JP2004217110A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering device having a superior durability and a high reliability for improving a lubricating state between a worm gear and a worm wheel constituting a gear reducer mechanism, producing no uncomfortable sound such as a tooth striking sound or vibration sound and the like. <P>SOLUTION: A worm 32 acting as a driving wheel is made of metal and a hard carbonic film is formed at the surface of a tooth part. In addition, a worm wheel 31 acting as a driven gear is provided with a cylindrical resin part 43 integrally formed by polyamide resin at an outer circumferential surface of a metallic core 42, and an outer circumferential surface of the resin part 43 has gear teeth 44. The hard carbonic film at the surface of the tooth of the worm 32 is formed by a plasma CVD process and a plasma ion implanting process and the like. The hard carbonic film is formed at the surface of the teeth at the worm 32 to cause a wetness for grease to be improved, enable a lubricating characteristic between the gears to be improved and then it is possible to provide an electric power steering device having a longer life and a higher reliability than those of the prior art. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an electric power steering device, and more particularly to an improvement in lubricity and wear resistance of a gear used in a gear reduction mechanism constituting a power assist unit.
[0002]
[Prior art]
In an electric power steering apparatus for a vehicle, since a relatively high-speed and low-torque electric motor is used, a gear reduction mechanism is incorporated between the electric motor and the steering shaft. As a gear reduction mechanism, a gear reduction mechanism using spur gears or other gears is also known, but a known worm composed of a worm and a worm wheel is used because a large reduction ratio can be obtained in one set. It is common to use a gear reduction mechanism.
[0003]
Such a worm gear reduction mechanism (hereinafter simply referred to as a reduction gear) includes a worm which is a drive gear connected to a rotating shaft of an electric motor, and a worm wheel which meshes with the worm.
[0004]
In such a reduction gear, if both the worm and the worm wheel are made of metal, there is an inconvenience that unpleasant noise such as rattling noise and vibration sound is generated at the time of operating the steering wheel. When made of metal, a blank disk made of a synthetic resin material is integrally formed on the outer periphery of a metal hub, that is, a metal core, as a worm wheel, and cutting and other means are performed on the circumference of the blank disk. The use of a worm wheel having synthetic resin teeth in which resin teeth are integrally formed with the teeth formed by using a worm wheel suppresses generation of unpleasant noise such as rattling noise and vibration noise.
[0005]
As a material of the resin tooth portion, a fiber reinforcing material such as glass fiber or carbon fiber is mixed with a base resin such as polyamide 6, polyamide 66, polyacetal, polyetheretherketone (PEEK), or polyphenylene sulfide (PPS). In addition to the above-mentioned materials, MC (monomer cast) nylon, polyamide 6, polyamide 66 and the like which do not contain a reinforcing material are also used.
[0006]
In consideration of dimensional stability and cost, MC nylon not containing a fiber reinforcing material is used, and polyamide 6, polyamide 66, polyamide 46, or the like containing glass fiber is used as the fiber reinforcing material (for example, Patent Document 1).
[0007]
In the worm gear reduction mechanism of the electric power steering device, the meshing surface of the worm and the worm wheel is lubricated in the internal space between the two ball bearings that hold the worm shaft in the gear case that houses the worm and the worm wheel. Is filled with grease. When a worm made of metal and a worm wheel with resin teeth are used, grease using mineral oil or poly-α-olefin oil in consideration of heat resistance is used as a base oil.
[0008]
In addition, a preload is applied to the ball bearings that support the worm shafts disposed at both ends of the worm, and the motor is prevented from rotating by moving the worm in the axial direction when a minute kickback input comes in from the tire side. In some cases, a rubber damper that acts to transmit only kickback information is attached to the handle side. In addition, as a rubber material used for a rubber damper, acrylic rubber represented by ethylene acrylic rubber having a small permanent set is most commonly used.
