JP2008151303A - Ball screw device and its screw groove machining method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ball screw device having long life and high quality, and to provide a screw groove machining method for producing the ball screw device at low cost. <P>SOLUTION: The ball screw device 100 comprises a screw shaft 10, a nut 20 to be threaded to the screw shaft 10, and a plurality of balls 30 for rolling between a screw groove 21 on the side of the nut 20 and a screw groove 11 on the side of the screw shaft 10. The screw groove 21 on the side of the nut 20 is formed into a sectionally Gothic arched shape, and a plurality of sectionally circular fine recessed portions 70 extending along the screw groove 21 are cut and machined in the surface of the screw groove. Thus, the high-quality ball screw device can be produced at low cost to achieve improved productivity and long life in a well balanced manner. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a ball screw device used as a machine element such as a steering device, a heavy-weight moving device, a processing machine, a precision positioning device, and a thread groove processing method thereof.

  In such a ball screw device, a nut having a spiral thread groove formed on the inner surface and a screw shaft having a spiral thread groove formed on the outer surface are screwed together via a plurality of balls (steel balls), By rotating the screw shaft, the nut is reciprocated along the screw shaft. Traditionally, it is an indispensable machine element for parts such as vehicles, steering devices, positioning devices, and various industrial manufacturing and processing machines. It has become one of the.

  Then, when processing and manufacturing a ball screw device having such a configuration, finish grinding processing (for example, a cross-sectional gothic arch) of a thread groove having an arc-shaped cross section preliminarily formed on the outer surface of the screw shaft and the inner surface of the nut is performed. However, the finish grinding of the thread groove on the inner surface side of the nut has more restrictions on the grinding process than the thread groove on the screw shaft side. It becomes.

  In other words, the most commonly used grinding method for thread groove finish grinding on the nut side is generally a quill provided on the main shaft (rotary shaft) of the thread grinder when the lead angle of the thread groove is 7 degrees or less. Attach a disc-shaped grindstone to the tip of the nut, insert the grindstone into the nut while rotating it at a high speed, and tilt the quill at the same angle as the lead angle of the thread groove and place the grindstone on the thread groove surface on the inner surface of the nut. By pressing, the thread groove is ground into a cross-sectional Gothic arch shape (hereinafter referred to as “normal grinding”).

  On the other hand, when the diameter of the nut is small, the nut length is long, or the lead angle is large, for example, when exceeding the limit of the tilt angle of the main spindle of a general screw grinding machine, for example, As shown in Japanese Patent Application Laid-Open No. H10-318, etc., the inner surface of the nut is ground while the inclination angle of the grindstone is smaller than the lead angle (hereinafter referred to as “interference grinding”).

Further, when the nut diameter is extremely small or the lead angle of the screw shaft is extremely large, for example, exceeding 17 degrees, and the finish grinding of the thread groove cannot be performed even by performing interference grinding as in Patent Document 1 etc. For example, finish grinding is performed using a grinding method called lapping using a lap bar having a male thread on the peripheral surface as shown in Patent Document 1 (FIG. 8) and Patent Documents 2 and 3 below. .
JP-A-6-55343 JP-A-6-218636 JP-A-9-168924

By the way, in the interference grinding as described above, it is merely a process close to a regular Gothic arch shape to be finally ground, and a regular Gothic arch shape is not obtained as in normal grinding. In particular, as the lead angle increases, the deviation from the regular shape increases, and as a result, the rolling fatigue life of the ball screw decreases as can be seen from FIG.
That is, FIG. 7 shows an example of the relationship between the lead angle and the life ratio, and a life ratio of 100% can be exhibited up to a lead angle of up to 7 degrees. When processed, the life ratio falls below 100%, and the life ratio decreases as the lead angle increases.

On the other hand, the above-described lapping has a problem that productivity is low because the processing time is long, and the production cost increases because the cost of the tool including the processing tool is high. Further, the lapping process has a problem that the variation in the thread groove shape is large compared to the above-described normal grinding and interference grinding.
Therefore, the present invention has been devised to solve the above-described problems, and its main purpose is to produce a long-life and high-quality ball screw device and the ball screw device at low cost. A thread groove processing method is provided.

