JP2009204141A - Ball screw - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ball screw suitable for preventing or restraining abnormal torque increase in a low speed operation, in a two point contact type ball screw. <P>SOLUTION: In this ball screw 1, the contact state between a raceway 7 and each load ball 4 is always brought into two point contact; spacer balls 8 rotating by themselves are interposed between the plurality of load balls 4; and the spacer ball 8 has an outer diameter keeping a distance not smaller than zero from the raceway 7 in a condition receiving a predetermined external load. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a ball screw configured such that a contact state between a load ball and a track is in contact at two points, and can be applied to a preloaded ball screw in general that requires low friction and low torque fluctuation. The present invention relates to a ball screw that is suitable for applications that do not require so much rated load and require high speed, low heat generation, and low torque.

  A ball screw as a ball rolling element includes a screw shaft having a spiral ball rolling groove on an outer peripheral surface, a nut having a spiral ball rolling groove on an inner peripheral surface, and a ball rolling groove of the screw shaft and the nut. Are configured with a plurality of load balls arranged in a track formed opposite to each other, and the contact state between the load ball and the track is generally a three-point or four-point contact. is there. On the other hand, in a bearing that is also a ball rolling element, the contact state between the raceway of the inner and outer rings and the load ball is usually two-point contact in the preload state.

  In a general ball screw, the reason why the contact state between the load ball and the track is a three-point or four-point contact type is to prevent the load ball from getting into the ball rolling groove. That is, in general, the ball screw has a problem that the torque increases particularly during low-speed operation (hereinafter, also referred to as “abnormal torque increase”) due to the spiral rolling of the ball rolling groove. This abnormal torque increase is considered to be caused by “clogging between the groove and the ball” in which the load ball bites into the ball rolling groove. Therefore, the contact state between the load ball and the track is a three-point or four-point contact type to increase the support point of the load ball, thereby preventing or suppressing biting. It is assumed that the torque increase due to the bite exceeds the torque increase due to contact of the third and fourth points.

However, in the three-point or four-point contact type ball screw, the load ball is supported at the three or four points with respect to the track, so that a slip component at the third or fourth point exists. Therefore, when the preload is increased, heat generation and wear due to friction become a problem due to the slip component. On the other hand, in the case of a ball screw of the two-point contact type, there is no slip component at the third and fourth points, so even if the preload is increased, heat generation and wear due to friction are three or four. This is advantageous compared to the point contact type.
For example, the technique described in Patent Document 1 maximizes the characteristics of the two-point contact type by using a gothic arc groove as the ball rolling groove and managing the axial clearance between the track and the ball in a predetermined manner. This makes it possible to provide a ball screw having a possible condition with stable quality.

By adopting the configuration described in this document, a two-point contact state can be realized with high accuracy in a two-point contact type ball screw. As a result, since the ball screw of the two-point contact type does not have a slip component at the third or fourth point, even if the preload is increased, the heat generation and wear due to friction are three or four. It becomes possible to suppress compared with the point contact type. Further, for example, in high-speed operation (shaft diameter of about 30 mm, 50 rpm or more), a low torque can be realized as compared with a normal preload ball screw in a three-point or four-point contact state.
JP 2004-257466 A

  By the way, the inventors of the present application conducted various element tests on the ball screw of the two-point contact type, and the abnormal torque increase that occurs particularly at low speed is not due to the biting into the rolling groove of the ball, but the third point. Since the ball is not restrained by the fourth contact point, it has been found that the main factor is that the ball becomes unstable and it is easy to compete with the adjacent load balls. The spacer balls relieve slippage between the balls by rotating in the direction opposite to that of the load balls.

  In the two-point contact type ball screw, the friction between the ball rolling groove and the load ball is smaller than that in the three-point or four-point contact state. For this reason, in the two-point contact type ball screw, the frictional fluctuation due to the competition between the balls 4 is relatively conspicuous. In particular, many ball screws for applications requiring operating characteristics have a small steel ball diameter. For this reason, the number of load balls in the circuit is increased, and the preload is relatively small when the operation characteristics are required, so that the torque increase due to the load ball being compressed by the preload is also reduced.

