JP2005329849A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To transmit power of a reduction gear of an electric power steering device with high efficiency. <P>SOLUTION: The electric power steering device transmits an auxiliary output by an electric motor to a steering mechanism of a vehicle via a reduction gear mechanism comprising a worm 8 and a worm wheel 7. The worm wheel comprises a metallic core pipe and a resin section integrated with this. Gear teeth 202 are formed on the outer periphery of the resin section. An engaging section of the worm with the worm wheel is lubricated with grease composition. The relation of shearing rate X(s<SP>-1</SP>) to apparent viscosity Y(Pas) at 25°C of the grease composition is within a region sandwiched by two curves expressed by Y=492.01×X<SP>-0.9713</SP>and Y=421.19×X<SP>-0.5063</SP>. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electric power steering device that transmits auxiliary output by an electric motor to a steering mechanism of a vehicle via a reduction gear mechanism, and more particularly, an outer periphery of a metal core bar made of a resin composition and on an outer peripheral surface. The present invention relates to an electric power steering apparatus that includes a driven gear integrally formed with a resin portion in which gear teeth are formed, and the driven gear is lubricated with grease.

  In recent automobile parts, home appliances, OA, AV equipment, etc., the use of resin parts instead of conventional metal parts is increasing due to demands for weight reduction and cost reduction. This trend is no exception in parts and machines that require lubrication, such as gears and cam bearings, and the use of these resins is progressing.

  The grease used for such a resin lubrication part was not particularly reviewed even when the material constituting the resin lubrication part was changed from metal to resin, and the grease used for conventional metal lubrication was used as it was. . An example of such grease is described in Japanese Patent Application Laid-Open No. 2002-37190. Recently, the demand for using a low-friction grease composition has been increasing for such resin-lubricated parts due to demands for improving efficiency. However, the grease used for the conventional metal lubrication part cannot realize low friction.

  An electric power steering device is an example of a device in which this problem is remarkable. Since an electric power steering device incorporated in an automobile uses a relatively high rotation and low torque electric motor, a speed reduction mechanism is incorporated between the electric motor and the steering shaft. As a reduction mechanism, a reduction gear for an electric power steering device (hereinafter, also simply referred to as “reduction gear”) including a worm and a worm wheel meshing with the worm, for example, because a large reduction ratio can be obtained by one set. ) Is common.

  In such a reduction gear, when both the worm wheel and the worm are made of metal, there is a problem that unpleasant noise such as rattling noise and vibration noise is generated when the handle is operated. Therefore, the worm wheel 11 is used to counter the noise by using a resin core 3 in which the gear teeth 10 are formed on the outer periphery of the metal core tube 1 by using an adhesive 8 or the like. doing.

  As the resin portion, for example, as described in JP-B-6-60674, a base resin such as polyamide 6, polyamide 66, polyacetal, polyether ether ketone (PEEK), polyphenylene sulfide (PPS), or the like is used, and glass fiber. MC (monomer cast) nylon, polyamide 6, polyamide 66, etc. that do not contain reinforcing agents are also used in addition to materials containing reinforcing agents such as carbon fiber and the like. Among these, in consideration of dimensional stability and cost, MC nylon containing no reinforcing agent, polyamide 6, glass 66 containing polyamide, polyamide 46 and the like are mainly used.

  In addition, the worm of the reduction gear is supported by a pair of ball bearings or other rolling bearings and connected to an electric motor, and the space between the pair of rolling bearings of the housing usually has both the worm and the worm wheel. Grease is filled for lubrication between gear teeth.

  In addition, preload is applied to the rolling bearing, and when a minute kickback input from the tire side comes in, the worm is moved in the axial direction to prevent the electric motor from rotating, and only the kickback information is given to the handle side. In order to convey, it is also known that a rubber damper is attached to the worm side of the rolling bearing, and the rubber used is generally an acrylic rubber represented by ethylene acrylic rubber having a small compression set. .