[0009]
[Patent Document 1]
Japanese Patent Publication No. 6-60674
[Problems to be solved by the invention]
However, mineral oil and poly-α-olefin oil, which are base oils of the above-mentioned grease, have good wettability to a metal drive gear (for example, a worm), but they are not sufficient, so that a resin driven gear (for example, The lubrication state between the tooth portion of the worm wheel) and the tooth portion of the metal drive gear was not sufficiently satisfactory.
[0011]
In addition, when glass fibers or carbon fibers are added to improve the wear resistance and mechanical strength of resin teeth, there is a disadvantage that the metal fibers are worn by the added fibers.
[0012]
As a result, a temporary oil film shortage occurs due to the operation of the electric power steering device for a long period of time, and the wear of the teeth of the metal drive gear gradually progresses, and the backlash at the meshing portion between the drive gear and the driven gear is increased. Rush (play) increases. In addition, the smooth operation of the drive gear is hindered by the increase of the backlash, and the steering feeling is deteriorated, and unpleasant noise such as rattling noise and vibration noise may be generated. Further, as the wear progresses, the teeth of the drive gear and the driven gear may be deformed or damaged, and may not function as a device.
[0013]
SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, and to form a metal drive gear constituting a gear reduction mechanism, and to form a hard carbon-based coating on a tooth surface to thereby make the surface of the tooth wettable to grease. The present invention is to provide an electric power steering device which is excellent in abrasion resistance and high in reliability while maintaining good lubrication and preventing wear of a metal drive gear.
[0014]
[Means for Solving the Problems]
The present invention solves the above-mentioned problem, and the invention of claim 1 is configured to transmit auxiliary power by an electric motor to a steering mechanism of a vehicle via a gear reduction mechanism, and the gear reduction mechanism includes at least one set. An electric power steering apparatus comprising: a driven gear having a driving gear and a driven gear having resin teeth meshing with the driving gear; wherein grease is interposed between the driving gear and the driven gear of the gear reduction mechanism. An electric power steering device, wherein a carbon-based coating is formed.
[0015]
The drive gear and the driven gear are any of a worm and a worm wheel, a helical gear, a spur gear, a bevel gear, and a hypoid gear.
[0016]
In addition, the hard carbon-based coating may use any of diamond-like carbon (DLC), metal-containing DLC, and carbon nitride.
[0017]
The thickness of the hard carbon-based coating is 0.5 to 10 μm, preferably 1 to 5 μm.
[0018]
Further, the hard carbon-based film is formed by plasma CVD method in which a hydrocarbon gas is decomposed by plasma, sputtering method using carbon as a target, method in which carbon material is vaporized by arc discharge, film method in which hydrocarbon ion is formed. It is preferable to use a hard carbon-based coating formed by any one of an ion beam evaporation method using a method and a plasma ion implantation method.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described.
[0020]
FIG. 1 is a front view illustrating an example of a configuration of an electric power steering device 10 suitable for carrying out the present invention, and is a column-type electric power steering device. In FIG. 1, reference numeral 11 denotes a steering wheel shaft, 12 denotes a steering wheel shaft housing, 13 denotes an electric motor, and 20 denotes a rack and pinion type motion conversion mechanism.
[0021]
Although not shown in FIG. 1, the steering shaft 11 includes an upper steering wheel shaft 11a and a lower steering wheel shaft 11b. The steering wheel shaft 11 is supported inside a steering wheel shaft housing 12 so as to be rotatable around an axis. The steering shaft housing 12 is fixed to a predetermined position in the vehicle interior such that a lower portion thereof is inclined forward. A steering wheel (not shown) is fixed to the upper end of the upper steering shaft 11a.
[0022]
Further, the upper steering wheel shaft 11a and the lower steering wheel shaft 11b are connected by a not-shown torsion bar, and the steering torque transmitted from the steering wheel to the lower steering wheel shaft 11b via the upper steering wheel shaft 11a is detected by the torsion bar. The output of the electric motor 13 is controlled based on the detected steering torque.
[0023]
The rack and pinion type motion conversion mechanism 20 is disposed substantially horizontally in the engine room at the front of the vehicle with the longitudinal direction being the left-right direction of the vehicle, and the rack shaft 21 is movable in the axial direction. It comprises a pinion shaft 22 including a pinion having a tooth portion that is supported obliquely with respect to the tooth portion of the rack shaft 21, and a cylindrical rack shaft case 23 that supports the rack shaft 21 and the pinion shaft 22. You.
