depth of the inner peripheral raceway surface is relatively small. Therefore, the balls rolling on the S-shaped groove surface are less likely to come into contact with the nut threads, allowing the balls to move smoothly.
• the ball screw device disclosed herein can reliably scoop up balls even without a tang.
• FIG. 1 is a schematic diagram of a ball screw device according to an embodiment, cut in the axial direction.
• FIG. 2 is an enlarged view of the S-shaped groove surface of the embodiment.
• FIG. 3 is a cross-sectional view of the ball screw device (a ball screw device whose corners are not cut) according to the embodiment taken along line VIII-VIII in FIG.
• FIG. 4 is a schematic diagram illustrating the vicinity of the center of the ball in FIG.
• FIG. 5 is a cross-sectional view of a ball screw device of a comparative example.
• FIG. 6 is a cross-sectional view of an embodiment of a rounded corner.
• FIG. 7 is a cross-sectional view of a C-chamfered corner in an embodiment.
• FIG. 8 is a cross-sectional view of the ball screw device (ball screw device with rounded corners) according to the embodiment taken along line VIII-VIII in FIG.
• FIG. 9 is a side view of the screw shaft of the ball screw device of the first modified example, viewed from the radial outside.
• FIG. 10 is a cross-sectional view of the ball screw device of the second modification taken along the S-shaped groove surface and the outer circumferential raceway surface, as viewed from the axial direction.
• FIG. 11A is an enlarged view of the S-shaped groove surface and its vicinity in Modification 2.
• FIG. 11B is an enlarged view of the S-shaped groove surface and its vicinity in Comparative Example 1.
• FIG. 12 is a view of the S-groove surface of the ball screw device of the third modification as viewed from the radially outer side.
• FIG. 13 is a view of the S-groove surface of Comparative Example 2 as viewed from the radially outer side.
• FIG. 14A is a schematic diagram of a cross section of a ball screw device according to Modification 3, taken at the center in the groove width direction of the outer circumferential raceway surface and the S-shaped groove surface, as viewed from the axial direction.
• FIG. 14B is a schematic diagram of the ball screw device of Comparative Example 2, in which the outer circumferential raceway surface and the S-shaped groove surface are cut at the center in the groove width direction, and the cross section is viewed from the axial direction.
• FIG. 14A is a schematic diagram of a cross section of a ball screw device according to Modification 3, taken at the center in the groove width direction of the outer circumferential raceway surface and the S-shaped groove surface, as viewed from the axial direction.
• FIG. 15 is a cross-sectional view perpendicular to the grooves of the outer peripheral raceway surface and the inner peripheral raceway surface of a ball screw device of Modification 4.
• FIG. 16 is a cross-sectional view perpendicular to the grooves of the outer peripheral raceway surface and the inner peripheral raceway surface of a ball screw device according to the fifth modification.
• FIG. 17 is a cross-sectional view perpendicular to the grooves of the outer peripheral raceway surface and the inner peripheral raceway surface of a ball screw device of Modification 6.
• FIG. 18 is an enlarged view of a cross section perpendicular to the grooves of the outer peripheral raceway surface and the inner peripheral raceway surface of the ball screw device of the seventh modification.
• FIG. 1 is a schematic diagram of a ball screw device according to an embodiment, cut in the axial direction.
• the ball screw device 100 is a device that converts rotational motion into linear motion, or linear motion into rotational motion.
• the ball screw device 100 includes a screw shaft 1, a nut 2, and a plurality of balls 3 (not shown in FIG. 1; see FIG. 3).
• the screw shaft 1 also has an S-shaped groove surface 4 formed thereon as a circulation portion.
• the direction parallel to the central axis X of the screw shaft 1 will be referred to as the axial direction.
• first direction X1 One side of the axial direction
• second direction X2 The direction perpendicular to the central axis X will be referred to as the radial direction.
• the nut 2 is cylindrically formed around the central axis X.
• An inner circumferential raceway 21 extending in a spiral direction is formed on the inner circumferential surface 20 of the nut 2.
• the inner circumferential raceway 21 is formed over the entire axial direction of the inner circumferential surface 20.
