JP2006077870A - Automobile ball screw - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an automobile ball screw stably less in noises, easy to manufacture with no cost increase, and free of a reduction in load capacity. <P>SOLUTION: A ball 4 is laid between rolling faces 6a, 7a of opposed ball screw grooves 6, 7 of a screw shaft 2 and a nut 3, respectively. The screw shaft 2 has the ball screw groove 6 rolled and the rolling face 6a subjected to surface finishing work such as superfinishing work after heat treatment. The nut 3 is supported by two bearings 12, 13. The rolling face 6a of the ball screw groove 6 of the screw shaft 2 undulates 2 μm or less in the rolling direction. One bearing 12 out of two bearings 12, 13 supporting the nut 3 is a four-point contact ball bearing whose rolling face is subjected to surface finishing work such as superfinishing work. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electric power steering device, a rear wheel steering device, and other ball screws for automobiles used for various other automobile applications.

  In the electric power steering apparatus, it is an important issue to reduce the noise level generated from the ball screw, and various countermeasures have been proposed so far. One example is a noise reduction measure for a noise generation mechanism that assumes that when a moment acts on a ball screw, the nut tilts and the load of the ball at both ends of the nut increases, so that noise is generated. This is because the effective diameter of the thread groove at both ends of the nut is made larger than that at the center of the nut, thereby changing the amount of clearance between the thread groove of the screw shaft and the thread groove of the nut to improve the load distribution. The noise level is reduced by reducing the load (for example, Patent Document 1).

Another example is to reduce the noise generated from the ball screw by differentiating the accumulated lead of the screw groove of the nut and the screw groove of the screw shaft and applying a preload to the ball interposed between both screw grooves. There is (for example, Patent Document 2).
Japanese Patent Laid-Open No. 11-11334 JP 2001-114117 A

Usually, in a product using a ball screw for automobiles, the moment amount acting on the ball screw and the inclination of the nut are not necessarily constant. For this reason, when the effective diameter of the thread groove at both ends of the nut is increased as in the noise reduction measures disclosed in Patent Document 1, the load distribution on each ball becomes non-uniform depending on the assembled state. There is also a possibility that the lifetime is shortened compared to the above. Further, the processing cost also increases as compared with a general ball screw.
Further, as a noise reduction measure disclosed in Patent Document 2, it is possible to apply a preload to the ball screw by making the accumulated leads of the screw grooves of the nut and screw shaft different from each other. Precise machining or matching of screw shaft and nut is required, which increases machining and assembly costs.
Furthermore, with the noise reduction measures disclosed in Patent Document 2, since only half of all balls can receive a load with respect to an axial load in one direction, the load capacity can be reduced, and the load capacity equivalent to that of the conventional product can be reduced. If you want to get it, you have to lengthen the nut.

  An object of the present invention is to provide a ball screw for a vehicle that can stably reduce the generation of noise, can be easily manufactured, can suppress an increase in cost, and does not cause a problem of a reduction in load capacity.

According to a first aspect of the present invention, there is provided a ball screw for automobiles in which a ball is interposed between rolling surfaces of opposing ball screw grooves of a screw shaft and a nut. In this case, the rolling surface is subjected to surface finishing after the heat treatment, and the nut is supported by two bearings. The surface finishing process is, for example, a super finishing process.
According to this configuration, since the ball screw groove of the screw shaft is rolled and the rolling surface is finished after heat treatment, the rolling direction undulation of the rolling surface in the ball screw groove is improved, Noise is reduced. If the radial load is applied to the nut for some reason, for example, the radial load generated when the nut is driven, the assembly accuracy, or the like, the noise reduction effect may not be sufficiently exhibited. The effect of reducing noise due to waviness becomes smaller as the radial load increases. However, since the nut is supported by the two bearings, the radial load applied to the nut is reduced. Therefore, the noise reduction effect due to the improvement of the swell is stably exhibited. In addition, since it reduces noise by rolling the screw shaft, surface finishing after heat treatment, and supporting the nut with two bearings, it is easy to manufacture and reduces noise while reducing costs. Therefore, there is no problem of reducing the load capacity.

