JP2014080991A - Reduction gear, and electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reduction gear capable of elongating a service life of a worm wheel by suppressing temperature rise of a tooth surface caused by friction by properly reducing a frictional coefficient in sliding between tooth surfaces by lubrication with an oil film, as a continuous oil film can be formed between the tooth surfaces of a worm and the worm wheel by surely preventing grease shortage only by applying a small amount of grease without complicating a structure, and to provide an electric power steering device having low steering torque and having high operation feeling.SOLUTION: A reduction gear 48 includes: a worm 11 and a worm wheel 12 engaged with each other; and a housing 5 accommodating the worm and the worm wheel, and is filled with grease having a viscosity of 0.3 Pa s or more and 2 Pa s or less at 100°C in a shear rate of 100 s. An electric power steering device includes: the reduction gear 48.

Description

  The present invention relates to a speed reducer and an electric power steering apparatus incorporating the speed reducer.

In general, in an electric power steering device, the rotation of the rotating shaft of an electric motor for assisting steering is reduced through a speed reducer, and the output is amplified and transmitted to a steering mechanism such as a rack and pinion mechanism. Torque assists the operation of the steering mechanism by the steering operation of the person.
As the speed reducer, a worm and a worm wheel that mesh with each other, a housing that accommodates the worm and the worm wheel, and a grease filled are generally used.

The filled grease forms an oil film interposed between the tooth surfaces of the worm and the worm wheel, thereby rotating the worm as the rotating shaft of the electric motor rotates and between the tooth surfaces due to the driven rotation of the worm wheel. It functions to assist sliding.
Therefore, by filling the grease, the steering torque of the electric power steering device can be reduced, and the driver's steering feeling can be improved.

As the grease, a grease whose characteristics such as viscosity are appropriately adjusted according to the application and use environment of the grease is used.
For example, as a grease for lubrication between the tooth surfaces of the worm and the worm wheel, such as a reduction gear of the electric power steering apparatus, the surface pressure applied between the tooth surfaces as the worm and the worm wheel rotate is resisted. In view of maintaining an oil film having a required thickness, it is general to use a hard material having a viscosity at 100 ° C. and a shear rate of 100 s ⁻¹ exceeding 2 Pa · s (for example, Patent Document 1). 2).

  However, since the conventional grease has low fluidity, it is easily removed from the meshing portion with the worm wheel by the worm and the sliding motion unique to the worm accompanying the rotation of the worm wheel. That is, once the grease is scraped out of the meshing portion by the sliding motion, the scraped grease remains around the meshing portion and does not automatically return to the meshing portion.

As a result, grease breakage occurs at the meshing portion, and the effect of assisting sliding between the tooth surfaces due to the oil film of the grease cannot be obtained. For example, in the case of an electric power steering device, the steering torque increases and friction The problem is that the tooth surface temperature rises and the worm wheel is damaged.
For this reason, it is common to fill grease not only in the meshing portion but also in almost the entire region of the housing, for example.

  In addition, a structure may be provided in the housing or the like for circulating the grease removed from the meshing portion with the rotation of the worm and the worm wheel and returning the grease to the meshing portion (for example, Patent Documents 3 to 5). Etc.).

JP-A-7-138857 Japanese Patent Laid-Open No. 9-169988 JP 2005-69476 A JP 2006-76443 A JP 2007-303487 A

  However, as described above, a large amount of grease is required to fill the entire area of the housing with grease, and such a large amount of grease is mixed into the rolling bearing incorporated in the speed reducer. This may affect the performance of the device and may cause problems such as leakage from the gaps in the housing and soiling the surroundings. There is also a problem that the amount of the grease increases, leading to an increase in cost of the speed reducer and the electric power steering device including the speed reducer.

Moreover, even if a large amount of grease is filled, if the grease is removed from the meshing part with the rotation, the conventional grease has low fluidity as described above. Therefore, the above-mentioned problem due to the lack of grease cannot be prevented.
On the other hand, when a mechanism for circulating the grease is provided in the housing or the like, the mechanism automatically returns the grease to the meshed portion of the worm and worm wheel as the worm and worm wheel rotate. Therefore, it is possible to reduce the total amount of grease filled in the housing and to prevent the above-mentioned problem due to the grease being exhausted.

