JP2008069846A - Planetary roller screw device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a planetary roller screw device having low friction on a sliding surface and excellent operation efficiency in conversion of rotary motion to axial linear motion. <P>SOLUTION: A planetary roller screw device is provided with a screw shaft 1 having a screw groove 1a formed on an outer circumference surface, a nut 2 having a screw groove 2a opposing to the screw groove 1a of the screw shaft 1 formed on an inner circumference surface, a roller row comprising a plurality of rollers 3 rollably put between both screw grooves 1a, 2a, and two gears 4, 4 fitted on the nut 2. Oil pockets comprising a lot of minute hollow parts are formed on at least one sliding surface of the screw groove 1a of the screw shaft 1, a screw groove 2a of the nut 2 and rolling surfaces 3a of the rollers 3. Arithmetical mean roughness Ra of a surface on which the oil pockets are formed is set to 0.4μm or less. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a planetary roller screw device.

  Conventional planetary roller screw devices include a screw shaft having a helical thread groove formed on the outer peripheral surface, a nut having a screw groove facing the screw groove of the screw shaft formed on the inner peripheral surface, and a gap between both screw grooves. A roller row composed of a plurality of rollers that are rotatably mounted on the roller, and a cage that rotatably supports the rollers at both ends of the roller row is interposed between the screw shaft and the nut. 2. Description of the Related Art A gear in which gears meshing with teeth formed on rollers at both ends in a row are fitted to a screw shaft or a nut is known (see, for example, Patent Document 1).

US Pat. No. 2,683,379 Japanese Patent No. 1594395 Japanese Patent No. 3357661 JP 2004-316699 A

The planetary roller screw device has a lower friction efficiency between the rolling element and the screw groove than the ball screw (that is, because the friction of the sliding surface is large), so that the operation efficiency for converting the rotational motion into the linear motion in the axial direction is low. Has its own problems. The causes of increased friction are as follows.
First, the first cause is slippage. In the planetary roller screw device, the roller rolls (rotates) and revolves around the screw shaft in a spiral shape. However, since the roller traveling direction and the rotation shaft are not perpendicular, the roller slides in the axial direction. is doing. In the ball screw, since the traveling direction of the ball, which is a rolling element, and the rotation axis are substantially perpendicular, slipping as large as that of the planetary roller screw device does not occur.

  Next, the second cause is the surface roughness of the sliding surface. In ball screws, the balls are super-finished, and the thread grooves, which are the sliding surfaces on which the balls come into sliding contact, are often subjected to grinding finish processing. Since it is finished with high accuracy of about 1/100 compared with that of the groove, it is possible to keep the lubrication state relatively good. On the other hand, in the planetary roller screw device, both the roller and the screw groove are subjected to grinding finish processing, and those having a relatively rough surface are brought into sliding contact with each other, so that the lubrication state is kept good. Is difficult.

As described above, since the sliding is large and the surface roughness of the sliding surface is rough, the planetary roller screw device has a friction on the sliding surface larger than that of the ball screw, resulting in lower operating efficiency. In particular, since the formation of an oil film becomes insufficient during low-speed operation, the operating efficiency tends to be lower than during high-speed operation. This low operating efficiency may cause an increase in heat generation and an increase in power consumption of the drive device.
Accordingly, the present invention solves the problems of the conventional planetary roller screw device as described above, and the planetary roller excellent in operating efficiency for converting the rotational motion into the linear linear motion with small friction on the sliding surface. It is an object to provide a screw device.

  In order to solve the above problems, the present invention has the following configuration. That is, in the planetary roller screw device according to the first aspect of the present invention, a screw shaft in which a spirally continuous screw groove is formed on the outer peripheral surface, and a screw groove facing the screw groove of the screw shaft on the inner peripheral surface. A formed nut and a plurality of rollers interposed so as to be freely rollable between the both screw grooves, and by the relative rotation movement of the screw shaft and the nut, via the rolling of the roller, In the planetary roller screw device in which the screw shaft and the nut move relative to each other in the axial direction, at least one of a screw groove of the screw shaft, a screw groove of the nut, and a rolling surface of the roller The oil reservoir is formed of a large number of minute recesses so that the arithmetic average roughness Ra is more than 0.05 μm and not more than 0.4 μm.

