Linear motion device, and guide shaft and moving body used in linear motion device

Abstract

This linear motion device is provided with: a guide shaft provided with a first ball rolling surface on the outer peripheral surface; a moving body which has a second ball rolling surface on the inner peripheral surface thereof so as to face the first ball rolling surface, and which moves relative to the guide shaft; the linear motion device includes a first ball rolling surface, a second ball rolling surface, and a ball that rolls in a space formed between the first ball rolling surface and the second ball rolling surface, and in the linear motion device, the average surface roughness Sa of the first ball rolling surface and / or the second ball rolling surface excluding a predetermined ball rolling direction component is 0.1 [mu] m or more.

Description

Technical Field

[0001] The present invention relates to a linear motion device and a guide shaft and a movable body used in the linear motion device. Background Art

[0002] Machine tools use linear motion devices such as ball screws and linear guides. However, to improve machining accuracy, these devices must operate smoothly and have a structure that is less prone to wear. To ensure smooth operation and a structure that is less prone to wear, a lubricant is preferably placed between the balls and the ball rolling surface.

[0003] In ball screws and other systems, lubricant cannot be retained on the ball rolling surface due to structural reasons. Therefore, a lubrication method is used in which lubricant is regularly supplied through a pipe installed in a nut, etc., and the lubricant is transferred to the ball rolling surface via the balls as the nut moves. Therefore, it is advantageous to retain more lubricant on the ball rolling surface for a longer period of time.

[0004] Furthermore, the ball rolling surfaces of the screw shaft and nut have fine irregularities as machining marks. Lubricant accumulates in the valleys of these irregularities, contributing to lubrication. Furthermore, the lower and smoother the peaks of these irregularities are, the less wear there is on the screw itself and the mating material.

[0005] The ball rolling surface is typically machined by grinding or lathe finishing. Conventionally, this machined surface is managed based on the surface roughness measured in the ball rolling direction. However, surface roughness alone cannot adequately assess lubricant retention. Specifically, even if the surface roughness of the ball rolling surface is within the specified range, it can sometimes be difficult to supply and retain oil at the boundary surface during ball rolling. This can increase friction and cause wear, particularly under low-speed operating conditions where an oil film is difficult to form.

[0006] In contrast, Patent Document 1 proposes numerically limiting the arithmetic mean height (Sa), maximum surface height (Sz), core level difference (Sk), average height of protruding peaks (Spk), and average depth of protruding valleys (Svk) of the machined surface of the ball rolling surface of the ball screw, thereby achieving a high-precision and long-life ball screw.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Publication No. 2019-39525 Summary of the Invention

[0010] Problems to be solved by the invention

[0011] However, the present inventors' research results indicate that even when the ratio of protruding peaks to protruding valleys, a factor related to the surface roughness of the machined surface, is within the above range, wear may not be adequately suppressed. Therefore, there is a need for a linear motion product that appropriately evaluates the surface properties of the ball rolling surface.

[0012] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a linear motion device including a ball rolling surface appropriately evaluated using a new evaluation method different from conventional ones, and a guide shaft and a movable body used in the linear motion device.

[0013] Means for solving problems

[0014] The direct-acting device of the present invention is characterized by having:

[0015] A guide shaft having a first ball rolling surface on an outer peripheral surface thereof;

[0016] a movable body having a second ball rolling surface on an inner peripheral surface thereof facing the first ball rolling surface and being movable relative to the guide shaft; and

[0017] a ball rolling in a space formed between the first ball rolling surface and the second ball rolling surface;

[0018] At least one of the first ball rolling surface and the second ball rolling surface has an average surface roughness Sa excluding a predetermined ball rolling direction component of 0.1 μm or more.

[0019] The guide shaft of the present invention is characterized in that it is used in a linear motion device, and the linear motion device has:

[0020] The guide shaft has a first ball rolling surface on its outer peripheral surface;

[0021] a movable body having a second ball rolling surface on an inner peripheral surface thereof so as to face the first ball rolling surface and being movable relative to the guide shaft; and

[0022] a ball rolling in a space formed between the first ball rolling surface and the second ball rolling surface;

[0023] In the first ball rolling surface, the average surface roughness Sa excluding a predetermined ball rolling direction component is 0.1 μm or more.

