JP7525905B2 - Rolling element screw device - Google Patents

Description

The present invention relates to a rolling element screw device such as a ball screw in which rolling elements are interposed between a screw shaft and a nut so as to be capable of rolling motion.

Industrial machines such as machine tools have an internal feed drive mechanism. Ball screws are often used in the feed drive mechanism. Ball screws are an important mechanical element that controls the performance of industrial machines through the feed drive mechanism.

A ball screw is a rolling screw mechanism. That is, a ball screw comprises a screw shaft, a nut, and a number of balls interposed between the screw shaft and the nut to enable rolling motion. A helical raceway surface is formed on the outer surface of the screw shaft. A helical raceway surface is formed on the inner surface of the nut that faces the raceway surface of the screw shaft. Balls are interposed between the raceway surface of the screw shaft and the raceway surface of the nut.

In conventional ball screws, the raceway surface of the screw shaft and the raceway surface of the nut are both shaped like a Gothic arch groove. The contact angle is set to 40 to 50 degrees (see Patent Document 1). This is to allow the nut to receive load on both its forward and return travels.

JP 2015-108400 A

Kazuo Miyaguchi, Mizuho Ninomiya, Yasumi Watanabe, Satoru Arai, Minoru Hamamura, Yoshiaki Kakino, "Study on Friction Torque Fluctuation When Ball Screw Motion Direction is Reversed (1st Report)," Journal of the Japan Society for Precision Engineering, Vol. 68, No. 6, 2002, pp. 833-837

A ball screw is basically designed to receive axial loads. However, in conventional ball screws, the contact angle is set to 40 to 50 degrees, so when an axial load acts on the ball screw, loads are generated inside the ball screw in the axial direction and in the direction perpendicular to the axis (radial direction). The latter radial load is of no use in receiving the axial load, so conventional ball screws have a problem in that their axial load rating is low. Furthermore, the radial position where the screw contacts the ball is smaller than the radial position where the nut contacts the ball, so the raceway surface has a convex curvature when viewed in the direction in which it extends. This convex curvature reduces the area of elastic contact, making it impossible to receive a large contact load. In other words, conventional ball screws have a problem in that their load rating is even smaller.

In addition, in conventional ball screws, the balls receive the load by contacting the screw shaft and the gothic arch groove of the nut at two points in total, one each. However, when the screw is rotating relative to the nut, a third contact point is involved depending on whether the rotation is forward or reverse (see Non-Patent Document 1). This third contact point causes sliding rather than rolling, which increases friction loss and generates a large amount of heat. When the ball screw generates heat, the metal of the screw shaft and nut expands, leading to changes and deterioration in feed accuracy.

Furthermore, in conventional ball screws, when the balls reverse their rolling direction, the contact state between the balls and the raceway changes from a three-point contact state to a two-point contact state to a three-point contact state, which creates a problem of a so-called free zone where friction torque decreases due to the two-point contact state. This free zone makes it complicated and difficult to correct trajectory control errors when the motion is reversed.

The present invention has been made in consideration of the above problems, and aims to provide a rolling element screw device that has a large axial load rating, generates little heat, and can suppress fluctuations in friction torque due to the free zone when the rolling direction is reversed.

In order to solve the above problems, one aspect of the present invention is a rolling element screw device comprising: a screw shaft having a helical thread on its outer surface, the screw shaft raceway surface being formed on the side of the thread; a nut having a helical nut thread on its inner surface, the nut raceway surface being formed on the side of the nut thread facing the screw shaft raceway surface; and a plurality of rolling elements interposed in a rollable manner between the screw shaft raceway surface and the nut raceway surface, wherein the outer diameter of the thread of the screw shaft is larger than the inner diameter of the nut thread of the nut.

According to the present invention, a rolling element screw device is obtained which has a large axial load rating, generates little heat, and suppresses fluctuations in friction torque due to the free zone when the rolling direction is reversed.

