Ball screws and ball screw devices

Abstract

The present invention provides a ball screw and ball screw device that have a simple configuration while suppressing heat generation during operation and maintaining positioning accuracy. [Solution] A ball screw comprising a screw shaft having an outer circumferential screw groove formed on its outer circumferential surface, a nut arranged around the screw shaft and having an inner circumferential screw groove formed on its inner circumferential surface opposite to the outer circumferential screw groove, and a plurality of balls housed in a rolling path formed by the opposing outer circumferential screw groove and inner circumferential screw groove, wherein the inner circumferential screw groove has an effective diameter of a constant φD, and the outer circumferential screw groove has a first region in which the effective diameter of the outer circumferential screw groove is a constant φA, and a second region in which the effective diameter of the outer circumferential screw groove is a constant φC (where φA > φC).

Description

[Technical Field]

[0001] The present invention relates to a ball screw and a ball screw device. [Background Art]

[0002] Linear motion devices such as ball screws and linear guides are used in machine tools, and it is necessary to ensure high rigidity of the linear motion device in order to improve machining accuracy.

[0003] In general, a ball screw has the advantage that it is easy to suppress backlash between the spiral groove of the nut and the spiral groove of the screw shaft, thereby ensuring high rigidity. In order to suppress backlash of the ball screw, preload is applied between the spiral groove of the nut and the spiral groove of the screw shaft.

[0004] On the other hand, ball screws used in machine tools generate heat due to friction during driving, which often affects the machining accuracy of machine tools. Most of the heat generation comes from two sources: one is heat generated during machining when driving with a preload applied for high positioning accuracy and rigidity, and the other is heat generated during high-speed movement for tool changing while the preload remains unchanged. Technologies such as shaft core cooling and nut cooling are effective for suppressing heat generation, but these technologies conflict with weight reduction, space saving and cost reduction. Therefore, there is a need for a technology that can suppress heat generation without relying on these methods.

[0005] Patent Document 1 discloses a ball screw adopting a preloading method that applies preloads in opposite axial directions to a first nut and a second nut screwed onto a screw shaft, wherein a Peltier element is attached to the spacer between the nuts to control the temperature change of the spacer, and a mechanism for increasing or decreasing the preload by utilizing thermal expansion and contraction of the spacer. According to the technology of Patent Document 1, since it is possible to apply preload at necessary timings, no preload is applied during high-speed feeding, which results in very low torque and thus reduced heat generation. [Prior Art Documents] [Patent Literature]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-70884 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, according to the technology in Patent Document 1, the use of a Peltier element results in a complex configuration. Furthermore, due to the nature of the Peltier effect, when cooling occurs on one side of the Peltier element, heat is generated on the opposite side, raising concerns that heat dissipation around the nut may hinder the cooling of surrounding components. In addition, a mechanism that heats the spacer to expand it and apply preload in order to obtain high rigidity in order to achieve machining accuracy may cause thermal expansion due to heat transfer from the spacer to the nut and from the nut to the shaft, potentially reducing machining accuracy.

[0008] The present invention has been made in view of the above problems, and aims to provide a ball screw and a ball screw device that have a simple configuration while suppressing heat generation during operation and maintaining positioning accuracy. [Means for solving the problem]

[0009] The ball screw of the present invention is A screw shaft having an external screw groove formed on its outer surface, A nut is arranged around the screw shaft, and has an inner screw groove formed on its inner surface opposite to the outer screw groove, A ball screw having a plurality of balls housed in a rolling path formed by opposing outer circumferential screw grooves and inner circumferential screw grooves, The aforementioned internal circumferential screw groove has a constant effective diameter of φD. The outer circumferential screw groove is characterized by having a first region in which the effective diameter of the outer circumferential screw groove is constant at φA, and a second region in which the effective diameter of the outer circumferential screw groove is constant at φC (where φA > φC). [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a ball screw and a ball screw device that have a simple configuration while suppressing heat generation during operation and maintaining positioning accuracy. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a cross-sectional view of the ball screw of this embodiment. [Figure 2] Figure 2 is a side view of the screw shaft according to this embodiment. [Figure 3] Figure 3 is a side view of a ball screw device using a ball screw according to this embodiment. [Figure 4] Figure 4 is a side view of a ball screw device using a modified ball screw. [Modes for carrying out the invention]