• the inner circumferential raceway 21 is continuous from the end of the inner circumferential surface 20 in the first direction X1 to the end in the second direction X2.
• the nut 2 of this embodiment is supported by other parts such as a housing so that it cannot rotate around the central axis X and is supported so that it can move freely in the axial direction.
• the screw shaft 1 comprises a screw shaft body 10 and a shaft portion 11.
• the shaft portion 11 is disposed in the first direction X1 relative to the screw shaft body 10.
• the shaft portion 11 is rotatably supported by other components, such as a bearing device (not shown). Torque generated by a motor (not shown) is input to the shaft portion 11.
• the screw shaft body 10 is formed in a cylindrical shape centered on the central axis X.
• Four outer peripheral raceway surfaces 13 and four S-shaped groove surfaces 4 are formed directly on the outer peripheral surface 12 of the screw shaft body 10.
• the outer peripheral raceway surfaces 13 and the S-shaped groove surfaces 4 are each formed by cutting the outer peripheral surface 12 of the screw shaft body 10.
• the screw shaft body 10 also has corners 15 where the groove surfaces (outer peripheral raceway surfaces 13 and S-shaped groove surfaces 4) and the outer peripheral surface 12 intersect.
• the outer raceway surface 13 extends in the same spiral direction as the inner raceway surface 21.
• the length of the outer raceway surface 13 is approximately one lead.
• the outer raceway surface 13 faces the inner raceway surface 21 in the radial direction.
• the space between the outer raceway surface 13 and the inner raceway surface 21 forms a raceway.
• a plurality of balls 3 are arranged in the raceway. Each ball 3 is in contact with the inner raceway surface 21 and the outer raceway surface 13, respectively, and is subjected to a load.
• FIG 2 is an enlarged view of the S-shaped groove surface of the embodiment.
• the S-shaped groove surface 4 has a pair of end portions 5 that connect to one end 13a and the other end 13b of the outer circumferential track surface 13, and an intermediate portion 6 that connects to the pair of end portions 5.
• the end portions 5 are formed in a straight line.
• the intermediate portion 6 is formed in a curved line.
• the depth of the intermediate portion 6 is greater than the depth of the end portions 5.
• the ball screw device 100 of this embodiment satisfies the following equation (5):
• Dw is the diameter of the ball 3.
• Rn in equation (5) is the radius of the arc 22 (see Figure 3) forming the groove-perpendicular cross section of the inner raceway surface 21 of the nut 2.
• the groove-perpendicular cross section is a cross section taken along a plane perpendicular (orthogonal) to the spiral direction in which the inner raceway surface 21 extends.
• ⁇ in equation (5) is the contact angle of the ball 3 with the inner raceway surface 21 of the nut 2.
• ⁇ 1 in equation (5) is the angle in the groove-perpendicular cross section that indicates the effective range of the inner raceway surface 21 of the nut 2.
• Dm in equation (5) is the diameter of a circle connecting the centers O of the multiple balls 3 arranged in the raceway.
• Dm is sometimes called BCD (Ball Center Diameter).
• Ds in equation (5) is the diameter of the outer peripheral surface 12 of the screw shaft 1 (screw shaft body 10).
• Q is the radial distance between the radially innermost part of the corner 15 of the screw shaft 1 and the outer circumferential surface of the screw shaft 1 when the apex of the corner 15 is ground off.
• k (k > 1) is a correction coefficient.
• Figure 3 is a cross-sectional view of a ball screw device according to an embodiment (a ball screw device in which the corners have not been cut) taken along line VIII-VIII in Figure 2.
• a ball screw device in which the corners have not been cut taken along line VIII-VIII in Figure 2.
• M1 denotes a line extending radially from the center axis X and passing through the axial center of the inner circumferential raceway surface 21.
• the inner circumferential raceway surface 21 of the nut 2 is formed symmetrically with respect to the line M1.
• the inner circumferential raceway surface 21 has a gothic arc shape and is composed of two arcs 22. Note that the present disclosure may also apply to an inner circumferential raceway surface having a circular arc shape.
• the center of the arcs 22 is designated e.
• a line that passes through the center e and is parallel to the line M1 is designated as line M2.