By the way, as the lubricating grease for the reduction gear portion, those using a metal soap as a thickener or a urea compound as a thickener have been used. In particular, in a rack and pinion type electric power steering device in which the reduction gear is used in a high temperature atmosphere, grease using a urea compound that is excellent in durability performance at high temperatures as a thickener is preferably used.
Japanese Patent Laid-Open No. 2002-37190 Japanese Patent Publication No. 6-60674

  Conventionally, various additives such as wax and polytetrafluoroethylene have been added to the grease in order to increase the power transmission efficiency of the reduction gear portion, which is useful for reducing friction of the reduction gear sliding portion. In general, attempts have been made to reduce the friction of the sliding portion in order to improve the power transmission rate of the reduction gear portion. However, the factor that affects the efficiency of the reduction gear is not only the friction of the sliding surface. Although the factors affecting the transmission efficiency of the reduction gear have not yet been fully elucidated, the present inventors have a clear correlation between the apparent viscosity of the grease and the transmission efficiency of the reduction gear. The present inventors have found that a grease composition having the apparent viscosity characteristic is effective in improving power transmission efficiency.

  If the apparent viscosity of the grease is high, the efficiency of the reduction gear decreases. This indicates that the agitation resistance of the grease in each drive unit in the apparatus is high, and that the power to be transmitted is expended in shearing the grease. The lower the apparent viscosity of the grease, the more advantageous for the power transmission efficiency of the reduction gear. However, if the apparent viscosity is too low, grease leakage will occur and the durability performance of the resin gear will be reduced.

  Therefore, the present invention provides a grease composition that is used in a power transmission unit and can maintain power transmission efficiency with high efficiency, and implements power transmission in a reduction gear of an electric power steering device with high efficiency. To provide a grease composition that can be used, to provide a power transmission device lubricated by the grease composition, and to provide an electric power steering mechanism in which a reduction gear mechanism is lubricated by the grease composition. , Respectively.

As a result of intensive studies by the present inventors, the relational expressions of shear rate X (s ⁻¹ ) at 25 ° C. versus apparent viscosity Y (Pa · s) are Y = 492.01 × X ^−0.9713 and Y = 421.19 × X. ^{It was found} that a grease composition having characteristics that fall within a region sandwiched between two curves indicated by ^-0.5063 is effective in increasing the power transmission efficiency in the reduction gear portion of the electric power steering.

That is, in the grease composition according to the present invention, the relational expressions of the shear rate X (s ⁻¹ ) at 25 ° C. and the apparent viscosity Y (Pa · s) are Y = 492.01 × X ^−0.9713 and Y = 421.19. It is characterized by having a characteristic that falls within a region sandwiched between two curves ^represented by × X ^−0.5063 .

The unit of the shear rate X is s ⁻¹ , and the unit of the apparent viscosity Y is Pa · s. This effect is exhibited especially without being affected by the composition of the grease, but when the heat resistance performance of the grease is emphasized, a diurea compound is particularly preferable as a thickener, and if the transmission efficiency in the reduction gear portion is emphasized, Lithium soap is preferred. Regarding diurea compounds, those in which the carbon structure introduced into the raw material monoamine is a linear saturated aliphatic alkyl group are preferred because they reduce friction on the sliding surface and are excellent in heat resistance.

  Moreover, as the base oil of the grease composition, all types known at present can be used. For example, usable base oils include mineral oils, ester synthetic oils typified by diesters and polyol esters, poly α-monoolefins, ethylene and α-olefin co-oligomers, synthetic hydrocarbon oils typified by polybutenes, alkyls. Examples thereof include ether synthetic oils typified by diphenyl ether and polypropylene glycol, silicone oils, and fluorinated oils. Among these, preferred base oils are synthetic hydrocarbon oil, mineral oil, and refined mineral oil. This is because an adverse effect such as stress cracking is not given to a resin having poor oil resistance such as ABS or PC. In view of low temperature fluidity, synthetic hydrocarbon oils and refined mineral oils are particularly preferable. It is also effective to add a wax such as polyolefin wax to reduce friction and prevent resin wear. Furthermore, the grease composition of the present invention can contain additives such as an antioxidant rust inhibitor, a metal corrosion inhibitor, an oily agent, an antiwear agent, an extreme pressure agent, and a solid lubricant as necessary.