  In this invention, it is preferable that the undulation in the rolling direction of the rolling surface in the ball screw groove of the screw shaft is 2 μm or less. When the undulation in the rolling direction is 2 μm or less, the noise reduction effect is further improved.

According to a second aspect of the present invention, there is provided a ball screw for automobiles in which a ball is interposed between rolling surfaces of ball screw grooves facing each other of a screw shaft and a nut, and the nut is supported by two bearings. Of the individual bearings, one of the bearings is a four-point contact ball bearing in which the rolling surface is finished. The surface finishing process is, for example, a super finishing process.
When the nut is supported by the two bearings, the radial load applied to the nut for some reason as described above is reduced. In this case, since one of the bearings is a four-point contact ball bearing, a thrust load can be applied, and the range of the axial clearance relative to the radial clearance can be suppressed. The backlash after assembling can be suppressed. The four-point contact ball bearing has a Gothic arch-shaped cross-section with a combination of two arcs on its rolling surface and is difficult to machine. Conventionally, surface finishing such as super-finishing has not been performed only by grinding. By applying surface finishing, the rotation of the inner ring is stabilized, and the radial load acting on the nut due to various factors can be further reduced. As a result, the generation of noise can be stably reduced. Further, since a four-point contact ball bearing is used, the nut can be supported with a compact configuration, unlike the case of using a double row bearing. Unlike the combination of two single-row angular ball bearings, two bearings are used to support the nut, and one of them is a four-point contact ball bearing, regardless of errors in assembly of the two bearings. Nut rotation support can be performed with higher accuracy. Therefore, the noise reduction effect is excellent.

  In the present invention, the four-point contact ball bearing preferably has a rolling surface waviness of 1 μm or less by surface finishing of the rolling surface. When the waviness is 1 μm or less, the noise reduction effect is further improved in combination with the support of the nut by the two bearings and the use of the surface-finished four-point contact ball bearing.

  In the first aspect of the present invention, one of the two bearings supporting the nut may be a four-point contact ball bearing having a finished rolling surface. In the case of this configuration, the noise reduction effect is further improved.

  In this case, the four-point contact ball bearing may have a rolling surface swell of 1 μm or less by surface finishing of the rolling surface. When the waviness of the rolling surface of the four-point contact ball bearing is 1 μm or less, the noise reduction effect is further improved.

The automotive ball screw according to the present invention is the automotive ball screw in which a ball is interposed between the rolling surfaces of the ball screw groove facing the screw shaft and the nut. The screw shaft is formed by rolling the ball screw groove. Since the rolling surface is finished after heat treatment and the nut is supported by two bearings, the generation of noise can be stably reduced, and the manufacturing is simple and the cost increase is suppressed. Therefore, the problem of reducing the load capacity can be prevented.
Another ball screw for an automobile of the present invention is an automobile ball screw in which a ball is interposed between rolling surfaces of ball screw grooves facing each other of a screw shaft and a nut. The nut is supported by two bearings. Since one of the individual bearings is a four-point contact ball bearing with a finished rolling surface, noise generation can be stably reduced, and manufacturing is simple and restrains cost increases. The load capacity reduction problem can be avoided.