• a straight line extending in the axial direction and passing through the center O of the ball 3 is defined as auxiliary line T1.
• a straight line passing through the center e of the arc 22 and parallel to auxiliary line T1 is defined as auxiliary line T2.
• the contact point between the ball 3 and the arc 22 is defined as h.
• a straight line passing through the contact point h and the center e of the arc 22 is defined as auxiliary line T3. Note that since the ball 3 is in contact at the contact point h, auxiliary line T3 passes through the center O of the ball 3.
• the contact angle between the ball 3 and the arc 22 is defined as ⁇ .
• Figure 4 is a schematic diagram of the area near the center of the ball in Figure 3. As shown in Figure 4, the distance between the center O of the ball 3 and the center e of the arc 22 is Z. The center O of the ball 3 and the center e of the arc 22 are on the auxiliary line T3, and the following equation (6) is obtained.
• the radial distance between the center O of the ball 3 and the center e of the arc 22 (the distance between auxiliary lines T1 and T2) is b.
• distance b can be calculated using the following equation (7-1). Furthermore, rearranging equation (7-1) yields equation (7-2).
• equation (8) can be obtained from equation (6) and equation (7-2).
• chamfered portion 23 As shown in Figure 3, in nut 2, the corner between inner circumferential surface 20 and inner circumferential raceway surface 21 is chamfered to form chamfered portion 23.
• the boundary position between this chamfered portion 23 and inner circumferential raceway surface 21 is designated as point f.
• a straight line that passes through point f and is parallel to auxiliary line T1 is designated as auxiliary line T4.
• auxiliary line T4 Furthermore, the intersection of straight line M2 and auxiliary line T4 is designated as U1.
• Equation (9-1) can be obtained from the trigonometric functions of the triangle with center e, point U1, and point f as its vertices. Furthermore, rearranging equation (9-1) gives equation (9-2).
• auxiliary line T5 intersects with the outline of ball 3 twice, and the intersection point farther from point f is called g.
• a line that passes through point g and is parallel to line M1 is called line M3.
• the intersection point between line M3 and auxiliary line T4 is called U2.
• auxiliary line T5 The angle at which straight line M3 and auxiliary line T5 intersect is ⁇ 3.
• the radial distance between auxiliary line T1 and point g is d.
• corner 15 If corner 15 is not cut, the vertex of corner 15 becomes scooping contact point a.
• the line passing through this scooping contact point a and center e is defined as L.
• the radial distance between auxiliary line T1 and scooping contact point a is defined as P.
• Figure 5 is a cross-sectional view of a ball screw device of a comparative example.
• the contact angle of ball 3 with screw shaft 1 is ⁇ 2.
• load F3 faces radially outward. In other words, ball 3 cannot be scooped up.
• Figure 6 is a cross-sectional view of a rounded corner in an embodiment.
• Figure 7 is a cross-sectional view of a C-chamfered corner in an embodiment.
• the corner 15 may be rounded by R-chamfering during the manufacture of the screw shaft 1.
• the corner 15 has no apex and is arc-shaped when viewed in cross section.
• the corner 15 may be C-chamfered during the manufacture of the screw shaft 1, resulting in a slope.
• the corner 15 may not be chamfered during the manufacture of the screw shaft 1, and may have a apex, but may become rounded due to deterioration over time. In this way, the corner 15 may be ground down and no apex may exist.
• Figure 8 is a cross-sectional view of a ball screw device according to an embodiment (a ball screw device in which the corners have been cut) taken along line VIII-VIII in Figure 2.
• the radial distance from the outer surface 12 of the screw shaft 1 to the scooping contact point a is Q.
• the radial distance P from the auxiliary line T1 to the scooping contact point a is given by the following equation (15).
• Equation (17) is the same as equation (5) mentioned above.
• the ball screw device 100 of this embodiment can reliably scoop up the balls 3 radially inward. It also makes it possible to grasp the critical angle of the angle ⁇ 1 of the effective range of the nut 2. In other words, it is possible to avoid a situation where the angle ⁇ 1 of the effective range of the nut 2 is set smaller than necessary in order to reliably scoop up the balls 3, causing the balls 3 to ride up onto the shoulder of the groove of the nut 2.