In a preferred form of the grease composition, the base oil contains at least one selected from polyalphaolefin oil or mineral oil in an amount of 70% by weight or more of the total amount of the base oil, and the thickener is a lithium soap or a diurea compound. It is characterized by that. Furthermore, the penetration is 200-300, and the base oil viscosity is 30-150 mm ² / s.

  The present invention further uses this grease composition in a power transmission section for the purpose of lubrication, and in particular, an electric power steering device that transmits auxiliary output from an electric motor to a steering mechanism of a vehicle via a reduction gear mechanism. The driven gear of the reduction gear mechanism is provided with a resin portion having gear teeth formed on the outer peripheral surface thereof, and the gear mechanism is lubricated with the above-described grease composition. It is what. More specifically, this driven gear is integrally provided with a resin portion having gear teeth formed on the outer periphery of a metal core tube. The resin constituting this resin part is preferably a polyamide resin. The power transmission unit to which the grease composition is applied may be a drive gear in addition to the driven gear, and examples of such a gear include a cylindrical worm gear, a helical gear, a spur gear, a bevel gear, and a hypoid gear. be able to. In addition, what is necessary is just to adjust the factor (material, viscosity, etc.) with respect to a composition so that the above-mentioned characteristic may be entered.

  As described above, according to the present invention, it is possible to provide a grease composition that is used in a power transmission unit and can maintain a high power transmission efficiency, and to increase power transmission in a reduction gear of an electric power steering device. To provide a grease composition that can be efficiently implemented, to provide a power transmission device that is lubricated by the grease composition, and to provide an electric power steering mechanism in which a reduction gear mechanism is lubricated by the grease composition Each of which can be provided.

  Next, an embodiment of the present invention will be described. FIG. 1 is a cross-sectional view showing a configuration of a main part of an electric power steering apparatus according to the present invention. Reference numerals 5a and 5b denote housings having a two-part structure of an input shaft side 5a and an output shaft side 5b. Inside the housings 5a and 5b, an input shaft 1, a torsion bar 3 disposed therein, and an output shaft 2 connected to the input shaft 1 through the torsion bar 3 are rotatable by bearings 6a, 6b and 6c. It is supported.

  The input shaft 1, the torsion bar 3, and the output shaft 2 are arranged coaxially. The input shaft 1 and the torsion bar 3 are spline-coupled, and the torsion bar 3 and the output shaft 2 are also spline-coupled. A steering wheel (not shown) is integrally attached to the left end side of the input shaft 1. Further, a pinion shaft 2a is formed integrally with the output shaft 2, and the pinion shaft 2a meshes with the rack 4 to constitute a known rack and pinion type steering mechanism.

  A worm wheel 7 that is coaxial with the output shaft 2 and rotates integrally therewith is fixed to the output shaft 2 and meshes with a worm 8 that is driven by an electric motor (not shown). As shown in FIGS. 2 and 3, the worm wheel 7 meshed with the worm 8 has a synthetic resin tooth 7 b integrally fixed to a metal hub 7 a (metal core tube) by an adhesive layer 200. Yes. Reference numeral 202 denotes gear teeth formed on the outer peripheral surface (resin portion) of the worm wheel. The worm and the worm wheel constitute a speed reduction mechanism for the electric power steering mechanism.

  As shown in FIG. 2, the worm 8 corresponds to a drive gear, and the worm wheel 7 corresponds to a driven gear. The rotational force of the electric motor is transmitted to the output shaft 2 via the worm 8 and the worm wheel 7, and a steering assist torque in an arbitrary direction is applied to the output shaft 2 by appropriately switching the rotation direction of the electric motor.