Further, the friction caused by the competition between the load balls varies depending on the use state (for example, speed, stroke, etc.) of the ball screw. For this reason, it is difficult to predict the frictional fluctuation due to the competition between the load balls, and in the two-point contact type ball screw, the competition between the adjacent load balls becomes an obstacle for driving the ball screw stably.
Here, in order to prevent competition between adjacent load balls, it is conceivable that a spacer (holding piece) is interposed between adjacent load balls as described in Patent Document 1.

However, the number of loaded balls of the ball screw is very large, for example, compared with that of a bearing that is a ball rolling element. Further, when a normal spacer (holding piece) is interposed, the spacer itself does not rotate. Also, in order to keep the spacer from falling off, it is necessary to insert the load ball and the spacer without any gaps in the circuit to some extent, so that a ball having a load ball with a small steel ball diameter that requires particularly operating characteristics. In the screw, the influence of the torque increase due to the competition between the load ball and the spacer is increased, and it is difficult to completely suppress the abnormal increase in torque.
Accordingly, the present invention has been made paying attention to such problems, and in a two-point contact type ball screw, a ball screw suitable for preventing or suppressing an increase in abnormal torque during low-speed operation is provided. It is intended to provide.

In order to solve the above-mentioned problems, the inventors of the present invention have conducted an experiment and studied. In a two-point contact type ball screw, when a predetermined spacer ball is interposed between adjacent load balls, the other It has been found that particularly excellent torque characteristics can be realized compared to the case where spacer balls are applied to the three-point or four-point contact type ball screws.
That is, the present invention provides a screw shaft having a spiral ball rolling groove on the outer peripheral surface, a nut having a spiral ball rolling groove on the inner peripheral surface, a ball rolling groove of the screw shaft, and the nut. In a ball screw comprising a plurality of load balls arranged in a raceway formed with opposing ball rolling grooves, the contact state between the raceway and the load ball is always a two-point contact. In addition, a spacer ball capable of rotating in the opposite direction to the load ball is interposed between the plurality of load balls, and under the condition that the load ball receives a predetermined external load, the spacer ball It is characterized by having an outer diameter that maintains a gap of zero or more.

According to the ball screw of the present invention, since the contact state between the track and the load ball is always a two-point contact, there is no slip component at the third point or the fourth point. Even in this case, heat generation and wear due to friction are more advantageous than those of the three-point or four-point contact type.
In addition, a spacer ball capable of rotating is interposed between a plurality of load balls, and this spacer ball is an outer surface that maintains a clearance from the track with no load or 1/10 or less of the dynamic load rating. Since it has a diameter, as will be apparent from the results of various element tests described later, it is possible to prevent or suppress an increase in abnormal torque during low-speed operation.

Here, in the ball screw according to the present invention, the ratio of the spacer balls to the load balls is such that when the number of spacer balls is S and the number of load balls is F, the ratio S / F is 0.5 ≦ It is preferable that S / F ≦ 1. Such a configuration is more preferable for preventing or suppressing an increase in abnormal torque during low-speed operation.
In the ball screw according to the present invention, the preloading method for applying a preload between the raceway and the load ball is a double nut spacer preloading method, an offset lead preloading method, or an interstitial offset lead preloading method. Is preferred. Such a configuration is suitable as a preloading method for applying a preload so that the contact state between the track and the load ball is always a two-point contact.

In the ball screw according to the present invention, it is preferable that a ball filling rate including the spacer balls is 98% or less. Such a configuration is suitable for preventing or suppressing an increase in abnormal torque during low-speed operation.
In the ball screw according to the present invention, the spacer ball is preferably made of a resin, and when the spacer ball is made of a resin, the resin is preferably a polyacetal resin. Such a configuration is suitable as a spacer ball for suitably preventing or suppressing an increase in abnormal torque during low-speed operation.

  As described above, according to the present invention, it is possible to provide a ball screw suitable for preventing or suppressing an increase in abnormal torque during low-speed operation in a two-point contact type ball screw.