• the value of the correction coefficient k may be set appropriately depending on the centrifugal force acting on the ball 3.
• the value of the correction coefficient k is preferably greater than 1.3 (k>1.3), and more preferably greater than 1.6 (k>1.6).
• the S-shaped groove surface 4 provided on the outer peripheral surface 12 of the screw shaft 1 in the embodiment, it is formed directly on the outer peripheral surface 12 of the screw shaft 1, but in the present disclosure, the S-shaped groove surface may be formed on a top.
• Variation 1 using a top will be described. Furthermore, the following explanation will focus on the differences from the embodiment.
• (Variation 1) 9 is a side view of the screw shaft of a ball screw device of Modification 1 as viewed from the radial outside.
• a mounting hole 16 recessed radially inward is formed in the outer peripheral surface 12 of the screw shaft 1 of the ball screw device 100A of Modification 1.
• a top 17 is attached to the mounting hole 16.
• An S-shaped groove surface 4 is formed on the radially outer surface of the top 17. The top 17 does not have a tang.
• the balls 3 are reliably scooped up.
• top 17 made of resin manufactured by injection molding may be used.
• a top 17 manufactured by metal powder injection molding may be used.
• a top 17 manufactured by cutting a metal material may be used.
• the outer peripheral raceway surface 13 of the screw shaft 1 is formed over the entire axial direction of the outer peripheral surface 12.
• the outer peripheral raceway surface 13 is continuous from the end of the outer peripheral surface 12 in the first direction X1 to the end in the second direction X2.
• the present disclosure may also utilize a screw shaft 1 whose outer peripheral surface 12 has been heat-treated.
• a common heat treatment method is to harden the outer peripheral surface 12 of the screw shaft 1 and then temper it. By performing this type of heat treatment, the outer peripheral surface 12 (outer peripheral raceway surface 13) of the screw shaft 1 can obtain the desired hardness and toughness.
• carburizing and quenching may be performed.
• the quenching method may also be induction hardening.
• the heat treatment method in the present disclosure may be a method other than those described above, and is not particularly limited.
• the S-shaped groove surface 4 may also be heat treated. This will prevent damage to the S-shaped groove surface 4.
• the surface hardness of the S-shaped groove surface 4 is preferably HRC 56 or higher.
• (Variation 2) 10 is a cross-sectional view of the ball screw device of Modification 2 taken along the S-shaped groove surface and the outer peripheral raceway surface, viewed from the axial direction.
• the portions of the S-shaped groove surface 4 that are both ends in the longitudinal direction and located at the groove bottom (the center in the groove width direction) will be referred to as one end 4a and the other end 4b.
• the one end 4a or the other end 4b is located at the boundary between the S-shaped groove surface 4 and the outer peripheral raceway surface 13.
• the portion of the S-shaped groove surface 4 that is the center in the longitudinal direction and located at the groove bottom (the center in the groove width direction) will be referred to as the center point 4c.
• the ball 3 located at the center point 4c of the S-shaped groove surface 4 will be referred to as the bottom ball 3A.
• a virtual line that is perpendicular to the central axis X and connects the central axis X to one end 4a of the S-shaped groove surface 4 is referred to as a first virtual line M11.
• a virtual line that is perpendicular to the central axis X and connects the central axis X to the other end 4b of the S-shaped groove surface 4 is referred to as a second virtual line M12.
• a virtual line that is perpendicular to the central axis X and connects the central axis X to the midpoint 4c of the S-shaped groove surface 4 is referred to as a third virtual line M13.
• a direction parallel to the third virtual line M13 and in which the central axis X is disposed as viewed from the S-shaped groove surface 4 is referred to as a depth direction M14.
• a direction perpendicular to both the axial direction (central axis X) and the depth direction M14 (third virtual line M13) is referred to as a cross direction M15.
• the angle ⁇ 10 formed by the first imaginary line M11 and the second imaginary line M12 is between 30° and 90°.
• the diameter Ds of the outer peripheral surface 12 of the screw shaft 1 is greater than 25 mm. This allows the balls 3 to be scooped up smoothly.
