JP2015128946A - Electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric power steering device having high reliability and excellent quietness when being applied to not only a column assist type electric power steering device but also a rack assist type electric power steering device used under severer conditions.SOLUTION: There is provided an electric power steering device which comprises a worm wheel having a gear tooth formed on the outer peripheral surface of a resin part formed of a resin composition containing a thermoplastic resin and an aramid fiber having a loss tangent at 40°C of 0.03 or more and a tensile modulus of elasticity of 20-90 GPa; a reduction gear formed of a metal worm; and a reduction gear mechanism in which a space between a tooth portion of the worm wheel and a tooth portion of the metal worm is filled with grease that contains poly-α-olefin oil as base oil and an urea compound as a thickening agent, and has a consistency of 200-300.

Description

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

  As a reduction mechanism used in an electric power steering apparatus incorporated in an automobile, a large reduction ratio can be obtained with one set, and the like, for example, is connected to a rotating shaft of an electric motor (not shown) as shown in FIG. Generally, a reduction gear 50 including a worm 52 and a worm wheel 51 that meshes with the worm 52 is used.

  In such a reduction gear 50, if both the worm wheel 51 and the worm 52 are made of metal, there is a problem that an unpleasant sound such as a rattling sound or a vibration sound is generated when the handle is operated. Therefore, a worm wheel 51 is used in which a resin core 63 made of resin and formed with gear teeth 64 on the outer peripheral surface is integrated with the outer periphery of a metal core tube 62, and a metal worm wheel is used. Measures against noise (in the sense that it is adopted).

  In the resin part 63, as shown in Patent Document 1, for example, a base resin such as polyamide 6, polyamide 66, polyacetal, polyether ether ketone (PEEK), polyphenylene sulfide (PPS), glass fiber, carbon fiber, or the like is used. MC nylon (registered trademark of Quadrant Polypenco Japan Co., Ltd.), polyamide 6, polyamide 66, etc. not containing glass fiber or carbon fiber are used in addition to a material containing a reinforcing material. Among these, MC nylon containing no reinforcing material, polyamide 6 containing glass fiber, polyamide 66, polyamide 46, and the like are mainly used from the balance between performance and cost.

  By the way, there are two types of electric power steering devices: a column assist type mounted in a vehicle compartment and a rack assist type mounted in an engine room. The above-mentioned mainstream materials are particularly column assist type electric power steering devices. In many cases.

Japanese Patent Publication No. 6-60674

  However, in the rack assist type installed in the engine room, the requirements for its performance include compatibility with a higher temperature environment (up to 120 ° C) and higher output compared to the column assist type. In addition, as with the column assist type, there is a strong demand for noise reduction.

  It is extremely difficult for the polyamide-based material described above to satisfy all the requirements of the rack assist type electric power steering in a balanced manner. Specifically, the material that has been applied to the column assist type is insufficient in durability at high temperatures (tooth surface wear durability, root bending fatigue strength), and changes in steering feeling due to the progress of abnormal wear are assumed. Is done.

  Further, glass fiber is used as a reinforcing material for the resin composition applied to the column assist type, but in such a glass fiber reinforced resin, strength / rigidity and dimensional stability are improved. Glass fiber has a strong attack on worms, and there is still room for improvement in terms of quietness. Furthermore, the glass fiber that has fallen from the gear tooth surface of the worm wheel due to the progress of normal wear due to normal use is interposed between the tooth portion of the worm wheel and the metal worm, thereby promoting the abrasive wear of the gear tooth surface. As a result, a change in steering feeling of the steering is assumed.

  In addition, grease is filled between the teeth of the worm wheel and the teeth of the metal worm, and depending on the characteristics of the selected grease, it can be expected to suppress the noise of the gear. Even when a high-quality grease is selected, with a conventional glass fiber reinforced resin worm wheel, the oil film is easily broken by the glass fibers present on the gear tooth surface, and a sufficient silent effect may not be obtained. Easy to assume.

  The present invention was made paying attention to such a situation, and the column assist type is naturally provided with high reliability even when applied to a rack assist type used under more severe conditions, And it aims at providing the electric power steering device excellent in silence.

  Regarding the problems with the glass fiber reinforced resin, MC nylon and polyphenylene sulfide resin are used as materials that do not contain a reinforcing material that is highly aggressive to worms, and it seems that it is possible to overcome the quietness, Assuming application to rack assist systems, which are expected to be used in harsh environments, resin materials that do not contain reinforcing materials are insufficient in durability, and products required for rack assist electric power steering systems Experiments have shown that the level is not reached.