However, when the mechanism is provided, the structure of the speed reducer, such as the housing, becomes complicated, and there is a problem that the cost of the speed reducer, the electric power steering device, etc. is increased.
The object of the present invention is to make it possible to form a continuous oil film between the tooth surfaces of the worm and the worm wheel without complicating the whole and reliably preventing the grease from running out by simply filling a small amount of grease. An object of the present invention is to provide a speed reducer that can satisfactorily reduce the friction coefficient of sliding between both tooth surfaces by lubrication with the oil film, suppress the temperature increase of the tooth surfaces due to friction, and extend the life of the worm wheel.

  It is another object of the present invention to provide an electric power steering apparatus having a steering torque as low as possible and excellent in operation feeling.

The invention according to claim 1 includes a worm (11) and a worm wheel (12) meshing with each other, and a housing (5) for housing the worm (11) and the worm wheel (12), and a shear rate of 100 ° C. The speed reducer (48) is characterized by being filled with grease having a viscosity at 100 s ⁻¹ of 0.3 Pa · s to 2 Pa · s.
The alphanumeric characters in the parentheses indicate corresponding components in the embodiments described later. The same shall apply hereinafter in this section.

  According to the configuration of the first aspect of the present invention, the relatively low-viscosity grease having the above-described viscosity is, for example, within a range that does not cause the outflow to the outside of the housing or the like when the speed reducer incorporated in the electric power steering apparatus is operated. Because of its high fluidity compared to the worm, even if the worm and the worm wheel are swept from the meshing part with the worm and the worm wheel by the sliding motion unique to the worm wheel, , Promptly flows into the meshing part with the rotation.

Therefore, without filling a large amount of grease almost entirely in the housing and without complicating the structure of the reduction gear, it is possible to prevent the occurrence of grease breakage in the meshing portion and to lubricate the meshing portion. It can always be filled with a sufficient amount of grease.
In addition, since a necessary amount of grease is always supplied to the meshing portion, a continuous oil film having a necessary thickness can be maintained against the surface pressure applied between the tooth surfaces as the worm and the worm wheel rotate. it can.

Therefore, the friction coefficient of sliding between the both tooth surfaces can be satisfactorily reduced by the lubrication by the oil film, the temperature increase of the tooth surfaces due to friction can be suppressed, and the life of the worm wheel can be extended.
In the present invention, the viscosity of the grease at 100 ° C. and a shear rate of 100 s ⁻¹ is represented by a value measured by a rotary rheometer (MCR301 manufactured by Anton Parr) using a cone plate. I will do it.

According to a second aspect of the present invention, there is provided an electric power steering apparatus including the speed reducer (48) according to the first or second aspect.
According to this configuration, by incorporating the speed reducer of the present invention, it is possible to provide an electric power steering apparatus that has a long service life, has a steering torque as low as possible, and is excellent in operation feeling.

  According to the present invention, it is possible to form a continuous oil film between the tooth surfaces of the worm and the worm wheel without complicating the whole, and reliably preventing the grease from running out by simply filling a small amount of grease. It is possible to provide a speed reducer that can satisfactorily reduce the friction coefficient of sliding between both tooth surfaces by lubrication with the oil film, suppress the temperature increase of the tooth surfaces due to friction, and extend the life of the worm wheel.

  Further, according to the present invention, it is possible to provide an electric power steering apparatus having a steering torque as low as possible and excellent in operation feeling.

1 is a schematic cross-sectional view of a reduction gear according to an embodiment of the present invention and a column type EPS as an electric power steering device incorporating the same. It is sectional drawing which follows the II-II line of FIG.

<Grease>
As described above, the grease filled in the speed reducer of the present invention needs to have a viscosity at 100 ° C. and a shear rate of 100 s ⁻¹ of 0.3 Pa · s to 2 Pa · s.
As described above, the hard grease whose viscosity exceeds the above range, once scraped from the meshing portion, stays around the meshing portion and does not automatically return to the meshing portion.

