Multi-row ball screw

A multi-row ball screw includes a nut having a plurality of spiral grooves on an inner peripheral surface thereof, a screw shaft having a plurality of spiral grooves on an outer peripheral surface thereof, and a plurality of balls disposed so as to be rollable on rolling paths constituted by the plurality of spiral grooves of the nut and the plurality of spiral grooves of the screw shaft. The spiral groove of the nut includes a U-shaped first spiral groove having left and right flanks that are respectively able to come into contact with two-row balls and separated from each other in an axial direction, and two second spiral grooves continuous with each row of the first spiral groove and each having left and right flanks that are able to come into two-point contact with the ball.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a National Phase Entry of PCT International Application No. PCT/JP2022/013763 filed on Mar. 23, 2022, which claims priority to Japanese Patent Application No. 2021-062357 filed on Mar. 31, 2021, the contents of all which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a multi-row ball screw, and more particularly, to a multi-row ball screw used in an electric injection molding machine, a press machine, or the like as a mechanical element that converts a rotational motion into a linear motion.

BACKGROUND ART

A ball screw generally includes a screw shaft formed with a spiral groove formed on an outer peripheral surface thereof, a cylindrical nut formed with a spiral groove formed on an inner peripheral surface thereof and mounted on an outer periphery of the screw shaft, and a plurality of balls incorporated between the spiral grooves of the screw shaft and the nut, and converts a rotational motion of one of the screw shaft and the nut to a linear motion of the other one of the screw shaft and the nut. For example, Patent Literature 1 discloses a low-friction and high-rigidity ball screw having a structure in which spiral grooves of a screw shaft and a nut are each formed in a wide U-shape (cross-sectional shape) and two-row balls are disposed in the spiral grooves.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, when accurately processing the U-shaped spiral groove of the nut as described in Patent Literature 1, a high degree of measurement technique is required. In a case of a ball groove having a Gothic shape, which is a general ball groove, a ball having the same diameter as a diameter of a ball to be used is fixed to a measurement terminal, an oblique diameter dimension is measured, and the ball can be processed to a target dimension. However, in the case of the U-shaped spiral groove, since there is no flank on one side, it is difficult to stably fix the ball of the measurement terminal to the spiral groove, and accurate dimension measurement is difficult.

The present invention has been made in view of the problems described above, and an object of the present invention is to provide a multi-row ball screw having a nut with high dimensional accuracy in a U-shaped spiral groove accommodating two-row balls.

Solution to Problem

The above object of the present invention is achieved by the following configuration.

A multi-row ball screw including:a nut having a plurality of spiral grooves on an inner peripheral surface thereof;a screw shaft having a plurality of spiral grooves on an outer peripheral surface thereof; anda plurality of balls disposed so as to be rollable on rolling paths constituted by the plurality of spiral grooves of the nut and the plurality of spiral grooves of the screw shaft,wherein each of the plurality of spiral grooves of the nut includes a U-shaped first spiral groove having left and right flanks that are respectively able to come into contact with two-row balls and separated from each other in an axial direction, and two second spiral grooves continuous with each row of the first spiral groove and each having left and right flanks that are able to come into two-point contact with the ball.

Advantageous Effects of Invention

According to the multi-row ball screw of the present invention, it is possible to accurately measure an oblique diameter dimension by the second spiral groove, and as a result, it is possible to grind the first spiral groove based on the accurate oblique diameter dimension, and it is possible to provide the multi-row ball screw having the nut in which a U-shaped first spiral groove has high dimensional accuracy.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a multi-row ball screw according to an embodiment of the present invention will be described in detail with reference to the drawings.

As shown inFIGS.1and2, a multi-row ball screw1according to the present embodiment includes a nut4having a plurality of spiral grooves40on an inner peripheral surface thereof, a screw shaft3having a plurality of spiral grooves30on an outer peripheral surface thereof, and a plurality of steel balls6disposed so as to be rollable on a rolling path20constituted by the spiral grooves40,30.

For example, seal rings10are fitted to inner peripheries of both end sides of the nut4to prevent leakage of lubricating oil between the nut4and the screw shaft3. The nut4is made of, for example, tough steel such as chromium molybdenum steel.