  The planetary roller screw device according to a second aspect of the present invention is the planetary roller screw device according to the first aspect, wherein the oil reservoir is formed by shot peening in which a shot material is sprayed at an injection speed of 100 m / s or more. The hardness of the shot material is equal to or greater than the hardness of the surface to be treated for shot peening, and the particle size is 40 μm or more and 200 μm or less.

Furthermore, the planetary roller screw device according to claim 3 of the present invention is the planetary roller screw device according to claim 1 or 2, wherein the oil sump includes two or more shots using different types of shot materials. It is formed by peening.
Furthermore, the planetary roller screw device according to claim 4 of the present invention is the planetary roller screw device according to any one of claims 1 to 3, wherein the axial position of the roller is constrained by the shaft or the nut. In addition, the oil reservoir is formed in one or both of the shaft and the nut that are not constraining the axial position of the roller and the roller.

  The planetary roller screw device of the present invention has a small friction on the sliding surface, and is excellent in operating efficiency for converting a rotational motion into an axial linear motion.

An embodiment of a planetary roller screw device according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a sectional view of a planetary roller screw device according to an embodiment of the present invention, taken along a plane parallel to the axial direction. In the drawings used for the following description, the same or corresponding parts are denoted by the same reference numerals.
As shown in FIG. 1, the planetary roller screw device of the present embodiment includes a screw shaft 1 having a substantially V-shaped screw groove 1 a formed in a spiral shape on the outer peripheral surface, and a screw groove 1 a of the screw shaft 1. A roller array comprising a nut 2 having a substantially V-shaped thread groove 2a opposed to the inner peripheral surface thereof, and a plurality of rollers 3 interposed between the thread grooves 1a and 2a so as to be able to roll. And two gears 4 and 4 fitted to the inner peripheral surface of the nut 2.

The outer peripheral surface of the roller row is formed in a screw shape so as to mesh with both screw grooves 1a, 2a, and the screw threads 1a, 2a and the roller 3 are all twisted in the same direction. The material of the screw shaft 1, nut 2, and roller 3 is not particularly limited, but steel such as SCM415H is preferable.
The gears 4 and 4 are disposed apart from each other in the axial direction, and mesh with the teeth 6 formed on the rollers 3 ′ and 3 ′ at both ends of the roller train. The screw shaft 1 and the nut 2 are moved relative to each other in the axial direction through the rolling of the roller 3 by the relative rotational movement of the screw shaft 1 and the nut 2.

  This planetary roller screw device is provided with two annular cages 5 and 5 and is interposed between the screw shaft 1 and the nut 2 and outside the rollers 3 ′ and 3 ′ at both ends in the axial direction. . The annular cages 5 and 5 are provided with a plurality of roller support holes 5a along the circumferential direction, and the protrusions 7 projecting outward in the axial direction provided in the rollers 3 ′ and 3 ′ at both ends are provided in the roller support holes 5a. The annular cages 5 and 5 support the rollers 3 ′ and 3 ′ so as to be rotatable. The roller support hole 5a may be a through hole or a bottomed hole.

  Further, a retaining ring 9 is attached to the nut 2 on the outer side in the axial direction of the annular cages 5 and 5 so as to contact the annular cages 5 and 5, and the protrusion 7 of the roller 3 ′ is connected to the annular cage 5. Is prevented from coming out of the roller support hole 5a. The retaining ring 9 also has a function of preventing the annular retainer 5 from being displaced in the axial direction relative to the nut 2 and preventing the annular retainer 5 from falling off the planetary roller screw device. The retaining ring 9 may be integral with the nut 2 or may be a separate member.

  Further, an oil sump (not shown) made up of a number of minute recesses is formed on at least one sliding surface of the screw groove 1a of the screw shaft 1, the screw groove 2a of the nut 2, and the rolling surface 3a of the roller 3. The arithmetic mean roughness Ra of the surface on which the oil sump is formed is 0.05 μm and 0.4 μm or less. The shape of the minute concave portion constituting the oil reservoir is not particularly limited, but a concave portion having a concave surface having an arcuate cross section as shown in FIG. 2 is preferable.