On the other hand, even if a sufficient amount of soft grease with a viscosity less than the above range is automatically supplied to the meshing part due to its own good fluidity, the housing can be damaged by vibration, centrifugal force due to rotation, or other external force. It tends to flow out and cannot maintain lubricity.
On the other hand, by selectively filling a grease having a viscosity within the above-mentioned range, which is moderately soft and easy to flow, as described above, the entire reduction gear is not complicated, and a small amount of grease is used. It is possible to reliably prevent the grease from running out by simply filling the worm and form a continuous oil film between the tooth surfaces of the worm and the worm wheel.

Therefore, the friction coefficient of sliding between the two tooth surfaces can be satisfactorily reduced by lubrication with the oil film, and it is possible to extend the life of the worm wheel by suppressing the temperature increase of the tooth surfaces due to friction.
The viscosity of the grease at 100 ° C. and a shear rate of 100 s ⁻¹ is preferably 0.5 Pa · s or more even within the above range in consideration of further improving the above effect, and 1.8 Pa · s. Hereinafter, it is particularly preferably 1.5 Pa · s or less.

A grease having a viscosity within the above range can be prepared by selecting a thickener composition to be used, adjusting the amount, adjusting the conductivity of the base oil, and the like.
Examples of the base oil on which the grease is based include synthetic hydrocarbon oils [for example, poly-α-olefin oil (PAO)], synthetic oils such as silicone oil, fluorine oil, ester oil, and ether oil, or mineral oil. 1 type, or 2 or more types. In particular, a synthetic hydrocarbon oil is preferable.

Examples of the thickener include one or two or more kinds of various thickeners such as a soap thickener, a urea thickener, an organic thickener, and an inorganic thickener.
Among these, the soap-type thickeners include, for example, metal soap type thickeners such as aluminum soap, calcium soap, lithium soap and sodium soap, mixed soap type thickeners such as lithium-calcium soap and sodium-calcium soap, aluminum One type or two or more types of complex type thickeners such as a complex, a calcium complex, a lithium complex, and a sodium complex are listed.

Examples of the urea thickener include polyurea and the like, and examples of the organic thickener include one or two of polytetrafluoroethylene (PTFE), sodium terephthalate, and the like. Furthermore, examples of the inorganic thickener include one or more of organic bentonite, graphite, silica gel, and the like.
For grease, solid lubricants such as fluororesin (PTFE, etc.), molybdenum disulfide, graphite, polyolefin wax (including amide, etc.), phosphorus-based and sulfur-based extreme pressure additives, tributylphenol as necessary One or more of antioxidants such as methylphenol, rust inhibitors, metal deactivators, viscosity index improvers, and oil agents may be added.

<Reduction gear and electric power steering device>
FIG. 1 is a schematic cross-sectional view of a speed reducer according to an embodiment of the present invention and a column type EPS as an electric power steering device incorporating the same. 2 is a cross-sectional view taken along the line II-II in FIG.
With reference to FIG. 1, in the electric power steering device of this example, a first steering shaft 2 as an input shaft to which a steering wheel 1 is attached and a second as an output shaft connected to a steering device 49. A steering shaft 3 is coaxially connected via a torsion bar 4.

  The housing 5 that supports the first and second steering shafts 2 and 3 is attached to a vehicle body (not shown). The housing 5 includes a sensor housing 6 and a gear housing 7 that are fitted together. Specifically, the gear housing 7 has a cylindrical shape, and an annular edge portion 7 a at the upper end thereof is fitted into an annular step portion 6 a on the outer periphery of the lower end of the sensor housing 6.

A worm gear mechanism 8 is accommodated in the gear housing 7, and a torque sensor 9, a control board 10, and the like are accommodated in the sensor housing 6. A reduction gear 48 is configured by housing the worm gear mechanism 8 in the gear housing 7.
The worm gear mechanism 8 is engaged with the worm wheel 12 that can rotate integrally with the intermediate portion of the second steering shaft 3 and is restricted from moving in the axial direction. The rotating shaft 32 is provided with a worm 11 (see FIG. 2) connected via a spline joint 33.