  Therefore, the present inventors analyzed in detail the end-of-life test of a sample gear made of a resin material that is currently used for a worm wheel of an electric power steering device, and also conducted a test using a worm wheel made of another developed prototype material. Repeated intensive researches. As a result, the reinforcing effect on the base resin is excellent, but from the standpoint of quietness, its application is not preferable, but it is less aggressive to the worm and hardly causes the oil film to break. In itself, when using aramid fiber with excellent vibration damping as a reinforcing material, and also using urea-based grease that is unlikely to cause oil film breakage and can also be expected to have a silencing effect, it is quiet and durable. It has been found that the reduction gear system can be applied to an electric power steering device.

That is, in order to solve the above problems, the present invention provides the following electric power steering device.
(1) In an electric power steering device that transmits auxiliary output from an electric motor to a vehicle steering mechanism via a reduction gear mechanism,
The reduction gear mechanism is made of a resin part made of a resin composition containing a thermoplastic resin and an aramid fiber having a loss tangent at 40 ° C. of 0.03 or more and a tensile elastic modulus of 20 to 90 GPa. Provided integrally with the outer periphery of the core tube, and provided with a reduction gear composed of a worm wheel formed with gear teeth on the outer peripheral surface of the resin part, and a metal worm meshing with the worm wheel,
Between the tooth part of the worm wheel and the tooth part of the metal worm, the base oil is poly-α-olefin oil, the thickener is a urea compound, and grease having a consistency of 200 to 300 is filled. An electric power steering device.
(2) The thermoplastic resin is at least one selected from polyamide resin, polyphenylene sulfide resin and polyether ether ketone resin, and the aramid fiber is polyurethane resin, polyester resin, polyether resin, vinyl ester resin, epoxy resin The electric power steering device according to (1), wherein the surface treatment is performed with one or more sizing agents selected from acrylic resin and bismaleimide resin.
(3) The electric power steering device according to (1) or (2), wherein the grease contains at least one selected from a montanic acid ester wax, a montanic acid ester portion saponified wax, and a polyethylene wax.

  In the electric power steering device of the present invention, in the reduction gear, the resin part constituting the worm wheel is made of a resin composition blended with an aramid fiber having a specific loss tangent and tensile elastic modulus, and further has a consistency of 200. By filling urea grease based on poly-α-olefin oil adjusted to ~ 300, the column assist type is of course high reliability even for rack assist type used under harsher conditions. And it comes to be excellent in quietness.

It is a schematic structure figure showing an example of a column assist type electric power steering device. It is a schematic block diagram which shows an example of a rack assist type electric power steering device. It is a perspective view which shows an example of the reduction gear of this invention. 6 is a graph showing the amount of wear with respect to the number of endurance cycles in the reduction gears of Examples 1 and 2 and Comparative Examples 1 and 2. It is a perspective view which shows the conventional reduction gear.

  Hereinafter, an embodiment of the present invention will be described in detail.

  The present invention can be applied to both a column assist type electric power steering device and a rack assist type electric power steering device.

  FIG. 1 is a schematic view showing an example of a column assist type electric power steering apparatus 10. A steering wheel shaft 11 is composed of an upper steering wheel shaft 11 a and a lower steering wheel shaft 11 b. The rudder wheel shaft housing 12 is fixed to a predetermined position in the vehicle interior so as to be inclined with its lower portion directed forward. A steering wheel (not shown) is fixed to the upper end of the upper steering wheel shaft 11a. Further, the upper steering wheel shaft 11a and the lower steering wheel shaft 11b are coupled by a torsion bar (not shown), 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 motor 13 is controlled based on the detected steering torque. A reduction gear 30 is connected to the motor 13, and the rotation of the motor 13 is transmitted to the lower steering wheel shaft 11b via the worm and the worm wheel.

  The rack-and-pinion motion conversion mechanism 20 is disposed substantially horizontally in the engine room at the front portion 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. A pinion shaft 22 including a pinion provided with a tooth portion that is supported obliquely with respect to the core and meshes with a tooth portion of the rack shaft 21, and a rack shaft 21 and a cylindrical rack shaft case 23 that supports the pinion shaft 22. Composed.

  FIG. 2 shows an example of the rack assist type electric power steering apparatus 100. The rack shaft 101, the first pinion 105 connected to the steering shaft (not shown), and the second pinion 106 on the motor 110 side are illustrated. And a reduction gear 30. The reduction gear 30 is composed of a worm and a worm wheel connected to a shaft of the motor 110, and a second pinion 106 is fixed to the worm wheel. The second pinion 106 is engaged with the rack shaft 101.