In order to solve the above-mentioned problem, the invention of claim 1
A screw shaft having a spiral thread groove on the outer peripheral surface, a nut having a spiral thread groove passing through the screw shaft and facing the screw groove on the screw shaft side on the inner peripheral surface, and the nut A ball screw device including a plurality of balls that roll between the screw grooves on the screw shaft side and the screw shaft side, wherein the screw groove shape on the nut side has a cross-sectional Gothic arch shape, and A ball screw device characterized in that a plurality of micro concave portions having an arcuate cross section extending along the thread groove are provided on the surface of the thread groove.

The invention of claim 2
2. The ball screw device according to claim 1, wherein a peak height between the adjacent minute recesses is 0.4 μm or more and 1.0 μm or less, and a ratio between a curvature R of the minute recess and the diameter Dw of the ball ( R / Dw) is a ball screw device characterized by being 0.3 or less.

The invention of claim 3
The ball screw device manufacturing method according to claim 1, wherein a cutting blade is attached to a rod body having a smaller diameter than the nut having a helical thread groove formed on an inner peripheral surface, and the rod body or the nut is attached to the nut. After inserting the rod body into the nut while spirally rotating along the thread groove of the nut and cutting the first micro concave portion on the surface of the thread groove of the nut by the cutting blade of the rod body, The second micro-recess is cut adjacent to the first micro-recess, and then the third and subsequent micro-recesses are sequentially cut adjacent to the second micro-recess to thread the nut. A thread groove machining method for a ball screw device, wherein the surface is cut into a cross-sectional Gothic arch shape having the plurality of minute recesses.

  According to the first aspect of the present invention, the screw groove shape on the nut side has a cross-sectional Gothic arch shape, and the surface of the screw groove has a plurality of minute concave portions having a circular arc shape extending along the screw groove. Therefore, the minute recesses serve as a reservoir for the lubricating oil, and the lubricity of the contact surface with the ball can be kept good. As a result, the lubricity is improved and a long life can be achieved.

Specifically, as defined in claim 2, the peak height between the adjacent minute recesses is 0.4 μm or more and 1.0 μm or less, and the curvature R of the minute recess and the diameter Dw of the ball If the ratio (R / Dw) is 0.3 or less, excellent productivity and long life can be achieved in a well-balanced manner as will be demonstrated later.
According to the third aspect of the present invention, even a thread groove having a large lead angle, which has been impossible in the past, can be cut into a regular Gothic arch shape. In addition, the variation in the thread groove shape as in the conventional lapping process is small, and it is possible to exhibit excellent productivity as compared with the conventional lapping process and to reduce the production cost.

Next, an embodiment of a ball screw device 100 according to the present invention will be described in detail with reference to the accompanying drawings.
1 and 2 show an embodiment of a ball screw device 100 according to the present invention. 1 is a longitudinal sectional view of a ball screw device 100 according to the present invention, and FIG. 2 is a partially enlarged view showing a portion A in FIG.

As illustrated, the ball screw device 100 includes a rod-shaped screw shaft 10, a cylindrical nut 20 that passes through the screw shaft 10, and a plurality of balls that roll between the nut 20 and the screw shaft 10. (Steel balls) 30, 30...
Further, spiral thread grooves 11 and 21 are formed on the outer peripheral surface of the screw shaft 10 and the inner peripheral surface of the nut 20 so as to face each other, and the spiral shape formed by these thread grooves 11 and 21 is formed. The plurality of balls 30, 30... Roll in the rolling path 40.

Further, two return tubes 50 and 50 are provided between the upstream and downstream sides of the spiral rolling path 40, and two return tubes 50 and 50 along the axial direction of the nut 20 together with a part of the rolling path 40 ( 2) circulation circuits 60, 60 are formed.
As shown in FIG. 2, the thread grooves 11 and 21 constituting the rolling path 40 are formed in a Gothic arch shape having a slightly sharp central portion, and are in contact with the ball 30 at two points, respectively. Thus, even if the axial clearance is adjusted to be extremely small, it can be turned lightly. In addition, by adopting such a cross-sectional Gothic arch shape, it is possible to adjust the preload to eliminate the axial gap and to have an appropriate rigidity suitable for the use conditions.