  Since the planetary roller screw device of the present embodiment is provided with an oil reservoir for holding lubricating oil on the sliding surface, an oil film is sufficiently formed between the sliding surfaces, and the lubrication state is kept good. Therefore, the friction of the sliding surface is small, and the operation efficiency for converting the rotational motion into the linear motion in the axial direction is excellent. Since the operation efficiency is excellent, heat generation and seizure are suppressed, and the power consumption of the drive device is also suppressed. Furthermore, since friction is reduced, wear is less likely to occur.

  When a low viscosity lubricant is used, or when a high viscosity is used even when a high viscosity lubricant is used, the formation of an oil film becomes insufficient and friction is likely to occur due to metal contact, especially at low speeds. Sometimes friction is likely to occur. However, if an oil reservoir is formed on the sliding surface, an oil film is sufficiently formed between the sliding surfaces even under conditions where the formation of the oil film is likely to be insufficient. The friction of is small. Therefore, the operation efficiency which converts rotational motion into axial linear motion is excellent.

  The method for forming the oil reservoir is not particularly limited, but shot peening in which a shot material is sprayed is preferable. The conditions for shot peening are not particularly limited, but the shot material injection speed is preferably 100 m / s or more. If the injection speed is less than 100 m / s, there is a risk that the kinetic energy of the shot material is insufficient and a favorable surface roughness cannot be obtained. The material of the shot material is preferably metal or ceramic.

  Furthermore, the hardness of the shot material is preferably equal to or greater than the hardness of the surface to be treated (sliding surface) on which shot peening is performed, and the particle size of the shot material is preferably 40 μm or more and 200 μm or less. If the particle size of the shot material is less than 40 μm, there is a possibility that the processing time becomes long, and if it exceeds 200 μm, it is difficult to make the arithmetic average roughness Ra 0.4 μm or less. May occur.

  However, it is preferable to form the oil reservoir by two or more shot peenings using different types of shot materials, rather than by one shot peening. As an example, a method for forming an oil sump by two shot peenings will be described. First, as the first shot peening, shot peening is performed using silicon carbide powder having a relatively sharp portion on the surface as a shot material. As a result, excessive shot peening is applied to the sliding surface to form minute concave portions and minute convex portions, so that the arithmetic average roughness Ra exceeds 0.4 μm and is close to 1 μm. Since metal contact is likely to occur, second shot peening is performed here. In the second shot peening, alumina powder and silica beads having a relatively smooth surface are used as a shot material, and the above-mentioned minute projections are flattened. Since the minute recesses remain as they are, many flat portions can be formed and deep minute recesses can be formed.

  In addition, this embodiment shows an example of this invention and this invention is not limited to this embodiment. For example, the sliding surface that forms the oil reservoir is not particularly limited, and any one of the sliding groove 1a of the screw shaft 1, the thread groove 2a of the nut 2, and the rolling surface 3a of the roller 3 is slid. It may be formed on a surface, may be formed on two of the three sliding surfaces, or may be formed on all three sliding surfaces.

  However, when the axial position of the roller 3 is restrained by the nut 2 as in the planetary roller screw device of this embodiment (the two gears 4 and 4 fitted to the nut 2 and the rollers at both ends of the roller train) Since the teeth 6 formed on the 3 ′ and 3 ′ mesh with each other, the axial position of the roller 3 is constrained by the nut 2). It is preferable to form an oil reservoir in one or both of the screw shaft 1 and the roller 3, that is, in one or both of the thread groove 1 a of the screw shaft 1 and the rolling surface 3 a of the roller 3.

  Then, since the friction of the sliding surface becomes smaller, the operation efficiency for converting the rotational motion into the linear motion in the axial direction becomes better. This is because the relationship between the roller 3 and the nut 2 is the same as that of the helical gear, so that the roller 3 is almost rolling with respect to the nut 2, but the roller 3 is circumferential with respect to the screw shaft 1. This is considered to be due to the sliding motion in the axial direction along with the rolling motion. Since most of the friction is due to the sliding motion between the roller 3 and the screw shaft 1, the friction can be reduced by forming an oil reservoir in one or both of the thread groove 1a of the screw shaft 1 and the rolling surface 3a of the roller 3. Can do.