[0024] The movable body of the present invention is characterized in that it is used in a linear motion device, and the linear motion device has:

[0025] A guide shaft having a first ball rolling surface on an outer peripheral surface thereof;

[0026] the movable body having a second ball rolling surface on an inner peripheral surface thereof facing the first ball rolling surface and being movable relative to the guide shaft; and

[0027] a ball rolling in a space formed between the first ball rolling surface and the second ball rolling surface;

[0028] In the second ball rolling surface, the average surface roughness Sa excluding a predetermined ball rolling direction component is 0.1 μm or more.

[0029] Effects of the Invention

[0030] According to the present invention, it is possible to provide a linear motion device including a ball rolling surface that is appropriately evaluated using a new evaluation method different from conventional ones, and a guide shaft and a movable body used in the linear motion device. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a cross-sectional view showing a ball screw as an example of a linear motion device.

[0032] Figure 2 (a) is an enlarged image showing the surface of the ball rolling surface processed by the conventional technology. Figure 2 (b) is a diagram schematically showing a state in which a ball B rolls on a ball rolling surface F processed by conventional technology.

[0033] Figure 3 (a) is an enlarged image showing the surface of the ball rolling surface having an average roughness Sa proposed by the present inventors. Figure 3 (b) is a diagram schematically showing a state in which a ball B rolls on a ball rolling surface F having an average roughness Sa proposed by the present inventors.

[0034] Figure 4 (a) is an enlarged image showing the surface of the ball rolling surface before shot peening treatment. Figure 4 (b) is an enlarged image showing the surface of the ball rolling surface after the shot peening treatment.

[0035] Figure 5 This is a schematic diagram for explaining how the shot peening material is ejected onto the ball rolling surface.

[0036] Figure 6 This figure shows an image of the surface properties of the ball rolling surface of a ball screw after shot peening treatment under different conditions, together with the average surface roughness Sa.

[0037] Figure 7This is a graph showing the average surface roughness Sa on the horizontal axis and the reduction rate of dynamic friction torque in the low-speed wear test on the vertical axis for each example.

[0038] Figure 8 (a) is the image before the FFT filter is applied based on the surface shape data, Figure 8 (b) is the image after application of the FFT filter based on the same surface texture data.

[0039] Figure 9 This figure shows an image of the surface properties of the ball rolling surface of the ball screw of the Example which was shot peened under different conditions, together with the average surface roughness Sa.

[0040] Figure 10 This is a graph showing the average surface roughness Sa on the horizontal axis and the reduction rate of dynamic friction torque in the low-speed wear test on the vertical axis for each example.

[0041] Figure 11 This is a graph showing the relationship between the rotational speed and the dynamic friction torque in a conventional ball screw and a ball screw having a ball rolling surface having an average surface roughness Sa of 0.1 μm or greater, excluding a component in the ball rolling direction. DETAILED DESCRIPTION

[0042] Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

[0043] Figure 1 It is a cross-sectional view showing a ball screw as an example of a linear motion device.

[0044] like Figure 1 As shown, the ball screw is composed of a nut (moving body) 1, a screw shaft (guide shaft) 2, a plurality of balls 3, an annular seal 4, tubes (circulation components) 5, 5' and a tube presser 6. A spiral groove (inner circumferential thread groove) 1a is formed on the inner circumferential surface of the nut 1, and a spiral groove (outer circumferential thread groove) 2a is formed on the outer circumferential surface of the screw shaft 2. The balls 3 are arranged between the rolling paths formed by the spiral groove 1a of the nut 1 and the spiral groove 2a of the screw shaft 2. A flange 11 is formed at one axial end of the nut 1. The surface of the spiral groove 1a of the nut (moving body) 1 is equivalent to the second ball rolling surface, and the surface of the spiral groove 2a of the screw shaft 2 is equivalent to the first ball rolling surface.