The nut4includes a circular plate-shaped flange8on one end side (a left side inFIG.1) of a cylindrical nut body4a, and the flange8is provided with bolt through holes8aas necessary. Accordingly, in the multi-row ball screw1, for example, the screw shaft3is connected to a driving unit (a shaft of an electric motor or the like) (not shown) and is rotationally driven in both forward and reverse rotational directions, while the nut4is used by fastening a driven member (a driven member such as an electric injection molding machine, a press machine, or a conveying machine) (not shown) to the flange8via the bolt through hole8a.

On an outer peripheral surface of the nut body4a, a plurality of flat surfaces9(only one is shown inFIG.1) each having a rectangular shape in a plan view are formed in a peripheral direction, and a pair of return tubes5are fastened to each of the flat surfaces9by a tube clamp7. Each of the return tubes5is formed by bending a steel pipe into a substantially U-shape, and both end portions of the tubes penetrate four through holes (through holes connected to a first spiral groove41) formed in the flat surface9and are connected to the rolling path20. Accordingly, two ball circulation paths (two circuits) are formed in which a large number of balls6move and circulate in the rolling path20via the respective return tubes5.

Referring also toFIGS.3to5, the spiral groove40of the nut4has a U-shaped first spiral groove41having left and right flanks41cthat are respectively able to come into contact with the two-row balls6and separated from each other in an axial direction, and two second spiral grooves42continuous with each row of the first spiral groove41and each having left and right flanks42cthat are able to come into two-point contact with the ball6. The terms “left” and “right” mean both sides along a central axis CL inFIG.2.

That is, the first spiral groove41has a cylindrical groove bottom41bbetween the left and right flanks41cbetween the adjacent ridge portions43, and is formed in a U shape that is wide in a left-right direction. Accordingly, in the first spiral groove41, the left flank41cand the right flank41crespectively support the two rows of balls6of the two circuits.

On the other hand, the second spiral groove42has left and right flanks42chaving substantially the same shape as that of the left and right flanks41cof the first spiral groove, and the left and right flanks42care formed in a Gothic arc shape capable of being in two-point contact with the ball6.

In the present embodiment, since the second spiral groove42is continuous with the first spiral groove41on an axially outer side of a ball scoop-up portion of a ball circulation path, the actual ball6does not circulate through the left and right flanks42c, but as will be described later, the same spherical fixture as the ball6or a measurement terminal having a spherical portion of the same size as the ball6is brought into contact with the left and right flanks42cat two points.

The first spiral groove41has a shape obtained by cutting off an intermediate ridge portion44(shaded portion inFIG.5) formed between the second spiral grooves42, and the first spiral groove41is formed by cutting off the intermediate ridge portion44after or at the same time as the formation of the second spiral groove42.

The spiral groove30of the screw shaft3is also configured as a wide U-shaped groove in which two balls6can be accommodated side by side in a direction of the central axis CL, and has substantially the same cross-sectional shape as the first spiral groove41of the nut4. Accordingly, the balls6in the spiral groove30are supported by flanks30cformed on both left and right sides. As the constituent material of the screw shaft3, for example, tough steel such as chromium molybdenum steel can be used.

The spiral groove30of the screw shaft3and the first spiral groove41of the nut4constitute the rolling path20such that the ridge portion31of the screw shaft3faces an axially intermediate portion of the first spiral groove41and the ridge portion43of the nut4faces an axially intermediate portion of the spiral groove30. Accordingly, the balls6are held between the flank30cof the screw shaft3and the flank41cof the nut4. Accordingly, the plurality of balls6incorporated between the spiral groove30and the first spiral groove41roll in the rolling path20as the screw shaft3rotates relative to the nut4, and the plurality of balls6roll in the rolling path20as the screw shaft3rotates, thereby smoothly moving the nut4in the axial direction.

A plurality of (four in the present embodiment) two-row rolling paths20are formed, and the multi-row ball screw1having the multi-row rolling paths20of the present embodiment (even rows which are at least two rows) is thereby configured.

Here, when forming the spiral groove40in the nut4, the first spiral groove41and the second spiral groove42are cut on the inner peripheral surface of the nut body4aby a lathe, and then heat treatment (quenching) is performed using a carburizing heat treatment device or a high-frequency heat treatment device.