• the effects of Modification 2 will be explained in comparison with Comparative Example 1.
• FIG. 11A is a schematic diagram showing an enlargement of the S-shaped groove surface and its vicinity in Modification 2.
• FIG. 11B is a schematic diagram showing an enlargement of the S-shaped groove surface and its vicinity in Comparative Example 1. Note that the angle ⁇ 10 of Modification 2 shown in FIG. 11A is 30°. On the other hand, in Comparative Example 1 shown in FIG. 11B, the angle ⁇ 10 of the S-shaped groove surface 1004 is approximately 15°, which does not meet the requirements of Modification 2 (angle ⁇ 10 of 30° to 90°). Furthermore, the configuration of Comparative Example 1 is identical to that of ball screw device 100B of Modification 2, except for the S-shaped groove surface 1004.
• M20 shown in Figures 11A and 11B is a circle (curve) connecting the centers O of the balls 3 rolling on the raceway (outer peripheral raceway surface 13).
• the balls 3 attempting to enter the S-shaped groove surface 4 from the outer peripheral raceway surface 13 are referred to as scooped balls 3B, 1003B (see ball 3B in Figure 11A and ball 1003B in Figure 11B).
• the balls 3 arranged on the S-shaped groove surface 4 move under the load of the scooping balls 3B.
• the scooping balls 3B receive a reaction force F25 (see Figure 11A) from the balls 3 arranged on the S-shaped groove surface 4.
• the scooping balls 3B are also pressed by the balls 3 rolling on the outer circumferential raceway surface 13, and receive a load F26 (see Figure 11A).
• the resultant force F27 which is the combination of the reaction force F25 and the load F26, becomes a load that moves toward the nut 2, as shown in Figure 11A.
• the angle formed by the reaction force F25 and the load F26 is ⁇ 11.
• the scooping ball 1003B receives a reaction force F1025 (see Figure 11B) from the balls 1003 arranged on the S-shaped groove surface 1004. Furthermore, the scooping ball 1003B receives a load F1026 (see Figure 11B) from the balls 1003 arranged on the outer circumferential track surface 13. Therefore, a resultant force F1027 (see Figure 11B) combining the reaction force F1025 and the load F1026 also acts on the scooping ball 1003B of Comparative Example 1. Furthermore, the angle formed by the reaction force F1025 and the load F1026 is ⁇ 1011.
• the scooping balls 3B, 1003B are subjected to a load F3 (see FIG. 5) which is a combination of the load F1 (see FIG. 5) received from the nut 2 and the load F2 (see FIG. 5) received from the screw shaft 1. If the resultant forces F27A, F1027A described above become greater than the load F3, the scooping balls 3B, 1003B will move toward the inner raceway surface 21 of the nut 2, preventing smooth scooping of the balls 3B, 1003B.
• the angle ⁇ 11 of Modified Example 2 is greater than the angle ⁇ 1011 of Comparative Example 1.
• the amount of load that the reaction force F25 and the load F26 cancel out when they oppose each other is greater than the amount of load that the reaction force F1025 and the load F1026 cancel out when they oppose each other. Therefore, the resultant force F27 is smaller than the resultant force F1027.
• Fig. 12 is a view of the S-shaped groove surface of the ball screw device of Modification 3 as viewed from the radial outside.
• the S-shaped groove surface 4 of the ball screw device 100C of Modification 3 is the same as that of the embodiment, and has a pair of end portions 5 and an intermediate portion 6 (see also Fig. 2).
• the end portions 5 are formed linearly when viewed from the radial outside. Therefore, the center line N5 passing through the center of the end portion 5 in the groove width direction is also linear.
• the intermediate portion 6 is formed in a curved shape. More specifically, the intermediate portion 6 is composed of a pair of R portions 7.
• the R portions 7 are formed in an R-shape (arc-like) when viewed from the radially outer side. Therefore, the center line N7 passing through the center of the R portions 7 in the groove width direction is also arc-shaped.
• the center line N7 is also part of an imaginary circle N6 centered at point N7a.
• the imaginary circle N6 (center line N7) has a curvature radius R7 and is tangent to the center line N5 at a tangent point N1. Furthermore, one imaginary circle N6 of the pair of R portions 7 is tangent to the other imaginary circle N6 at point N2.