[0045] In this embodiment, the seals 4 provided at both ends of the nut 1 are contact seals whose inner peripheries are in contact with the outer periphery of the screw shaft 2 . The outer periphery of the seal 4 and the inner periphery of the nut 1 are sealed by O-rings 7 .

[0046] This ball screw has two circulation circuits constituted by a rolling path of the balls 3 (a rolling path formed by the spiral groove 1 a and the spiral groove 2 a ) and tubes 5 and 5 ′.

[0047] Flat surfaces 12, 12 are formed on the outer periphery of the nut 1 outside the flange 11 for accommodating the two tubes 5, 5'. Tube mounting holes 16, 16' are formed in the flat surface 12 for attaching the tubes 5, 5'. The ends of the tubes 5, 5', bent into a U-shape, are inserted into the tube mounting holes 16, 16' and secured with tube retainers 6, 6, respectively.

[0048] Lubricant is supplied to the nut 1 via a lubricant supply passage 17. The lubricant supply passage 17 extends radially through the inner and outer circumferences of the nut 1. It has an outward-facing opening at its outer end and an inward opening at its inner end. This opening is located near an offset point on the inner circumference of the nut 1 (or between the first and second nuts described later). Here, "near" refers to a range within ±1 pitch of the spiral groove 1a along the axial direction from the offset point. While the example shows the lubricant supply passage 17 being arranged in rows of spiral grooves 1a separated by offset points, it can also be arranged between the spiral grooves 1a or at the bottom of the Gothic arc of the spiral groove 1a.

[0049] However, the ball rolling surfaces of the screw shaft and the nut in conventional ball screws are finished by grinding or turning, resulting in surfaces having a surface roughness mainly in the ball rolling direction component.

[0050] Figure 2 (a) is an enlarged image showing the surface of a ball rolling surface processed by conventional technology, and the depth of the concavities and convexities is represented by grayscale. Figure 2 (b) is a diagram schematically showing a state in which a ball B rolls on a ball rolling surface F processed by conventional technology, and reference numeral L represents a lubricant.

[0051] Figure 2 The surface roughness of the ball rolling surface F shown in (a) is small except for the surface roughness of the ball rolling direction. Figure 2 As shown in (b), the lubricant L is scraped off by the rolling balls B, making it difficult to retain the lubricant L at the boundary surface with the ball rolling surface F after the balls B have rolled. Therefore, under low-speed operating conditions where it is difficult to form a lubricant film, friction increases, making wear more likely to occur.

[0052] exist Figure 3 (a) shows an enlarged image of the surface of a ball rolling surface having an average roughness Sa proposed by the present inventors, and the depth of the concavities and convexities is represented by grayscale shades. Figure 3(b) is a diagram schematically showing a state in which a ball B proposed by the present inventors rolls on a ball rolling surface F having an average roughness Sa, and reference symbol L represents a lubricant.

[0053] According to the proposal of the present inventors, Figure 3 As shown in (a), by ensuring a large surface roughness Sa excluding the ball rolling direction component, as shown in Figure 3 As shown in (b), lubricant L can be retained in the micro-concave portions of the ball rolling surface F. This oil storage effect and dynamic pressure effect can enhance the ability to form a lubricant film at the boundary between the ball B and the ball rolling surface F, thereby reducing friction and suppressing wear.

[0054] Here, as a method of forming surface roughness other than the component in the ball rolling direction, the present inventors subjected the ball rolling surfaces of the screw shaft and the nut to shot peening and verified the effect thereof.

[0055] Figure 4 (a) is an enlarged image showing the surface of the ball rolling surface before shot peening (immediately after grinding). Figure 4 (b) is an enlarged image of the ball rolling surface after further shot peening. While the depth of the unevenness is represented by grayscale, the surface primarily exhibits surface roughness, with the ball rolling component resulting from grinding being in the vertical direction.

[0056] (Shot peening treatment)

[0057] An example of shot peening is shown below. For example, a raw material made of high carbon steel is processed into the shape of a screw shaft or nut, and after heat treatment, the ball rolling surface is formed by cutting and grinding, and then shot peening is performed. In this shot peening process, the shot material collides with the ball rolling surface at high speed, Figure 4 The tops of the stripe-like processing marks produced by cutting or grinding as shown in (a) are flattened, and a large number of fine irregularities can be formed.