1 is a cross-sectional view taken along an axis of a rolling body screw device according to a first embodiment of the present invention. FIG. 2 is an enlarged view of part II in FIG. 3A is a plan view of the rolling body screw device, FIG. 3B is a side view of the rolling body screw device showing only the upper half of the nut, and FIG. 3C is a view taken along the arrow c in FIG. 3B. FIG. 2 is a conceptual diagram showing the relationship between the inclination of the side of a screw thread and the inclination of a grindstone. FIG. 5 is a cross-sectional view taken along the axis of a rolling body screw device according to a second embodiment of the present invention. FIG. 11 is a cross-sectional view taken along the axis of a rolling body screw device according to a third embodiment of the present invention.

Hereinafter, a rolling element screw device according to an embodiment of the present invention will be described with reference to the accompanying drawings. However, the rolling element screw device according to the present invention can be embodied in various forms and is not limited to the embodiments described in this specification. This embodiment is provided with the intention of enabling a person skilled in the art to fully understand the scope of the invention by fully disclosing the specification.
First Embodiment

Figure 1 is a cross-sectional view along the axis of a ball screw 1 as a rolling element screw device according to a first embodiment of the present invention. Figure 2 is an enlarged view of part II in Figure 1. 2 is a screw shaft, 2a is the axis of the screw shaft, 3 is a nut, and 5a and 5b are balls as rolling elements.

As shown in FIG. 1, a helical thread 10 is provided on the outer surface of the screw shaft 2. The thread 10 of the screw shaft 2 has a pair of left and right side surfaces 10b, 10c and a crest 10a (see FIG. 2). A first screw shaft raceway surface 11 is formed on one side surface 10c of the thread 10 (the side surface on the right side of the thread 10 in FIG. 1). A second screw shaft raceway surface 12 is formed on the other side surface 10b of the thread 10 (the side surface on the left side of the thread 10 in FIG. 1). The outer diameter of the thread 10 of the screw shaft 2, i.e., the diameter of an imaginary cylinder in contact with the crest 10a of the thread 10, is D1.

The nut 3 comprises a first nut part 3a and a second nut part 3b which are connected in the axial direction. The first nut part 3a is provided with a hole 8a into which the screw shaft 2 is inserted. A helical nut thread 6a is formed on the inner surface of the hole 8a. The nut thread 6a has a pair of left and right side surfaces 7b, 7c and a crest 7a, similar to the thread 10 (see FIG. 2). A first nut raceway surface 21 facing the first screw shaft raceway surface 11 of the screw shaft 2 is formed on one side surface 7b (left side surface in FIG. 1) of the pair of side surfaces 7b, 7c of the nut thread 6a. The first nut raceway surface 21 is formed only on one side surface 7b of the nut thread 6a (see FIG. 2). Note that one nut raceway surface 21 may be formed on each of the pair of left and right side surfaces 7b, 7c of the nut thread 6a.

The second nut portion 3b is provided with a hole 8b into which the screw shaft 2 is inserted. A helical nut thread 6b is formed on the inner surface of the hole 8b of the second nut portion 3b, similar to the first nut portion 3a. The nut thread 6b has a pair of left and right side surfaces and a crest. A second nut raceway surface 22 that faces the second screw shaft raceway surface 12 of the screw shaft 2 is formed on the other side surface (on the right side in FIG. 1) of the pair of side surfaces of the nut thread 6b. The second nut raceway surface 22 is formed only on the other side surface (on the right side in FIG. 1) of the nut thread 6b. The second nut raceway surface 22 may be formed on each of the pair of side surfaces of the nut thread 6b.

The inner diameters of the nut threads 6a and 6b, i.e., the diameters of the imaginary cylinders that are in contact with the crests 7a of the nut threads 6a and 6b, are both D2. The outer diameter D1 of the threads 10 of the screw shaft 2 is larger than the inner diameter D2 of the nut threads 6a and 6b of the nut 3. The nut threads 6a and 6b enter the adjacent threads 10 of the screw shaft 2.

A first ball 5a is interposed between the first screw shaft raceway surface 11 of the screw shaft 2 and the first nut raceway surface 21 of the first nut portion 3a. The first ball 5a is present only on the first screw shaft raceway surface 11 side of the thread 10 of the screw shaft 2, and is not present on the second screw shaft raceway surface 12 side. The first ball 5a is subjected to a load acting on the first nut portion 3a in only one direction (the direction of the arrow (1) in FIG. 1).