  The reduction gear worm 8 is supported by a pair of ball bearings and other rolling bearings and is connected to the 110 electric motor 100. The space between the pair of rolling bearings 120 of the housing includes a worm 8 and a worm wheel 7. In order to lubricate between the gear teeth (engagement portion), the grease according to the present invention is filled. Furthermore, preload is applied to the rolling bearing, and when a minute kickback input from the tire side is input, the worm 8 is moved in the axial direction so that the electric motor 100 does not rotate, and only the kickback is provided on the handle side. In order to convey information, a rubber damper 130 is attached to the rolling bearing worm side. As rubber to be used, acrylic rubber represented by ethylene acrylic rubber having a small compression set is generally used.

  In addition to the grease composition according to the present invention, a known grease composition is preferably applied to each part of the electric power steering including the torque detector. In many cases, rust prevention performance is superior to that of rust prevention oil alone.

  The torque detection unit includes a sensor shaft portion 11 formed on the right end side in FIG. 1 of the input shaft 1, detection coils 13 and 14 disposed inside the housing 5a, and a cylindrical member 12 disposed therebetween. Composed.

  FIG. 4 is a perspective view showing the configuration of the torque detector. A sensor shaft portion 11 made of a magnetic material is formed on the outer side of the input shaft 1 near the right end in FIG. 1. Nine) ridges 11a are formed at equal intervals along the circumferential direction, and groove portions 11b wider than the width t1 of the ridges 11a are formed between the ridges 11a.

  Further, on the outside of the sensor shaft portion 11, a cylindrical member 12 made of a conductive and non-magnetic material, for example, aluminum, which is close to the sensor shaft portion 11, is arranged coaxially with the sensor shaft portion 11, The extension 12 e of the cylindrical member 12 is fixed to the outside of the end 2 e of the output shaft 2.

  The cylindrical member 12 includes a first window row composed of a plurality of rectangular windows 12a arranged at equal intervals in the circumferential direction at positions facing the protrusions 11a on the surface of the sensor shaft portion 11 described above. A second window row comprising a plurality of rectangular windows 12b having the same shape as the window 12a and having different phases in the circumferential direction is provided at a position shifted in the axial direction from the first window row. .

  The outer periphery of the cylindrical member 12 is surrounded by a yoke 15 around which detection coils 13 and 14 of the same standard are wound. That is, the detection coils 13 and 14 are arranged coaxially with the cylindrical member 12, the detection coil 13 surrounds the first window row portion formed of the window 12a, and the detection coil 14 includes the second window row portion formed of the window 12b. Siege. The yoke 15 is fixed inside the housing 5a, and the output lines of the detection coils 13 and 14 are connected to a circuit board disposed inside the housing 5a.

Next, the power transmission efficiency of the reduction gear unit was measured by a desktop testing machine using an actual EPS (electric power steering) machine. Test conditions are shown below.
Resin gear material: Polyamide 6 containing 30% glass fiber
Worm material: S45C
Oscillating surface pressure: 80Mpa
Reduction gear part grease amount: about 15g
Test atmosphere temperature: 100 ° C
Sliding speed: 0.1 to 1.4 m / s, selected and evaluated as needed Grease 1: Grease having a composition in which the thickener is lithium soap and the base oil is poly-α-olefin. The base oil kinematic viscosity is 40 mm ² / s at 40 ° C. Several greases with different penetrations were prepared to provide several levels of apparent viscosity. The mixing penetration is preferably 200 to 300.
Evaluation grease 2: Grease having a composition in which the thickener is aliphatic diurea and the base oil is polyolefin. The base oil kinematic viscosity is 40 mm ² / s at 40 ° C. Several greases with different penetrations were prepared to provide several levels of apparent viscosity. In addition, since the friction on the sliding surface of the reduction gear is slightly higher than that of lithium soap grease, 5% polyethylene wax was added to improve the sliding characteristics.