[0012] Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. Figure 1 is a cross-sectional view of a ball screw 10 according to this embodiment. Figure 2 is a side view showing the screw shaft 11. The ball screw 10 comprises a screw shaft 11, a nut 13 arranged around the screw shaft 11, and a plurality of balls 15. The ball screw of this embodiment is suitably used in machine tools such as multi-tasking lathes and machining centers.

[0013] The nut 13 is positioned around the screw shaft 11. The inner surface of the nut 13 has a spirally formed inner screw groove 19 and circulation grooves that connect both ends of the inner screw groove 19. The effective diameter φD of the inner screw groove 19 is constant.

[0014] The ends of the internal thread groove 19 are connected to the circulation groove, forming a single closed, annular rolling groove. In other words, the nut 13 has an annularly formed rolling groove.

[0015] A helical outer circumferential thread groove 17 is formed on the outer circumferential surface of the screw shaft 11 so as to face the inner circumferential thread groove 19 of the nut 13. The inner circumferential thread groove 19 and the outer circumferential thread groove 17 are each formed to a depth approximately equal to the radius of a ball 15.

[0016] Between a rolling groove formed by the inner circumferential thread groove 19 and a circulation groove, and the outer circumferential thread groove 17, a rolling path is defined in which a plurality of balls 15 are rollably accommodated.

[0017] Along with the relative rotation of the screw shaft 11 with respect to the nut 13, the plurality of balls 15 circulate within the circulation circuit, thereby enabling the nut 13 to perform relative linear motion with respect to the screw shaft 11 in the axial direction of the screw shaft 11.

[0018] In FIG. 2, the screw shaft 11 is divided into a region A (a first region) and a region B (a third region) along the axial direction by a first boundary point BD1, and is further divided into the region B and a region C (a second region) along the axial direction by a second boundary point BD2.

[0019] When the effective diameter of the outer circumferential thread groove 17 in the region A is φA, the effective diameter of the outer circumferential thread groove 17 in the region B is φB, and the effective diameter of the outer circumferential thread groove 17 in the region C is φC, the following formula (1) is satisfied. φA>φB>φC (1)

[0020] The outer circumferential thread grooves 17 in the regions A and C are formed such that the effective diameters φA and φC are constant over two or more threads, respectively. That is, the term "constant" means that the effective diameter is φA or φC over two or more threads of the outer circumferential thread groove 17. It is preferable that the regions A and C are each formed longer than the axial length of the nut 13. On the other hand, in the outer circumferential thread groove 17 of the region B, the effective diameter φB gradually decreases from the region A side toward the region C side over two or more threads.

[0021] Provided that the pitch and lead of the outer circumferential thread grooves 17 in the regions A, B and C are equal.

[0022] Since the effective diameter φD of the internal thread groove 19 is constant and equation (1) is satisfied, the difference in effective diameter between the internal thread groove 19 and the external thread groove 17 in region A (φD-φA) is smaller than the difference in effective diameter between the internal thread groove 19 and the external thread groove 17 in region C (φD-φC). Therefore, the preload of the ball screw 10 in region A is greater than the preload of the ball screw 10 in region C. Clearly, the preload of the ball screw 10 in region B is less than the preload of the ball screw 10 in region A and greater than the preload of the ball screw 10 in region C.

[0023] Furthermore, the preload applied to the ball screw 10 may include fixed-position preload. Fixed-position preload generally involves interposing a spacer of a predetermined thickness between two nuts and applying pressure to fix the two nuts in a direction that moves them apart or toward each other. By adjusting the thickness of the spacer, it is possible to adjust the amount of preload in areas A, B, and C.

