JP2011174532A - Ball screw - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ball screw minimized in abrasion without deteriorating high-speed performance and torque characteristics, in a two-point contact ball screw. <P>SOLUTION: The ball rolling grooves 5, 6 of a screw shaft 2 and a nut 3 are pre-loaded. Thereby, contact of balls 4 with a raceway 7 is performed in the form of a two-point contact. Lubricant containing at least a urea-based thickener is interposed between the balls 4 and the ball rolling grooves 5, 6. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a ball screw that requires low friction and low torque fluctuation, and more particularly, to a ball screw that requires a rated load and requires high speed, low heat generation, and low torque.

The ball screw has a ball rolling groove formed on the screw shaft (hereinafter referred to as a screw shaft side rolling groove) and a ball rolling groove formed on the nut (hereinafter referred to as a nut side rolling groove). ), A circular arc groove, a gothic arc groove, or a combination thereof is used. In addition, as a contact mode between the raceway formed by the opposing screw shaft side rolling groove and the nut side rolling groove and the ball, there are a two-point contact type, a three-point contact type, and a four-point contact type. Usually, these contact types are determined by making the shapes of the screw shaft side rolling groove and the nut side rolling groove into a circular arc groove, a gothic arc groove, or a combination thereof.
For example, gothic arc grooves are generally employed in a four-point contact type. One of the advantages of the four-point contact type with the Gothic arc groove is that the measurement ball using a ball at the tip is stabilized in the ball rolling groove, so that the effective diameter can be managed with high accuracy.

On the other hand, the circular arc groove is generally employed in a two-point contact type (see, for example, Patent Document 1). One of the advantages of the two-point contact method is that even when the preload is increased, heat generation and wear due to friction are less than that of the four-point contact method. In the two-point contact type, for example, a double nut type ball screw creates a gap in the axial direction and uses a spacer or the like that adds (or subtracts) a width that allows for a preload to the axial gap. Preload is applied by moving the screw shaft relative to each other in the axial direction. Therefore, it is necessary to accurately manage the axial gap.
The axial gap refers to a gap in the axial direction of the screw shaft formed by the ball and the ball rolling groove before the preload is applied between the track and the ball.

  On the other hand, Patent Document 2 discloses a ball screw that reduces friction loss and adopts a two-point contact method as in Patent Document 1. The ball screw disclosed in Patent Document 2 uses a Gothic arc shape for the ball rolling groove, ensures management of the effective diameter of the screw shaft and nut, reduces friction loss, and improves accuracy during assembly. At the same time, the quality of accuracy of the ball rolling groove is improved. As a result, according to this Patent Document 2, in high-speed operation (screw shaft diameter is about 30 mm and the test rotation speed is 50 rpm or more), the ball screw of the normal preload (three-point contact type or four-point contact type) is used. It is said that low torque can be realized.

Here, FIG. 7 is a graph showing the relationship between the preload amount and the preload dynamic torque in each preload system. The preload method for each specimen was Reference Example 1 (three-point (Z) contact type), Reference Example 2 (four-point (P) contact type), and Reference Example 3 (two-point contact type). The numerical value shown in parentheses on the horizontal axis is the preload (kg).
In FIG. 7, the “currently calculated value” is a result calculated from the relational expression between the preload amount (preload load) and the preload dynamic torque shown in the following formulas (1) and (2). In the following formulas (1) and (2), T represents preload dynamic torque (N · cm), Fao represents preload (kg), L represents lead (mm), and K represents preload friction. Indicates a coefficient, and β indicates a lead angle.

As the ball screw, “BS3205 (diameter of screw shaft 2: 32 mm, lead: 5 mm, reference diameter of ball: 3.175 mm (1/8 inch)”, number of circuits: 2.5 windings, 1 row × manufactured by NSK Ltd. 2) ”, the lubricant was“ ISO VG # 68 ”, and the test was performed at a test rotation speed of 100 rpm. Note that the contact angle of each specimen in the preload state was set to 45 °.
As shown in FIG. 7, the ball screw of Reference Example 3 in which the contact state between the ball rolling groove and the ball is preloaded by the two-point contact method is the ball screw of Reference Example 1 and Reference Example 2 in which another preload method is adopted. Compared to the above, it is shown that the torque is small even when the preload is increased. This indicates that the friction loss of the ball screw of Reference Example 3 is small.