  While changing the sliding speed of the reduction gear, the power transmission efficiency of the reduction gear is measured, and screened at the efficiency of 85-87%, 87-89%, 89-91% based on the measured value. An area diagram was created by plotting the results on a graph of shear rate versus apparent viscosity.

In FIG. 5, the region I is a region between Y = 421.19 × X ⁻⁰⁵⁰⁶³ and Y = 378.14 × X ^−0.5396 , and the region II is Y = 378.14 × X ^−0.5396 and Y = 369. 27 × X ^−0.6082 , and region III is a region between Y = 369.27 × X ^−0.6082 and 492.01 × X ^−0.9713 .

  The grease in which the diagram of the shear rate versus the apparent viscosity was within the range shown in the region I shown in FIG. 5 exhibited an efficiency of 85-87%. Similarly, region II showed an efficiency of 87-89% and region III showed an efficiency of 89-91%. Further, for example, a grease having a shear rate versus apparent viscosity diagram in the regions I to III exhibited power transmission efficiency in the range of 85 to 91% depending on the sliding speed of the reduction gear portion. Simultaneously with the above evaluation test, the endurance test of the reduction gear was carried out using the same testing machine. The test method is shown below.

  A cross-knurled, degreased outer diameter 45 mm, width 13 mm S45C core tube is placed in a mold fitted with a sprue and a disk gate, and polyamide 6 containing 30% by weight of glass fiber is injection molded. A worm wheel blank material having an outer diameter of 60 mm and a width of 13 mm was prepared, and then the outer periphery of the resin portion was cut to form gear teeth to prepare a worm wheel specimen.

  The manufactured worm wheel specimen is incorporated into the reduction gear of an actual electric power steering device, and the test grease is applied evenly to the gear tooth surface of the worm wheel and the gear tooth surface of the worm, and maintained at an ambient temperature of 100 ° C. Steering was performed at a sliding speed of 1.2 m / s, and the amount of wear on the gear teeth after 300,000 times of steering was measured.

As a result, in the grease having the apparent viscosity characteristics included in the regions I, II, and III in FIG. 5, the increase in the backlash of the gear after the test was 2.5 degrees or less, whereas 492.01 × X ⁻ It was found that the grease whose shear rate vs. apparent viscosity is located lower than ^0.9713 has a backlash increase of 2.5 degrees or more and is inferior in durability. This is presumably because grease with an apparent viscosity that is too low tends to flow out of the sliding portion and hardly contributes to lubrication. The power transmission efficiency in the reduction gear portion depends not only on the apparent viscosity of the grease but also on the frictional resistance of the sliding portion. Therefore, the region diagram shown in FIG. 5 changes according to the friction coefficient of the sliding portion. Therefore, in the present invention, the power transmission efficiency was accidentally divided into three levels of 85 to 87%, 87 to 89%, and 89 to 91%. However, the efficiency values themselves are not absolute, And change according to the test conditions. However, it is advantageous in terms of efficiency that the apparent viscosity is in the above range (regions I, II, III).

  As grease base oil, it is preferable to use mineral oil or poly-α-olefin oil with relatively low polarity so as not to cause changes in the shape of resin gears and rubber dampers, rigidity reduction, etc. Therefore, a small amount of polar lubricating oil can be mixed and used. In this case, if the amount of nonpolar lubricating oil used is 80% by weight or more of the total base oil, the swelling and modification of the resin is negligible and can be ignored.