  Of these, the worm wheel 12 includes an annular cored bar 12a coupled to the second steering shaft 3 so as to be integrally rotatable, and a synthetic resin member 12b that surrounds the cored bar 12a and has teeth formed on the outer peripheral surface portion. And. For example, the cored bar 12a is inserted into the mold when the synthetic resin member 12b is molded. And the metal core 12a and the synthetic resin member 12b are combined and integrated by the resin molding in the inserted state. The second steering shaft 3 is rotatably supported by first and second rolling bearings 13 and 14 that are arranged with the worm wheel 12 sandwiched between the upper and lower sides in the axial direction.

  The outer ring 15 of the first rolling bearing 13 is fitted and held in a bearing holding hole 16 provided in the cylindrical projection 6 b at the lower end of the sensor housing 6. Further, the upper end surface of the outer ring 15 is in contact with the annular step portion 17, so that the movement of the outer ring 15 in the axial direction relative to the sensor housing 6 is restricted. On the other hand, the inner ring 18 of the first rolling bearing 13 is fitted to the second steering shaft 3 by an interference fit. The lower end surface of the inner ring 18 is in contact with the upper end surface of the cored bar 12 a of the worm wheel 12.

  The outer ring 19 of the second rolling bearing 14 is fitted and held in the bearing holding hole 20 of the gear housing 7. Further, the lower end surface of the outer ring 19 is in contact with the annular step portion 21, so that the downward movement in the axial direction with respect to the gear housing 7 is restricted. On the other hand, the inner ring 22 of the second rolling bearing 14 is attached to the second steering shaft 3 in a state in which the inner ring 22 can rotate integrally and the relative movement in the axial direction is restricted. Further, the inner ring 22 is sandwiched between a step portion 23 of the second steering shaft 3 and a nut 24 that is fastened to a screw portion of the second steering shaft 3.

  The torsion bar 4 passes through the first and second steering shafts 2 and 3. An upper end 4 a of the torsion bar 4 is connected to the first steering shaft 2 by a connecting pin 25 so as to be integrally rotatable, and a lower end 4 b is connected to the second steering shaft 3 by a connecting pin 26 so as to be integrally rotatable. As described above, the lower end of the second steering shaft 3 is connected to a steering device such as a rack and pinion mechanism via an intermediate shaft (not shown). The connecting pin 25 connects a third steering shaft 27 disposed coaxially with the first steering shaft 2 so as to be integrally rotatable with the first steering shaft 2. The third steering shaft 27 passes through the tube 28 constituting the steering column.

The upper portion of the first steering shaft 2 is rotatably supported by the sensor housing 6 via a third rolling bearing 29 made of, for example, a needle roller bearing. The reduced diameter portion 30 at the lower part of the first steering shaft 2 and the hole 31 at the upper part of the second steering shaft 3 are configured so as to restrict the relative rotation of the first and second steering shafts 2 and 3 within a predetermined range. They are fitted with a predetermined play in the rotational direction.
Referring to FIG. 2, worm 11 is rotatably supported by fourth and fifth rolling bearings 34 and 35 held by gear housing 7. Inner rings 36 and 37 of the fourth and fifth rolling bearings 34 and 35 are fitted into corresponding constricted portions of the worm 11. The outer rings 38 and 39 are respectively held in bearing holding holes 40 and 41 of the gear housing 7. The gear housing 7 includes a cylindrical portion 7 b that houses the worm 11.

  The outer ring 38 of the fourth rolling bearing 34 that supports the one end 11 a of the worm 11 is positioned in contact with the step 42 of the gear housing 7. On the other hand, the movement of the inner ring 36 toward the other end 11b is restricted by contacting the positioning step 43 of the worm 11. Further, the inner ring 37 of the fifth rolling bearing 35 that supports the vicinity of the other end portion 11 b (joint side end portion) of the worm 11 is brought into contact with the positioning step portion 44 of the worm 11, whereby one end portion 11 a. Movement to the side is restricted.