Thus, the ball screw with two-point contact has the same preload load (preload amount) in a steady state (when rotating in the same direction) compared to other preload methods (three-point contact / four-point contact). ) But low torque can be realized, so torque characteristics are excellent.
Here, in the ball screw of the two-point contact type, the ball is supported at two points on the track, so that the ball is in a plane perpendicular to the rolling direction of the ball (hereinafter referred to as a groove orthogonal cross section). It is easy to move, and the so-called zigzag phenomenon, in which the arrangement of balls in the orbit is staggered, is likely to occur.

The friction generated when the ball screw is operated includes friction between the ball rolling groove and the ball and friction due to competition between the balls. In the two-point contact type ball screw, since the friction between the ball rolling groove and the ball is small, the friction variation due to the competition of the balls is more remarkable. In addition, the friction due to the competition between the balls changes depending on the use situation (speed, stroke, etc.), so that it is difficult to predict and becomes an obstacle to stably driving the ball screw.
In Cited Document 2, a configuration in which a holding piece or a spacer ball is inserted between the balls is proposed, whereby the competition between the balls is reduced, and as a result, the staggered phenomenon can be reduced.

JP 2002-276765 A JP 2004-257466 A

However, in the ball screw with two-point contact, in addition to the staggered phenomenon, when the ball screw is swung at a low speed, the ball moves at a low speed in the cross section of the groove at a low speed. There was a problem that the wear of the grooves and balls progressed.
Specifically, the ball screw is naturally used in reciprocating motion, and in particular, repetitive reciprocating motion with a short stroke at the same position is generally called swinging operation. When this rocking operation is performed at a low speed, it is difficult for the lubricating oil to be caught in the vicinity of the contact point between the ball rolling groove and the ball, and wear due to poor lubrication is likely to occur. In particular, in the ball screw of two-point contact, the movement of the ball in the cross section perpendicular to the groove is present as compared with other preload systems.

  In a two-point contact ball screw, since the raceway is twisted in a spiral shape, the contact between the ball and the screw shaft side rolling groove, the contact between the ball and the nut side rolling groove, and the ball center are all screws. The spiral movement around the axis of the axis is performed, but the radii at each point are different, so the spirals are not parallel to each other. Therefore, while rolling, the ball is pulled in the spiral direction at each contact point, and although it is minute, it moves in the ball rolling groove in a direction perpendicular to the rolling direction and bites in a wedge shape. If the biting into the ball rolling groove reaches a certain steady state while rolling, the ball will continue rolling operation with sliding there. The slip ratio at this time increases as the lead angle increases.

FIG. 6 is an enlarged view of a main part for explaining the movement of the ball in the cross section of the groove of the two-point contact ball screw. FIG. 6A shows the case of normal operation (thrust load direction and movement direction are reversed). , (B) shows the case of reverse operation (thrust load direction and motion direction are the same).
As shown in FIG. 6 (a), in a ball screw of two-point contact, in the case of normal operation, the ball 4 moves into the nut-side rolling groove 6 and bites into it, as shown in FIG. 6 (b). In the reverse operation, the ball 4 moves into the screw shaft side rolling groove 5 and bites in.

As described above, in the ball screw 1 preloaded by the two-point contact, a minute movement of the ball 4 in the cross section perpendicular to the groove occurs during the low-speed and swinging operation. The minute movement of the ball 4 is a very small stroke of several μm and is very slow. And this minute motion occurs at both ends of the stroke, and the number of times becomes very large in the swing operation.
That is, in other rolling elements, the lubricant is likely to be caught in the boundary surface by increasing the speed. However, in the ball screw 1 of the two-point contact type, the fretting wear is caused by the minute movement of the ball 4. Is likely to result in poor lubrication.

However, the ball screw preloaded by the two-point contact type has a small increase in the preload torque with respect to the increase in the preload amount, as shown in FIG. In other words, when the preload torque is adjusted to 10 N · cm, the P preload (four-point contact) and the Z preload (three-point contact) have a preload amount of about 0.5 to 1 μm, whereas the two-point contact type. In this case, the preload amount is about 3 μm. For this reason, if the amount of preload is increased to some extent, it is considered that there is little concern about preload loss even if it is worn. However, due to problems such as reduced rigidity, wear must be reduced as much as possible.
The present invention has been made paying attention to such a problem, and an object of the present invention is to propose a ball screw having both low friction characteristics and wear resistance in a two-point contact type ball screw.