[0058] Shot peening materials used in shot peening can be particles made of steel or ceramics such as SiC. Particle sizes of 30 to 60 μm are preferred, with 40 to 50 μm being more preferred. The smaller the particle size, the finer the resulting unevenness. The shape of the shot peening material can be various, in addition to spherical. The peening pressure is preferably 0.3 to 0.5 MPa, more preferably 0.4 to 0.5 MPa.

[0059] In addition, the shot blasting material is preferably ejected at an angle to the ball rolling surface so that the shot blasting material collides perpendicularly to the tangent line of the cross section of the ball rolling surface. Figure 5 This is a cross-sectional view of a Gothic arch-shaped ball rolling surface. It is preferable to spray the shot peening material while swinging the spray nozzle 10 diagonally upward or downward in the cross-section so that a line A passing through the center of the spray nozzle 10's spray port intersects perpendicularly with a tangent line L on the ball rolling surface. This allows the shot peening material to collide perpendicularly with the ball rolling surface, effectively forming irregularities.

[0060] Furthermore, since the ball rolling surface is a spiral groove, to uniformly sandblast the spiral groove surface, it is best to rotate the blasting target (screw shaft or nut) while blasting the target axially while moving the spray nozzle in sync with the rotation of the target. Alternatively, multiple spray nozzles can be used to simultaneously sandblast the arcuate surfaces on both sides of the spiral groove. This allows for efficient sandblasting of the spiral groove surface.

[0061] Compare Figure 4 As shown in (a) and (b), shot peening creates numerous depressions and indentations, resulting in a surface containing components other than those in the ball rolling direction. However, the present inventors' research has determined that shot peening alone does not improve the kinetic friction torque reduction rate of the ball rolling surface. It should be noted that the ball rolling surface processing is not limited to shot peening; similar effects can be achieved through sandblasting, shot peening, surface rolling, tumbling, and laser texturing.

[0062] Figure 6 Images of the surface properties of the ball rolling surface of a nut shot peened under varying conditions are shown, along with the average surface roughness Sa. Specifically, a nut for a ball screw device was produced using high-carbon steel, formed into a predetermined shape using conventional methods, quenched, and then fine-polished on the ball rolling surface. The three-dimensional roughness profile and surface roughness waveform were then measured using a surface roughness meter to determine the average surface roughness Sa.

[0063] In ISO 25178, an international standard defining surface roughness evaluation methods, the core level difference is defined as Sk, the average height of the protruding peaks as Spk, and the average depth of the protruding valleys as Svk. The portion higher than the core is the protruding peak, and the portion lower than the core is the protruding valley. The ratio of the protruding peaks to the protruding valleys can be calculated using the load curve. The arithmetic mean roughness Sa is the value calculated using the equation in the figure above, expressed in μm, by cutting a reference length L along the mean line of the roughness curve according to ISO 25178. The x-axis is the direction of the mean line of the cut section, and the y-axis is the direction of the longitudinal magnification. When the roughness curve is expressed as y = f(x), the value is expressed in μm using the equation in the figure above.

[0064] A low-speed wear test was performed under the following conditions using a ball screw having a nut treated under different conditions.

[0065] Ball screw specifications: shaft diameter 32mm, thread groove lead 5mm

[0066] Lubricant used: VG68 oil

[0067] Screw shaft speed: 1min -1 ,

[0068] Stroke: 150mm

[0069] Reciprocating times: 10 times

[0070] Figure 7 This is a graph showing the average surface roughness Sa of the ball rolling surface of the ball screw with a nut in each embodiment, and the vertical axis is the dynamic friction torque reduction rate in the low-speed wear test. The dynamic friction torque reduction rate is the ratio of the dynamic friction torque of the ball screw after the low-speed wear test to the dynamic friction torque of the ball screw before the low-speed wear test being 100%. Figure 7 It is clear that no clear correlation is found between the average surface roughness Sa and the dynamic friction torque reduction rate. The present inventors examined this test result.

