JP2003148586A - Rolled ball screw shaft - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rolled ball screw shaft which has a good precision groove and a low cost by restraining the stress by sudden plastic deformation generated when rolling of a ball screw shaft is started and minimizing the generation of an incomplete screw part. SOLUTION: A spiral ball groove 1a is formed on an outer periphery of this rolled ball screw shaft by rolling. The width T of the leading end chamfered part 6a of the ball screw shaft is set to 1/2-1 of the lead L of the ball groove 1a. Therefore, the good precision groove 1a can be formed by restraining the stress by sudden plastic deformation generated when rolling is started by setting the small diameter d1 of the leading end chamfered part 6a to groove bottom diameter d or less of the ball groove 1a.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolled ball screw shaft applied to, for example, a ball screw for electric power steering.

[0002]

2. Description of the Related Art A ball screw has a plurality of balls formed between a spiral thread groove formed on the outer peripheral surface of a screw shaft and a spiral thread groove formed on the inner peripheral surface of a nut fitted onto the screw shaft. Is arranged to convert the rotational power of the screw shaft (or nut) into thrust of the nut (or screw shaft) via the ball. In the screw shaft of such a ball screw, there are a method by grinding and a method by rolling as a method for accurately processing the thread groove, and the rolling is superior in terms of productivity. For example, in a ball screw for an electric power steering device which is an automobile part, it is desired to reduce the weight and the manufacturing cost.

FIG. 6 is a schematic view of an apparatus for rolling a ball screw shaft, FIG. 7 is a front view showing the shape of a shaft end portion of a raw material, and FIG.
FIG. 3 is a schematic view showing a state in which a material bites into a roll die. When the thread groove of the ball screw shaft is manufactured by rolling, as shown in FIG. 6, the columnar material 60 placed on the rest 50 is sandwiched between a pair of round roll dies 70, 80, and the desired shape is obtained. While pressing it with pressure, it sends it in the axial direction. The pair of roll dies 70 and 80 rotate at the same peripheral speed in the directions of arrows P and Q, the shaft of one roll die 70 is fixed, and the shaft of the other roll die 80 is the fixed side die 7.
It moves in the direction of arrow R with respect to the axis of 0 to pressurize the material 60 to transfer the thread groove formed on the outer circumference of the roll dies 70, 80.

As shown in FIG. 7, at the shaft end of the material 60,
A cylindrical portion 61 whose outer diameter A is larger than the thread groove bottom diameter and smaller than the material outer diameter D, and which is continuous with this cylindrical portion 61 and expands to the material outer diameter D side at a taper angle θ (1 to 3 °). A tapered portion 62 that opens is formed. Further, the axial length W of the columnar portion 61 is set to 2/3 of the width of the roll dies 70 and 80. Further, a C chamfered portion 63 is formed at the shaft end to facilitate the biting of the material 60 into the roll dies 70 and 80.

By forming the shaft end portion of the material 60 into such a shape, the shaft end portion of the material 60 gently bites into the roll dies 70 and 80 as shown in FIG. The thread groove 65 that it has can be formed by rolling. Further, an incomplete threaded portion 66 having a width of the thread 64 wider than that of the regular thread and a depth of the thread groove 65 shallower than that of the regular thread is formed at the shaft end portion (Japanese Patent Laid-Open No. 2000-519).
84 publication).

[0006]

However, by forming the shaft end shape of the ball screw shaft as described above, when one end of the material bites into the roll die, the stress due to the rapid plastic deformation is relieved, and the desired lead is obtained. Although it is possible to form a thread groove having good accuracy such as accuracy, an incomplete thread portion is present as a result. Since this incomplete thread portion is an obstacle to weight reduction, it is necessary to cut and remove it after the completion of rolling, but the material cost of the cutting portion and the addition of cutting man-hours increase the manufacturing cost. It was

The present invention has been made in view of the above circumstances. By devising the chamfered shape of the material tip of the rolled ball screw shaft, the stress caused by the rapid plastic deformation generated at the start of rolling is suppressed. In addition, the object of the invention is to provide a low-cost rolled ball screw shaft that can obtain a thread groove with good accuracy by minimizing the generation of incomplete threaded portions.