  An embodiment of the present invention will be described with reference to the drawings. The automotive ball screw 1 includes a screw shaft 2, a nut 3, and a plurality of balls 4, and the nut 3 is provided with a circulation portion 5A. The screw shaft 2 is a shaft in which a spiral ball screw groove 6 is formed on the outer peripheral surface. The nut 3 is externally fitted to the screw shaft 2 in a loosely fitted state, and a spiral ball screw groove 7 is formed on the inner peripheral surface. The surface of each ball screw groove 6, 7 becomes a rolling surface 6 a, 7 a of the ball 4. The ball 4 is accommodated in a rolling path 8 formed between the ball screw grooves 6 and 7 of the screw shaft 2 and the nut 3 facing each other. The circulation part 5A is a means in which both ends are open on the inner surface of the ball screw groove 7 of the nut 3 and a circulation path 9 is formed for circulating the ball 4 of the rolling path 8 so that the ball 4 is picked up at one end and returned at the other end. Due to the circulation path 9, the rolling path 8 becomes a continuous circulation path of one round. The circulating portion 5A is formed of the piece member 5 in this embodiment. A plurality of piece members 5 are provided on the nut 3.

The screw shaft 2 has a ball screw groove 6 formed by rolling, and the rolling surface 6a is subjected to surface finishing. Specifically, the screw shaft 2 is subjected to surface finishing on the rolling surface 6a for the material manufactured in the steps of (1) rolling, (2) heat treatment, (3) bending correction, and (4) scale reduction. It is supposed to have been applied. This surface finishing process is, for example, a super finish process. Superfinishing is also called superfinishing. The undulation in the rolling direction of the rolling surface 6a, which is the surface-finished surface, is preferably 2 μm or less.
Note that the finishing process of the rolling surface 6a is not limited to the super-finishing process, and any process may be used as long as it can improve the rolling direction waviness and roughness. .

  The nut 3 is supported by two bearings 12 and 13. These bearings 12 and 13 may be single-row bearings or double-row bearings as long as they can load radial loads and thrust loads. Here, of these two bearings 12 and 13, one bearing 12 is a four-point contact ball bearing shown in FIG. 2, and the other bearing 13 is a deep groove ball bearing. The four-point contact ball bearing used as the bearing 12 includes an inner ring 14, an outer ring 15, and a plurality of balls 16 interposed between the inner and outer rings 14, 15. The rolling surface 14a formed on the outer peripheral surface of the inner ring 14 and the rolling surface 15a formed on the inner peripheral surface of the outer ring 15 have a pair of centers of curvature that are offset equidistantly in the axial direction with respect to the axial center of the bearing. It is formed in a circular arc surface, so-called Gothic arch shape. Therefore, although this four-point contact ball bearing is a single row bearing, it can carry a radial load and a thrust load in both directions. Moreover, the range of the axial clearance with respect to the radial clearance can be suppressed, and the backlash after the ball screw 1 is assembled to, for example, the electric power steering device can be suppressed. The fixing method for the bearings 12 and 13 in the axial direction is not mentioned here, but any fixing method may be used.

According to the ball screw 1 for an automobile having this configuration, the ball screw groove 6 of the screw shaft 2 is rolled and the surface finish is applied to the rolling surface 6a after the heat treatment, and the nut 3 is supported by the two bearings 12 and 13. Therefore, the rolling direction undulation of the rolling surface 6a in the ball screw groove 6 of the screw shaft 2 can be improved, and the radial load applied to the nut 3 can be reduced. For this reason, even if a radial load is generated on the ball screw 1 due to some factor or the radial load is large, noise generated from the ball screw 1 can be reduced. When the undulation in the rolling direction of the rolling surface 6a in the ball screw groove 6 of the screw shaft 2 is 2 μm or less, the noise reduction effect is further improved.
Further, of the two bearings 12 and 13 that support the nut 3, one of the bearings 12 is a four-point contact ball bearing in which the rolling surfaces 14 a and 15 a are subjected to surface finishing. The rolling direction waviness of the rolling surfaces 14a and 15a in the bearing can also be improved, and the noise reduction effect becomes remarkable. When the waviness of the rolling surfaces 14a and 15a of the four-point contact ball bearing 12 is 1 μm or less, the noise reduction effect is further improved.