[0024]
The pinion shaft 22 and the lower part of the lower steering wheel shaft 11b are connected by two universal joints 25 and 26. A worm gear reduction mechanism 30 described later is arranged in an intermediate portion of the lower steering wheel shaft 11b, and is configured so that a steering assist force is supplied from the electric motor 13 to the lower steering wheel shaft 11b.
[0025]
FIG. 2 is a partial cross-sectional view showing the configuration of the worm gear reduction mechanism 30 of the electric power steering device 10 described above. Reference numeral 31 denotes a worm wheel, 32 denotes a worm that meshes with the worm wheel 31, and 33 denotes a gear case. The worm 32 has worm shafts 32a and 32b integrally formed at both ends thereof. The worm shafts 32a and 32b are rotatably supported by ball bearings 34a and 34b mounted on the gear case 33, respectively. The worm shaft 32b is splined or serrated to the drive shaft 13a of the electric motor 13.
[0026]
The hub of the worm wheel 31, that is, the core metal 42 is connected to the lower steering wheel shaft 11 b, and the rotation of the electric motor 13 is transmitted to the lower steering wheel shaft 11 b via the worm 32 and the worm wheel 31.
[0027]
FIG. 3 is a perspective view showing the configuration of the worm wheel 31 of the worm gear reduction mechanism 30 according to the embodiment of the present invention. A cylindrical resin portion 43 integrally formed of synthetic resin is provided on the processed surface, and gear teeth 44 are formed on the outer peripheral surface of the resin portion 43. On the other hand, the worm 32 is made of metal like the conventional worm.
[0028]
The resin portion 43 of the worm wheel 31 uses, as a base resin, a polyamide resin such as polyamide 6, polyamide 66, or polyamide 46 in consideration of durability. These resins alone exhibit sufficient durability, act favorably against abrasion of the metal worm 32 meshing with the worm wheel 31, and function sufficiently as a reduction gear. However, if the gear teeth 44 are used under more severe use conditions, the gear teeth 44 are expected to be worn or broken. Therefore, it is preferable to add a reinforcing material to improve reliability.
[0029]
As the reinforcing material, glass fiber, carbon fiber, potassium titanate whisker, aluminum borate whisker, and the like are preferable, and those subjected to a surface treatment with a silane coupling agent in consideration of the adhesiveness to the polyamide resin described above are more preferable.
[0030]
Further, these reinforcing materials can be compounded by combining a plurality of types.
Considering the impact strength, it is preferable to add a fibrous reinforcing material such as glass fiber or carbon fiber. Further, considering the damage of the metal worm 32 meshing with the worm wheel 31, it is preferable to mix the whisker-like reinforcing material with the fibrous reinforcing material. When these reinforcing materials are combined and combined, the combination ratio differs depending on the types of the fibrous reinforcing material and the whisker-like reinforcing material, and is appropriately selected in consideration of impact strength and damage to the worm 32.
[0031]
It is preferable that these reinforcing materials have a compounding ratio of 5 to 40% by weight, particularly 10 to 30% by weight of the whole resin. If the compounding ratio of the reinforcing material is less than 5% by weight, the mechanical strength is not improved, which is not preferable. On the other hand, if the compounding ratio of the reinforcing material exceeds 40% by weight, the worm 32 is likely to be damaged, and abrasion of the worm 32 is promoted, and durability as a reduction gear may be insufficient, which is not preferable.
[0032]
The worm 32 is not particularly limited, and may be made of metal as in the conventional case.
A hard carbon-based coating (not shown) is formed on the tooth surface of the worm 32 to improve the grease wettability. Further, since the tooth surface is coated with the hard carbon-based coating, even when meshing with the fiber-reinforced resin teeth, wear of the tooth surface of the worm 32 is suppressed, and backlash does not occur. It operates stably for a long time.