Thereafter, processing for forming the first spiral groove41to a predetermined dimension is performed. The processing is performed by turning, but grinding may be performed after turning. At this time, dimension measurement for determining a processing amount is performed, a cutting amount and a grinding amount are calculated from a dimension measurement result, and turning and grinding are performed.

In the dimension measurement, as shown inFIG.3, an oblique diameter dimension D of the second spiral groove42(a dimension between grooves facing each other at a phase of 180° in a state where the groove advances by 0.5 lead) is measured using the measurement terminal. Therefore, at least one of the second spiral grooves42remains 0.5 leads (indicated by a dimension L inFIG.3) or more.

In addition, the oblique diameter dimension D of a portion of the second spiral groove42is the same dimension as an oblique diameter dimension of the first spiral groove41. That is, since the first spiral groove41has a shape obtained by cutting off the intermediate ridge portion44between the two second spiral grooves42, the oblique diameter dimension D between the grooves of the second spiral groove42represents the oblique diameter dimension of the first spiral groove41.

Further, at the time of measurement, the ball-shaped fixture or the measurement terminal is stably held by the left and right flanks42cof the second spiral groove42, so that the oblique diameter dimension D can be accurately measured.

Since the two spiral grooves are disposed at a phase of 180°, the other second spiral groove42is formed at an opposite side to the one second spiral groove42by 180° at the two same positions in the axial direction. Therefore, it is sufficient that there is a groove short enough to set a measurement instrument and the measurement terminal, and a groove length L1of the second spiral groove42is equal to or larger than a diameter of the ball.

As a result of the dimension measurement, the grinding amount necessary for a finished dimension is determined, and the processing is performed as shown inFIGS.6and7. A quill50used for grinding has a pair of grinding stones51matching the shape of the flank41cof the first spiral groove41, and is set to be inclined along a lead angle θ of the first spiral groove41with respect to the central axis CL, and by rotating the grinding stone51at a high speed, the flanks41cof the adjacent first spiral grooves41and the flanks42cof the adjacent second spiral grooves42, which are on both left and right sides with respect to the ridge portion43, are machined.

By performing processing while measuring the oblique diameter dimension D of the second spiral groove42in this manner, it is possible to form the nut4having the first spiral groove41with high dimensional accuracy.

The present invention is not limited to the embodiment described above, and modifications, improvements, and the like can be appropriately made.

For example, in the embodiment described above, the second spiral groove42for measuring the oblique diameter dimension may be formed in the screw shaft3. Accordingly, by accurately machining the oblique diameter dimension of the screw shaft3, it is possible to form the screw shaft3having the spiral groove30with high dimensional accuracy.

As described above, the following matters are disclosed in the present specification.

(1) A multi-row ball screw including:a nut having a plurality of spiral grooves on an inner peripheral surface thereof;a screw shaft having a plurality of spiral grooves on an outer peripheral surface thereof; anda plurality of balls disposed so as to be rollable on rolling paths constituted by the plurality of spiral grooves of the nut and the plurality of spiral grooves of the screw shaft,wherein each of the plurality of spiral grooves of the nut includes a U-shaped first spiral groove having left and right flanks that are respectively able to come into contact with two-row balls and separated from each other in an axial direction, and two second spiral grooves continuous with each row of the first spiral groove and each having left and right flanks that are able to come into two-point contact with the ball.

According to this configuration, it is possible to accurately measure an oblique diameter dimension by the second spiral groove, and as a result, it is possible to process the first spiral groove based on the accurate oblique diameter dimension, and it is possible to provide the multi-row ball screw having the nut in which a U-shaped spiral groove has high dimensional accuracy.

(2) The multi-row ball screw according to (1),wherein second spiral groove is formed by 0.5 leads or more.

According to this configuration, in the second spiral groove, the oblique diameter dimension of the second spiral groove can be accurately measured.

(3) The multi-row ball screw according to (1) or (2),wherein a groove length of the second spiral groove is equal to or larger than a diameter of the ball.

According to this configuration, a measurement terminal can be set, and the oblique diameter dimension can be measured in the second spiral groove.

The present application is based on a Japanese patent application (Japanese Patent Application No. 2021-062357) filed on Mar. 31, 2021, and the contents thereof are incorporated herein by reference.