3 (A) and 3 (B) show cross-sectional curves at each manufacturing stage in which the rolling direction undulation for one lead of the screw shaft rolling surface 6a is evaluated. Of these, FIG. 3 (A) shows a cross-sectional curve after heat treatment, bending correction, and scale reduction (before surface finishing), and FIG. 3 (B) shows a cross-sectional curve after surface finishing. Show.
3 (A) and 3 (B), it can be seen that the rolling direction waviness generated by the heat treatment is improved by the surface finishing process. That is, the rolling direction undulation before the surface finishing shown in FIG. 3A is 2.5 μm, but the rolling direction undulation after the surface finishing is 2 μm or less (1.5 μm in FIG. 3B). It has become.

  In general, four-point contact ball bearings are used in the low rotation range, and the rolling surface has a Gothic arch shape, which makes it difficult to process. Conventionally, the rolling surface is only grinding. The surface finish is not applied. However, here, the rolling surfaces 14a and 15a of the bearing 12 are subjected to surface finishing after grinding, and in particular, the rolling surfaces 14a and 15a have a shape such as roughness and waviness in the rolling direction of the balls 16. The error component has been improved.

  4A and 4B show the surface roughness in the axial direction on the rolling surface 14a of the inner ring 14 of the bearing 12 depending on the presence or absence of surface finishing. The sample is a four-point contact ball bearing having an inner diameter φ40, an outer diameter φ75, and a width of 16 mm. As can be seen, the surface roughness of the rolling surface after the surface finishing process (FIG. 4B) is approximately ½ compared to the rolling surface before the surface finishing process shown in FIG. Has been improved. The surface roughness in the rolling direction is preferably Ra0, 1 or less, and the maximum amplitude of rolling direction waviness is preferably regulated to 1 μm or less. The surface finish processing in this case is also super finish processing or lapping processing, but is not limited to this, and any processing method that can improve rolling direction waviness and roughness may be used.

As noise reduction measures, (1) Rolling screw shafts with super processing or the like to suppress rolling direction waviness to 2 μm or less, and (2) Four-point contact ball bearings are used in the nut support bearings. Proposals have been made to suppress the rolling undulation of the rolling surface to 1 μm or less.
Actually, when the nut is used in a state where no radial load is applied, the effect of reducing the noise of the configurations (1) and (2) is great.
However, even when (1) and (2) are used alone or both, if a radial load is applied to the nut due to some factor (for example, radial load or assembly accuracy generated when the nut is driven), noise is reduced. It was found that the effect was small. Therefore, as in the above embodiment, the nut 3 is supported by the two bearings 12 and 13 including the four-point contact ball bearings by adopting one or both of the above (1) and (2). The load of radial load on the threaded portion of 1 was reduced. Thus, it has been found that the effects (1) and (2) are maintained, and noise can be reduced.

  FIG. 5 is an explanatory diagram showing the results of a noise comparison test. The figure compares the noise level of the ball screw 1 of this embodiment with the noise level of other ball screws, where the vertical axis indicates the noise level and the horizontal axis indicates the classification of each ball screw. Each is shown. In the same figure, the data enclosed by the broken line (sign ▲) is for a ball screw that supports the nut with two bearings (one of which is a four-point contact ball bearing and the other is a deep groove ball bearing). The other data (symbols ◆) indicate the ball screw that supports the nut with one 4-point contact ball bearing.

In the horizontal axis, section A is a screw shaft whose surface has been subjected to surface finishing (see FIG. 3B) and a four-point contact ball bearing which has been subjected to surface finishing (FIG. 4). A ball screw using (see (B)) is shown.
Section B is a ball that uses a surface finish processed as a screw shaft (see FIG. 3 (B)) and a 4-point contact ball bearing that does not have a surface finish processed (see FIG. 4 (A)). Indicates screw.
For category C, balls using a screw shaft not subjected to surface finishing (see FIG. 3A) and a four-point contact ball bearing subjected to surface finishing (see FIG. 4B) Indicates screw.
Section D shows a ball screw that uses a screw shaft and a four-point contact ball bearing that are not subjected to surface finishing (see FIGS. 3A and 4A).