[0008]

[Means for Solving the Problems] In order to achieve the purpose,
According to a first aspect of the present invention, in a rolled ball screw shaft in which a spiral ball screw groove is formed on the outer periphery by rolling, the width of the tip chamfered portion of the ball screw shaft is the lead of the ball screw groove. The configuration is set to 1/2 to 1.

In this way, in the chamfered portion formed at the tip of the ball screw shaft, by setting the width of the chamfered portion to 1/2 to 1 of the lead of the ball screw groove, one of a pair of roll dies can be obtained. Since the thread groove is started to be formed by one roll die and then formed by two roll dies, it is possible to relieve the abrupt machining stress generated at the start of plastic working, and improve the rolling accuracy. At the same time, it is possible to minimize the incomplete threaded portion formed in the chamfered portion.

When the small diameter of the chamfered portion of the tip of the ball screw shaft is set to be equal to or smaller than the groove bottom diameter of the ball screw groove, the working stress at the initial stage of rolling can be further suppressed. be able to.

Further, when the land portion between the ball screw grooves is formed by the rolling surface as in the invention described in claim 3, not only can the edge generated in the groove shoulder portion be prevented, but at the same time when the screw groove is formed. , The shoulder can be formed into a desired shape.

Further, as in the invention described in claim 4, when a pre-processed cutting groove is left at the groove bottom of the ball screw groove and the ball screw groove is used as a rolling surface, the plastic working stress due to rolling is generated. Can be suppressed, the rolling processing accuracy can be improved, and deformation such as bending of the screw shaft can be suppressed.

According to the fifth aspect of the present invention, the rolling ball screw shaft has a hollow portion having a through hole at the axial center thereof, so that the temperature rise during operation can be suppressed by the air cooling effect, and It is possible to reduce the weight of the ball screw device.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to FIG.
5 through FIG. FIG. 1 is a plan view showing a rolling device for a rolled ball screw shaft according to the present invention. The cylindrical material 1 of the ball screw shaft moves axially in the bushes 4 and 5 arranged on both sides of the pair of roll dies 2 and 3.
A plurality of bushes 4 and 5 are appropriately arranged according to the length of the material 1 to be rolled, and the axial center of the material 1 is regulated.
Smoothly bites between the pair of roll dies 2 and 3,
The material 1 is guided so that it can be moved in the axial direction without shaking.

Of the roll dies 2 and 3, one of the roll dies 2 is a fixed side having a fixed shaft center, and the other roll die 3 is a movable side that can advance and retreat in the radial direction of the screw shaft 1. . Spiral threads 2a and 3a having a desired shape are formed on the outer peripheral surfaces of the roll dies 2 and 3, and the thread groove 1a and the land portion 1b are formed on the outer circumference of the material 1 by plastic working while rotating in the same direction. To form. here,
If the thread groove 1a and the land portion 1b are simultaneously rolled by the pair of roll dies 2 and 3, the thread groove 1a
Can be formed in a smooth arc shape.

Normally, if a contact ellipse generated when a ball (not shown) rolls over the shoulder portion causes excessive peeling pressure and early peeling or abnormal wear, it is not preferable. The edges of the shoulders are removed by the above method to form an arc shape. Further, after forming the thread groove by rolling, the convex portion that plastically flows to the outer diameter side is deleted, and the outer diameter is cut by grinding or the like in order to correct the bending of the screw shaft caused by the rolling process.
In the present embodiment, the thread groove 1a and the land portion 1b are machined at the same time, so that the desired shoulder shape can be formed, and the plastic working stress of rolling can be relaxed to suppress the deformation of the screw shaft. Therefore, such processing steps are unnecessary or can be minimized. Therefore, it is possible to obtain a highly accurate rolled screw shaft at low cost.

The material 1 moves from left to right as shown by the arrow in the figure, but the thread ridges 2b and 3b of the roll dies 2 and 3 at the rolling start portion are smaller in diameter than the other thread ridges 2a and 3a. The outer tangent line of the tops of the threads 2b and 3b has a smooth arc shape or a tapered shape. Therefore, the tip portion 6 of the material 1 bites the roll dies 2 and 3 without receiving a strong impact.