• the curvature radius R7 of the center line N7 of the R portions 7 satisfies the following equation (18).
• Figure 13 is a view of the S-shaped groove surface of Comparative Example 2 as viewed from the radially outer side.
• the S-shaped groove surface 2004 of Comparative Example 2 differs from Modified Example 3 in that it has an intermediate portion 2006 instead of intermediate portion 6.
• the intermediate portion 2006 has a pair of R portions 2007 and a straight portion 2008 disposed between the pair of R portions 2007.
• dashed lines B2001 and B2002 in Figure 13 are the boundaries between the end portion 5 and the intermediate portion 2006 of the S-shaped groove surface 2004.
• boundaries B1 and B2 of Modified Example 3 are also shown to make it easier to understand the difference from boundary B2001 of Comparative Example 2.
• the size of the S-shaped groove surface 2004 in the cross direction M15 of Comparative Example 2 is the same as that of the S-shaped groove surface 4 of Modification Example 3.
• the depth of the S-shaped groove surface 2004 in Comparative Example 2 is also the same as that of the S-shaped groove surface 4 of Modification Example 3 (see Figures 14A and 14B).
• Center line N2007 which passes through the center of R portion 2007 in the groove width direction, is part of imaginary circle N2006 centered at point N2007a.
• Imaginary circle N2006 (center line N2007) has a radius of curvature R2007 and is tangent to center line N5 at point N2001.
• the radius of curvature R2007 of center line N2007 of R portion 2007 is less than the value obtained by multiplying diameter Dw of ball 3 by 1.3, and does not satisfy equation (18). Therefore, center line N2007 has a smaller diameter than center line N7 of variant example 3.
• the straight portion 2008 is formed in a straight line. Therefore, the center line N2008 passing through the center of the straight portion 2008 in the groove width direction is also straight.
• the center line N2008 is tangent to each of the two imaginary circles N2006 at point N2002.
• the ball 3 is scooped up radially inward at the boundaries between the end portion 5 and the middle portion 6 of the S-shaped groove surface 4 (see boundaries B1, B2 and boundaries B2001, B2002 in Figure 13).
• boundaries B1, B2 in Modification 3 are positioned closer to both ends of the S-shaped groove surface 4 than boundaries B2001, B2002 in Comparative Example 2. This is because the radius of curvature R7 of center line N7 is larger, and therefore the point of contact N1 with center line N5 is closer to both ends of the S-shaped groove surface 4.
• the ball 3 is scooped up closer to both ends of the S-shaped groove surface 4 in Modification 3 than in Comparative Example 2.
• Figure 14A is a schematic diagram of a ball screw device of Modification Example 3, with the outer circumferential raceway surface and S-shaped groove surface cut at the center in the groove width direction, and the cross section viewed from the axial direction.
• Figure 14B is a schematic diagram of a ball screw device of Comparative Example 2, with the outer circumferential raceway surface and S-shaped groove surface cut at the center in the groove width direction, and the cross section viewed from the axial direction.
• Balls 3C and 2003C shown in Figures 14A and 14B are balls that are scooped up at boundaries B1 and B2001.
• the ball 3 that enters the S-shaped groove surface 4 from the raceway (outer raceway surface 13) receives a radially inward load at boundaries B1 and B2001 (see Figures 12 and 13) and is scooped up radially inward. Therefore, as shown in Figures 14A and 14B, before reaching boundaries B1 and B2001, in other words, the center O of the ball 3 moving along end 5, roughly coincides with imaginary line M20.
• ball 3C is subjected to load F3 (see Figure 5), which is the combination of load F1 (see Figure 5) and load F2 (see Figure 5), and is directed radially inward. Furthermore, ball 3C is subjected to a resultant force F, which is the combination of reaction force F25 received from balls 3 arranged on S-groove surface 4 and resultant force F27 received from balls 3 arranged on outer circumferential raceway surface 13.
• the angle formed by reaction force F25 and load F26 is ⁇ 12.
• the intermediate portion 6 of the present disclosure may include, in addition to the pair of R portions 7, a straight portion 2008 (FIG. 13) disposed between the pair of R portions 2007.