Further, as shown in the figure, among the screw grooves 11 and 21 having the cross-sectional Gothic arch shape, the surface of the screw groove 21 on the nut 20 side has an arcuate cross section extending along the screw groove 21. A plurality of minute recesses 70 are cut so as to continue in the width direction.
FIG. 3 and FIG. 4 show an example of the cutting method of the screw groove 21 on the nut 20 side.

  That is, first, as shown in FIG. 3, a cutting blade 90 having a sharp tip is attached to a rod body 80 having a smaller diameter than the nut 20, and the rod body 80 or the nut 20 side is connected to the thread groove 21 of the nut 20. The rod body 80 is inserted into the nut 20 while being rotated in a spiral manner, and the first micro-recess 70a is cut into the thread groove surface of the cylindrical body by the cutting blade 90 of the rod body 80 ( FIG. 4 (1) → (2)).

  Next, the rod body 80 is again extracted from the nut 20, and the phase of the rod body 80 with respect to the axis of the nut 20 or (and) the position in the normal direction with respect to the axis is changed, and then the first minute The second micro concave portion 70b is cut adjacent to the concave portion 70a (FIG. 4 (2) → (3)), and thereafter the cutting blade 90 is similarly adjacent to the second micro concave portion 70b. The third and subsequent minute recesses 70 c, 70 d... Are sequentially cut (FIG. 4 (3) → (6)) to form a plurality of minute recesses 70 on the surface of the thread groove 21 of the nut 20.

As a result, as shown in FIG. 2, a plurality of micro concave portions 70 having an arc-shaped cross section extending along the screw groove 21 are cut to the surface of the screw groove 21 on the nut 20 side so as to be continuous in the width direction. Can do.
The size (size) of each of the minute recesses 70, 70... Cut into the surface of the thread groove 21 in this way is not particularly limited. For example, as shown in FIGS. The height (depth) between the minute recesses 70 and 70 adjacent to each other is set to “0.4 μm” to “1.0 μm”, and the ratio of the curvature R of the minute recess 70 to the diameter Dw of the ball 30 (R When / Dw) is set to “0.3” or less, excellent productivity and long life can be achieved in a well-balanced manner.

That is, first, FIG. 5 shows the relationship between the life ratio and the productivity per hour of the ball screw device in which the curvature R and the peak height of the minute recess 70 are sequentially changed.
The ball screw device used in this test is a double nut type ball screw, shaft diameter φ 40 mm, lead 40 mm, lead angle 17 degrees, ball diameter 6.35 mm, preload amount 620 N, curvature R 0.8 mm, R /Dw=0.126.

  And since this ball screw device has a lead angle of 17 degrees, as can be seen from FIG. 7, it can theoretically be expected to have a life extension effect of 1.67 times that of a conventional interference grinding ball screw. According to the test results shown in FIG. 5, when the height of the crest between the adjacent minute recesses 70 is set to 1 μm or less, the life extension effect is about 2-3 times that of the ball screw device for interference grinding. You can see that

This is because, as described above, the plurality of minute recesses 70 formed in the screw groove 21 act as an oil reservoir for the lubricating oil, and the lubricity of the contact surface with the ball 30 is maintained well to prevent the oil film from being cut. It is thought to prevent it.
Also, as can be seen from the figure, it can be seen that the life ratio becomes extremely low when the peak height exceeds 1 μm, but this is the case when the ball 30 contacts the surface of the thread groove 21. This is presumably because the contact surface pressure at the top of the peak of the minute recess 70 suddenly increases and breaks early.

Therefore, it is desirable that the height of the peak is low. However, if the number of the minute recesses 70 is increased in order to keep it low, the number of cutting processes of the minute recesses 70 is increased, and the productivity per hour is lowered. In order to maintain productivity equivalent to that of the ball screw device, it is desirable that the peak height be at least 0.4 μm as described above.
On the other hand, FIG. 6 shows the relationship between the curvature R of the micro-recess 70 when the height between the micro-recesses 70, is 0.4 μm, the life ratio with respect to the ball screw device for normal grinding, and the productivity per hour. It is shown. In addition, since the magnitude | size of the curvature R of this micro recessed part 70 is generally represented by ratio with respect to the finishing radius of a to-be-processed object, the horizontal axis was set to R (curvature) / Dw (ball diameter) in FIG.