A cylindrical plate made of ethylene acrylic rubber (diameter 10 mm, thickness 5 mm) is prepared, the thickener is diurea, the base oil is polyol ester oil (30 mm ² / s @ 40 ° C.) and the poly α-olefin oil (30 mm ² / s). @ 40 ° C.) was mixed in a grease composition and left in a 100 ° C. constant temperature bath for 100 hours. Thereafter, a flat plate made of ethylene acrylic rubber was taken out, and the dimensional change in thickness was measured. FIG. 6 shows how the dimensional change rate of the rubber changes with respect to the use amount of various polyol ester oils. From FIG. 6, the ratio of the ester oil to the total amount of the base oil is 30% by weight or less. If it exists, the dimensional change rate is 0.3% or less, and it can be seen that it can be almost ignored. More preferably, it is 20% by weight or less.

The base oil kinematic viscosity in the present invention is preferably 30 to 150 mm ² / s at 40 ° C. If the base oil kinematic viscosity at 40 ° C. is less than 30 mm ² / s, a sufficient oil film thickness cannot be secured on the rocking surface, which may affect the durability of the resin gear. Further, when the base oil kinematic viscosity exceeds 150 mm ² / s, the low temperature pour point rises, and when the electric power steering is used in a cold region, the lubrication state of the resin gear deteriorates and the transmission efficiency decreases. there is a possibility.

  In the embodiment described above, the driving gear and the driven gear are constituted by the cylindrical war gear. However, the combination of the spur gear shown in FIG. 7, the helical gear shown in FIG. 8, the bevel gear shown in FIG. The hypoid gear shown in FIG.

It is sectional drawing which shows the structure of the principal part of the electric power steering apparatus concerning this invention. It is AA 'sectional drawing of FIG. It is a perspective view of the drive mechanism (worm) and the driven mechanism (worm wheel) which comprise the deceleration mechanism of an electric power steering. It is a perspective view which shows the structure of the torque detection part of the electric power steering apparatus shown in FIG. It is a characteristic view which shows the relationship between a shear rate and an apparent viscosity. It is a characteristic view which shows the relationship of the ratio (weight%) of the ester oil to the base oil whole quantity. It is a perspective view which shows embodiment which comprised the drive gear and the driven gear from the bevel gear. It is a perspective view which shows embodiment which comprised the drive gear and the driven gear from the helical gear. It is a perspective view which shows embodiment which comprised the drive gear and the driven gear from the helical gear. It is a perspective view which shows embodiment which comprised the drive gear and the driven gear from the hypoid gear.

Explanation of symbols

  7 is a worm wheel, 7a is a metal core tube, 7b is a resin part, 8 is a worm, 100 is an electric motor, and 202 is a gear tooth.

Claims (6)

An electric power steering device that transmits auxiliary output by an electric motor to a steering mechanism of a vehicle via a reduction gear mechanism,
The outer peripheral surface of the gear constituting the reduction gear mechanism is provided with a resin portion in which gear teeth are formed, and the gear mechanism is lubricated with a grease composition,
The relationship between the shear rate X (s ⁻¹ ) of this grease composition at 25 ° C. and the apparent viscosity Y (Pa · s) is shown by Y = 492.01 × X ^−0.9713 and Y = 421.19 × X ^−0.5063 An electric power steering device characterized in that the electric power steering device falls within a region sandwiched between two curved lines.   2. The electric power steering apparatus according to claim 1, wherein the gear is a driven gear that engages with a driving gear, and the driven gear is formed by integrating the resin portion on the outer periphery of a metal core tube.   The base oil of the grease composition contains at least one selected from polyalphaolefin oil or mineral oil in an amount of 70% by weight or more of the total amount of the base oil, and the thickener is a lithium soap or a diurea compound. The device described.   The apparatus according to any one of claims 1 to 3, wherein a penetration degree of the grease composition is 200-300. The apparatus according to any one of claims 1 to 4, wherein the grease composition has a base oil viscosity of 30 to 150 mm ² / s. 3. The apparatus according to claim 2, wherein the resin constituting the resin portion is a polyamide resin, and the driven gear and the drive gear are a cylindrical worm gear, a helical gear, a spur gear, a bevel gear, or a hypoid gear.

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