[0024] Methods for measuring the effective diameter of the internal thread groove 19 and the external thread groove 17 include measurement using three needles, measurement using a microscope, and measurement using measuring terminals. For example, measurement using three needles involves using three needles with precisely defined diameters, placing two pin gauges on one side of the thread groove and one pin gauge on the opposite side, and measuring the outer dimension to determine the effective diameter. Details are specified in JIS B0271. Alternatively, measurement using measuring terminals can be performed using a passameter with balls (of the same diameter as the balls 15 used in this ball screw 10, for example) attached to two measuring terminals, each fixed by welding. This method involves placing the ball of one measuring terminal of the passameter on an arbitrary part of the thread groove, then placing the ball of the other measuring terminal on a part of the thread groove 180° ahead in phase, and determining the effective diameter by how much the measured value deviates from the reference value.

[0025] In this embodiment, the effective diameter of the screw shaft 11 of the ball screw 10 differs in each region, so the preload applied to the ball screw 10 changes, and the frictional torque changes depending on the position where the nut 13 attached to the screw shaft 11 strokes.

[0026] For example, in the stroke range of a nut 13 where high support rigidity and positioning accuracy are required, normal friction occurs when the nut 13 is moved at a relatively low speed VA within region A. On the other hand, in the stroke range of region C, the amount of heat generated can be suppressed even when the nut 13 is moved at a speed VC higher than the speed VA in region A.

[0027] In other words, when the ball screw 10 of this embodiment is used in the feed mechanism of a machine tool or the like, excessive torque is not applied when the nut 13 moves through region C, so the amount of heat generated due to friction can be suppressed compared to when the nut 13 moves through region A. To put it another way, the nut 13 is not overheated when it passes through region C and enters region A, and furthermore, when passing through region A, the nut 13 is moving at a low speed, so even if the torque is large, heat generation is suppressed, thereby suppressing heat generation of the nut 13 throughout the entire range and ensuring positioning accuracy.

[0028] Furthermore, region B can be used as a deceleration region when the nut 13 moves from region A to region C, or as an acceleration region when it moves from region C to region A. Therefore, even if the nut 13 passes through region B at a speed VB that is higher than speed VA and lower than speed VC, heat generation can be suppressed.

[0029] Furthermore, the screw shaft 11 may have regions C formed on both sides of region A, or regions A formed on both sides of region C. It is preferable to have region B between region A and region C.

[0030] Figure 3 is a side view showing a ball screw device 100 incorporating a ball screw 10. In Figure 3, the screw shaft 11 is hollow and has a flow path 18 that runs through its entire length. The right end of the screw shaft 11 is rotatably supported by a hollow first support portion 110 via a first bearing 111, and the left end of the screw shaft 11 is rotatably supported by a hollow second support portion 120 via a second bearing 121.

[0031] The interior of the first support section 110 is connected to the inlet pipe 112 and the flow path 18, and the rest of the interior is sealed. The interior of the second support section 120 is connected to the discharge pipe 122 and the flow path 18, and the rest of the interior is sealed. The inlet pipe 112 is connected to a coolant source (not shown) that stores coolant at a temperature lower than room temperature, and the discharge pipe 122 is connected to a coolant recovery section (not shown). The first support section 110, the inlet pipe 112, and the coolant source constitute a supply device that supplies coolant to the flow path 18 from the outside.

[0032] The screw shaft 11 is divided into region A and region B along the axial direction by a first boundary point BD1, and further divided into region B and region C along the axial direction by a second boundary point BD2. Region A is located on the side of the first support portion 110, and region C is located on the side of the second support portion 120.

[0033] When the screw shaft 11 is rotated by a drive source (not shown), the nut 13 moves axially according to the rotation angle, generating heat in the screw shaft 11. In response, low-temperature coolant from a coolant source enters the flow path 18 through the inlet pipe 112 and the inside of the first support part 110, and heat exchange occurs with the screw shaft 11 as it passes through the flow path 18. As a result, the screw shaft 11 is cooled, preventing thermal expansion and suppressing friction between the inner screw groove 19 and the outer screw groove 17.