A second ball 5b is interposed between the second screw shaft raceway surface 12 of the screw shaft 2 and the second nut raceway surface 22 of the nut 3. The second ball 5b is present only on the second screw shaft raceway surface 12 side of the thread 10 of the screw shaft 2, and is not present on the first screw shaft raceway surface 11 side. The second ball 5b is subjected to a load acting on the second nut portion 3b only in the other direction (the direction of the arrow (2) in FIG. 1).

As shown in FIG. 2, the shape of the first screw shaft raceway 11 formed on the thread 10 of the screw shaft 2 is formed into a circular arc groove of a single arc. The radius of this circular arc groove is slightly larger than the radius of the first ball 5a. The first screw shaft raceway 11 and the first ball 5a contact each other at point P1 at the bottom of the groove. Since the thread 10 has a lead angle, to be precise, in a perpendicular cross-sectional view of the thread 10, the shape of the first screw shaft raceway 11 is formed into a circular arc groove. The shape of the first screw shaft raceway 11 may be a gothic arch groove that contacts the first ball 5a at two points.

The side surface 10c of the thread 10 of the screw shaft 2 (the line connecting the side surface of the root portion 10-1 and the side surface of the crest portion 10-2 of the thread 10 in FIG. 2) is inclined with respect to a line 2b perpendicular to the axis 2a of the screw shaft 2 (FIG. 2 shows the axis 2a parallel to the axis 2a of the screw shaft 2 in FIG. 1). The width of the root portion 10-1 of the thread 10 of the screw shaft 2 is greater than the width of the crest portion 10-2. Even if the side surface 10c is inclined, the first screw shaft raceway surface 11 makes point contact with the first ball 5a at point P1 at the bottom of the groove. In other words, the contact angle α described later is maintained at approximately 90°. Therefore, the arc length of the first screw shaft raceway surface 11 from the point P1 to the groove end N1 on the root portion 10-1 side (lower side in FIG. 2) is longer than the arc length of the first screw shaft raceway surface 11 from the point P1 to the groove end N2 on the apex portion 10-2 side (upper side in FIG. 2).

It is desirable that the groove width (area represented by central angle θ1 in Figure 2) of the first screw shaft raceway surface 11 of the screw shaft 2 is set so that the contact major axis radius (area represented by central angle θ2 in Figure 2) does not deviate from the groove width when an allowable surface pressure of 4,200 MPa acts on the contact point P1 between the first ball 5a and the first screw shaft raceway surface 11.

The side surface 10b of the thread 10 of the screw shaft 2 is inclined in the opposite direction to the side surface 10c. A second screw shaft raceway surface 12 is formed on the side surface 10b of the thread 10. The shape of the second screw shaft raceway surface 12 is formed into a circular arc groove of a single arc, similar to the first screw shaft raceway surface 11. In a perpendicular cross-sectional view of the thread 10, the thread 10 is formed symmetrically.

The shape of the first nut raceway surface 21 formed on the nut thread 6a of the first nut portion 3a is also formed into a circular arc groove of a single arc. The radius of this circular arc groove is slightly larger than the radius of the first ball 5a, as with the first screw shaft raceway surface 11. The first nut raceway surface 21 and the first ball 5a contact each other at point P2 at the bottom of the groove. Since the nut thread 6a has a lead angle, to be precise, in a perpendicular cross-sectional view of the nut thread 6a, the shape of the first nut raceway surface 21 is formed into a circular arc groove. The shape of the first nut raceway surface 21 may be a gothic arch groove that contacts the first ball 5a at two points.

Similarly to the first screw shaft raceway surface 11, the groove width of the first nut raceway surface 21 of the first nut portion 3a is desirably set so that the contact major axis radius does not deviate from the groove width when an allowable surface pressure of 4,200 MPa acts on the contact point P2 between the first ball 5a and the first nut raceway surface 21.

The side surface 7b of the nut thread 6a of the first nut portion 3a is also inclined with respect to a line 2b perpendicular to the axis 2a of the screw shaft 2. The width of the root portion 6-1 of the nut thread 6a is greater than the width of the crest portion 6-2. Even if the side surface 7b is inclined, the first nut raceway surface 21 makes point contact with the first ball 5a at point P2 at the bottom of the groove. In other words, the contact angle α described later is kept close to 90°. For this reason, the arc length of the first nut raceway surface 21 from point P2 to the groove end M1 on the root portion 6-1 side (upper side of FIG. 2) is longer than the arc length of the first nut raceway surface 21 from point P2 to the groove end M2 on the crest 6-2 side (lower side of FIG. 2).