[0071] Here, we explain why the inventors focused on the reduction rate of dynamic friction torque. Ball screws are often used with a preload applied to increase rigidity and eliminate backlash. The magnitude of the preload is correlated with the dynamic friction torque, and the amount of preload can be estimated based on the dynamic friction torque. As ball screw wear progresses, the preload decreases, leading to a decrease in rigidity. The reduction in preload can be estimated by measuring the reduction in dynamic friction torque. It can be said that the smaller the reduction in dynamic friction torque, the higher the wear resistance of the ball screw.

[0072] (Influence of low-speed wear test)

[0073] The inventors speculate that the main reason for not finding a clear correlation between average surface roughness Sa and the kinetic friction torque reduction rate is that the ball rolling direction component of grinding varies depending on the ball screw. Specifically, the inventors speculate that the measured average surface roughness includes both a ball rolling direction component due to the finish of the ball rolling surface and a component other than the ball rolling direction due to shot peening. When the finish of the ball screw varies significantly, the average surface roughness value becomes unstable, and thus no correlation with the wear test results can be obtained.

[0074] Based on the above assumptions, the inventors used the surface roughness evaluation method specified in ISO 25178. They applied an FFT filter (frequency-based processing) to the surface property data within the ball rolling direction ±10° and at a wavelength of 648.554 to 0.625 μm. The result was the average surface roughness Sa, excluding the component in the ball rolling direction. This is referred to as the average surface roughness Sa after the specified component in the ball rolling direction has been removed.

[0075] It should be noted that the application of the FFT filter visually determines the direction where the grinding mark is 0°, and removes the surface roughness components within ±10° of this direction. Furthermore, if the wavelength range is 648.554 to 0.625 μm, it can correspond to the frequency of surface irregularities found in typical grinding and cutting processes on the ball rolling surface of a ball screw.

[0076] Figure 8 (a) shows the image before applying the FFT filter based on the surface property data, and Figure 8 (b) shows an image after applying an FFT filter based on the same surface texture data. The depth of the unevenness is represented by the grayscale, but if we compare Figure 8 From (a) and (b), it can be seen that the application of the FFT filter effectively removes the ball rolling direction component. In other words, the purpose of applying the FFT filter is to evaluate only the roughness of the tiny pits (recesses) on the ball rolling surface that retain the lubricant.

[0077] Figure 9 This diagram shows an image of the surface properties of the ball rolling surface of a nut after shot peening using a ceramic shot peening material with a particle size of 30 μm to 63 μm at a shot peening pressure of 0.3 MPa to 0.5 MPa, together with the average surface roughness Sa. Figure 10 The graph shows the average surface roughness Sa of the ball rolling surface of the ball screw with a nut according to each embodiment, with the horizontal axis representing the average surface roughness Sa and the vertical axis representing the dynamic friction torque reduction rate in the low-speed wear test. The conditions of the low-speed wear test are as follows.

[0078] Ball screw specifications: shaft diameter 32mm, thread groove lead 5mm

[0079] Lubricant used: VG68 oil

[0080] Screw shaft speed: 1min -1 ,

[0081] Stroke: 150mm

[0082] Reciprocating times: 10 times

[0083] from Figure 10 A clear correlation is observed between the average surface roughness Sa (excluding the ball rolling direction component) and the kinetic friction torque reduction rate. This correlation confirms that, for ball rolling surfaces with an average surface roughness Sa (excluding the ball rolling direction component) of 0.1 μm or greater, the kinetic friction torque reduction rate decreases, effectively suppressing wear. For ball rolling surfaces with an average surface roughness Sa (excluding the ball rolling direction component) of 0.1 μm or greater, this can be achieved by adjusting the shot peening particle size, shot peening pressure, and other factors.

[0084] exist Figure 11 The figure shows the relationship between the rotational speed and the dynamic friction torque of a conventional ball screw and a ball screw having a ball rolling surface with an average surface roughness Sa of 0.1 μm or more, excluding the ball rolling direction component. -1 Under the following low-speed operating conditions, the dynamic friction torque is lower than in the previous conditions. Therefore, it is confirmed that even under low-speed operating conditions, where lubricant film formation is generally difficult, the ability to form a lubricant film is improved, reducing friction. As mentioned above, the research focused on the ball rolling surface of the ball screw nut, but the same research is also applicable to the surface of the ball, the ball rolling surface of the screw shaft, or the ball rolling surface of the slider of a linear guide.

