Multi-start ball screw

Abstract

A multi-row ball screw (1) is provided with: a nut (4) having a multi-row spiral groove (40) on the inner peripheral surface; a screw shaft (3) having a multi-row spiral groove (30) on the outer peripheral surface; and a plurality of balls (6) disposed so as to be able to roll on a rolling path (20) constituted by the multi-row spiral groove (40) of the nut (4) and the multi-row spiral groove (30) of the screw shaft (3). The spiral groove (40) of the nut (4) comprises: U-shaped first spiral grooves (41) having left / right flanks (41c) separated in the axial direction allowing for contact with each of two-row balls (6), and second two-row spiral grooves (42) having left / right flanks continuous with each row of the first spiral grooves (41) and allowing for two-point contact with the balls (6). Obtained thereby is a multi-row ball screw having a nut that has high dimensional accuracy in U-shaped spiral grooves for accommodating two-row balls.

Description

[Technical Field]

[0001] The present invention relates to a multi-start ball screw, and more particularly to a multi-start ball screw used in electric injection molding machines, press machines, and the like as a mechanical element that converts rotational motion into linear motion. [Background technology]

[0002] A ball screw generally has a screw shaft with a spiral groove formed on its outer circumferential surface, a cylindrical nut with a spiral groove formed on its inner circumferential surface and attached to the outer periphery of the screw shaft, and multiple balls embedded between the spiral grooves, and converts the rotational motion of either the screw shaft or the nut into the linear motion of the other. For example, Patent Document 1 discloses a low-friction, high-rigidity ball screw in which the spiral grooves of the screw shaft and the nut are configured to be wide and U-shaped (cross-sectional shape), and two balls are arranged in these spiral grooves. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 56-147954 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, advanced measurement technology is required to accurately machine a U-shaped spiral groove in a nut, as described in Patent Document 1. In the case of a Gothic-shaped ball groove, which is a common ball groove, a ball with the same diameter as the ball to be used is fixed to a measuring terminal, the inclined diameter dimension is measured, and the groove can be machined to the target dimension. However, in the case of a U-shaped spiral groove, there is no flank on one side, making it difficult to stably fix the ball of the measuring terminal to the spiral groove, and accurate dimensional measurement has been difficult.

[0005] The present invention has been made in view of the above-mentioned problems, and its object is to provide a multiple-start ball screw having a nut with a U-shaped spiral groove that houses two balls and has high dimensional accuracy. [Means for solving the problem]

[0006] The above object of the present invention can be achieved by the following configuration. a nut having multiple spiral grooves on its inner circumferential surface; a screw shaft having multiple spiral grooves on its outer circumferential surface; a plurality of balls rollably disposed in a rolling path formed by the multiple spiral grooves of the nut and the multiple spiral grooves of the screw shaft; A multi-start ball screw comprising: The multi-start ball screw has a multi-start helical groove of the nut that includes a U-shaped first helical groove having left and right flanks spaced apart in the axial direction and capable of contacting two balls, respectively, and a two-start second helical groove that is continuous with the first helical groove and has left and right flanks that can make two-point contact with the balls. [Effects of the Invention]

[0007] According to the multiple-start ball screw of the present invention, the second spiral groove enables accurate measurement of the inclined diameter dimension, and as a result, the first spiral groove can be ground based on the accurate inclined diameter dimension, making it possible to provide a multiple-start ball screw having a nut with a U-shaped first spiral groove having high dimensional accuracy. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a perspective view of the appearance of a multiple-start ball screw according to an embodiment of the present invention; [Figure 2] FIG. 2 is a cross-sectional view of the multi-start ball screw of FIG. 1. [Figure 3] FIG. 4 is a cross-sectional view of a main part showing a spiral groove of a nut. [Figure 4] FIG. 4 is an enlarged cross-sectional view of a first spiral groove shown in FIG. 3. [Figure 5] FIG. 4 is an enlarged cross-sectional view of the second spiral groove shown in FIG. 3. [Figure 6]FIG. 4 is a schematic cross-sectional view showing the state of grinding of the nut. [Figure 7] FIG. 10 is a schematic front view showing the state of grinding the nut. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, a multi-start ball screw according to an embodiment of the present invention will be described in detail with reference to the drawings. As shown in Figures 1 and 2, the multi-thread ball screw 1 of this embodiment comprises a nut 4 having multiple spiral grooves 40 on its inner surface, a screw shaft 3 having multiple spiral grooves 30 on its outer surface, and a plurality of steel balls 6 that are rollably arranged in a rolling path 20 formed by both spiral grooves 40, 30.

[0010] In addition, for example, seal rings 10 are fitted on the inner periphery of both ends of the nut 4 to prevent leakage of lubricating oil between the nut 4 and the screw shaft 3. The nut 4 is made of a tough steel such as chrome molybdenum steel.