The side surface 7c of the nut thread 6a of the first nut portion 3a is inclined in the opposite direction to the side surface 7b. The first nut raceway surface 21 is not formed on the side surface 7c of the nut thread 6a.

The angle α between the line connecting the contact points P1 and P2 and the line 2b perpendicular to the axis 2a of the screw shaft 2, i.e., the contact angle α, is set in the range of 90°±30°, preferably in the range of 90°±20°, and more preferably in the range of 90°±10°. If the first screw shaft raceway surface 11 and the first nut raceway surface 21 are shaped into a gothic arch groove, the contact angle α is conveniently set to a line connecting the bottoms of the gothic arch groove.

As shown in FIG. 1, the nut thread 6b of the second nut portion 3b is formed symmetrically with the nut thread 6a of the first nut portion 3a.

Between the first nut portion 3a and the second nut portion 3b, there is a spacer 4 as a preload applying means for applying a preload to the first ball 5a and the second ball 5b. The spacer 4 presses the first nut portion 3a in one direction (the direction of the arrow (1) in FIG. 1) and compresses the first rolling element between the first nut portion 3a and the screw shaft 2. The spacer 4 also presses the second nut portion 3b in the other direction (the direction of the arrow (2) in FIG. 1) and compresses the second rolling element between the second nut portion 3b and the screw shaft 2. Note that a spring may be used instead of the spacer 4 as a preload applying means.

The nut 3 may be a single nut in which the first nut portion 3a and the second nut portion 3b are integrated. By shifting the nut thread 6a of the first nut portion 3a and the nut thread 6b of the second nut portion 3b in the center of the single nut, a preload can be applied to the first ball 5a and the second ball 5b.

In addition, the first nut portion 3a and the second nut portion 3b may be arranged in a left-right reversed position from that shown in FIG. 1, i.e., the first nut portion 3a may be arranged on the right side and the second nut portion 3b on the left side, and a preload may be applied to the first ball 5a and the second ball 5b so that the first nut portion 3a and the second nut portion 3b pull against each other.

Figure 3 shows the circulation structure of the ball screw 1 of this embodiment. Figure 3(a) is a plan view of the ball screw 1, Figure 3(b) is a side view of the ball screw 1 showing only the upper half of the nut 3, and Figure 3(c) is a view seen from the arrow c in Figure 3(b).

The first nut portion 3a is provided with two rows of circulation paths, each of which is, for example, 2.5 turns. The first nut portion 3a is provided with two return pipes 23 connected to one end and the other end of the circulation paths. The return pipes 23 scoop up the first balls 5a that roll to one end of the circulation paths and return them to the other end of the circulation paths. The second nut portion 3b is provided with two rows of circulation paths, each of which is, for example, 2.5 turns. The second nut portion 3b is also provided with two return pipes 25 connected to one end and the other end of the circulation paths. 24 is a key for preventing the first nut portion 3a and the second nut portion 3b from rotating.

Note that the number of turns and rows of the circulation path are just an example and are set appropriately according to the rated load. Also, instead of the return pipe system, other circulation systems such as the top system, end cap system, end deflector system, and return plate system may be used.

The configuration of the ball screw 1 of this embodiment has been described above. The ball screw 1 of this embodiment has the following effects.

The contact angle α can be set to an angle that is favorable for receiving axial loads, and the contact area between the balls 5a, 5b and the screw shaft 2 is large, so the axial rated load can be increased. In conventional ball screws, the contact area between the screw shaft and balls is small, but the contact area between the nut and balls is large. Therefore, the rated load is determined by the smaller screw shaft side. On the other hand, in this embodiment, the difference in contact area between the two is reduced or eliminated, so the rated load is increased.

The radius (radius centered on the screw shaft) of the contact position between the screw shaft 2 and the balls 5a, 5b is approximately the same as the radius of the contact position between the nut 3 and the balls 5a, 5b, so even if the balls 5a, 5b roll, in the case of a Gothic arch groove, a third contact point is not generated, and even in the circular arc groove of this embodiment, the angle between the two lines connecting the two contact points P1, P2 and the ball center O is approximately 180 degrees. This makes it possible to reduce slippage (friction) and suppress heat generation in the ball screw due to friction.