Hereinafter, embodiments of the present invention will be described.
FIG. 1 is a plan view showing a part of a configuration of a ball screw according to an embodiment of the present invention in a cross section. In FIG. 1, the nut is broken along a plane including the axial center of the nut. Show. FIG. 2 is a diagram for explaining the relationship between the track and the load ball in a state where a preload is applied to the ball screw shown in FIG.

  As shown in FIG. 1, this ball screw 1 includes a screw shaft 2 having a spiral ball rolling groove 5 on the outer peripheral surface, and a spiral ball rolling groove facing the ball rolling groove 5 of the screw shaft 2. 6 on the inner peripheral surface, and on a raceway 7 formed by a cylindrical nut 3 screwed to the screw shaft 2, a ball rolling groove 5 of the screw shaft 2 and a ball rolling groove 6 of the nut 3. And a plurality of load balls 4 that are movably loaded. Then, the nut 3 screwed into the screw shaft 2 via the plurality of balls 4 and the screw shaft 2 are relatively moved in the axial direction via the rolling of the balls 4.

Here, a spacer ball 8 made of resin is interposed between the plurality of balls 4 so as to be able to rotate, and the competition between the balls 4 is suppressed by the spacer ball 8, thereby improving the operability. Deterioration is prevented and friction and damage of the ball 4 are reduced.
Specifically, as the spacer ball 8, in the present embodiment, the track 7 is a load of 1/10 or less of the dynamic load rating (a load other than the preload acting on the load ball 4; hereinafter referred to as "external load"). The outer diameter is maintained to be zero (zero) or more (that is, the outer diameter is such that the spacer ball 8 does not bear the load under such load conditions).

  In general, the external load acting on the ball screw 1 is designed to be a sufficiently small value as compared with 1/10 of the dynamic load rating of the ball screw. Accordingly, when an external load that is 1/10 of the dynamic load rating is applied, the clearance with the track 7 can be maintained at zero (zero) or more (hereinafter, “corresponding to an external load of 1/10 of the dynamic load rating”). If it is set as the outer diameter of the spacer ball 8, there is no practical problem.

  However, the size of the outer diameter of the spacer ball 8 is not limited to this, and a predetermined size can be selected according to the use condition of the ball screw. For example, when the external load is substantially negligible, the spacer ball 8 is not loaded (ie, the external load is not applied to the load ball 4 and only the preload is applied). And the outer diameter of the track 7 may be any diameter that is substantially equal to zero. That is, the maximum diameter of the spacer ball 8 may be a size that does not prevent rotation of the spacer ball 8 in the direction opposite to the load ball 4 when the spacer ball 8 receives a load between the spacer ball 8 and the track 7.

  Furthermore, it is possible to select an outer diameter smaller than a size that can maintain the clearance with the track 7 at zero or more when an external load of 1/10 of the dynamic load rating is applied. Even when the outer diameter of the spacer ball 8 is smaller than the diameter corresponding to an external load that is 1/10 of the dynamic load rating, it may be within a range in which there is no problem due to an excessively large gap with the track 7. . For example, if the gap between the spacer ball 8 and the track 7 is too large, the ball may become staggered in the load groove (spiral groove) or in the circulation path, which may hinder the ball from moving in the traveling direction. If there is no. Specifically, considering the dimensional tolerance of the load ball 4, it was confirmed that there is no problem even if the outer diameter of the spacer ball 8 is about −30 μm relative to the design size of the load ball 4.

  Accordingly, the outer diameter of the spacer ball 8 is such that the gap between the spacer ball 8 and the track 7 is substantially equal to zero when there is no load (that is, no external load is applied to the load ball 4 and only preload is applied). 8 is the maximum diameter that can be set as the outer diameter. For the outer diameter of the spacer ball 8, a value between this maximum diameter and a diameter corresponding to −30 μm with respect to the design dimension of the load ball 4 is selected in accordance with the actual use conditions such as the size of the external load. it can.