  The outer ring 39 is urged toward the fourth rolling bearing 34 by a preload adjusting screw member 45. The screw member 45 is screwed into a screw hole 46 formed in the gear housing 7 to apply a preload to the pair of rolling bearings 34 and 35 and to position the worm 11 in the axial direction. 47 is a lock nut engaged with the screw member 45 in order to fix the screw member 45 after the preload adjustment.

Referring to FIG. 1, the steering device 49 includes, for example, a pinion 50 provided on the second steering shaft 3 so as to be integrally rotatable via an intermediate shaft, and a rack that meshes with the pinion 50 and extends in the left-right direction of the vehicle. The rack and pinion mechanism includes a shaft 51.
The rack shaft 51 is supported so as to be linearly reciprocable in the axial direction. Tie rods 52 are coupled to both ends of the rack shaft 51, respectively. Each tie rod 52 is connected to a corresponding steering wheel 53 via a knuckle arm (not shown).

When the steering wheel 1 is operated and the first and second steering shafts 2 and 3 are rotated, the rotation is converted into a linear motion of the rack shaft 51 along the left-right direction of the automobile by the pinion 50 and the rack shaft 51. Thus, the steering wheel 53 is steered.
Referring to FIG. 2, the gear housing 7 includes a substantially cylindrical portion 7 c that is disposed with a space A at a constant interval between the outer periphery 12 c of the worm wheel 12.

The grease having a viscosity at 100 ° C. and a shear rate of 100 s ⁻¹ of 2 Pa · s or less is preferably selectively filled in the space A, the meshing portion B between the worm 11 and the worm wheel 12, or the like.
Accordingly, the rolling bearings 13 and 14 in which the excess grease is incorporated in the speed reducer 48 while keeping the effect of using the low-viscosity grease satisfactorily while minimizing the filling amount of the grease. , 34, 35, and the like, which can affect the performance of the speed reducer 48, or can cause problems such as leakage from the gaps in the housing 5 and contamination of the surroundings. Further, since the amount of grease can be reduced, the cost of the reduction gear 48 and the electric power steering apparatus including the reduction gear 48 can be reduced.

  In addition, this invention is not limited to embodiment of FIG. 1, FIG. For example, the configuration of the speed reducer of the present invention can be applied to a speed reducer for devices other than the electric power steering device. In addition, various modifications can be made within the scope of the matters described in the claims of the present invention.

<Example 1>
Using a synthetic hydrocarbon oil having a kinematic viscosity at 40 ° C. of 48 mm ² / S as a base oil, a grease containing 11% by mass of a Li complex as a thickener was prepared. The viscosity of the grease at 100 ° C. and a shear rate of 100 s ⁻¹ was measured with a rotary rheometer (MCR301 manufactured by Anton Parr) using a cone plate as described above. s.

<Example 2>
Using the same synthetic hydrocarbon oil as in Example 1 as the base oil, a grease containing 7% by mass of aliphatic diurea as a thickener was prepared. The viscosity of the grease at 100 ° C. and a shear rate of 100 s ⁻¹ was measured in the same manner as 0.4 Pa · s.
<Example 3>
Using the same synthetic hydrocarbon oil as in Example 1 as a base oil, a grease containing 9% by mass of aliphatic diurea as a thickener was prepared. The viscosity of the grease at 100 ° C. and a shear rate of 100 s ⁻¹ was measured in the same manner and found to be 0.7 Pa · s.

<Example 4>
Using the same synthetic hydrocarbon oil as in Example 1 as a base oil, a grease containing 15% by mass of aliphatic diurea as a thickener was prepared. The viscosity of the grease at 100 ° C. and a shear rate of 100 s ⁻¹ was measured in the same manner and found to be 1.4 Pa · s.
<Comparative example 1>
Using the same synthetic hydrocarbon oil as in Example 1 as the base oil, a grease containing 20% by mass of aliphatic diurea as a thickener was prepared. The viscosity of the grease at 100 ° C. and a shear rate of 100 s ⁻¹ was measured in the same manner to be 2.2 Pa · s.