[0011] The nut 4 has a disk-shaped flange 8 on one end side (left side in FIG. 1) of the cylindrical nut body 4a, and the flange 8 is provided with a bolt through hole 8a as needed. Therefore, in use, the multiple-start ball screw 1 is, for example, connected to a driving means (such as the shaft of an electric motor) not shown in the figures and rotated in both forward and reverse directions, while the nut 4 is fastened to the flange 8 via the bolt through hole 8a to a driven member (such as an electric injection molding machine, press machine, or conveying machine) not shown in the figures.

[0012] On the outer peripheral surface of the nut body 4a, flat surfaces 9 that are rectangular in plan view are formed at multiple locations in the circumferential direction (only one location is shown in FIG. 1), and a pair of return tubes 5 are fastened to each of these flat surfaces 9 with tube clamps 7. The return tubes 5 are made by bending a steel pipe into a roughly U-shape, and both ends of the tube pass through four through holes (through holes connected to the first spiral groove 41) formed in the flat surfaces 9, connecting them to the rolling paths 20. This forms two ball circulation paths (two circuits) through which a large number of balls 6 move and circulate within the rolling paths 20 via each return tube 5.

[0013] 3 to 5, the spiral groove 40 of the nut 4 has a U-shaped first spiral groove 41 having left and right flanks 41c spaced apart in the axial direction and capable of contacting two balls 6, respectively, and two second spiral grooves 42 that are continuous with the first spiral groove 41 and have left and right flanks 42c that can come into two-point contact with the ball 6. Note that left and right mean both sides in the direction along the central axis CL in FIG. 2.

[0014] That is, the first spiral groove 41 is formed in a U-shape that is wide in the left-right direction, with a cylindrical groove bottom 41b between left and right flanks 41c between adjacent protrusions 43. Therefore, in the first spiral groove 41, the left flank 41c and the right flank 41c support the two rows of balls 6 of the two circuits, respectively.

[0015] On the other hand, the second spiral groove 42 has left and right flanks 42c having substantially the same shape as the left and right flanks 41c of the first spiral groove, and the left and right flanks 42c are formed in a Gothic arc shape that can come into two-point contact with the ball 6.

[0016] In this embodiment, the second spiral groove 42 is continuous with the first spiral groove 41 axially outside the ball scooping portion of the ball circulation path, so the actual ball 6 does not circulate on the left and right flanks 42c, but as described below, a spherical jig the same size as the ball 6 or a measurement terminal having a spherical portion the same size as the ball 6 is brought into two-point contact.

[0017] In addition, the first spiral groove 41 has a shape in which the intermediate protrusion portion 44 (shaded portion in Figure 5) formed between the second spiral grooves 42 is cut away, and the first spiral groove 41 is formed by cutting away the intermediate protrusion portion 44 after or at the same time as forming the second spiral groove 42.

[0018] The spiral groove 30 of the screw shaft 3 is also configured as a wide U-shaped groove that can accommodate two balls 6 lined up in the direction of the central axis CL, and has a cross-sectional shape that is approximately the same as that of the first spiral groove 41 of the nut 4. Therefore, the ball 6 in the spiral groove 30 is supported by flanks 30c formed on both the left and right sides. The screw shaft 3 can be made of a tough steel such as chrome-molybdenum steel, for example.

[0019] The spiral groove 30 of the screw shaft 3 and the first spiral groove 41 of the nut 4 form a rolling path 20, with the protrusion 31 of the screw shaft 3 facing an axially intermediate portion of the first spiral groove 41 and the protrusion 43 of the nut 4 facing an axially intermediate portion of the spiral groove 30. As a result, the ball 6 is sandwiched between the flank 30c of the screw shaft 3 and the flank 41c of the nut 4. Therefore, the multiple balls 6 incorporated between the spiral groove 30 and the first spiral groove 41 roll within the rolling path 20 as the screw shaft 3 rotates relative to the nut 4, and by rotating the screw shaft 3, the multiple balls 6 roll within the rolling path 20, causing the nut 4 to move smoothly in the axial direction.

[0020] In addition, multiple two-thread rolling paths 20 (four in this embodiment) are formed, thereby forming a multi-thread ball screw 1 having multiple rolling paths 20 (at least two, an even number of threads) in this embodiment.

[0021] Here, when forming the spiral groove 40 in the nut 4, the first spiral groove 41 and the second spiral groove 42 are cut on the inner surface of the nut body 4a using a lathe, and then heat treatment (hardening) is performed using a carburizing heat treatment device or a high-frequency heat treatment device. Thereafter, processing is performed to form the first spiral groove 41 to a predetermined dimension. Processing is performed by turning, but grinding may be performed after turning. At this time, dimensional measurements are performed to determine the amount of processing, and the cutting amount and grinding amount are calculated from the dimensional measurement results, and then the turning and grinding are performed.