When the rolling direction is reversed, the contact state between the balls 5a and 5b remains in a two-point contact state, so fluctuations in friction torque due to the free zone can be suppressed.

A first screw shaft raceway surface 11 and a second screw shaft raceway surface 12 are formed on each of a pair of side surfaces 10b, 10c of the thread 10 of the screw shaft 2, and the first ball 5a of the first nut portion 3a is provided only on the first screw shaft raceway surface 11 side of the screw shaft 2, and the second ball 5b of the second nut portion 3b is provided only on the second screw shaft raceway surface 12 side of the screw shaft 2, so that the lead angle of the screw shaft 2 can be reduced. In addition, a preload can be applied to the first ball 5a and the second ball 5b.

A spacer 4 is provided between the first nut portion 3a and the second nut portion 3b, so that a preload can be applied to the first nut portion 3a and the second nut portion 3b.

Since the side surfaces 10b, 10c of the thread 10 of the screw shaft 2 are inclined with respect to a line 2b perpendicular to the axis 2a of the screw shaft 2, it is easy to machine the screw shaft raceway surfaces 11, 12 on the side surfaces 10b, 10c of the thread 10. As shown in Figure 4, by inclining the side surfaces 10b, 10c of the thread 10 of the screw shaft 2 from the solid line to the dashed line in the figure, the grindstone T for grinding the first screw shaft raceway surface 11 can be inclined from the solid line to the dashed line, and the first screw shaft raceway surface 11 can be ground with the peripheral surface T1 of the grindstone T.

Since the root portion 10-1 of the thread 10 of the screw shaft 2 is thicker than the top portion 10-2, it is possible to improve the strength of the thread 10 of the screw shaft 2. Similarly, it is possible to improve the strength of the nut threads 6a, 6b of the nut 3.
Second Embodiment

Figure 5 is a cross-sectional view of a ball screw 31 as a rolling body screw device according to a second embodiment of the present invention. 2 is a screw shaft, 2a is the axis of the screw shaft, 3 is a nut, 3a is a first nut portion, 3b is a second nut portion, 5a is a first ball, 5b is a second ball, and 4 is a spacer. The basic configuration of the ball screw 31 of the second embodiment is the same as that of the ball screw 1 of the first embodiment, so the same reference numerals are used and the description thereof is omitted.

In the ball screw 1 of the first embodiment, the side surfaces 10b, 10c of the thread 10 of the screw shaft 2 are inclined with respect to the line 2b perpendicular to the axis 2a of the screw shaft 2, whereas in the ball screw 31 of the second embodiment, the side surfaces 10b, 10c of the thread 10 of the screw shaft 2 are parallel to the line 2b perpendicular to the axis 2a of the screw shaft 2. Similarly, in the ball screw 1 of the first embodiment, the side surfaces 7b, 7c of the nut threads 6a, 6b of the nut 3 are inclined with respect to the line 2b perpendicular to the axis 2a of the screw shaft 2, whereas in the ball screw 31 of the second embodiment, the side surfaces 7b, 7c of the nut threads 6a, 6b of the nut 3 are parallel to the line 2b perpendicular to the axis 2a of the screw shaft 2.

In the ball screw 31 of the second embodiment, the side surfaces 10b, 10c of the thread 10 and the side surfaces 7b, 7c of the nut threads 6a, 6b are not inclined, which has the disadvantage that it is difficult to machine the screw shaft raceway surfaces 11, 12 and the nut raceway surfaces 21, 22. However, apart from this point, the same effects as those of the ball screw 1 of the first embodiment are achieved.
Third Embodiment

Figure 6 is a cross-sectional view of a ball screw 40 as a rolling body screw device according to a third embodiment of the present invention. 41 is a screw shaft, 41a is the axis of the screw shaft, 42 is a nut, and 43 is a ball. In the ball screw 1 of the first embodiment and the ball screw 31 of the second embodiment, the ball screws 1 and 31 are configured to receive loads in two directions, forward and backward, but in the ball screw 40 of the third embodiment, the ball screw 40 is configured to receive loads in one direction (the downward Z-axis direction indicated by the arrow (3) in Figure 6).