On the other hand, a chamfered portion 6a is formed on the tip portion 6 of the material 1 as shown in FIG. In the figure, chamfered corners,
Although it is formed in a so-called C chamfer, it is not limited to this and may be a smooth arc shape. It is preferable that the chamfered angle 6a of the chamfered portion 6a be set to be small in terms of smooth biting of the material 1, but if the chamfered portion 6a is made too small, the width of the chamfered portion 6a becomes large, and the incomplete threaded portion 1c is accordingly increased. Will increase and hinder weight reduction. Also,
When cutting and removing this incomplete screw portion 1c after rolling,
The material cost of the cutting portion and the addition of the cutting man-hour increase the manufacturing cost, which is not preferable.

On the other hand, if the chamfering angle φ is too large, the incomplete threaded portion 1c can be suppressed, but a sharp machining stress is generated at the start of rolling, which is not preferable because the rolling precision is deteriorated. The range of 30 to 45 ° is preferable.
FIG. 2A shows that when the chamfering angle φ is set to 30 °,
Further, FIG. 2B shows the case where the chamfer angle φ is set to 45 °.

The small diameter d1 of the chamfered portion 6a is smaller than the groove bottom diameter d of the thread groove 1a. This is the rolling start point of the roll dies 2 and 3, and the end face of the material 1 is relaxed so as not to collide with the threads 2b or 3b of the roll dies 2 and 3 and generate a large impact or excessive stress due to plastic deformation. Work.

The width T of the chamfered portion 6a is set to 1/2 to 1 of the lead L of the thread groove 1a. The reason for this will be described below. When rolling the screw shaft, at the beginning of the rolling process of the raw material 1, the raw material 1 comes into contact with only one roll die 2 (3) of the pair of roll dies 2 and 3, and plastic deformation starts. Such plastic deformation by the roll dies 2 (3) at the initial stage of rolling continues until the processing of the material 1 progresses by 1/2 of the lead L of the screw shaft, and thereafter contacts the two roll dies 2, 3. Rolling processing of material 1 proceeds. Therefore, when the width T of the chamfered portion of the material 1 is T <1 / 2L, not only the incomplete thread portion 1c but also the regular thread groove 1a is processed by the one roll die 2 (3), which is preferable. Absent. The incomplete thread portion 1c here means the thread groove 1a.
And the part where the screw shaft outer diameter size and shape are different from the regular ones.

The width T of the chamfered portion 6a of the material 1 is T>
In the case of L, the state in which the rolling of one lead of the material 1 is completed by the two roll dies 2 and 3, that is, the incomplete screw portion 1c
Is completed with the two roll dies 2 and 3, and the normal thread groove 1a is obtained with both roll dies 2 and 3 in sufficient contact.
Thus, the effect of alleviating the plastic working stress by the chamfered portion 6a formed on the tip portion 6 of the material 1 is reduced by half.

In the present embodiment, a rolled screw shaft in which a spiral screw groove 1a is formed by rolling on the outer diameter of the columnar material 1 is shown as an example. For example, an electric power steering device for automobile parts is used. In order to reduce the weight of the ball screw, a so-called hollow screw shaft having a through hole at the shaft center of the screw shaft may be used. In the hollow screw shaft, there are a case where a thread groove is formed on the pipe material by the pair of roll dies described above, and a case where the through hole is formed by a boring tool after the thread groove is formed on the solid material. In particular, when thread-forming a thread groove on a pipe material, the spiral thread groove is pre-processed by turning, etc., leaving the rolling allowance, and then the ball-contact portion of the thread groove is formed by rolling. Good to shape.

Generally, the cross-sectional shape of the thread groove 1a is a groove formed of a single circle having a radius of curvature slightly larger than the ball radius,
A Gothic arc groove having two offset centers of curvature is appropriately adopted to contact the ball at a predetermined angle. Here, in the case of a Gothic arc groove, the shape of the groove bottom has a corner where two arcs intersect, so the surface pressure during rolling becomes high, and from the aspect of wear life of the roll die, pre-processing It is preferable to form the thread groove by cutting.