The ball screw specifications, bearing specifications, and test conditions in the noise comparison test are as follows.
[Ball screw specifications]
Bearing: φ25.4
Lead: 6mm
Ball diameter: 5/32
Number of turns x number of rows: 1 x 4
[Bearing specifications]
4-point contact bearing: φ40 × φ75 × 16
Deep groove bearing: 6908 (φ40 × φ62 × 12)
〔Test conditions〕
Radial load: 120N
Nut rotation speed: 550rpm
Microphone position: 300mm from the nut

  From this noise comparison result, it can be seen that the noise level of the ball screw 1 is reduced by supporting the nut 3 with the two bearings 12 and 13 as in the embodiment. Further, when surface finishing is applied to the rolling surfaces of the screw shaft 2 and the four-point contact ball bearing 12, surface finishing processing is further applied to both the rolling surfaces of the screw shaft 2 and the four-point contact ball bearing 12. It can be seen that the noise level of the ball screw 1 is further reduced in the case where the above is applied.

  In the above embodiment, the circulation portion 5A is a piece type formed by the piece member 5, but the present invention can be applied regardless of the circulation method of the circulation portion 5A. The circulation system of the circulation unit 5A is roughly divided into an internal circulation system and an external circulation system, but any system may be used. The internal circulation system is a system in which the balls 4 are circulated inside the nut 3, and there is an end cap type in which a circulation part is formed by an end cap in addition to the above-mentioned piece type. The external circulation system is a system in which the ball 4 is circulated outside the main body of the nut 3. As a representative example, there are a return tube type in which a circulation part is formed by a return tube and a guide plate type in which a circulation part is formed in a guide plate. is there. The present invention can be applied to each of these circulation types.

FIG. 6 shows an example of a wheel steering apparatus equipped with the automotive ball screw of the present invention. This steering device is an electric power steering device for an automobile. In the figure, the housing 21 has a bracket (not shown) and is fixed to the vehicle body. A screw shaft 2 of the ball screw 1 serving as a steering shaft passes through the housing 21, and tie rods 23 and 24 are connected to both ends of the ball screw shaft 2.
A handle shaft 25 is provided so as to extend obliquely upward from the vicinity of one end of the housing 21, and a handle is connected to the upper end of the handle shaft 25. The handle shaft 25 is rotatably supported, and the rotation of the handle shaft 25 is transmitted as a moving force in the axial direction from the lower end thereof to the ball screw shaft 2 via a wrap pinion mechanism 26 that is a conversion mechanism. The rack and pinion mechanism 26 includes a rack portion 27 formed by a part in the longitudinal direction of the ball screw shaft 2 and a pinion (not shown) provided at the lower end of the handle shaft 25. It meshes with the rack of the rack portion 27 inside. A steering torque detector (not shown) for detecting the steering torque is provided for the handle shaft 25.

  The housing 21 is formed in a cylindrical shape, and is configured by connecting end members 21b and 21c to both ends of a central cylindrical body 21a. A stator 29 of the electric motor 28 is provided in the axial center of the housing 21. The stator 29 includes a core and a stator coil. On the inner peripheral side of the stator 29, a rotor 30 of the electric motor 28 is provided via a gap. The rotor 30 is formed in a cylindrical shape by a magnet or a magnetic material, and is attached to the outer periphery of the extension portion 3a of the nut 3 of the ball screw 1 so as to rotate integrally with the nut extension portion 3a. The electric motor 28 is controlled according to the detected value of the steering torque detector by a motor control circuit (not shown).

  One end of the ball screw shaft 2, in this example, the end on the handle shaft 25 side is supported by a bush (not shown) on the back surface of the rack portion 27 in the housing 21, and the other end is attached to the housing 21. It is supported by the guide sleeve 35 in the thick cylindrical shaft support 21d provided. The shaft support portion 21 d is provided on the end member 21 b of the housing 21.

