JP2003183735A - Method and apparatus for heat-treating ball screw - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for heat-treating a ball screw, which can harden the surface of a formed groove portion of a nut, by induction hardening. <P>SOLUTION: In a process of heat-treating the nut 2 which forms the ball screw, this apparatus is characterized by arranging the first coil 11 for low-frequency heating, in the outer diameter side of the nut 2, and the second coil 12 for high-frequency heating in the interior diameter side of the nut 2. Then, the heat treatment method comprises, at first, passing a low-frequency current to the first coil 11, to heat the nut 2 from the outer diameter side, leaving it for a predetermined time after heating by the first coil 11, passing a high-frequency current to the second coil 12 arranged in the interior diameter side of the nut 2, to rapidly heat the surface of the formed groove portion, stopping the heating by the second coil 12 after heating the surface of the formed groove portion up to a quenching temperature, and rapidly cooling the nut 2 with cooling water. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method and a heat treatment apparatus for a ball screw for hardening the surface of an inner diameter side thread groove forming portion of a nut constituting a ball screw.

[0002]

2. Description of the Related Art In machine tools, a ball screw is used for a slide driving portion. This ball screw has a mechanism in which a large number of balls are interposed between a screw shaft and a nut, and the balls circulate while rolling in a screw groove to transmit the motion. The sliding friction of the screw is replaced by rolling friction. It is a thing.

The structure of this ball screw will be described with reference to FIG. FIG. 4 is a view partially showing an example of the structure of the tube type ball screw.

As shown in FIG. 4, the ball screw has a screw shaft 1 and a nut 2, and the screw shaft 1 and the nut 2 are formed with a thread groove for receiving the ball 3. A large number of balls 3 are filled between the screw shaft 1 and the nut 2 via respective screw grooves. When the screw shaft 1 is rotated, the ball 3 is caught between the screw shaft 1 and the nut 2, and the ball 3 advances while rolling between them along the screw groove.
It circulates via the return tube 4.

Since such a ball screw is required to have high durability, surface hardening is performed by heat treatment on the thread groove forming portions of the ball screw shaft and the net.

As a surface hardening method for this screw shaft, there is a method in which a shaft subjected to thread groove processing is manufactured from a bar material of low carbon alloy steel, and this shaft is subjected to carburizing heat treatment. That is, according to this method, the low carbon alloy steel has a small amount of carbon (less than 0.2% by weight), and the hardness required for rolling devices such as bearings and ball screws cannot be obtained even by simply quenching. In this method, the carbon content of the surface layer is increased by carburizing and quenching is performed to obtain a hardness that can be used as a rolling device even when a low carbon alloy steel is used as a material.

Further, in the case of using a medium carbon alloy steel which can obtain a sufficient hardness only by quenching and tempering without carburizing,
There is a method in which a shaft having a thread groove is manufactured from a rod of medium carbon alloy steel, induction hardening is performed on the shaft, and then tempering is performed to perform surface hardening.

On the other hand, in the case of the nut of the ball screw, the outer shape is cylindrical and the thread groove is formed in a spiral shape on the inner diameter side thereof. After being machined, it is heat-treated and ground to be finished as a part. Here, as the material, for example, chromium steel having a carbon content of about 0.15% by weight (JIS SC
R415), chrome molybdenum steel (SCM415)
Low carbon alloy steels such as those mentioned above are used, and carburizing and quenching is usually performed as a heat treatment method with emphasis on the wear resistance of the rolling surface of the nut. As a result, the amount of carbon in the surface layer increases and the required hardness can be obtained by quenching.

[0009]

However, when the surface hardening of the nut of the ball screw is carried out by carburizing and quenching, the strain accompanying the heat treatment becomes large, and when the thread groove spirally formed on the inner diameter side is ground and finished. Since the machining allowance of becomes large, the machining efficiency becomes poor.