The invention according to claim 1 for achieving the above object 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 the screw shaft. In a ball screw comprising: a side rolling groove, and a plurality of balls arranged in a track formed so as to face the nut side rolling groove,
A lubricant is interposed between the ball that has been preloaded on the ball and the raceway and brought into contact with the raceway at two points, the screw shaft side rolling groove, and the nut side rolling groove, The lubricant is characterized by containing at least a urea-based thickener.

According to the first aspect of the present invention, a lubricant containing at least a urea-based thickener is interposed between the ball, the screw shaft side rolling groove, and the nut side rolling groove. The wear can be reduced while maintaining the low friction characteristics of the ball screw preloaded by the two-point contact method.
The invention according to claim 2 is characterized in that, in the ball screw according to claim 1, the holding piece impregnated with the lubricant is disposed between the plurality of balls.
According to the second aspect of the present invention, since the holding piece is disposed between the plurality of balls, a sufficient amount of lubricant is provided between the balls and the screw shaft side rolling grooves and the nut side rolling grooves. Can be interposed.

The invention according to claim 3 is characterized in that, in the ball screw according to claim 1 or 2, a solid lubricating film is applied to the ball.
According to the invention of claim 3, the wear resistance is further improved by forming a solid lubricating film on the surface of the ball. As a result, it is possible to provide a two-point contact type ball screw with further improved wear resistance.

  According to the present invention, it is possible to provide a ball screw with improved wear resistance while maintaining low friction characteristics in a two-point contact type ball screw.

It is a plane sectional view of a ball screw in one embodiment of the present invention. It is explanatory drawing of the preload addition part in one Embodiment of this invention. It is explanatory drawing of the axial direction clearance gap in one Embodiment of this invention. It is a principal part enlarged view explaining the ball | bowl and holding piece part in FIG. It is a graph which shows the relationship between the rocking | fluctuation stroke at the time of 100,000 reciprocations in one Embodiment of this invention, and a ball wear amount. It is a principal part enlarged view explaining the micro motion of the ball | bowl in the groove | channel orthogonal cross section of the ball screw of a two-point contact. It is a graph which shows the relationship between the preload amount and preload dynamic torque of each preload system in the conventional ball screw.

Hereinafter, an embodiment of a transport device according to the present invention will be described with reference to the drawings.
FIG. 1 is a plan view showing a part of a configuration of a ball screw 1 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. It shows. FIG. 2 is an explanatory diagram for explaining the relationship between the track and the ball in a state where a preload is applied to the ball screw 1.

  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 6 facing the screw shaft side rolling groove 5. Rolled into a raceway 7 formed by a cylindrical nut 3 screwed onto the screw shaft 2, a screw shaft side rolling groove 5 and a nut 3 side rolling groove 6 on the inner peripheral surface. A large number of balls 4. Then, the nut 3 screwed to the screw shaft 2 via a large number of balls 4 and the screw shaft 2 are relatively moved in the axial direction via the rolling of the balls 4.

A flange 22 for fixing the nut 3 to a table or the like is provided at one end in the axial direction of the nut 3. 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 pass through the nut 3 to reach the track 7, and the balls 4 rolling in the track 7 circulate through the circulation passage 9.

Therefore, the ball 4 moves in the raceway 7 and turns around the screw shaft 2 a plurality of times, and then at one end of the circulation passage 9 at one end of the raceway 7 (intersection of the end of the circulation passage 9 and the raceway 7). The ball 4 scooped up from the portion (opening portion) into the circulation passage 9 passes through the circulation passage 9 and returns from the other end portion (opening portion) of the circulation passage 9 to the other end of the track 7. It is. 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.
The ball screw 1 employs gothic arc grooves for the ball rolling grooves 5 and 6. 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.