  The nut 3 of the ball screw 1 is rotatably supported in the housing 21 by two bearings 12 and 13 and the rotor 30 of the electric motor 28 is fitted on the outer diameter surface of the nut extension 3a. ing. The rotation of the electric motor 28 is transmitted through the ball screw 1 as a force for moving the ball screw shaft 2 in the axial direction.

According to the electric power steering device, when the vehicle is in a straight traveling state and the rotation of the handle is stopped, a torque signal is not output from a steering torque detector (not shown) of the handle shaft 25, and the motor control means (Not shown) causes the electric motor 28 to stop rotating. Therefore, this wheel steering device is in a state where no auxiliary steering force is output.
When the steering wheel is steered, a torque signal is output from the steering torque detector of the handle shaft 25, and the electric motor 28 rotates the rotor 30 under the control of the motor control circuit. When the rotor 30 is rotated, the nut 3 of the ball screw 1 is rotated together with the rotor 30, the steering shaft composed of the ball screw shaft 2 is moved in the axial direction, and an auxiliary steering force is generated. In this way, the steering force of the steering wheel is supplemented by the electric motor 28.

  Thus, when the ball screw 1 of the said embodiment is used for a wheel steering device, generation | occurrence | production of a vibration and a noise is reduced.

  In addition to the electric power steering device, the wheel steering device to which the automotive ball screw of the present invention is applied includes an ARS system (Active Rear Stear) of the vehicle, that is, a vehicle that is steered by the front wheels. Thus, the automotive ball screw of the present invention can also be applied to a ball screw in a wheel steering device such as a system for electronically controlling the steering of the angle of the rear wheel.

(A), (B) is the front view and longitudinal cross-sectional view of the ball screw for motor vehicles concerning one Embodiment of this invention, respectively. It is sectional drawing of the 4-point contact ball bearing used for the ball screw. The change of the axial waviness before and after the surface finishing process is shown on the ball screw groove rolling surface of the screw shaft of the ball screw, where (A) shows before processing and (B) shows after processing. It shows the surface roughness in the axial direction before and after surface finishing on the rolling surface of the inner ring of the four-point contact ball bearing used for the ball screw. (A) shows before processing and (B) shows after processing. Show. It is explanatory drawing which shows the noise comparison result which compared the same ball screw with another ball screw. It is a fracture front view of the wheel steering device which applied the ball screw.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Ball screw 2 ... Screw shaft 3 ... Nut 4 ... Ball 6 ... Ball screw groove 6a of screw shaft ... Rolling surface 7 ... Ball screw groove 7a of nut ... Rolling surface 12 ... Bearing (4-point contact ball bearing)
13 ... Bearing (Deep groove ball bearing)

Claims (6)

In an automotive ball screw in which a ball is interposed between rolling surfaces of ball screw grooves facing each other of a screw shaft and a nut,
The screw shaft is a ball screw for automobiles in which a ball screw groove is formed by rolling and surface finishing is performed on a rolling surface after heat treatment, and the nut is supported by two bearings.   2. The automotive ball screw according to claim 1, wherein the rolling surface waviness of the rolling surface in the ball screw groove of the screw shaft is 2 μm or less. In an automotive ball screw in which a ball is interposed between rolling surfaces of ball screw grooves facing each other of a screw shaft and a nut,
An automotive ball screw in which the nut is supported by two bearings, and one of the two bearings is a four-point contact ball bearing having a finished rolling surface.   4. The automotive ball screw according to claim 3, wherein the four-point contact ball bearing has a rolling surface with a waviness of 1 μm or less by surface finishing of the rolling surface.   3. The automotive ball screw according to claim 1, wherein one of the two bearings supporting the nut is a four-point contact ball bearing having a surface finish processed on a rolling surface. 4.   6. The automotive ball screw according to claim 5, wherein the four-point contact ball bearing has a rolling surface with a waviness of 1 μm or less by surface finishing of the rolling surface.

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