Further, it is possible to harden the surface of the thread groove forming portion of the nut by induction hardening, but in this case, since the thread groove is formed on the inner diameter side of the nut,
It is necessary to insert the coil in the nut. However, when the inner diameter of the nut is small, a sufficient heating capacity cannot be obtained from the coil inserted in the nut, the heating time becomes long, and the quenching layer cannot be formed on the surface. Also, the durability of the coil itself is poor. Under such circumstances, the method of hardening the surface of the thread groove forming portion of the nut by induction hardening has not been put into practical use.

The present invention has been made to solve the above problems, and provides a heat treatment method and a heat treatment apparatus for a ball screw, which can perform surface hardening of a thread groove forming portion of a nut by induction hardening. With the goal.

[0012]

DISCLOSURE OF THE INVENTION The present invention is a method for heat treating a ball screw for hardening the surface of an inner diameter side thread groove forming portion of a nut constituting a ball screw, the method being arranged on the outer diameter side of the nut. Low-frequency current is passed through the first coil
A step of heating the nut from its outer diameter side and a high frequency current is passed through a second coil arranged on the inner diameter side of the nut after a lapse of a predetermined time so that the temperature of the thread groove forming portion of the nut becomes the quenching temperature. It is characterized by including a step of heating the nut from the inner diameter side until reaching the temperature and a step of cooling and quenching the nut after heating from the inner diameter side of the nut is completed.

Further, the present invention is a heat treatment apparatus for a ball screw, comprising a first coil arranged on the outer circumference of a nut constituting the ball screw and a second coil arranged on the inner circumference of the nut. , And control means for independently supplying an alternating current having a corresponding frequency to the first coil and the second coil, respectively.

In the heat treatment method for the ball screw of the present invention, first, a low frequency current is passed through the first coil arranged on the outer diameter side of the nut. As a result, an eddy current is induced near the outer diameter side surface of the nut, and the nut is heated from the outer diameter side for a predetermined time. Then, after a lapse of a predetermined time, that is, after the entire nut is heated by the first coil, a high-frequency current is passed through the second coil arranged on the inner diameter side of the nut, and the surface of the thread groove forming portion of the nut An eddy current is induced in the nut to heat the nut from the inner diameter side. Here, since the high frequency current is passed through the second coil, the heating portion of the thread groove forming portion is only the surface layer, and the entire nut is heated. The surface layer of is heated to the quenching temperature in a very short time. When the surface layer of the thread groove forming portion is heated to the quenching temperature, the nut is cooled (quenched).

By the above series of treatments, that is, induction hardening,
In the thread groove forming portion on the inner diameter side of the nut, a uniform surface hardened layer can be formed along the contour of the thread groove forming portion, and excellent durability can be imparted to the nut. Therefore, by using this nut, a ball screw having excellent durability can be constructed. Further, since the heating time by the second coil is short, the problem of coil life is eliminated even in the case of a small diameter nut. The tempering performed after quenching may be the same as the conventional one.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram schematically showing a main part of an apparatus for realizing a heat treatment method for a ball screw according to an embodiment of the present invention, and FIG. 2 is a thread groove forming part of a nut by the apparatus of FIG. FIG. 3A is a diagram showing a heat pattern for an inner diameter side surface layer portion including, FIG. 3A is a diagram schematically showing a surface hardened layer formed on a thread groove forming portion of a nut by heat treatment of the apparatus of FIG.
FIG. 3B is a diagram showing a state of stress distribution under load in the surface hardened layer formed in the thread groove forming portion of the nut by the heat treatment of the apparatus of FIG.

In the ball screw, surface hardening is performed on each of the thread groove forming portions of the ball screw shaft and the net by heat treatment. Regarding the production of this screw shaft, as in the conventional method, a rod material of a low carbon alloy steel having a low carbon content (less than 0.2% by weight) is thread grooved, and the processed product is carburized to reduce the carbon content of the surface layer. Increase and quench. As a result, the surface of the screw shaft is obtained. Also, when using medium carbon alloy steel that can obtain sufficient hardness only by quenching without carburizing, manufacture a shaft with thread groove processing from the rod of this medium carbon alloy steel and By subjecting the steel to induction hardening, a surface-hardened screw shaft can be obtained.