[0033]
The hard carbon-based coating is not particularly limited, and is not particularly limited as long as it is a carbon-based hard coating such as diamond-like carbon (DLC), metal-containing DLC, and carbon nitride.
[0034]
Examples of the method for forming the hard carbon-based coating include a plasma CVD method in which a hydrocarbon gas is decomposed by plasma to form a film, a sputtering method using carbon as a target, a method in which a carbon material is vaporized by arc discharge, and a method in which a film is formed by arc discharge. Known formation methods such as an ion beam evaporation method using a hydrocarbon ion and a plasma ion implantation method can be used.
[0035]
The thickness of the hard carbon-based coating is preferably 0.5 to 10 μm, and more preferably 1 to 5 μm. Further, from the viewpoint of controlling the film forming time and gas pressure of the hard carbon-based film, the film thickness is desirably 1 to 3 μm. If the film thickness is smaller than 0.5 μm, the resin surface is partially exposed, and the wettability to grease is not sufficiently improved, which is not preferable. On the other hand, when the film thickness is more than 10 μm, the internal stress of the hard carbon-based coating increases, and self-destruction is apt to occur. As a result, the effect of improving the wettability to grease is not long, which is not preferable.
[0036]
Next, a description will be given of grease for maintaining a good lubrication state between the teeth of the resin worm wheel and the teeth of the worm coated with the carbon-based hard coating.
[0037]
The grease used in the present invention is a mineral oil, a polyalphaolefin oil, a hydrocarbon-based compound such as an alkyl polyphenyl ether as a base oil, a urea compound composed of an amine and an isocyanate, Li soap, Li complex soap, Ba soap, Ba complex soap or the like can be used as a thickener.
[0038]
These hydrocarbon-based base oils do not have sufficient wettability to metal materials such as steel, and furthermore, the worm surface is worn by the fibers of the teeth of a fiber-reinforced resin worm wheel, which may cause As a result, oil film breakage gradually occurs due to long-term use, and wear of the tooth portion progresses, whereby backlash gradually increases, which is one of the causes of a decrease in the function of the electric power steering device. I was
[0039]
On the other hand, by forming a hard carbon-based coating on the surface of the tooth portion of the metal worm, a low-polarity dense structure composed of only carbon (C) and hydrogen (H) is formed on the outermost surface of the tooth portion. A membrane will be present. Due to the presence of this film, the wettability of the base oil of relatively low-polarity grease is greatly improved, and a stable oil film is formed on the teeth. An electric power steering apparatus with less wear, less occurrence of backlash, and excellent durability and reliability can be provided.
[0040]
Further, the grease may be added with an oil agent such as a fatty acid, an antioxidant such as an amine or phenol, an antirust agent such as Ca sulfonate, an extreme pressure additive such as MoDTC, and the like. .
[0041]
Next, fabrication of a worm specimen, measurement of wettability, and evaluation of durability will be described.
[0042]
(1) Fabrication of worm specimen A worm specimen finished by cutting from a steel (SC) material was degreased and cleaned, and then set in an unbalanced magnetron sputtering apparatus manufactured by Kobe Steel Co., Ltd. A coating was formed.
[0043]
(2) Formation of Hard Carbon-Based Coating After the ion bombardment with argon, the coating process on the worm specimen (Example) was carried out in order to enhance the adhesion of DLC, such as Cr (chrome), W (tungsten), (Carbon) An intermediate layer having a thickness of 0.5 to 1.0 μm is formed, and a carbon layer (comprising C or CH) having a thickness of 1.0 to 1.5 μm is formed thereon. A carbon-based hard coating (DLC) having a thickness of 2 μm was formed.
[0044]
(3) Measurement of wettability with grease base oil For measurement of wettability with grease base oil, a circle having a diameter of 30 mm made of the same steel steel (SC material) as the worm was used in place of the worm test piece described above. The test was performed using a plate test piece. First, after the above-mentioned disc test piece was degreased and washed, an example and three kinds of comparative examples were prepared.