Further, particularly in the case of a hollow screw shaft, since the deformation of the shaft due to the rolling process is larger than that of the solid shaft, the spiral screw groove including the shoulder portion is formed in the pre-process in this way, and the rolling process is performed. If only the contact portion with the ball is formed by the forming process, the pressing force required for the rolling process can be minimized and the deformation of the screw shaft can be suppressed. Further, it is possible to minimize the plastic flow of the material to the shoulder portion and the land portion described above, and it is possible to form a highly accurate thread groove.

Usually, in the ball screw shaft, in order to improve the rolling fatigue life, a hardened layer is formed on the surface of the screw groove in the range of 58 to 63 HRC. For this hardened layer, a method by high-frequency induction heating that can form a desired hardened layer depth at a relatively low cost in a short time is generally adopted, but especially in the rolling screw shaft, processing stress remains inside. The residual stress is released by such quenching treatment, and the deformation such as the bending of the shaft is large. Therefore, various quenching treatments with less deformation have been studied.However, as described above, if the thread groove is formed in advance by pre-processing and then only the ball contact portion is formed by rolling, complicated quenching treatment is performed. There is no need to straighten the screw shaft after hardening.

Next, the state in which the material 1 bites the pair of roll dies 2 and 3 will be described with reference to FIGS.

FIG. 3 shows the case where the chamfer angle φ is 30 ° and the small diameter d1 of the chamfer is set to be larger than the groove bottom diameter d such that d1> d. The width T of the chamfered portion 6a is the lead L of the thread groove 1a.
Is set to 1/2 to 1. In the case of this embodiment, the chamfered portion 6a of the material 1 first comes into contact with the thread 2b of the one roll die 2 and only half of the lead L of the screw shaft is used.
Shows the moment when the plastic working of No. 3 progressed and just contacted the other roll die 3. Roll dies 2, 3 threads 2
After forming the incomplete threaded portion 1c on the chamfered portion 6a from half circumference to the entire circumference by b and 3b, the incomplete threaded portion is formed on the outer diameter of the material 1 as well. Move to. After that, the threads 2a and 3a of the roll dies 2 and 3 are sequentially engaged with the incomplete thread portion 1c of the chamfered portion 6a, and the incomplete thread portion of the outer diameter of the material 1 is formed into the regular thread groove 1a. .

FIG. 4 shows a case where the chamfering angle φ is 30 ° and the small diameter d1 of the chamfered portion is set smaller than the groove bottom diameter d such that d1 <d. The width T of the chamfered portion 6a is set to 1/2 to 1 of the lead L of the thread groove 1a. In the case of this embodiment, the chamfered portion 6a of the blank 1 indicates the moment when the screw thread 3b of one roll die 3 is first contacted. After this,
Similar to FIG. 3 described above, the incomplete thread portion 1c is formed on the chamfered portion 6a by the other threads 2b and 3b of the roll dies 2 and 3, and the incomplete thread portion is also formed on the outer diameter of the material 1. . After that, the threads 2a and 3a of the roll dies 2 and 3 are sequentially engaged with the incomplete thread portion 1c of the chamfered portion 6a, and the incomplete thread portion of the outer diameter of the material 1 is formed into the regular thread groove 1a. . The chamfered portion small diameter d1 is smaller than the groove bottom diameter d and d1 <d
If the setting is made, at the rolling start point, the corners of the chamfered portion 6a do not come into contact with the slopes of the threads 2b and 3b of the roll dies 2 and 3, but the chamfered portion 6a comes into contact with the chamfered portion 6a. The processing stress at the initial stage of manufacturing can be reduced.

FIG. 5 shows a case where the chamfering angle φ is set to 45 ° and the chamfered portion small diameter d1 is set smaller than the groove bottom diameter d so that d1 <d. The width T of the chamfered portion 6a is set to 1/2 to 1 of the lead L of the thread groove 1a. In the case of this embodiment, the chamfered portion 6a of the blank 1 indicates the moment when the thread 3b of the roll die 3 is first contacted. The subsequent molding is the same as in the above-described embodiment, but the chamfering angle φ is 45 °.
And the small diameter d1 of the chamfered portion is set smaller than the groove bottom diameter d such that d1 <d. Therefore, the width dimension of the chamfered portion 6a can be made smaller than the case of 30 °. Similar to the embodiment, at the rolling start point, the chamfered portion 6a
Since the corners of the above do not contact the slopes of the threads 2b and 3b of the roll dies 2 and 3, but the surface of the chamfered portion 6a, the processing stress at the initial stage of rolling can be reduced.