For the production of the nut, medium carbon alloy steel is used as a raw material. Here, the medium carbon alloy steel used has a hardness of HRC6 as the hardness of the hardened layer after quenching and tempering.
Any material that can obtain a hardness of 0 or more may be used. For example, chrome steel, chrome molybdenum steel, or other alloys can be used. Considering machinability, the carbon content is preferably 0.2 to 0.9% by weight (more preferably 0.4 to 0.7% by weight). Then, the above-mentioned medium carbon alloy steel is machined to produce a nut having a cylindrical outer shape and a thread groove spirally formed on the inner diameter side thereof. This nut is transferred to a heat treatment process for hardening the surface of the thread groove forming portion.

In this heat treatment step, as shown in FIG. 1, the low-frequency heating first coil 11 is attached to the outer diameter side of the nut 2.
And the second coil 12 for high frequency heating is arranged on the inner diameter side of the nut 2. The second coil 12 is a nut 2
Of the spiral groove having a pitch equal to the lead of the screw groove 2a of the nut 2 so as to face the screw groove 2a.
Is positioned within. First coil 11 and second coil 12
Is connected to a control device (not shown), and this control device controls to supply a low frequency current to the first coil 11 and a high frequency current to the second coil 12 independently. This control device is configured to perform switching control of current supply to the first coil 11 and the second coil 12 using, for example, one variable frequency current source and a switch. This switching control is executed according to a heat pattern shown in FIG. 2 described later. Further, instead of the variable frequency current source, a dedicated current source may be provided individually for each coil 11, 12.

In this heat treatment step, specifically,
As shown in FIG. 2, first, a low-frequency current is passed through the first coil 11, and an eddy current is induced near the outer diameter side surface of the nut 2. As a result, the nut 2 is heated from the outer diameter side. Then, when the entire nut 2 is heated to the predetermined temperature Tw by the first coil 11 (time t1), the supply of the low frequency current to the first coil 11 is stopped, and the nut 2 is kept for a predetermined period (time t1 to t2). Period) is naturally cooled.

Next, the second nut arranged on the inner diameter side of the nut 2
A high frequency current is passed through the coil 12 (at time t2). As a result, eddy current concentrates on the surface of the inner diameter side portion of the nut 2 where the thread groove 2a is formed (hereinafter referred to as thread groove forming section), so that the surface layer of the thread groove forming section is rapidly heated. During this period, the part of the nut 2 that is not heated by the second coil 12 (that is, the part other than the part that becomes the surface hardened layer) is being naturally cooled. Here, although the temperature is somewhat lowered by natural cooling, the entire nut 2 is already heated, so the second coil 12
As a result, the surface layer temperature of the thread groove forming portion reaches the quenching temperature Tw in a very short time (at time t3). This quenching temperature Tw
Is appropriately selected depending on the type of steel (carbon content, etc.).

When the surface layer of the thread groove forming portion is heated to the quenching temperature, the supply of the high frequency current to the second coil 12 is stopped and the heating by the second coil 12 is stopped. Then, the nut 2 is rapidly cooled with cooling water. After that, tempering is performed by an appropriate method. For the heating for tempering, either the first coil 11 or the second coil 12 may be used, or another one may be used. As a result, as shown in FIG. 3A, the thread groove 2a is formed in the thread groove forming portion.
The hardened surface layer 2b having a substantially uniform depth is formed along the contour of the. In this embodiment, about HRC (Rockwell C
(Scale hardness) 60 surface hardened layer 2b having a substantially uniform depth
Is formed. In addition, the temper hardness (HRC 20 to 40) is secured in the other parts.