[0045]
In the embodiment, the disk test piece was set in the same unbalanced magnetron sputtering apparatus as the worm test piece, and the carbon-based hard coating having a total coating thickness of 2 μm was formed under the same conditions as the DLC coating formation conditions described above. (DLC).
[0046]
In Comparative Example 1, a carbon-based hard coating (DLC) having no carbon-based hard coating (DLC) was formed. In Comparative Example 2, a carbon-based hard coating (DLC) having the same conditions as the above-described DLC coating, but having a total coating thickness of 0.2 μm was used. In Comparative Example 3, a carbon-based hard coating (DLC) was formed under the same conditions as the above-described DLC coating, but with a total coating thickness of 10 μm.
[0047]
With respect to the above Examples, Comparative Examples 1, 2 and 3, a drop of the poly-α-olefin oil (8 mm ² / s at 100 ° C.) used as a base oil of grease was applied after 3 seconds. The contact angle was measured. The measurement was performed at room temperature using DIGIDROP manufactured by GBX.
[0048]
Column A in FIG. 4 shows the measurement result of the contact angle after 3 seconds. The contact angle was 16 degrees in the example (the one in which the hard carbon-based coating was formed), and the contact angle was 30 degrees in Comparative Example 1. In Example 2, the contact angle was 23 degrees, and in Comparative Example 3, the contact angle was 16 degrees. In the examples, the contact angle of the base oil is smaller than that of the comparative example 1, which indicates that the wettability is excellent. The comparative example 2 having a small film thickness has a slightly larger base oil contact angle and a small effect of improving the wettability as compared with the example having a large film thickness (in which a hard carbon-based coating is formed) and the comparative example 3. I understand.
[0049]
(4) Evaluation of Durability The worm 32 inside the gear case 33 of the worm gear reduction mechanism 30 shown in FIG. 1 and FIG. 2 was replaced with a worm specimen (carbon-based hard coating (DLC) formed on the tooth surface). Example), a hard carbon-based coating having no hard carbon-based coating (Comparative Example 1), a hard carbon-based coating having a thickness of 0.2 μm (Comparative Example 2), and a hard carbon-based coating having a thickness of 20 μm (Comparative Example 3) were sequentially replaced and mounted, and the steering operation described below was performed to evaluate durability.
[0050]
First, a base grease (thickener amount) containing poly-α-olefin oil (8 mm ² / s at 100 ° C.) as a base oil and a diurea compound as a thickener is provided on the worm surface and the outer peripheral surface of the resin portion of the worm wheel. : 13% by weight) and filled with a grease further formulated with an antioxidant, and evaluated. To the grease, 2,4% by weight of 4,4'-dioctyldiphenylamine was added as an antioxidant, and 0.5% by weight of neutral calcium sulfonate was added as a rust preventive.
[0051]
In the durability test, the atmosphere temperature was controlled to 80 ° C, steering was performed 100,000 times, and the wear of the worm wheel passed 40 μm or less from the initial state. Was observed.
[0052]
Column B in FIG. 4 shows the results of the durability test. In Examples (having a hard carbon-based coating having a thickness of 2.0 μm), the wettability with respect to a low-polarity polyalphaolefin oil as a grease base oil was obtained. A good and stable oil film was formed, and almost no wear occurred even after 100,000 steering operations.
[0053]
On the other hand, in Comparative Example 1 (with no hard carbon-based coating), the wettability was not good and wear gradually progressed, and the wear amount exceeded the allowable limit of 40 μm after 30,000 steering operations.
[0054]
In Comparative Example 2 (having a hard carbon-based coating having a thickness of 0.2 μm), the wettability was improved and the occurrence of abrasion was small as compared with Comparative Example 1, but the film thickness was too thin to improve. The effect was not stabilized, and the amount of wear exceeded the allowable limit of 40 μm after 50,000 steering operations.
[0055]
Comparative Example 3 (having a hard carbon-based coating having a thickness of 0.2 μm) had a sufficient improvement effect, but the thickness was too large and the internal stress was high, and the hard carbon-based coating was obtained after 20,000 steering operations. Has cracked.