• FIG. 15 is a cross-sectional view perpendicular to the grooves of the outer peripheral raceway surface and the inner peripheral raceway surface of a ball screw device of Modified Example 4.
• the cross-sectional view perpendicular to the grooves is a cross-sectional view taken along a plane perpendicular (orthogonal) to the spiral direction in which the outer peripheral raceway surface 13 and the inner peripheral raceway surface 21 extend.
• a virtual line N3 shown in FIG. 15 is a straight line perpendicular to the central axis X of the screw shaft 1, connecting the central axis X with the center O of the ball 3. As shown in FIG.
• the depth ⁇ 13 of the outer peripheral raceway surface 13 is greater than the depth ⁇ 21 of the inner peripheral raceway surface 21. While the outer peripheral raceway surface 13 and the inner peripheral raceway surface 21 shown in FIG. 15 have the same Gothic arc shape as the embodiment, they may have a circular arc shape in the present disclosure.
• the depth from the outer peripheral surface 12 to the groove bottom 13c of the outer peripheral raceway surface 13 is ⁇ 13.
• the groove bottom 13c is the center of the outer peripheral raceway surface 13 in the groove width direction, and is the intersection of two arcs 113 that make up the Gothic arc shape.
• the depth from the inner circumferential surface 20 to the groove bottom 21a of the inner circumferential raceway surface 21 is ⁇ 21.
• the groove bottom 21a is the center of the inner circumferential raceway surface 21 in the groove width direction, and is the intersection of two arcs 121 that make up the Gothic arc shape.
• Arc 113 and arc 121 have the same radius of curvature, and the outer peripheral raceway surface 13 and inner peripheral raceway surface 21 have the same shape in cross section perpendicular to the groove. Therefore, the contact angle ⁇ 15 between the outer peripheral raceway surface 13 and ball 3 is the same as the contact angle ⁇ 16 between the inner peripheral raceway surface 21 and ball 3. Furthermore, the distance ⁇ 13A from contact point 13d between the outer peripheral raceway surface 13 and ball 3 to groove bottom 13c is the same as the distance ⁇ 21A from contact point 21b between the inner peripheral raceway surface 21 and ball 3 to groove bottom 21a.
• the depth ⁇ 13 of the outer peripheral raceway surface 13 is greater than the depth ⁇ 21 of the inner peripheral raceway surface 21. Therefore, the distance ⁇ 13B from contact point 13d to the outer peripheral surface 12 of the screw shaft 1 is greater than the distance ⁇ 21B from contact point 21b to the inner peripheral surface 20 of the nut 2.
• the depth ⁇ 13 of the outer peripheral raceway surface 13 is greater than the depth ⁇ 21 of the inner peripheral raceway surface 21 can be adopted because the present disclosure satisfies the prerequisite that the screw shaft 1 is provided with an S-shaped groove surface 4.
• the balls 3 are scooped up radially outward by the S-shaped groove surface 4 and go beyond the threads of the screw shaft 1.
• the screw shaft 1 is provided with an S-shaped groove surface 4.
• Fig. 16 is a cross-sectional view perpendicular to the grooves of the outer peripheral raceway surface and the inner peripheral raceway surface of a ball screw device of Modified Example 5.
• a ball screw device 100E of Modified Example 5 differs from Modified Example 4 in that an initial contact angle ⁇ 17 between the outer peripheral raceway surface 13 and the ball 3 is larger than an initial contact angle ⁇ 18 between the inner peripheral raceway surface 21 and the ball 3.
• the ball screw device 100E of Modified Example 5 has in common with Modified Example 4 the depth ⁇ 13 of the outer peripheral raceway surface 13 being larger than the depth ⁇ 21 of the inner peripheral raceway surface 21.
• the initial contact angle will be explained.
• the no-load state for the balls is when the nut 2 is moved axially relative to the screw shaft 1, causing the balls 3 to contact the outer raceway surface 13 and the inner raceway surface 21, and the load acting on the balls 3 is zero.
• the contact angle between the outer raceway surface 13 and the balls 3 is initial contact angle ⁇ 17
• the contact angle between the inner raceway surface 21 and the balls 3 is initial contact angle ⁇ 18.