  In addition, as the spacer ball resin according to the present invention, an engineering plastic such as polyacetal resin (POM) can be suitably used. In this embodiment, the spacer ball 8 is a spacer made of Delrin (trademark registered). A ball is interposed. Furthermore, in the ball screw according to the present invention, the ratio between the spacer balls 8 and the balls 4 in one circuit is such that the ratio S / F is S when the number of spacer balls 8 is S and the number of balls 4 is F. The range of 0.5 ≦ S / F ≦ 1 is preferable, and the ball filling rate with respect to the total length of the ball circulation path is desirably 98% or less. Therefore, in this embodiment, the ratio S / F between the spacer balls 8 and the balls 4 is “1”, and the ball filling rate with respect to the total length of the ball circulation path is 94%.

The ball screw 1 is provided with a flange 22 at one end in the axial direction of the nut 3 for fixing the nut 3 to a table or the like. Between the flange 22 and the screw shaft 2 and between the other axial end of the nut 3 and the screw shaft 2 are closed by a dustproof seal 10.
A cutout portion 21 is formed on the outer peripheral surface of the nut 3, and a circulation passage 9 made of a tube bent in a substantially U shape is disposed in the cutout portion 21. The circulation passage 9 is fixed to the notch 21 with a circulation passage presser 81. Both ends of the circulation passage 9 constitute a “circuit” in which the nut 4 passes through the nut 3 to the track 7, and the balls 4 rolling in the track 7 circulate through the circulation passage 9.

  In other words, the ball screw 1 is moved by the above “circuit” so that the ball 4 moves in the track 7 and rotates around the screw shaft 2 a plurality of times before one end of the track 7 (the end of the circulation passage 9 and the track 7). At one end (opening) of the circulation passage 9 at a point where the ball 4 is picked up from the ball rolling grooves 5 and 6 to the circulation passage 9, that is, from the load zone to the non-load zone. As a transition point), the scooped-up ball 4 is scooped up into the circulation passage 9, and the scooped ball 4 passes through the circulation passage 9 to the other end (opening portion) of the circulation passage 9. As the ball return portion (the point at which the ball 4 is returned from the circulation passage 9 to the ball rolling grooves 5, 6, that is, the point at which transition from the non-load zone to the load zone) is made from the ball return portion to the other end of the track 7. Returned to Note that one circulation passage 9 is arranged in the axial direction for each of the first nut 31 and the second nut 32, and two circulation passages are provided in total.

Here, the nut 3 uses a double nut type, and is applied with a preload. Specifically, the nut 3 includes a first nut 31 and a second nut 32 that are arranged in the axial direction, and a spacer 33 that is interposed between the nuts 21 and 22. Then, by the interposition of the spacers 33, a preload is applied to the balls 4 in the track 7 in the direction of the arrow shown in the figure, and each ball 4 faces one point of the ball rolling groove 6 of the nut 3 and this. It is in contact at two points, one point of the ball rolling groove 5 of the screw shaft 2 at the position where In the direction perpendicular to the line connecting the two points, the ball 4 is not in contact with both ball rolling grooves 5 and 6, or even if in contact, no preload is applied (see FIG. 2).
That is, this ball screw 1 adopts a double nut spacer preload system as a preload application system, and applies a preload by shifting the leads between the left and right circuits, thereby making contact between the ball 4 and the track 7 two-point contact. It has a format.

As for the groove type of the ball rolling grooves 5 and 6, the third point contact is not caused when the nut 3 and the screw shaft 2 move relative to each other in the axial direction via the rolling of the ball 4. The groove type is set.
Specifically, in the example of the present embodiment, Gothic arc grooves are employed for the ball rolling grooves 5 and 6, respectively. That is, the cross-sectional shape of the ball rolling grooves 5 and 6 is a substantially V-shape combining two identical arcs having different curvature centers. Preferably, the axial gap Δ before applying a preload between the ball 4 and the track 7 is 0.5% Dw or more and 5% Dw or less when the diameter of the ball 4 is Dw. Further, the radius of curvature of the arc of the gothic arc groove is in the range of 52% Dw to 60% Dw when the diameter of the ball 4 is Dw. Here, the “axial gap Δ” is a gap in the axial direction of the screw shaft 2 between the ball 4 and the ball rolling grooves 5 and 6 before the preload is applied.