  Further, the nut 3 is a double nut type and is preloaded. Specifically, the nut 3 is formed by integrating a first nut 31 and a second nut 32 arranged in the axial direction and a spacer 33 interposed between the nuts 21 and 22. Then, by the interposition of the spacer 33, a preload is applied to the balls 4 in the track 7 in the direction of the arrow shown in FIG. It contacts at two points, one point of the screw shaft side rolling groove 5. In a direction perpendicular to the line connecting the two points, the ball 4 is not in contact with both the ball rolling grooves 5 and 6, or even if in contact, no preload is applied (see FIG. 2). That is, the ball screw 1 adopts an offset lead preload structure as a preload application structure, and applies a preload, thereby making the contact between the ball 4 and the track 7 a two-point contact type.

  In the ball screw 1 of the present embodiment, the holding piece 8 may be disposed between all the balls 4 as shown in FIG. 4 shown as a partially enlarged view of FIG. The holding piece 8 has a shape in which both bottom surfaces of the cylinder are concave surfaces (ball holding surfaces) 71. The diameter of this cylinder is slightly smaller than the diameter of the ball 4. Further, the concave surface 71 has a surface shape formed by combining two spherical surfaces having the same radius of curvature, and the cross section thereof has a Gothic arc shape. The concave surface 71 is formed such that the radius of the ball 4 is smaller than the radius of the concave surface 71, and the intersection position of the radii of the concave surface 71 is the center position of the ball 4. Therefore, the concave surface 71 of the holding piece 8 and the ball 4 can be in line contact. Thereby, the ball 4 can contact the concave surface 71 of the holding piece 8 with extremely low friction. Therefore, the sliding resistance between the ball 4 and the holding piece 8 can be reduced. Therefore, the circulation performance of the holding piece 8 is improved, and the deterioration of the operability due to the contact between the balls 4 and the friction and damage of the balls 4 can be remarkably reduced.

FIG. 5 is a graph showing results obtained by varying the amount of wear of the ball with respect to the amount of swing stroke during 100,000 reciprocations by varying the preload method and the lubricant used. The preload method was a two-point contact method or a four-point contact method. As the lubricant, “ISO VG # 68” was prepared.
As shown in FIG. 5, the wear amount of the ball screw 1 of Comparative Example 1 preloaded in a two-point contact system using oil as a lubricant increases rapidly during a small stroke. On the other hand, the ball screw 1 of Comparative Example 2 preloaded (P preloaded) in a four-point contact type has little change due to the stroke. Since the number of swings is the same, the difference in the amount of wear is likely to be a difference in the contact type and lubricity between the ball rolling groove 5.6 and the ball 4. The amount of wear this time is large in a small stroke range in which the revolution of the ball 4 about its axis is less than one rotation. That is, it is conceivable that the amount of wear increases when the lubrication performance decreases in addition to the instability of the ball 4 originally shown in FIG.

  On the other hand, it can be seen from FIG. 5 that the wear amount of the ball screw 1 using grease as the lubricant is kept small. In particular, the ball screw 1 using grease as a lubricant has less wear than the ball screws 1 of Comparative Example 1 and Comparative Example 2. Furthermore, of these ball screws 1, the ball screw 1 of the first embodiment of the two-point contact method is superior to the ball screw 1 of the second embodiment of the four-point contact type when the swing stroke is 8 (mm) or more. The wear amount of the ball 4 is suppressed to 0.1 μm or less when the swing stroke is in the range of 0 to 150 mm.

  The ball screw of the two-point contact type has a high degree of freedom because the ball is held only at two points in the ball groove. For this reason, for example, in the case of rocking operation (repeated with a small stroke), it is considered that the ball moves minutely in the ball groove in addition to the revolution accompanying the movement of the nut. The wear of the ball groove or ball is considered to be fretting wear due to this minute movement.

  The minute movement of the ball is a phenomenon peculiar to the two-point contact type ball screw. In order to reduce this phenomenon, when the anti-fretting grease is used, the above-described effects can be obtained. Anti-fretting grease is a urea-based thickener that uses high-grade synthetic oil as the base oil. By using such anti-fretting grease, it is considered that the lubricant can easily enter a minute space and is effective in reducing fretting wear.

  As described above, the present invention uses the grease, not oil, as a lubricant for the ball screw 1 preloaded by the two-point contact type, so that the low friction specific to the ball screw 1 pre-loaded by the two-point contact type is achieved. The wear resistance is improved while maintaining the characteristics. In particular, the effect of “low friction” is largely due to the preload of the ball screw of the two-point contact type, so as to reduce the phenomenon peculiar to the ball screw of the two-point contact type as described above, as anti-fretting grease, A remarkable effect could be obtained by using a lubricant containing a urea-based thickener.