Although the embodiment of the present invention has been described above, the present invention is not limited to the embodiment and is merely an example, and does not depart from the scope of the present invention. Of course, the present invention can be implemented in various forms, and the scope of the present invention is shown by the description of the scope of claims, and the equivalent meanings described in the scope of claims and all within the scope Including changes.

[0032]

As described in detail above, in the rolled ball screw shaft according to the present invention, in the chamfered portion formed at the tip of the ball screw shaft, the width of the chamfered portion is 1 / By setting it to 2 to 1, one of the pair of roll dies starts to form the thread groove and then two roll dies to form the thread groove. The processing stress can be relaxed, the rolling processing accuracy can be improved, and the incomplete threaded portion formed in the chamfered portion can be suppressed to the minimum. Further, by setting the small diameter of the chamfered portion to be equal to or smaller than the groove bottom diameter of the ball screw groove, the processing stress at the initial stage of rolling can be further suppressed.

[Brief description of drawings]

FIG. 1 is a plan view showing a state in which a rolled ball screw shaft according to the present invention is being rolled.

FIG. 2 (a) is a partial enlarged view showing an embodiment of a rolled ball screw shaft according to the present invention. (B) It is a partial enlarged view showing other embodiment of the rolling ball screw shaft concerning the present invention.

FIG. 3 is a schematic view showing a rolled ball screw shaft blank according to the present invention at the start of rolling.

FIG. 4 is a schematic view showing a rolled ball screw shaft blank according to the present invention at the start of rolling.

FIG. 5 is a schematic view showing a rolled ball screw shaft blank according to the present invention at the start of rolling.

FIG. 6 is a schematic view showing an apparatus for rolling a conventional rolled ball screw shaft.

FIG. 7 is a front view showing the shape of a shaft end portion of a conventional rolled ball screw shaft blank.

FIG. 8 is a schematic view showing the start of rolling of a conventional rolled ball screw shaft blank.

[Explanation of symbols]

1 ... Material 1a .... Screw groove 1b --- Land section 1c .... Incomplete thread 2, 3 ... Roll dies 2a, 2b ... 3a, 3b ... 4, 5 ... Bush 6 ... ・ ・ ・ Tip 6a .... Chamfer d ・ ・ ・ ・ ・ ・ Screw groove bottom diameter d1 ... ・ Small diameter of chamfer L ・ ・ ・ ・ ・ ・ Screw groove lead Width of chamfer φ ... Chamfer angle 50 ... Rest 60 ・ ・ ・ ・ Material 61 .... Cylindrical part 62 ... ・ Tapered part 63 ... Chamfer 64 ... 65 ・ ・ ・ ・ ・ ・ Screw groove 66 ... Incomplete thread 70, 80 ... Roll dies A ・ ・ ・ ・ ・ ・ Cylinder outer diameter D ... Material outer diameter W: Length of column

Claims (5)

[Claims] 1. In a rolled ball screw shaft having a spiral ball screw groove formed on the outer periphery by rolling, the width of the chamfered portion of the tip of the ball screw shaft is 1 / the width of the lead of the ball screw groove.
A rolled ball screw shaft characterized by being set to 2-1. 2. The rolled ball screw shaft according to claim 1, wherein the small diameter of the chamfered end of the ball screw shaft is set to be equal to or smaller than the groove bottom diameter of the ball screw groove. 3. The rolled ball screw shaft according to claim 1, wherein the land portion between the ball screw grooves is formed by a rolling surface. 4. The rolled ball screw shaft according to claim 1, wherein a pre-processed cutting groove portion is left on the groove bottom portion of the ball screw groove, and the ball screw groove is a rolling surface. 5. The rolled ball screw shaft according to claim 1, wherein the rolled ball screw shaft is hollow and has a through hole at the axial center thereof.