Next, functions and effects of the ball screw 1 will be described.
As described above, since the contact state between the track 7 and the ball 4 is always a two-point contact, the ball screw 1 has high operation (small torque fluctuation), low friction torque (low heat generation), and abrasion resistance. Excellent wear resistance.
That is, in this ball screw 1, since the contact state between the track 7 and the ball 4 is always two-point contact, there is no slip component at the third or fourth point, so the preload is increased. Even in this case, heat generation and wear due to friction are more advantageous than those of the three-point or four-point contact type. The ball screw 1 is provided with a spacer ball 8 that can rotate between a plurality of balls 4, and the spacer ball 8 has a clearance of 1/10 of the dynamic load rating with respect to the track 7. Therefore, the advantage that the friction loss between the ball rolling grooves 5 and 6 and the ball 4 in the two-point contact type is small can be further extracted. That is, as is apparent from the results of the examples described later, it is possible to prevent or suppress an increase in abnormal torque during low-speed operation.

Further, according to this ball screw 1, the ratio between the spacer balls 8 and the balls 4 in each circuit is such that when the number of spacer balls is S and the number of load balls is F, the ratio S / F is “1”. Therefore, an abnormal torque increase during low-speed operation can be suitably prevented or suppressed.
Further, according to this ball screw 1, since the preloading method for applying the preload between the track 7 and the ball 4 is a double nut spacer preloading method, the contact state between the track 7 and the ball 4 is always two points. It is suitable for applying a preload so as to be in contact.

Moreover, according to this ball screw 1, since the ball filling rate with respect to the total length of the ball circulation path is 94%, it is suitable for preventing or suppressing an increase in abnormal torque during low-speed operation.
Moreover, according to this ball screw 1, since the spacer ball 8 is made of resin and the resin is a polyacetal resin, it is suitable as a spacer ball for suitably preventing or suppressing an increase in abnormal torque during low-speed operation.

[Example]
Hereinafter, the embodiment of the above-described ball screw according to the present invention will be described in detail with reference to test data in comparison with a conventional two-point contact type ball screw.
First, before describing an embodiment of a ball screw according to the present invention, characteristics of a conventional two-point contact type ball screw will be described with reference to test data. In the following, the test data shown in each figure is a test result having the following [Test Conditions] in common except that differences are shown in the explanation in each figure.

[Test conditions]
Ball screw designation: Ball screw manufactured by NSK Ltd., 32 x 05 x 700 C3 (Ball 1/8 inch)
Shaft diameter: φ32mm
Lead: 5mm
Number of circuits: 2.5 windings x 2 rows (The ball rolling groove is a gothic arc groove shape, realizing a two-point contact system with double nut spacer preload)
Testing machine: NSK Ltd. ball screw torque testing machine Preload: 500N ~
Shaft rotation speed: 1 to 500 rpm
Single unit gap is 3% Dw (Dw = steel ball diameter of ball 4)
Lubrication: Lubricant # 68

FIG. 3 is a graph showing the relationship between the amount of preload and the preload torque when the contact type is changed by overball size preload.
As shown in the figure, the ball screw of the two-point contact type has the characteristic that the torque is small even if the preload is increased, compared to the three-point contact type or the four-point contact type. I understand. This indicates that the two-point contact type ball screw has a smaller friction loss than the three-point contact type and the four-point contact type. However, the conventional two-point contact type ball screw has the following weak points.

  That is, in the figure, for example, when adjusting to a preload torque of 10 N · cm, when a three-point contact method or a four-point contact method is adopted, the preload amount is about 0.5 to 1 μm. On the other hand, when two-point contact is adopted, the preload amount is about 3 μm. In general, when the preload amount of the ball screw is large, fluctuation due to competition between the balls also increases. Therefore, when the preload torque is constant, torque fluctuation is larger in the conventional two-point contact type than in the three-point contact type or the four-point contact type. Further, in a two-point contact type ball screw with a small base torque, when there is a torque fluctuation due to competition between the load balls, the torque fluctuation becomes very conspicuous.