As can be seen from the figure, it has been confirmed that the life ratio is drastically reduced when the curvature R of the minute recess 70 exceeds 30% (0.3) of the diameter of the ball 30.
This is because when the curvature R of the minute recess 70 is larger than the diameter (Dw) of the ball 30, it becomes closer to machining with a total bite rather than point cut, and the groove shape and spiral in the direction perpendicular to the groove are increased by increasing cutting resistance. This is because the roundness of the direction becomes worse.

Further, as the size of the curvature R of the minute concave portion 70 is reduced as in the case of the above-described peak height, the number of cutting processes increases and the productivity is lowered. Therefore, at least the productivity equivalent to that of a ball screw device for normal grinding is maintained. In order to achieve this, it is understood that R (curvature) / Dw (ball diameter) needs to be “0.126”.
Accordingly, as can be seen from these test results, the peak height (depth) between the adjacent minute recesses 70 and 70 is set to the range of “0.4 μm” to “1.0 μm” as described above, and R When (curvature) / Dw (ball diameter) is set to “0.3” or less, the life ratio can be maintained at almost 100% regardless of the lead angle as shown in FIG. Productivity and long service life can be achieved in a well-balanced manner.

In addition, although this cross-sectional arc-shaped micro recessed part 70 has a step part (crest) in the width direction of the screw groove 21, since the rolling direction of the ball 30 is smoothly continuous, the rolling performance of the ball screw device 100 is achieved. Will not be adversely affected.
In the present embodiment, an example of a tube ball type ball screw has been described. However, the present invention does not depend on the type of the circulating portion, and can be applied to, for example, a top type, an end cap type, or a type having no circulation path. .

1 is a longitudinal sectional view showing an embodiment of a ball screw device 100 according to the present invention. It is the elements on larger scale which show the A section in FIG. It is explanatory drawing which shows one Embodiment of the method of this invention. It is explanatory drawing which shows one Embodiment of the method of this invention. It is a graph which shows the relationship between a life ratio and productivity, and a mountain height. It is a graph which shows the relationship between life ratio and productivity, and R (curvature) / Dw (ball diameter). It is a graph which shows the relationship between a lead angle and a life ratio.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 ... Ball screw 10 ... Screw shaft 11 ... Screw groove (screw shaft side)
20 ... Nut 21 ... Thread groove (Nut side)
DESCRIPTION OF SYMBOLS 30 ... Ball 40 ... Rolling path 50 ... Return tube 60 ... Circulation circuit 70 ... Micro recessed part 80 ... Rod body 90 ... Cutting blade

Claims (3)

A screw shaft having a spiral thread groove on the outer peripheral surface, a nut having a spiral thread groove passing through the screw shaft and facing the screw groove on the screw shaft side on the inner peripheral surface, and the nut A ball screw device comprising a plurality of balls that roll between a screw groove on the side and a screw groove on the screw shaft side,
A ball screw device characterized in that a screw groove shape on the nut side has a Gothic arch shape in cross section, and a plurality of minute concave portions having an arc cross section extending along the screw groove on a surface of the screw groove. The ball screw device according to claim 1,
The peak height between the adjacent minute recesses is 0.4 μm or more and 1.0 μm or less, and the ratio (R / Dw) between the curvature R of the minute recess and the diameter Dw of the ball is 0.3 or less. There is a ball screw device. It is a processing method of the thread groove of the ball screw device according to claim 1,
A cutting blade is attached to a rod body having a smaller diameter than the nut in which a spiral thread groove is formed on the inner peripheral surface,
The rod body or the nut is spirally rotated along the thread groove of the nut, the rod body is inserted into the nut, and a first microscopic surface is formed on the surface of the thread groove of the nut by the cutting blade of the rod body. After cutting the recess,
Cutting the second minute recess adjacent to the first minute recess,
Further thereafter, the third and subsequent minute recesses are sequentially cut adjacent to the second minute recesses, and the thread groove surface of the nut is cut into a cross-sectional Gothic arch shape having a plurality of the minute recesses. A thread groove machining method for a ball screw device.

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