  On the other hand, when the axial position of the roller 3 is constrained by the screw shaft 1 as in the planetary roller screw device shown in FIG. 3 (the two gears 4 and 4 fitted to the screw shaft 1 and the roller train). Since the teeth formed on the rollers 3 ′ and 3 ′ at both ends mesh with each other, the axial position of the roller 3 is not constrained by the screw shaft 1). It is preferable to form an oil reservoir in one or both of the nut 2 and the roller 3 which are the other members, that is, in one or both of the thread groove 2a of the nut 2 and the rolling surface 3a of the roller 3.

  As in the case described above, the relationship between the roller 3 and the screw shaft 1 is the same as that of the helical gear, so that the roller 3 is almost rolling with respect to the screw shaft 1, but the roller 3 is a nut 2. On the other hand, it slides in the axial direction along with the rolling motion in the circumferential direction. Since most of the friction is due to the sliding motion between the roller 3 and the nut 2, the friction can be reduced by forming an oil reservoir in one or both of the thread groove 2a of the nut 2 and the rolling surface 3a of the roller 3. .

Hereinafter, the structure of the planetary roller screw device of FIG. 3 will be described in detail. The configuration and operation of the planetary roller screw device of FIG. 3 are almost the same as those of the planetary roller screw device of FIG.
The planetary roller screw device of FIG. 3 has a screw shaft 1 having a substantially V-shaped screw groove 1a formed on the outer peripheral surface thereof and a substantially V-shaped cross section facing the screw groove 1a of the screw shaft 1. The screw shaft 2 is fitted to a screw shaft 1 and a nut 2 having a screw groove 2a formed on the inner peripheral surface, a roller train composed of a plurality of rollers 3 interposed between the screw grooves 1a and 2a so as to be able to roll. Two gears 4 and 4 are provided. The outer peripheral surface of the roller row is formed in a screw shape so as to mesh with both screw grooves 1a and 2a, and the twist directions of both screw grooves 1a and 2a are the same, but the twist direction of the roller 3 is the both screws. The direction of twisting of the grooves 1a and 2a is opposite.

  The gears 4 and 4 are arranged apart from each other in the axial direction, and mesh with the teeth formed on the rollers 3 'and 3' at both ends of the roller train. Further, a retaining ring 9 is attached to the outer side in the axial direction of the annular cages 5 and 5 on the screw shaft 1 so as to come into contact with the annular cages 5 and 5, and the protrusion 7 of the roller 3 ′ is connected to the annular cage 5 is prevented from coming out of the roller support hole 5a. Further, the retaining ring 9 has a function of preventing the annular cage 5 from being displaced in the axial direction with respect to the screw shaft 1 and preventing the annular cage 5 from falling off the planetary roller screw device. The retaining ring 9 may be integral with the screw shaft 1 or may be a separate member.

〔Example〕
The present invention will be described more specifically with reference to the following examples. A planetary roller screw device having substantially the same configuration as that of the planetary roller screw device of FIG. 1 was prepared, and its performance was evaluated. The specifications of the planetary roller screw device used are as follows.
Effective thread groove diameter: 29.840 mm screw shaft, 50 mm nut, 10 mm roller
Lead: Screw shaft 10mm, nut 10mm, roller 2mm
Number of threads: 5 screw shafts, 5 nuts, 1 roller Effective meshing length: 25 mm
Number of rollers: 6 Load: 3750N
Lubricating oil: Daphne Mechanic Oil 68 manufactured by Idemitsu Kosan Co., Ltd.

The following shot peening was performed on the sliding surface of such a planetary roller screw device. First, first shot peening was performed using silicon carbide powder having an average particle diameter of 45 μm as a shot material to form minute convex portions and minute concave portions. The conditions are as follows.
Injection speed: 200m / s
Injection pressure: 0.39 MPa
Injection time: 10min
Injection distance: 15-40mm

  The injection distance is a distance between the shot material injection port and the material to be processed (sliding surface). The arithmetic average roughness Ra before the first shot peening was 0.4 μm or less and the maximum height Ry was 2 to 3 μm. However, excessive shot peening was applied by such first shot peening, The gloss of the moving surface was lost, the arithmetic average roughness Ra exceeded 0.4 μm, and the maximum height Ry was 6 μm or more.

  Next, the second shot peening was performed on the sliding surface subjected to the first shot peening using silica beads having an average particle diameter of 45 μm as a shot material. The conditions are the same as in the first shot peening. Due to the second shot peening, the minute projections generated by the first shot peening are scraped off and the flat parts increase (the minute depressions remain as oil pools), and the gloss is restored to the sliding surface. Ra was 0.4 μm or less.