[0056]
The embodiment described above is an example in which the present invention is applied to a metal worm of a worm gear reduction mechanism of a column-type electric power steering device and a worm wheel provided with resin teeth meshing with the metal worm. Is not limited to the application of the above-mentioned column type electric power steering device to the worm gear reduction mechanism, but is another electric power steering device, which is composed of a metal gear and a gear having resin teeth meshing therewith. Needless to say, the present invention can be applied to various gear reduction mechanisms.
[0057]
In addition to other general gear mechanisms, for example, a worm and a worm wheel, a well-known helical gear, a spur gear, a bevel gear, a metal gear composed of a hypoid gear, and the like, and a resin gear meshing therewith. It goes without saying that the present invention can also be applied to a gear mechanism composed of gears. The helical gears, spur gears, bevel gears, hypoid gears, and the like exemplified above have a well-known configuration, and are not illustrated here.
[0058]
【The invention's effect】
As described above, the present invention relates to a gear mechanism including at least a metal drive gear and a driven gear having resin teeth meshing with the metal drive gear, which constitutes a gear reduction mechanism of an electric power steering apparatus. By forming a hard carbon-based coating on the surface of the teeth, the wettability to relatively low-polarity oils such as mineral oil, poly-α-olefin oil, and alkyl polyphenyl ether is improved. A grease based on such a material, that is, a relatively low-polarity oil such as a mineral oil, a poly-α-olefin oil, or an alkyl polyphenyl ether, was provided with a metal gear and resin teeth meshing with the metal gear. By filling the space between the gears, it is possible to improve the wear resistance of the gears and improve the lubricity between the gears by improving the wettability. Can be provided.
[Brief description of the drawings]
FIG. 1 is a front view illustrating an example of a configuration of an electric power steering device suitable for carrying out the present invention.
FIG. 2 is a partial sectional view showing a configuration of a worm gear reduction mechanism of the electric power steering device shown in FIG.
FIG. 3 is a perspective view showing a configuration of a worm wheel of the worm gear reduction mechanism.
FIG. 4 is a view showing the results of measurement of a worm wettability and a durability test.
[Explanation of symbols]
Reference Signs List 10 electric power steering device 11 steering wheel shaft 11a upper steering wheel shaft 11b lower steering wheel shaft 12 steering wheel shaft housing 13 electric motor 13a drive shaft 20 rack-pinion type motion conversion mechanism 21 rack shaft 22 pinion shaft 23 rack shaft cases 25, 26 universal joint 30 Worm gear reduction mechanism 31 Worm wheel 32 Worm 32a, 32b Worm shaft 33 Gear case 34a, 34b Ball bearing 42 Hub (core metal)
43 resin part 44 gear teeth

Claims (5)

The auxiliary power by the electric motor is configured to be transmitted to a steering mechanism of the vehicle via a gear reduction mechanism, the gear reduction mechanism includes at least one set of drive gears and a driven gear having resin teeth meshing with the drive gears, In an electric power steering device in which grease is interposed between a driving gear and a driven gear of a gear reduction mechanism,
An electric power steering apparatus, wherein at least the drive gear has a hard carbon-based coating formed on a tooth surface. The electric power steering apparatus according to claim 1, wherein the drive gear and the driven gear of the gear reduction mechanism are any of a worm and a worm wheel, a helical gear, a spur gear, a bevel gear, and a hypoid gear. The electric power steering device according to claim 1, wherein the hard carbon-based coating is any one of diamond-like carbon (DLC), metal-containing DLC, and carbon nitride. The electric power steering apparatus according to claim 1, wherein the thickness of the hard carbon-based coating is 0.5 to 10 m, preferably 1 to 5 m. The hard carbon-based coating uses a plasma CVD method in which a hydrocarbon gas is decomposed with plasma to form a film, a sputtering method using carbon as a target, a method in which a carbon material is vaporized by arc discharge, and a method in which hydrocarbon ions are used. The electric power steering apparatus according to claim 1, wherein the electric power steering apparatus is a hard carbon-based coating formed by one of an ion beam deposition method and a plasma ion implantation method.

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