  The reduction gear 30 is a worm in which a resin portion 43 made of a resin composition and having gear teeth 44 formed on the outer peripheral end face thereof is integrated with the outer periphery of a metal core tube 42 as shown in FIG. A wheel 31 and a worm 32 are provided. The worm 32 may be made of metal.

  The thermoplastic resin that is the base material of the resin composition that forms the resin portion 43 of the worm wheel 31 is a polyamide-based resin, that is, a polyamide 6T / polyamide 6 that is a semi-aromatic polyamide in addition to polyamide 6, polyamide 66, and polyamide 46. 6-6, polyamide 6T / 6I, polyamide 6T / 6I / 6-6, polyamide 6T / M-5T, polyamide 9T, and the like can be suitably used. Polyphenylene sulfide resins and polyether ether ketone resins can also be used.

  The above resin composition has a viscoelastic property as a reinforcing material, specifically, a loss tangent (tan δ) value measured at 40 ° C. and 100 Hz of 0.03 or more, and a tensile elastic modulus of 20 to 90 GPa polyamide fiber is blended. In general, a material with a small loss tangent value is inferior in vibration absorption performance (vibration suppression, quietness). From the viewpoint of quietness, a material with a loss tangent value that is too small is not preferable, and a certain level or more is required. is there. As a result of investigations on silence, the present inventors have found that the loss tangent value of an aramid fiber applied as a reinforcing material is 0.03 (measured value at 40 ° C., 100 Hz) or more. It has been found that the fiber reinforced material can realize the necessary and sufficient silence as a worm wheel material. The loss tangent value is preferably 0.04 or more.

  Moreover, since aramid fiber is used as a reinforcing material that replaces glass fiber, a certain level or more should be secured for the elastic modulus of the aramid fiber. In this regard, the present inventors have found that a tensile elastic modulus of 20 GPa or more is required as a reinforcing material for the worm wheel material by repeating the gear durability test. That is, when the tensile elastic modulus of the aramid fiber is less than 20 GPa, the reinforcing effect is insufficient, which is not preferable as a reinforcing material for the worm wheel material.

  Furthermore, the present inventors have found that there is an appropriate range for the elastic modulus of the aramid fiber applied to the worm wheel, and the appropriate range for the tensile elastic modulus is 20 to 90 GPa. If the tensile elastic modulus of the aramid fiber exceeds 90 GPa, not only the vibration damping property is impaired, but also the strength as an aramid fiber reinforced material when blended in a resin is not preferable. The reason for this is not clear so far, but aramid fibers having a high elastic modulus exceeding 90 GPa have a higher affinity with the base resin than that of fibers having a relatively low elastic modulus. Therefore, it is assumed that a strong interface structure is not formed between the aramid fiber having a high elastic modulus and the resin, so that a sufficient reinforcing effect may not be exhibited. The tensile elastic modulus is preferably 40 to 80 GPa.

  The aramid fiber is selected from polyurethane resin, polyester resin, polyether resin, vinyl ester resin, epoxy resin, acrylic resin, and bismaleimide resin in order to improve the reinforcing effect by improving the adhesion with the thermoplastic resin. The surface treatment may be performed with a sizing agent of more than one species.

  The aramid fiber is blended within a range of 10 to 40% by mass of the total amount of the resin composition. When it is less than 10% by mass, a sufficient reinforcing effect is not exhibited, which is not preferable. Moreover, when it exceeds 40 mass%, since the improvement effect by an increase is hardly recognized, it is unpreferable. A preferable compounding amount is 10 to 25% by mass.

  In addition to the aramid fiber, various additives may be added to the resin composition. For example, in order to prevent deterioration due to heat at the time of molding and use, an iodide heat stabilizer or a phenolic resin is used. The amine-based antioxidants may be added alone or in combination.

The reduction gear 30 is filled with grease in order to maintain a good lubrication state between the tooth portion of the worm wheel 31 and the tooth portion of the worm 32. In the present invention, however, the base oil mainly comprises poly-α-olefin oil. Further, as a thickener, a urea compound composed of an amine and an isocyanate is blended, and a grease having a consistency of 200 to 300 is used. The kinematic viscosity of the base oil is preferably ²⁰ to 90 mm ² / s (40 ° C.) in consideration of fluidity from a low temperature to a high temperature. Further, in order to improve the lubricity of the poly-α-olefin oil, it may be a mixture of diester oil or aromatic ester oil, and the mixing amount in that case is 30% by mass or less based on the whole base oil. It is.