• the initial contact angle ⁇ 19 of Comparative Example 3 is larger than the initial contact angle ⁇ 17 of Modified Example 5.
• the radial component 16B of the load F16 is larger than the radial component 14B of the load F14. Therefore, the load F16 applied to the outer circumferential raceway surface 13 of Comparative Example 3 is larger than the load F14 applied to the outer circumferential raceway surface 13 of Modified Example 5.
• the load (surface pressure) acting on the outer circumferential raceway surface 13 is reduced.
• contact point 13d in Modification 5 is positioned radially outward of contact point 13e in Comparative Example 3. This means that there is a higher possibility that the balls 3 will ride up onto the groove shoulder of the screw shaft 1.
• the depth ⁇ 13 of the outer peripheral raceway surface 13 is greater than the depth ⁇ 21 of the inner peripheral raceway surface 21. Therefore, even in Modification 5, there is a low possibility that the balls 3 will ride up onto the groove shoulder of the screw shaft 1.
• Fig. 17 is a cross-sectional view perpendicular to the grooves of the outer peripheral raceway surface and the inner peripheral raceway surface of a ball screw device of Modified Example 6.
• a ball screw device 100F of Modified Example 6 differs from Modified Example 4 in that the radius of curvature R121 of the arc 121 of the inner peripheral raceway surface 21 is larger. That is, in Modified Example 6, the radius of curvature R113 of the arc 113 of the outer peripheral raceway surface 13 is smaller than the radius of curvature R121 of the arc 121 of the inner peripheral raceway surface 21. Note that the depth ⁇ 13 of the outer peripheral raceway surface 13 is larger than the depth ⁇ 21 of the inner peripheral raceway surface 21, which is common to Modified Example 4.
• the contact points 21b, 21b on the inner circumferential raceway surface 21 of the nut 2 will be closer to the imaginary line N3 than the contact points 13d, 13d on the outer circumferential raceway surface 13. Therefore, in variant example 6, the contact angle ⁇ 20 between the outer circumferential raceway surface 13 (arc 113) and the ball 3 is greater than the contact angle ⁇ 21 between the inner circumferential raceway surface 21 (arc 121) and the ball 3. Therefore, according to Modification 6, the load (surface pressure) acting on the outer circumferential raceway surface 13 is reduced, just like Modification 5.
• Figure 18 is an enlarged view of a cross section perpendicular to the groove of the outer peripheral raceway surface and the inner peripheral raceway surface of the ball screw device of Modified Example 7.
• the imaginary line N30 is a line drawn in the axial direction from a circle connecting the centers of the multiple balls 3 in the raceway.
• the diameter of the inner peripheral surface 20 of the nut 2 is taken as Dn.
• the ball screw device 100G of Modified Example 7 satisfies the following formulas (19) and (20).
• (Dm - Ds)/2 represents the distance Dms (see Figure 18) between the outer peripheral surface 12 of the screw shaft 1 and the imaginary line N30. If this distance Dms is 0.25 mm or less, the outer peripheral surface 12 of the screw shaft 1 is positioned near the center O of the ball 3. In other words, the distance from the groove bottom 13c (see Figure 15) of the outer peripheral raceway surface 13 to the outer peripheral surface 12, in other words, the depth of the outer peripheral raceway surface 13, is large. Therefore, the ball 3 is less likely to ride up onto the groove shoulder of the screw shaft 1.
• (Dn - Dm)/2 in equation (20) represents the distance Dmn (see Figure 18) from the inner peripheral surface 20 of the nut 2 to the imaginary line N30.
• the larger this distance Dmn the smaller the distance from the groove bottom 21a (see Figure 15) of the inner peripheral raceway surface 21 to the inner peripheral surface 20 of the nut 2; in other words, the smaller the depth of the inner peripheral raceway surface 21. Therefore, the balls 3 rolling on the S-shaped groove surface 4 are less likely to come into contact with the threads of the nut 2, allowing the balls 3 to move smoothly.

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  • 2025-03-25 JP JP2025545279A patent/JPWO2025225246A1/ja active Pending
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