Next, FIGS. 4 and 5 show dynamic torque characteristics of a conventional two-point contact type ball screw. 4 shows dynamic torque characteristics at a high speed (100 rpm), and FIG. 5 shows dynamic torque characteristics at a low speed (10 rpm).
As is clear from a comparison between FIG. 4 and FIG. 5, torque fluctuations are large at low speeds. This variation is a long-period (up to several times) torque increase called “that is,”. As shown in FIG. 5, there are also peaks with a period of about 3 rotations. The cause of this variation (that is,) is considered to be due to the competition between the balls in the load zone (in the track 7 formed by the ball rolling grooves 5 and 6). For this reason, in particular, when the circulation path 9 shown in FIG. The fluctuation (that is,) becomes large.

Next, FIG. 6 shows a result of changing the magnitude of torque fluctuation according to the shaft rotation speed (ball revolution speed) in a conventional two-point contact type ball screw.
As can be seen from the figure, the torque of the conventional two-point contact ball screw becomes extremely large at low speed. This is because, at high speed, the load ball moves at a certain speed, so when entering the load zone, there is a gap between the load balls due to the repulsive force caused by the collision between the load balls, whereas at low speed This is because the influence of gravity acts greatly, and the load balls are gathered on the lower side (load zone side), so that the balls can easily compete with each other in the load zone. As described above, the conventional two-point contact type ball screw has a small loss between the ball rolling grooves 5 and 6 and the ball 4 at high speed (close to pure rolling), but at low speed, the ball 4 There is a feature that torque fluctuation due to competition between each other is large.

  7 and 8 show the torque characteristics of a conventional two-point contact type ball screw. FIG. 7 shows a conventional two-point contact type ball screw in which a spacer ball is not interposed in the circuit and a “total ball” filled only with load balls is provided on the tube (circulation passage 9). Shows a torque characteristic in a posture in which is positioned above the axis with respect to the direction of gravity. Further, FIG. 8 shows a conventional two-point contact type ball screw in a posture of “total ball” and “below the tube (the circulation passage 9 is positioned below the axis with respect to the direction of gravity)”. Torque characteristics are shown.

As shown in FIG. 7, in a conventional two-point contact type ball screw that does not interpose a spacer ball, torque fluctuation caused by the competition between balls is observed at a low speed of 10 rpm.
In addition, as shown in FIG. 8, the torque characteristics in the “total ball, under the tube” do not have the torque fluctuation as in the case of the “total ball, on the tube”, but the short-cycle torque fluctuation (hereinafter, this Short cycle torque fluctuations are also referred to as “beards”). Note that the “beard” has a characteristic that is different from “that is” because its period is as short as about 0.1 rotation of the shaft. In the present specification, a “whisker” is defined as one that eliminates the torque increase within 0.2 rotations of the shaft, and “that is” is defined as a value exceeding that.

In the two-point contact type ball screw, when the circulation path 9 is located below the axis with respect to the direction of gravity, a gap is easily formed between the balls 4 in the load zone at a low speed. Therefore, the torque increase due to the competition between the balls 4 in the load zone is relatively small. However, in the torque characteristics of the “total ball”, since the ball 4 also enters the circulation passage 9 without a gap, a short period torque increase (the “beard” described above) is caused by interference between the scooping portion and the ball returning portion. ) Is considered to occur.
Therefore, the two-point contact type ball screw of the present invention aims to improve the torque characteristics due to the competition between the balls 4 at low speed in the conventional two-point contact type ball screw as described above.

  FIG. 9 is a diagram showing torque characteristics in the embodiment of the ball screw of the two-point contact type according to the present invention. This example is different from the conventional ball screw having the torque characteristics shown in FIG. The torque characteristics “on the tube” when the ball 4 and the spacer ball 8 are interposed in the circuit at a ratio of 1: 1 are shown. As shown in the figure, a self-rotating spacer ball 8 having an outer diameter that maintains a clearance of zero (zero) or more with the track 7 with a load of 1/10 or less of the dynamic load rating is interposed between the load balls. It can be seen that by wearing, the competition between the balls 4 is eliminated, and torque fluctuation is suppressed to a small level. It can be confirmed that the same is true even when the outer diameter of the spacer ball 8 is in a range from a size where the gap between the spacer ball 8 and the track 7 is almost zero to the outer diameter of the load ball −30 μm.