  The above base oils and thickeners were selected because the present inventors considered the usage environment and usage of the product and the compatibility with the resin composition for worm wheels applied in the present invention. As a result of intensive studies, we have decided.

  That is, as a result of repeated optimization studies on the hardness (consistency) of grease composed of the above components, it has been found that the most desirable performance is exhibited when the consistency is 200 to 300. Here, it is not preferable that the consistency is less than 200, because not only the steering feeling that is too hard and light is lost, but also the transmission efficiency at a low temperature is lowered to a level that cannot be overlooked. On the other hand, when the consistency exceeds 300, an oil film breakage is likely to occur, and the oil film breakage is not preferable, so that it is not possible to expect a gear noise suppression effect due to grease. A preferred consistency is 265-295.

  Various additives can be added to the grease as necessary. For example, amine-based and phenol-based antioxidants, rust preventives such as Ca sulfonate, extreme pressure additives such as MoDTC, montanic acid ester wax, montanic acid ester partly saponified wax, polyethylene wax, oleic acid and other oily improvers Of these, waxes are preferred.

  As described above, the grease used in the present invention contains a urea-based thickener and various additives in an appropriate amount to a poly-α-olefin oil having an appropriate kinematic viscosity, and is used for a reduction gear system of an electric power steering. To achieve the best grease performance, the consistency is adjusted so that it falls within the above range.

  In the present invention, spur gears, helical gears, bevel gears, hypoid gears and the like can be used as the types of the reduction gear 30 in addition to those shown in FIG. The resin part in which the gear teeth are formed is integrated.

  Examples The present invention will be further described below with reference to examples and comparative examples, but the present invention is not limited thereby.

(Examples 1-3, Comparative Examples 1-8)
Assuming a worm wheel material, a resin composition comprising a thermoplastic resin and a reinforcing material shown in Tables 1 and 2 was molded to produce a plate-shaped test piece. In Examples 1 to 3 and Comparative Examples 1, 2 and 6 to 8, various aramid fibers having different viscoelastic modulus characteristics and tensile elastic modulus were added to polyamide 66 (Ube Industries "UBE nylon 66"). The resin composition which mix | blended was used. In Comparative Example 3, glass fiber reinforced polyamide resin (30% glass fiber reinforced polyamide 66: Toray “Amilan CM3006G-30”) was used. In Comparative Example 4, MC nylon (Quadrant Polypenco Japan Co., Ltd. “MC901” was used. In Comparative Example 5, polyphenylene sulfide (DIC Co., Ltd. “PPS Z-200-E5”) was used.

In addition, a grease having a different consistency was prepared as indicated by using a poly-α-olefin oil of 80 mm ² / s (40 ° C.) as a base oil and a urea compound.

  And (1) Evaluation of the aggressiveness to a worm axis | shaft shown below, (2) Evaluation of the sound generated at the time of rotation, and (3) Durability evaluation were performed.

(1) Evaluation of aggressiveness to the worm shaft,
For the purpose of grasping the worm wheel's aggressiveness to the worm, a ball-on-disk test was conducted using each of the plate-like test pieces of Examples 1, 2 and Comparative Examples 1 to 5, and the metal ball after the test was completed The damage state of was evaluated. That is, in this test, grease was interposed between the plate-shaped test piece and the metal ball, the metal ball was rotated at an ambient temperature of 23 ° C., and the surface state of the metal ball was observed after a predetermined time.

  The results are shown in Table 1, but in Comparative Example 3 which is a glass fiber reinforcement, damage marks due to glass fibers were clearly recognized on the surface of the metal ball after the test was completed, whereas it was an aramid fiber reinforcement. In the examples, the opponent material was not attacked, and the surface condition of the metal ball after the test was extremely good. From this result, according to the worm wheel made of the material of the embodiment, unlike the conventional product using the glass fiber reinforcing material, the generation of unpleasant noise caused by the glass fiber sliding while attacking the worm during the steering operation is completely performed. Obviously, it can be prevented. In addition, since the plate-shaped test piece of the comparative examples 4 and 5 was only resin, the damage of metal balls was not seen.

(2) Evaluation of sound generated during rotation A S45C core tube having an outer diameter of 45 mm and a width of 13 mm, which has been subjected to cross knurling, is placed in a mold equipped with a sprue and a disk gate, and an injection molding machine is used. The molding compositions made of the resin compositions or resins of the examples and comparative examples are integrally molded to form a worm wheel blank material having an outer diameter of 60 mm and a width of 13 mm, and then the outer periphery of the resin portion is cut to form gear teeth. A wheel was produced.