Next, the wear characteristics due to the difference in the thickener were measured under the following conditions. The results are shown in Table 1. In Table 1, the wear characteristics were compared by the amount of decrease in the diameter of the ball 4 (wear amount).
(Implementation conditions)
Ball screw: Nippon Seiko Co., Ltd., “BS3205”
Screw shaft diameter: 32 [mm]
Lead: 5 [mm]
Standard diameter of the ball: 3.175 [mm]
Preload method: Two-point contact type Test rotation speed: 5 [rpm]
Swing stroke: 5 [mm]
Number of oscillations: 100,000 times Lubricant: Grease 1 and Grease 2

  As shown in Table 1, by using a lubricant (grease) that uses a urea system rather than a lithium soap system that is generally used as a thickener, the two-point contact type ball screw 1 can be operated at low speed. It was shown that the amount of wear during dynamic operation is reduced. “Grease 1” in Table 1 corresponds to the urea-based lubricant shown in FIG. Therefore, when “grease 2” is used as a lubricant for measuring the “amount of wear of the ball with respect to the swing stroke amount during 100,000 reciprocations”, when the “swing stroke” is 5 mm in FIG. It shows a wear amount of 3 μm.

Further, the ball 4 may be provided with a solid lubricating film. By forming a solid lubricating film on the surface of the ball 4, it is possible to provide a ball screw of a two-point contact type with improved wear resistance while further reducing the amount of wear and maintaining low friction characteristics.
Furthermore, in this embodiment, the holding piece 8 is interposed between the balls 4. Therefore, since the competition between the balls 4 is suppressed by the holding piece 8, it is possible to more suitably suppress the generation of friction torque.

  In addition, the ball screw which concerns on this invention is not limited to the said embodiment, A various deformation | transformation is possible unless it deviates from the meaning of this invention. For example, in the above embodiment, the holding piece 8 is disposed between the balls 4, but this holding piece 8 is to hold the lubricant used in the present embodiment on the surface of the ball 4. Even if the holding piece 8 is not used, a sufficient effect can be obtained by merely interposing the lubricant used in the present embodiment between the ball 4 and the track 7.

Moreover, in the said embodiment, although the holding piece 8 is arrange | positioned between all the balls 4, as for the ball screw 1 of this invention, the holding piece 8 is arrange | positioned between all the balls 4. It is not limited to. For example, a method of sandwiching a small ball or other cages can be used.
Moreover, in the said embodiment, although the tube-type circulation path 9 is used, it is not limited to this, You may use the circulation path of another type.

In the above embodiment, the offset lead preload is used as the preload method, but the present invention is not limited to this, and the following method can be applied as the preload method of the ball screw 1.
For example, the lead near the center of the single nut can be increased by a preload amount to give a preload. In this embodiment, two nuts are used and a spacer thicker than the gap between the nuts by a preload amount is inserted, but conversely, a thin spacer can be inserted to give a preload. Further, in a multi-thread screw, preload can be applied by shifting the space between the screw shaft and the space between the nuts.

DESCRIPTION OF SYMBOLS 1 Ball screw 2 Screw shaft 3 Nut 4 Ball 5 Ball rolling groove (of screw shaft) 6 Ball rolling groove (of nut) 7 Track 8 Holding piece 9 Circulating passage 10 Dust-proof seal 21 Notch 22 Flange 31 First nut 32 Second nut 33 Spacer 71 Concave surface 81 Circulation passage presser

Claims (3)

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 on the screw shaft, and a ball rolling groove on the nut. In a ball screw comprising: a plurality of balls arranged in tracks formed opposite to each other;
A lubricant is interposed between the ball that has been preloaded on the ball and the raceway and brought into contact with the raceway at two points, the ball rolling groove of the screw shaft, and the ball rolling groove of the nut. The ball screw is characterized in that the lubricant contains at least a urea-based thickener.   The ball screw according to claim 1, wherein a holding piece is disposed between the plurality of balls.   The ball screw according to claim 1, wherein a solid lubricating film is formed on the surface of the ball.

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