In heating the second coil 12,
Since the eddy current concentrates on the surface layer as the frequency increases, it is possible to control the depth of the surface hardened layer 2b by controlling the frequency of the high frequency current. For example, when forming a shallow hardened layer on a small material. The frequency of the high frequency current may be set higher. In the present embodiment, for example, as shown in FIG. 3B, when the thread groove 2a is a groove of a Gothic arc, shear that occurs when a load is applied in the normal direction in the vicinity of the contact position P with the ball 3 The depth of the surface hardened layer 2b to be formed is determined according to the distribution state of the force σ. Here, the depth of the surface hardened layer 2b is the maximum shearing force σ.
The depth is preferably determined to include the position of max, and the frequency of the high frequency current, the heating pattern, etc. are determined so as to satisfy this condition. However, if the required performance (wear resistance, durability, etc.) is obtained, it is not necessary to limit the depth to such a value,
It may be set appropriately.

A tube type ball screw (shown in FIG. 4) is obtained by combining the nut 2 thus obtained with the above-mentioned carburized and quenched screw shaft. The screw shaft may be a combination of induction hardened medium carbon alloy steel.

As described above, according to this embodiment, the second
By induction hardening using the coil 12, a uniform surface hardened layer 2b can be formed along the contour of the thread groove forming portion on the inner diameter side of the nut 2, and the nut 2 can have excellent durability. . Therefore, by using this nut 2, a ball screw having excellent durability can be constructed. Further, since the heating time by the second coil 12 is short, the problem of coil life is eliminated even in the case of a small diameter nut.

Needless to say, the principle of the present invention can be applied to other types of ball screws such as a top type ball screw.

[0028]

As described above, according to the present invention,
A low-frequency current is applied to the first coil arranged on the outer diameter side of the nut to heat the nut from the outer diameter side, and after a predetermined time has passed, a high frequency wave is applied to the second coil arranged on the inner diameter side of the nut. Applying an electric current, heating the nut from the inner diameter side until the temperature of the thread groove forming part of the nut reaches the quenching temperature,
After the heating from the inner diameter side of the nut is completed, the nut is cooled and hardened, so by induction hardening with the second coil, a uniform surface hardened layer is formed in the thread groove forming portion on the inner diameter side of the nut along the contour thereof. It is possible to give the nut excellent durability. Therefore, by using this nut, a ball screw having excellent durability can be constructed. Further, since the heating time by the second coil is short, it is possible to prevent the life of the coil from being shortened even in the case of a small diameter nut.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a main part of an apparatus for realizing a heat treatment method for a ball screw according to an embodiment of the present invention.

FIG. 2 is a diagram showing a heat pattern for an inner diameter side surface layer portion including a thread groove forming portion of a nut by the apparatus of FIG.

3 (a) is a diagram schematically showing a surface hardened layer formed on a thread groove forming portion of a nut by heat treatment of the apparatus of FIG. 1,
FIG. 2B is a diagram showing a stress distribution state under load in the surface hardened layer formed in the thread groove forming portion of the nut by the heat treatment of the apparatus of FIG. 1.

FIG. 4 is a diagram partially showing an example of the structure of a tube type ball screw.

[Explanation of symbols]

2 nuts 2a thread groove 2b Surface hardened layer Three balls 11 first coil 12 Second coil

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Claims (2)

[Claims] 1. A method of heat treating a ball screw for hardening the surface of an inner diameter side thread groove forming part of a nut constituting a ball screw, wherein the first coil arranged on the outer diameter side of the nut has a low temperature. A step of applying a high-frequency current to heat the nut from the outer diameter side, and a high-frequency current is applied to the second coil arranged on the inner diameter side of the nut after a predetermined time elapses, so that the thread groove forming portion of the nut is formed. A ball screw characterized by comprising the step of heating the nut from the inner diameter side thereof until the temperature reaches the quenching temperature, and the step of cooling and quenching the nut after the heating from the inner diameter side of the nut is completed. Heat treatment method. 2. A first coil arranged on the outer circumference of a nut constituting a ball screw, a second coil arranged on the inner circumference of the nut, the first coil and the second coil. And a control means for independently supplying an alternating current having a corresponding frequency to the heat treatment apparatus for a ball screw.