  Then, this worm wheel and a metal worm are assembled in an aluminum case made by simulating a gearbox of an actual electric power steering device, filled with grease, and then the worm wheel is rotated at a constant speed. The sound generated when the reduction gear was rotating was evaluated. This evaluation was performed in an anechoic chamber, and the generated sound was measured with a microphone. A reduction gear of 5 dB or more with respect to the current reduction gear system was determined to be acceptable.

  The results are shown in Tables 1 and 2. In the examples in which the resin composition containing an aramid fiber having viscoelastic properties and tensile elastic modulus according to the present invention was used, and the consistency of the grease was 200 to 300, pass / fail judgment was made. Results satisfying the criteria were obtained. On the other hand, in Comparative Example 1 in which the loss tangent of the aramid fiber was less than 0.03 and Comparative Example 3 in which the glass fiber was blended, the pass / fail criterion was satisfied even when the grease consistency was in the range of 200 to 300. Not. Moreover, even if it uses the resin composition containing the aramid fiber which has the viscoelastic property and tensile elasticity modulus which concern on this invention, in the comparative examples 6-8 whose grease consistency is outside the range of 200-300, a pass / fail criterion The results satisfying the above were obtained. That is, in Comparative Example 6 in which the consistency of the grease is less than 200, the rotational torque is so large that it is difficult to rotate at a desired speed. Therefore, before Comparative Example 6 evaluates the generated sound, The increase was judged to be a level that would hinder actual steering, and was rejected. Further, in Comparative Examples 7 and 8 where the consistency of the grease exceeded 300, the effect of reducing the generated sound with respect to the current product was small, and in Comparative Example 8, it was worse than the current, and was rejected.

(3) Durability evaluation (2) Assembling the reduction gear in the same way as the evaluation of sound generated during rotation, controlling the ambient temperature to 80 ° C, conducting repeated durability tests of 200,000 cycles, damage (tooth cracks, etc.) In addition, a case where the wear was not more than a predetermined amount was determined to be acceptable.

  The results are shown in Tables 1 and 2. The worm wheel of the example did not show any cracks even after enduring 200,000 cycles, had little wear, and operated without problems. In contrast, Comparative Example 1 using an aramid fiber having a tensile modulus of elasticity exceeding 90 GPa, Comparative Example 2 using an aramid fiber having a loss tangent exceeding 0.03 but a tensile modulus of elasticity less than 20 GPa, and no reinforcing fiber In Comparative Examples 4 and 5, sufficient durability is not obtained. In Comparative Examples 4 and 5, since tooth root cracks were observed, the wear was not evaluated.

  FIG. 4 is a graph showing the amount of wear with respect to the number of cycles for the reduction gears of Examples 1 and 2 and Comparative Examples 1 and 2. As shown in the figure, the reduction gears of Examples 1 and 2 have low wear and excellent durability even after endurance of 200,000 cycles, but in Comparative Example 1, the amount of wear continues to increase after exceeding 100,000 cycles. , Exceeding the reference value in about 140,000 cycles. In Comparative Example 2, tooth root cracks are generated.

30 Reduction gear 31 Worm wheel 32 Worm 42 Core tube 43 Resin part 44 Gear teeth

Claims (3)

In the electric power steering device that transmits the auxiliary output by the electric motor to the steering mechanism of the vehicle via the reduction gear mechanism,
The reduction gear mechanism is made of a resin part made of a resin composition containing a thermoplastic resin and an aramid fiber having a loss tangent at 40 ° C. of 0.03 or more and a tensile elastic modulus of 20 to 90 GPa. Provided integrally with the outer periphery of the core tube, and provided with a reduction gear composed of a worm wheel formed with gear teeth on the outer peripheral surface of the resin part, and a metal worm meshing with the worm wheel,
Between the tooth part of the worm wheel and the tooth part of the metal worm, the base oil is poly-α-olefin oil, the thickener is a urea compound, and grease having a consistency of 200 to 300 is filled. An electric power steering device.   The thermoplastic resin is at least one selected from polyamide resin, polyphenylene sulfide resin and polyether ether ketone resin, and the aramid fiber is polyurethane resin, polyester resin, polyether resin, vinyl ester resin, epoxy resin, acrylic resin 2. The electric power steering apparatus according to claim 1, wherein the surface treatment is performed with at least one sizing agent selected from the group consisting of bismaleimide resin and bismaleimide resin. 3. The electric power steering apparatus according to claim 1, wherein the grease contains at least one selected from a montanate ester wax, a montanate ester partly saponified wax, and a polyethylene wax.

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