FIG. 10 is a diagram showing torque characteristics in an embodiment of the ball screw of the two-point contact type according to the present invention. The resin spacer balls 8 are interposed between the balls 4 at a ratio of 1: 1. Shows the torque characteristics “under the tube”.
As is clear from the figure, when a spacer ball 8 having an outer diameter that maintains a gap with the track 7 with a load of 1/10 or less of the dynamic load rating is interposed between the balls 4, Occurrence of “beard” is suppressed. This is considered to be because the interference between the balls between the scooping portion and the ball return portion is alleviated by the spacer ball 8. It can be confirmed that the same is true even when the outer diameter of the spacer ball 8 is in a range from a size where the gap between the spacer ball 8 and the track 7 is almost zero to the outer diameter of the load ball −30 μm.

FIG. 11 is a diagram showing the torque characteristics in the embodiment of the ball screw of the two-point contact type according to the present invention, and the clearance with the track 7 is zero (zero) or more with a load of 1/10 or less of the dynamic load rating. Self-rotating spacer balls 8 having an outer diameter to be maintained are interposed between the balls 4 at a ratio of 1: 1.
The generation ratio of “beard” when the ratio of the resin spacer balls 8 in the spacer balls 8 (other spacer balls are steel balls) is changed is shown. The generation ratio of “beard” is based on the number of occurrences of “beard” generated in one reciprocation in the case of the total ball shown in FIG. 8, and in FIG. The spacer balls are steel balls (resin spacer ratio: 0%), and in the symbol P, all the interposed spacer balls are resin (resin spacer ratio: 100%).

  As can be seen from the figure, the number of occurrences of “whiskers” decreases as the proportion of the spacer balls made of resin in the interposed spacer balls 8 increases. That is, as the ratio of the resin spacer balls present in the circulation passage 9 is increased, the soft portion such as resin is more effective in suppressing the generation of “beard”. Thus, the effect of suppressing the occurrence of “beard” is particularly effective when the spacer ball 8 is made of a soft material such as resin.

  FIG. 12 shows torque fluctuations when the filling rate of the spacer balls 8 interposed between the balls 4 is changed in the two-point contact type ball screw of the present invention. That is, the graph shown in the figure shows the torque characteristics when the number of balls packed in one circuit (the path length of the raceway 7 formed by the ball rolling grooves 5 and 6 and the circulation passage 9) is changed. It is. Note that the ball filling rate is 100% when the balls are inserted into the circulation path of one circuit without any gap.

  Here, in a conventional two-point contact type ball screw that includes a spacer (holding piece) that does not rotate, a gap that does not drop out in the circuit so that the spacer does not drop out between the balls 4. It is necessary to fill so that For this reason, the ball filling rate must be 95% or more (so that the spacer (holding piece) does not fall out between the balls), so that it is difficult to obtain good operating characteristics especially during low-speed operation. is there.

  On the other hand, when the spacer ball 8 that can rotate is interposed like the ball screw 1 of the two-point contact type according to the present invention, the problem that the spacer ball 8 falls off between the balls 4 can be eliminated. it can. And when this self-rotating spacer ball 8 is interposed, the effect is that the rotation direction of the ball 4 which is a load ball can be rotated in the reverse direction, and that a gap can be created in the circulation path. Can be held together. Therefore, it is possible to improve the low-speed operation characteristics of the two-point contact type ball screw.

Here, in the ball screw of the two-point contact type of the present invention, it is preferable that the ball filling rate with respect to the total length of the ball circulation path is 98% or less. In particular, as can be seen from FIG. If so, it is more preferable. In other words, as can be seen from the figure, even when the spacer ball 8 is interposed, the torque characteristic is poor when the filling rate is larger than 90%, similarly when the spacer (holding piece) that does not rotate is interposed. (Torque fluctuation increases).
As described above, according to the ball screw 1, an increase in abnormal torque during low-speed operation can be prevented or suppressed in the two-point contact type ball screw.