[0034] Meanwhile, the coolant whose temperature has risen due to heat exchange is discharged from the flow path 18 and collected in the coolant recovery section via the inside of the second support section 120 and the discharge pipe 122.

[0035] According to this embodiment, since the first support portion 110 is located on the side of the screw shaft 11 that is close to region A where the temperature rise is relatively high, region A can be efficiently cooled by the relatively low-temperature coolant flowing in from the coolant source.

[0036] Furthermore, a sensor for detecting the temperature of region A of the screw shaft 11 and an on-off valve that receives a signal from the sensor and opens and closes the connection between the coolant source and the inlet pipe 112 based on that signal can also be installed. In this example, while the sensor detects that region A is below a predetermined temperature, the on-off valve is closed by the signal from the sensor, and only when the sensor detects that region A has been heated above the predetermined temperature does the on-off valve open, allowing coolant to flow from the coolant source to the inlet pipe. This prevents region A from being overcooled.

[0037] However, region C may be located on the side of the first support portion 110, and region A may be located on the side of the second support portion 120.

[0038] (modified version) Figure 4 is a side view of a modified ball screw device 100'. In Figure 4, the left end of the screw shaft 11 is rotatably supported by a hollow first support part 110 via a first bearing 111, and the left end of the screw shaft 11 is rotatably supported by a second bearing 121. A sealed channel 18 is formed on the left end side of the screw shaft 11.

[0039] The inside of the first support section 110 is in communication with the flow path 18 and the inflow pipe 112, and the rest is sealed.

[0040] The screw shaft 11 is divided into region A and region B along the axial direction by a first boundary point BD1, and further divided into region B and region C along the axial direction by a second boundary point BD2. Region A is located on the side of the first support portion 110, and region C is located on the side of the second bearing 121.

[0041] Regions B and C generate relatively less heat compared to region A due to their lower pre-pressure. According to this modified example, low-temperature coolant enters the flow path 18 from the coolant source through the inlet pipe 112 and the inside of the first support part 110, and heat exchange occurs in region A of the screw shaft 11, thereby cooling only region A, and thus enabling miniaturization of the ball screw device 100'.

[0042] The present invention is not limited to the embodiments described above. Within the scope of the present invention, any component of the embodiments described above can be modified. Furthermore, any component can be added or omitted in the embodiments described above. For example, the present invention can be applied to various ball screws, such as circulating ball screws, tube ball screws, deflector ball screws, and end cap ball screws. [Explanation of Symbols]

[0043] 100, 100' Ball Screw Device 10 Ball screws 11 Screw shaft 13 nuts 17 Outer circumference thread groove 19 Internal thread groove Area A (first area) Area B (3rd area) C area (second area)

Claims

1. A screw shaft having an external screw groove formed on its outer surface, A nut is arranged around the screw shaft, and has an inner screw groove formed on its inner surface opposite to the outer screw groove, A ball screw having a plurality of balls housed in a rolling path formed by opposing outer circumferential screw grooves and inner circumferential screw grooves, The aforementioned internal circumferential screw groove has a constant effective diameter of φD. The ball screw is characterized in that the outer circumferential thread groove has a first region in which the effective diameter of the outer circumferential thread groove is constant at φA, and a second region in which the effective diameter of the outer circumferential thread groove is constant at φC (where φA > φC).

2. The ball screw according to claim 1, characterized in that the outer circumference screw groove has a third region between the first region and the second region, the effective diameter of the outer circumference screw groove being φB (where φA > φB > φC).

3. The ball screw according to claim 2, characterized in that the outer circumference screw groove of the third region gradually decreases as the effective diameter φB moves from the first region side toward the second region side.

4. A ball screw device using a ball screw according to any one of claims 1 to 3, The screw shaft has a passage through which coolant flows, A ball screw device characterized by having a supply device that supplies coolant to the flow path from the outside.

5. The ball screw device according to claim 4, characterized in that the flow path is formed corresponding to the first region.

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