The screw shaft 41 is arranged in the Z-axis direction. It prevents rattling due to the gravity of a movable body (not shown) attached to the nut 42.

In the ball screw 40 of the third embodiment, a screw shaft raceway surface 45 is also formed on the side of the helical thread 44 of the screw shaft 41. A nut raceway surface 47 facing the screw shaft raceway surface 45 is formed on the side of the helical nut thread 46 of the nut 42. A plurality of balls 43 are interposed between the screw shaft raceway surface 45 and the nut raceway surface 47 in a rollable manner. The outer diameter of the thread 44 of the screw shaft 41 is larger than the inner diameter of the nut thread 46 of the nut 42, and the contact angle is set in the range of 90°±30°.

According to the ball screw 40 of the third embodiment, similar to the ball screw 1 of the first embodiment and the ball screw 31 of the second embodiment, the axial load rating is large, heat generation is low, and fluctuations in friction torque due to the free zone when the rolling direction is reversed can be suppressed.

The present invention is not limited to the above-described embodiment, but may be embodied in other embodiments without departing from the spirit and scope of the present invention.

For example, in the above embodiment, balls are used as rolling elements, but rollers may also be used as rolling elements.

This specification is based on Japanese Patent Application No. 2019-141986, filed on August 1, 2019, the entire contents of which are incorporated herein by reference.

1, 31, 40...ball screw (rolling body screw device), 2...screw shaft, 3a...first nut portion (nut), 3b...second nut portion (nut), 4...spacer, 5a...first ball (rolling body), 5b...second ball (rolling body), 6a, 6b...nut thread, 6-1...root of nut thread, 6-2...top of nut thread, 7b, 7c...side of nut thread, 10...thread, 10-1...root of thread, 10-2...top of thread portion, 10b, 10c...side surface of thread, 11...first thread shaft raceway surface (thread shaft raceway surface), 12...second thread shaft raceway surface (thread shaft raceway surface), 21...first nut raceway surface (nut raceway surface), 22...second nut raceway surface (nut raceway surface), 41...thread shaft, 43...ball (rolling element), 44...thread, 45...thread shaft raceway surface, 46...nut thread, 47...nut raceway surface, D1...outside diameter of thread, D2...inside diameter of nut thread, α...contact angle

Claims (7)

A screw shaft having a helical thread on an outer surface thereof and a screw shaft raceway surface formed on a side surface of the thread;
A nut having a helical nut thread on an inner surface thereof, and a nut raceway surface facing the screw shaft raceway surface is formed on a side surface of the nut thread;
A plurality of rolling elements are rollably interposed between the screw shaft raceway surface and the nut raceway surface,
The outer diameter of the thread of the screw shaft is larger than the inner diameter of the nut thread of the nut,
A first screw shaft raceway surface and a second screw shaft raceway surface are formed on each of a pair of side surfaces of the thread of the screw shaft,
The nut includes a first nut portion having a first nut raceway surface facing the first screw shaft raceway surface of the screw shaft, and a second nut portion having a second nut raceway surface facing the second screw shaft raceway surface of the screw shaft,
The first rolling element in the first nut portion is present only on the first screw shaft raceway surface side of the screw shaft,
A rolling element screw device , characterized in that a second rolling element in the second nut portion is present only on the second screw shaft raceway surface side of the screw shaft . 2. The rolling element screw device according to claim 1 , characterized in that a spacer for applying a preload to the first rolling element and the second rolling element is provided between the first nut portion and the second nut portion. 3. The rolling element screw device according to claim 1 , wherein the nut is a single nut having the first nut portion and the second nut portion. The rolling element screw device according to any one of claims 1 to 3, characterized in that the side surface of the thread of the screw shaft and/or the side surface of the nut thread of the nut are inclined with respect to a line perpendicular to the axis of the screw shaft. 5. The rolling element screw device according to claim 4 , characterized in that a root portion of the thread of the screw shaft and/or a nut thread of the nut is thicker than a top portion. 6. The rolling element screw device according to claim 1, wherein the screw shaft raceway surface and the nut raceway surface have a circular arc groove shape. 7. The rolling element screw device according to claim 1, wherein the contact angle is set in the range of 90°±30°.

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