<Comparative example 2>
Using a synthetic hydrocarbon oil having a kinematic viscosity at 40 ° C. of 30 mm ² / s as a base oil, a grease containing 28% by mass of Ba complex as a thickener was prepared. The viscosity of the grease at 100 ° C. and a shear rate of 100 s ⁻¹ was measured in the same manner as 2.3 Pa · s.

<Comparative Example 3>
Using the same synthetic hydrocarbon oil as in Example 1 as a base oil, a grease containing 4% by mass of aliphatic diurea as a thickener was prepared. The viscosity of the grease at 100 ° C. and a shear rate of 100 s ⁻¹ was measured in the same manner and found to be 0.25 Pa · s.
<Real machine test>
The grease prepared in each of the above Examples and Comparative Examples is a space A between the substantially cylindrical portion 7c of the gear housing 7 and the outer periphery 12c of the worm wheel 12 of the electric power steering apparatus shown in FIGS. 11 and the worm wheel 12 are filled in the meshing portion B, and the electric power steering apparatus is continuously driven.

Then, the temperature of the tooth surface was measured after a certain time had elapsed from the start of driving at room temperature, and the temperature increase of the tooth surface was evaluated.
Next, the driving was continued, and the time until the synthetic resin member 12b of the worm wheel 12 was damaged due to the temperature rise was measured to determine the lifetime of the worm wheel 12.
Further, it was confirmed whether or not the grease leaked from the gear housing 7 until the end of the service life.

  The results are shown in Table 1.

From the results of Examples 1 to 4 and Comparative Examples 1 to 3 in Table 1, the grease runs out by filling with grease having a viscosity at 100 ° C. and a shear rate of 100 s ⁻¹ of 0.3 Pa · s to 2 Pa · s. In this way, a continuous oil film can be formed between the tooth surfaces of the worm and the worm wheel, so that the friction coefficient of sliding between the both tooth surfaces can be reduced well by lubrication with the oil film, and the tooth surface due to friction can be reduced. It was found that the life of the worm wheel can be extended by suppressing the temperature rise.

Moreover, considering that the effect is further improved from the results of Examples 1 to 4, the viscosity of the grease at 100 ° C. and a shear rate of 100 s ⁻¹ is 0.5 Pa · s or more even within the above range. It was found that it was preferably 1.8 Pa · s or less, particularly 1.5 Pa · s or less.

  1: steering wheel, 2: first steering shaft, 3: second steering shaft, 4: torsion bar, 4a: upper end, 4b: lower end, 5: housing, 6: sensor housing, 6a: annular step, 6b: cylinder 7: gear housing, 7a: annular edge, 7b: cylindrical portion, 7c: substantially cylindrical portion, 8: worm gear mechanism, 9: torque sensor, 10: control board, 11: worm, 11a: one end, 11b: other end, 12: worm wheel, 12a: metal core, 12b: synthetic resin member, 12c: outer periphery, 13: rolling bearing, 14: rolling bearing, 15: outer ring, 16: bearing holding hole, 17: stepped portion , 18: inner ring, 19: outer ring, 20: bearing retaining hole, 21: stepped portion, 22: inner ring, 23: stepped portion, 24: nut, 25: connecting pin, 26: connecting pin, 27: steering shaft, 28: Tube 29: Rolling bearing, 0: reduced diameter portion, 31: hole, 32: rotating shaft, 33: spline joint, 34: rolling bearing, 35: rolling bearing, 36: inner ring, 37: inner ring, 38: outer ring, 39: outer ring, 40, 41: Bearing holding hole, 42: stepped portion, 43: stepped portion, 44: stepped portion, 45: screw member, 46: screw hole, 47: lock nut, 48: speed reducer, 49: steering device, 50: pinion, 51 : Rack shaft, 52: tie rod, 53: steering wheel, M: electric motor, A: space, B: meshing part

Claims (2)

A worm and a worm wheel that mesh with each other and a housing that accommodates the worm and the worm wheel, and filled with grease having a viscosity at 100 ° C. and a shear rate of 100 s ⁻¹ of 0.3 Pa · s to 2 Pa · s. A reduction gear characterized by that.   An electric power steering apparatus comprising the speed reducer according to claim 1.

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