[0022] In the dimension measurement, a measuring probe is used to measure the inclined diameter dimension D of the second spiral groove 42 (the dimension between opposing grooves at a phase of 180° when the grooves are advanced by 0.5 leads) as shown in Fig. 3. For this reason, at least one of the second spiral grooves 42 is left with a lead of 0.5 or more (shown as dimension L in Fig. 2). The inclined diameter dimension D of the second spiral groove 42 is the same as the inclined diameter dimension of the first spiral groove 41. That is, since the first spiral groove 41 has a shape in which the intermediate protrusion portion 44 between the two second spiral grooves 42 is removed, the inclined diameter dimension D between the grooves of the second spiral groove 42 represents the inclined diameter dimension of the first spiral groove 41. During measurement, the ball-shaped jig or measuring terminal is stably held by the left and right flanks 42c of the second spiral groove 42, so that the inclined diameter dimension D can be measured accurately.

[0023] The two spiral grooves are arranged 180° out of phase with each other, so that at the same axial position, one second spiral groove 42 is formed 180° away from the other second spiral groove 42. For this reason, it is sufficient that the groove is short enough to allow the measurement jig or measurement terminal to be set, and the groove length L1 of the second spiral groove 42 is sufficient as long as it is equal to or greater than the diameter of the ball.

[0024] As a result of the above dimensional measurements, the amount of grinding required for the finished dimensions is determined, and processing is carried out as shown in Figures 6 and 7. The quill 50 used for grinding has a pair of grinding stones 51 that match the shape of the flanks 41c of the first spiral groove 41, and is set at an angle relative to the central axis CL along the lead angle θ of the first spiral groove 41. By rotating these grinding stones 51 at high speed, each flank 41c of the adjacent first spiral groove 41, which is on both the left and right sides of the protrusion 43, and the left and right flanks 42c of the adjacent second spiral groove 42 are processed. In this way, by machining while measuring the inclined diameter dimension D of the second spiral groove 42, it is possible to form the nut 4 having the first spiral groove 41 with high dimensional accuracy.

[0025] The present invention is not limited to the above-described embodiments, and modifications and improvements are possible as appropriate.

[0026] For example, in each of the above-described embodiments, the second spiral groove 42 for measuring the inclined diameter dimension may be formed on the screw shaft 3. This makes it possible to form the screw shaft 3 having the spiral groove 30 with high dimensional accuracy by machining the inclined diameter dimension of the screw shaft 3 with high precision.

[0027] As described above, the present specification discloses the following: (1) A nut having multiple spiral grooves on its inner circumferential surface; a screw shaft having multiple spiral grooves on its outer circumferential surface; a plurality of balls rollably disposed in a rolling path formed by the multiple spiral grooves of the nut and the multiple spiral grooves of the screw shaft; A multi-start ball screw comprising: The multi-start ball screw has a multi-start helical groove of the nut that includes a U-shaped first helical groove having left and right flanks spaced apart in the axial direction and capable of contacting two balls, respectively, and a two-start second helical groove that is continuous with the first helical groove and has left and right flanks that can make two-point contact with the balls. According to this configuration, the second spiral groove enables accurate measurement of the inclined diameter dimension, and as a result, the first spiral groove can be machined based on the accurate inclined diameter dimension, making it possible to provide a multi-start ball screw having a nut with a U-shaped spiral groove having high dimensional accuracy.

[0028] (2) The multi-start ball screw according to (1), wherein the second spiral groove is formed with a lead of 0.5 or more. According to this configuration, the inclined diameter of the second spiral groove can be accurately measured.

[0029] (3) The multi-start ball screw according to (1) or (2), wherein the groove length of the second spiral groove is equal to or greater than the diameter of the ball. According to this configuration, the measurement terminal can be set and the inclined diameter dimension can be measured in the second spiral groove.

[0030] This application is based on a Japanese patent application (Patent Application No. 2021-062357) filed on March 31, 2021, the contents of which are incorporated by reference into this application. [Explanation of symbols]

[0031] 1 Multi-start ball screw 3 Screw shaft 4 nuts 20 Rolling Path 30, 40 spiral groove 41 1st spiral groove 42 2nd spiral groove CL center axis D Diameter

Claims

1. a nut having multiple spiral grooves on its inner circumferential surface; a screw shaft having multiple spiral grooves on its outer circumferential surface; a plurality of balls rollably disposed in a rolling path formed by the multiple spiral grooves of the nut and the multiple spiral grooves of the screw shaft; A multi-start ball screw comprising: The multiple spiral grooves of the nut include a U-shaped first spiral groove having left and right flanks spaced apart in the axial direction and capable of contacting two balls, respectively, and two second spiral grooves that are continuous with the first spiral groove and have left and right flanks capable of two-point contact with the balls, a multiple-start ball screw, wherein the second spiral groove is continuous with the first spiral groove axially outside a ball scooping portion of the ball circulation path;

2. 2. The multi-start ball screw according to claim 1, wherein the second spiral groove is formed with a lead of 0.5 or more.

3. 3. The multi-start ball screw according to claim 1, wherein the groove length of the second spiral groove is equal to or greater than the diameter of the ball.

Images