  The planetary roller screw device in which the oil reservoir was formed on such a sliding surface was driven to measure the operation efficiency for converting the rotational motion into the linear motion in the axial direction. In Example 1, an oil sump is formed only on the rolling surface of the roller, and in Example 2, an oil sump is formed on the thread groove of the screw shaft and the rolling surface of the roller. . In the comparative example, an oil sump is not formed.

  The results are shown in the graph of FIG. The operation efficiency of the planetary roller screw device of the comparative example decreased as the operation speed decreased. In contrast, the planetary roller screw device of Example 1 was superior in operating efficiency to the comparative example because the lubrication state was good even during low-speed operation. In addition, the planetary roller screw device of Example 2 was more excellent in operating efficiency than Example 1 because oil pools were formed in the thread groove of the screw shaft and the rolling surface of the roller.

  Next, in the planetary roller screw device of the second embodiment, by changing the processing time (injection time) of the second shot peening, various slides having different arithmetic average roughness Ra are prepared, and the rotational speed is 10 rpm. The operating efficiency during low-speed operation was measured. The results are shown in the graph of FIG. As a result, when the arithmetic average roughness Ra exceeded 0.4 μm, the operating efficiency was the same level as that of the comparative example. When the processing time of the second shot peening is too short, the arithmetic average roughness Ra is substantially the same level as that of the one not subjected to the second shot peening (only the first shot peening), but the operating efficiency is It was worse than the comparative example. Therefore, the processing time for the second shot peening is preferably set to a time such that the arithmetic average roughness Ra is 0.4 μm or less.

  The lower limit value of the arithmetic average roughness Ra is preferably more than Ra 0.05 μm as the surface roughness of the raceway surface of the bearing not based on shot peening (see, for example, Japanese Patent Application Laid-Open No. 10-311337). The lower limit Ra of more than 0.15 μm is more preferable. In addition, when only the second shot peening is performed without performing the first shot peening, a deep minute recess having a depth of 3 μm or more is not formed, and the operation efficiency of such a planetary roller screw device is almost the same as that of the comparative example. It was the same level.

It is sectional drawing which fractured | ruptured the planetary roller screw apparatus which is one Embodiment of this invention by the plane parallel to an axial direction. It is a schematic diagram explaining an oil sump. It is sectional drawing which fractured | ruptured the planetary roller screw apparatus of the modification of this embodiment by the plane parallel to an axial direction. It is a graph which shows the relationship between the operating speed (rotational speed) of a planetary roller screw device, and work efficiency. It is a graph which shows the relationship between arithmetic mean roughness Ra of the sliding surface of a planetary roller screw device, and working efficiency.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Screw shaft 1a Thread groove 2 Nut 2a Thread groove 3 Roller 3 'End roller 3a Rolling surface 4 Gear

Claims (4)

A screw shaft in which a spiral groove is formed on the outer peripheral surface, a nut formed on the inner peripheral surface of the screw shaft opposite to the screw groove of the screw shaft, and a roll between the screw grooves. A plurality of interposed rollers, and the screw shaft and the nut move relative to each other in the axial direction through the rolling of the roller by the relative rotational movement of the screw shaft and the nut. In the planetary roller screw device,
At least one of the screw groove of the screw shaft, the screw groove of the nut, and the rolling surface of the roller is composed of a large number of minute recesses so that the arithmetic average roughness Ra exceeds 0.05 μm and is 0.4 μm or less. A planetary roller screw device characterized in that an oil reservoir is formed.   The oil reservoir is formed by shot peening in which shot material is sprayed at an injection speed of 100 m / s or more, and the hardness of the shot material is equal to or greater than the hardness of the surface to be treated to which shot peening is applied. The planetary roller screw device according to claim 1, wherein the diameter is 40 μm or more and 200 μm or less.   The planetary roller screw device according to claim 1 or 2, wherein the oil reservoir is formed by two or more shot peenings using different types of shot materials.   The axial position of the roller is constrained by the shaft or the nut, and one or both of the roller and the member that does not constrain the axial position of the roller among the shaft and the nut, The planetary roller screw device according to any one of claims 1 to 3, wherein an oil sump is formed.

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