[0085] More specifically, with conventional ball screws, the amount of lubricant flowing between the balls and the ball rolling surface decreases as the speed decreases, resulting in a thinner lubricant film. This increases the metal-to-metal contact area and tends to increase dynamic friction torque. High-precision, long-life ball screws with a ball rolling surface that has an average surface roughness Sa of 0.1 μm or greater, excluding the ball rolling direction component, facilitate the formation of an oil film between the balls and the ball rolling surface, even when driven at low speeds. This suppresses increases in metal-to-metal contact area and dynamic friction torque.

[0086] The average surface roughness Sa excluding the ball rolling direction component is preferably 0.4 μm or less. If it exceeds 0.4 μm, the operation performance and operating sound of the linear motion device will be adversely affected.

[0087] The present invention is not limited to the above-described embodiment. Within the scope of the present invention, any of the components of the above-described embodiment may be modified. In addition, any component of the above-described embodiment may be added or omitted. For example, in addition to being applicable to various ball screws such as recirculating joint ball screws, tubular ball screws, deflector ball screws, and end cap ball screws, the present invention may also be applied to direct-acting devices such as linear guides having a slider as a moving body.

[0088] While various embodiments have been described above, the present invention is not limited to these examples. It is obvious that a person skilled in the art would be able to devise various variations or modifications within the scope of the claims, and these variations or modifications naturally fall within the technical scope of the present invention. Furthermore, the various components of the above embodiments may be arbitrarily combined without departing from the spirit of the invention.

[0089] In addition, this application is based on the Japanese patent application (Japanese Patent Application No. 2023-033833) filed on March 6, 2023, and the contents are incorporated herein by reference.

[0090] Description of Reference Numerals

[0091] 1: Nut (moving body)

[0092] 2: Screw shaft (guide shaft)

[0093] 3: Ball

[0094] 4: Seals

[0095] 5, 5': tube

[0096] 6: Tube pressing piece

Claims

1. A direct-acting device, characterized in that: have: A guide shaft having a first ball rolling surface on an outer peripheral surface thereof; a movable body having a second ball rolling surface on an inner peripheral surface thereof facing the first ball rolling surface and being movable relative to the guide shaft; as well as a ball rolling in a space formed between the first ball rolling surface and the second ball rolling surface; At least one of the first ball rolling surface and the second ball rolling surface has an average surface roughness Sa excluding a predetermined ball rolling direction component of 0.1 μm or more.

2. The direct-acting device according to claim 1, characterized in that: The average surface roughness Sa excluding the ball rolling direction component is calculated based on the surface property data of at least one of the first ball rolling surface and the second ball rolling surface by applying an FFT filter under the conditions of ±10° in the ball rolling direction and a wavelength of 648.554~0.625μm.

3. The direct-acting device according to claim 1 or 2, characterized in that: The direct-acting device is at least 50 minutes -1 Use at the following speeds.

4. A guide shaft, characterized in that: For a direct-acting device having: The guide shaft has a first ball rolling surface on its outer peripheral surface; a movable body having a second ball rolling surface on an inner peripheral surface thereof facing the first ball rolling surface and being movable relative to the guide shaft; as well as a ball rolling in a space formed between the first ball rolling surface and the second ball rolling surface; In the first ball rolling surface, the average surface roughness Sa excluding a predetermined ball rolling direction component is 0.1 μm or more.

5. A mobile object, characterized in that: For a direct-acting device having: A guide shaft having a first ball rolling surface on an outer peripheral surface thereof; The movable body includes a second ball rolling surface on an inner peripheral surface thereof facing the first ball rolling surface, and moves relative to the guide shaft; as well as a ball rolling in a space formed between the first ball rolling surface and the second ball rolling surface; In the second ball rolling surface, the average surface roughness Sa excluding a predetermined ball rolling direction component is 0.1 μm or more.

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