The ball screw according to the present invention is not limited to the above-described embodiment or examples, and various modifications can be made without departing from the spirit of the present invention.
For example, in the above embodiment, a double nut spacer preloading system is adopted as a preloading structure as a preloading system for making the contact state between the track 7 and the ball 4 always a two-point contact type, and between the left and right circuits. Although the example in which the preload is applied by shifting the lead has been described, the present invention is not limited to this. An offset lead preloading method or the like may be used, and when a multi-threaded ball screw is used, an interstrip offset lead preloading method or the like can be adopted.

  The spacer balls 8 interposed between the balls 4 are desirably inserted alternately with the balls 4 at a ratio of 1: 1, but are not limited to the alternate insertion, and the insertion ratio is 1: For example, when the assembly time is shortened, the low-speed operation characteristic can be improved according to the ratio of the intervention, even when the intervention time is random. Further, the ball screw using a tube as the circulation passage has been described as an example, but the present invention can also be applied to a ball screw of an end cap type or a top type.

It is a plane sectional view of a ball screw of the two-point contact type in one embodiment of the present invention. It is explanatory drawing of the preload addition part made into the two-point contact type in one Embodiment of this invention. It is a graph which shows the relationship between the amount of preload when a contact type is changed, and preload torque. It is a figure which shows the dynamic torque characteristic of the conventional two-point contact type ball screw. It is a figure which shows the dynamic torque characteristic of the conventional two-point contact type ball screw. In the conventional two-point contact type ball screw, it is a figure which shows the result of the magnitude | size of a torque fluctuation | variation changing with shaft rotation speed (ball revolution speed). It is a figure which shows the torque characteristic in the conventional ball screw of a two-point contact type. It is a figure which shows the torque characteristic in the conventional ball screw of a two-point contact type. It is a figure explaining the torque characteristic in the Example of the ball screw of the two-point contact type of this invention. It is a figure explaining the torque characteristic in the Example of the ball screw of the two-point contact type of this invention. It is a figure explaining the torque characteristic in the Example of the ball screw of the two-point contact type of this invention. It is a figure explaining the torque characteristic in the Example of the ball screw of the two-point contact type of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Ball screw 2 ... Screw shaft 3 ... Nut 4 ... Ball (load ball)
5 ... Ball rolling groove (of screw shaft) 6 ... Ball rolling groove of (nut) 7 ... Track 8 ... Spacer ball 9 ... Circulation path 10 ... Dust-proof seal 21 ... notch 22 ... flange 31 ... first nut 32 ... second nut 33 ... spacer 81 ... circulation passage retainer

Claims (6)

A screw shaft having a spiral ball rolling groove on the outer peripheral surface, a nut having a spiral ball rolling groove on the inner peripheral surface, and the ball rolling groove of the screw shaft and the ball rolling groove of the nut are opposed to each other. A ball screw having a plurality of load balls arranged in a track formed as follows:
The contact state between the track and the load ball is always a two-point contact, and a spacer ball capable of rotating in the opposite direction to the load ball is interposed between the plurality of load balls. The ball screw, wherein the spacer ball has an outer diameter that maintains a clearance of zero or more with respect to the track under a condition that the ball receives a predetermined external load.   The ratio between the spacer balls and the load balls is such that the ratio S / F is in the range of 0.5 ≦ S / F ≦ 1, where S is the number of spacer balls and F is the number of load balls. The ball screw according to claim 1.   The preloading method for applying a preload between the track and the load ball is a double nut spacer preloading method, an offset lead preloading method, or an interstitial offset lead preloading method. The ball screw described.   4. The ball screw according to claim 1, wherein a ball filling rate including the spacer balls is 98% or less.   The ball screw according to claim 1, wherein the spacer ball is made of resin.   The ball screw according to claim 5, wherein the resin is a polyacetal resin.

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